Displays

Little is known about active volcanism in the remote regions of the global ocean. Here, we resort to long‐range acoustic measurements to study the July/August 2004 eruption at Isolde, a submarine volcanic cone in the Tristan da Cunha archipelago, South Atlantic Ocean. Underwater sound phases associated with the event were recorded as far as Cape Leeuwin, Western Australia, where a bottom-moored hydrophone array is operated as part of the International Monitoring System (IMS). IMS hydrophone data in combination with local seismic observations suggest that the center of activity is located east of Nightingale Island, where a recent seafloor mapping campaign aboard R/V Maria S Merian (MSM20/2) has revealed a previously unknown, potentially newly formed stratocone. Transmission loss modeling via the parabolic equation approach indicates that low-frequency sound phases travel at shallow depths near and within the Antarctic Circumpolar Current, thereby avoiding bathymetric interference along the 10,265 km source-receiver path. Our study highlights the potential of the IMS network for the detection and study of future eruptions both at Isolde and elsewhere. Implications for test-ban treaty monitoring and volcano early warning will be discussed.

How to cite: Metz, D., Grevemeyer, I., Jegen, M., Geissler, W., and Vergoz, J.: Hydroacoustic measurements of the 2004 submarine eruption near Nightingale Island, Tristan da Cunha , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12289, https://doi.org/10.5194/egusphere-egu2020-12289, 2020

The end-to-end calibration from the hydrophone ceramic element input to the digitizer output of CTBT IMS Hydroacoustic (HA) hydrophone stations is measured in a laboratory environment before deployment. After the hydrophones are deployed permanently with the Underwater System (UWS) hydrophone triplets, the response of the digitizer component can be measured by activating remotely a relay which excludes the hydrophone ceramic, preamplifier and riser cable, and feeds a pre-stored known waveform into the digitizer circuit via a digital-to-analogue converter. Analysis of these underwater calibration sequences makes it possible to verify the stability of the digitizer response over time and obtain useful information for investigations which require an accurate knowledge of the system response. Results are presented showing the stability of the UWS electronics response over time and one case, pertaining to the H10S triplet of HA10 Ascension Island, where changes in the calibration response appeared after the onset of electronic noise in one hydrophone channel with cross-talk to the other two channels.

How to cite: Zampolli, M., Haralabus, G., Stanley, J., and Nielsen, P.: Analysis of CTBT IMS Hydroacoustic hydrophone station underwater system electronics calibration sequences, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5481, https://doi.org/10.5194/egusphere-egu2020-5481, 2020

Hydroacoustic activity of the submarine Monowai Volcanic Centre (MVC) is repeatedly observed at two distant triplet hydrophone stations, south of Juan Fernandez Islands (H03S, 9,159km) and north of Ascension Island (H10N, 15,823km). T-phase converted energy recorded at the broadband seismic station Rarotonga on Cook Island (RAR, 1,845km) is used as a reference for the cross-correlation analysis. A detailed processing scheme for the calculation of the daily cross-correlation functions (CCF) of the hydroacoustic and seismic data is provided. Preprocessing is essential to account for the non-identical measurements and sensitivities as well as the different sample rates. Further postprocessing by systematic data selection has to be applied before stacking CCFs in order to account for the non-continuous activity of the MVC source. Daily volcanic activity is determined for the period from 2006 until 2018 using the signal-to-noise ratio of the CCFs assuming sound propagation in the SOFAR channel. Monthly stacked CCFs with clear volcanic activity are used to study seasonal variations in sound propagation between the MVC and the hydrophone stations. In winter, however, a faster than expected signal is observed at H10N which is hypothesized to (partial) propagation through the formed sea ice along the path near Antarctica.

How to cite: Smets, P., Weemstra, K., and Evers, L.: Long-range hydroacoustic observations of the Monowai Volcanic Centre as a proxy for seasonal variations in sound propagation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18440, https://doi.org/10.5194/egusphere-egu2020-18440, 2020

Underwater noise is recognised as a form of marine pollutant and there is evidence that over exposure to excessive levels of noise can have effects on the wellbeing of the marine ecosystem. Consequently, the variation in the ambient sound levels in the deep ocean has been the subject of a number of recent studies, with particular interest in the identification of long-term trends. We describe a statistical method for performing long-term trend analysis and uncertainty evaluation of the estimated trends from deep-ocean noise data. This study has been extended to include  measured data  from four monitoring stations located in the Indian (Cape Leeuwin & Diego Garcia), Pacific (Wake Island) and Southern Atlantic (Ascension Islands) Oceans over periods spanning between 8 to 15 years. The data were obtained from the hydro-acoustic monitoring stations of the Preparatory Commission for the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO). The monitoring stations provide information at a sampling frequency of 250 Hz, leading to very large datasets, and at acoustic frequencies up to 105 Hz.

The analysis method uses a flexible discrete model that incorporates terms that capture seasonal variations in the data together with a moving-average statistical model to describe the serial correlation of residual deviations. The trend analysis is applied to time series representing daily aggregated statistical levels for four frequency bands to obtain estimates for the change in sound pressure level (SPL) over the examined period with associated coverage intervals. The analysis demonstrates that there are statistically significant changes in the levels of deep-ocean noise over periods exceeding a decade. The main features of the approach include (a) using a functional model  with terms  that represent both long-term and seasonal behaviour of deep-ocean noise, (b) using a statistical model to capture the serial correlation of the residual deviations that are not explained by the functional model, (c) using daily aggregation intervals derived from 1-minute  sound pressure level averages, and (d) applying a non-parametric approach to validate the uncertainties of the trend estimates that avoids the need to make an assumption about the distribution of the residual deviations.

The obtained results show the long term trends vary differently at the four stations. It was observed that low frequency noise generally dominated the significant trends in these oceans. The relative differences between the various statistical levels are remarkably similar for all the frequency bands. Given the complexity of the acoustic environment, it is difficult to identify the main causes of these trends. Some possible explanations for the observed trends are discussed. It was however observed some stations are subjected to strong seasonal variation with a high degree of correlation with climatic factors such as sea surface temperature, Antarctic ice coverage and wind speed. The same seasonal effects is less pronounced in station located closer to the equator.

How to cite: Cheong, S.-H., Robinson, S. P., Harris, P. M., Wang, L. S., and Livina, V.: Long-term trend analysis of deep-ocean acoustic noise data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21819, https://doi.org/10.5194/egusphere-egu2020-21819, 2020

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS) is a world-wide network of stations and laboratories designed to detect nuclear explosions underground, in the oceans and in the atmosphere. The IMS incorporates four technologies: seismic, hydroacoustic and infrasound (collectively referred to as waveform technologies), and radionuclide (particulate and noble gas). The focus of this presentation is the hydroacoustic component of the IMS, which consists of 6 hydroacoustic stations employing hydrophones deployed in the oceans and 5 near-shore seismic stations, called T-phase stations, located on islands or continental coastal regions. The purpose of T-phase stations is to detect water-borne hydroacoustic pressure waves converted into seismic waves that propagate on the earth’s crust and are detected by land seismometers. However, the conversion process from in-water pressure to near-interface seismic waves is complex and strongly dependent on the properties of the local underwater and geological environment. To further understand this conversion process, state-of-the-art hybrid seismo-acoustic wave propagation models have been applied to simplified environments and to scenarios representative of the conditions encountered at IMS T-phase stations to compute broadband pressure time-series in the water and particle  velocity components on-land. Transfer functions from in-water pressure to on-land seismic particle velocity and vice versa were estimated both from modelling results and from real data acquired in locations where the hydrophones and (non-IMS) seismic stations were within 50-km distance. The presented results have been used to give a first assessment of the feasibility of characterizing the hydroacoustic phase of an in-water event by on-land seismic recordings at IMS T-phase stations, subject to limited a-priori environmental information and limiting factors, such as band-width and instrumental and/or environmental noise.

How to cite: Nielsen, P., Zampolli, M., Le Bras, R., Haralabus, G., Stevens, J., and Hanson, J.: Estimates of seismo-acoustic transfer functions relevant to CTBT IMS T-phase stations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4594, https://doi.org/10.5194/egusphere-egu2020-4594, 2020

The amplitude of ground motions caused by earthquakes and subsurface explosions generally decreases with distance from the epicenter. However, in the near-source region, other factors, e.g., near surface geology, topography, and the source radiation pattern, may significantly vary the amplitude of ground motions. Although source location and magnitude (or yield), can be rapidly determined using distant seismic stations, without a dense seismological network in the epicentral region, the ability to resolve such variations is limited.

Besides seismic waves, earthquakes and subsurface explosions generate infrasound, i.e., inaudible acoustic waves in the atmosphere. The mechanical ground motions from such sources, including the effects from the above mentioned factors, are encapsulated by the acoustic pressure perturbations over the source region. Due to the low frequency nature of infrasound and facilitated by waveguides in the atmosphere, such perturbations propagate over long ranges with limited attenuation and are detected at ground-based stations. In this work we demonstrate a method for resolving ground motions and the source mechanism from remotely detected infrasound. This is illustrated for the 2010 Mw 7.0 Port-au-Prince, Haiti earthquake, and the 6th and largest nuclear test conducted by the Democratic People's Republic of Korea in 2017.

Such observations are made possible by: (1) An advanced array processing technique that enables the detection of coherent wavefronts, even when amplitudes are below the noise level, and (2) A backprojection technique that maps infrasound detections in time to their origin on the Earth's surface.

Infrasound measurements are conducted globally for the verification of the Comprehensive Nuclear-Test-Ban Treaty and together with regional infrasound networks allow for an unprecedented global coverage. This makes infrasound as an earthquake disaster mitigation technique feasible for the first time and contributes to the Treaty's verification capacity.

How to cite: Shani-Kadmiel, S., Averbuch, G., Smets, P., Assink, J., and Evers, L.: Seismically induced ground motions and source mechanism passively retrieved from remote infrasound detections, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18451, https://doi.org/10.5194/egusphere-egu2020-18451, 2020

The IDC advances its methods and continuously improves its automatic system for the infrasound technology. The IDC focuses on enhancing the automatic system for the identification of valid signals and the optimization of the network detection threshold by identifying ways to refine signal characterization methodology and association criteria. Alongside these efforts, the IDC and its partners also focuses on expanding the capabilities in NDC-in-a-Box (NiaB), which is a software package specifically aimed at the CTBTO user community, the National Data Centres (NDC).

An objective of this study is to illustrate the latest efforts by IDC to increase trust in its products, while continuing its infrasound specific effort on reducing the number of associated infrasound arrivals that are rejected from the automatic bulletins when generating the reviewed event bulletins. A number of ongoing projects at the IDC will be presented, such as: - improving the detection accuracy at the station processing stage by introducing the infrasound signal detection and interactive review software DTK-(G)PMCC (Progressive Multi-Channel Correlation) and by evaluating the performances of detection software; - development of the new generation of automatic waveform network processing software NET-VISA to pursue a lower ratio of false alarms over GA (Global Association) and a path for revisiting the historical IRED. The IDC identified a number of areas for improvement of its infrasound system, those will be shortly introduced.

How to cite: Mialle, P.: IDC Infrasound technology path to continuous improvement, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17691, https://doi.org/10.5194/egusphere-egu2020-17691, 2020

The International Monitoring System (IMS) is in place for the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Part of the IMS are 60 infrasound arrays, of which 51 currently provide real-time infrasound recordings from around the world. Those arrays play a central role in the characterization of the global infrasonic wavefield and localization of infrasound sources.

Power Spectral Density (PSD) estimates give insight into the noise levels per station and array. The IMS global low and high noise model curves have been determined in a study by Brown et al. [2014] using a distribution of computed PSDs. All the IMS infrasound arrays, except IS23, have been included in the determination of the atmospheric ambient noise curves. IS23 is located at Kerguelen Island and exist of 15 elements that have been divided into five 100 meter aperture triplets arrays. The array is located at one of the noisiest locations in the world, due to the high wind conditions that exist year-round. The resulting high noise floor appears to hamper infrasound detection at this island array.

In this work, the effects of meteorological, oceanographic, and topographical conditions on the infrasound recordings at IS23 are studied. Five years of infrasound data is analyzed, as recorded by IS23, by using various processing techniques. Contributions within different frequency bands are evaluated. The infrasound detections are explained in terms of the stratospheric winds and ocean wave activity. Understanding and characterization of the low-frequency recordings of IS23 are of importance for successfully including this array for verification of the CTBT.

How to cite: den Ouden, O. F. C., Assink, J. D., Smets, P. S. M., and Evers, L. G.: A climatology of infrasound detections at Kerguelen Island, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7047, https://doi.org/10.5194/egusphere-egu2020-7047, 2020

Seismo Wave Company is ongoing improving metrological processes and quality surveys to guarantee the best Infrasound sensors technology. In accordance with our quality approach, a running-in step for infrasound sensors has been investigated and implemented. Once the metrology process is completed (acoustical and electrical calibration, self-noise measurement), objective is to keep monitoring on sensitivity of MB3a sensors during several days, using the in-situ electrical calibration capability. For this purpose, a new bench has been designed and characterized in our laboratory. Different sensitivity assessment methods have been compared. Testing conditions, bench design, methodology and results are laid out in this poster.

How to cite: Guilbert, E. and Hue, A.: Test bench development dedicated to microbarometers run-in, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21841, https://doi.org/10.5194/egusphere-egu2020-21841, 2020

The analysis of hydroacoustic signals originating from marine volcanic activity recorded by a remote hydroacoustic (HA) station, HA11 at Wake Island, of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS) is presented in this study. The events studied pertain to an eruption series at Kadovar Island, Papua New Guinea during the period January to February 2018. Local visual observations determined that the Kadovar volcano began to erupt at the summit of the island, and then created new vent spots near the coast. The events included the collapse of a lava dome on 9 February 2018. Directions-of-arrivals of the hydroacoustic signals detected at HA11 were evaluated using a cross-correlation technique, this allowed discrimination between hydroacoustics signals originating from the Kadovar volcanic activity and other numerous hydroacoustic signals generated by general seismic activity in the Pacific. Discrimination between volcanic activity and seismicity was achieved by examining the time-frequency characteristics of the hydroacoustic signals, i.e. associating short duration broadband bursts with volcanic eruptions, in line with criteria generally applied for such events. Episodes of high volcanic activity with as many as 80 detections per hour were identified on two occasions, separated by a one-month period of relative quiet. Some of the hydroacoustic signals were characterized by broadband frequency content and high received levels (i.e. ca. 30 dB higher than the ocean microseismic background). It was found that corresponding non-hydroacoustic signals could not be identified by other regional IMS stations, this providing an indication of the likely submarine origin of these events. Long duration bursts recorded on the day when the lava dome collapsed have been identified and characterized in time-frequency space. This study provides a further example of the added value of CTBT IMS hydroacoustic station remote monitoring of marine volcanic events.

How to cite: Matsumoto, H., Zampolli, M., Haralabus, G., Stanley, J., Robertson, J., and Meral Özel, N.: Study of a high activity eruption sequence of Kadovar volcano, Papua New Guinea, using data recorded by the CTBT International Monitoring System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6415, https://doi.org/10.5194/egusphere-egu2020-6415, 2020

Fireballs are very bright meteors with magnitudes of at least -4. They can spark a lot of public interest. Especially, if they can be seen during daytime over populous areas. Social Media allows us to be informed about almost everything, worldwide, and in all areas of life in real-time. In the age of intensive use of these media, information is freely available seconds after the sighting of a fireball.

This is the basis of the alert system which is part of NEMO, the NEar real-time MOnitoring system, for bright fireballs. It uses Social Media, mainly Twitter, to be informed about a fireball event in near real-time. In addition, the system accesses various data sources to collect further information about the detected fireballs. The sources range from meteor networks, the data from weather satellites or lightning detectors to the infrasound data of the IMS (International Monitoring System) operated by the CTBTO (Comprehensive Nuclear-Test-Ban Treaty Organisation).

Since large meteoroids or asteroids can be detected by these infrasound sensors when they enter the Earth's atmosphere, this network provides the possibility to detect fireballs worldwide and during day and night. From the infrasound data the energy of the object that caused the fireball can be determined and hence, its size and mass can be calculated. By combining all available information about the fireball from different data sources the amount of scientific knowledge about the event can be maximized.

NEMO was under development for about 2.5 years. Since the beginning of the year the system is in operation at the European Space Agency, as part of its Space Safety Programme. In this presentation we will give an overview about NEMO, its working principle and its relation to the IMS.

How to cite: Ott, T., Drolshagen, E., Koschny, D., Drolshagen, G., Pilger, C., Mialle, P., Vaubaillon, J., and Poppe, B.: NEMO - The NEar real-time MOnitoring system for bright fireballs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7367, https://doi.org/10.5194/egusphere-egu2020-7367, 2020

International Monitoring System (IMS) is designed to detect and locate nuclear test explosions as part of Comprehensive Nuclear Test-Ban Treaty (CTBT) verification regime. This network can be also used for civil applications, such as the remote monitoring of volcanic activity.

Events related to volcanic eruptions, which are listed in the International Data Centre (IDC) bulletins, are typically detected by infrasound stations of the IMS network. Infrasound station IS44 and primary seismic station PS36 are situated in Kamchatka, Russian Federation, in the vicinity of several active volcanoes. These two stations recorded seismo-acoustic events generated by volcanic eruptions. In addition to atmospheric events, the IMS network has the potential of detecting underwater volcanic activity. Under favourable conditions, the hydroacoustic stations located in the Pacific Ocean and PS36 may detect underwater events close to the shore of Kamchatka Peninsula.

The aim of this presentation is to show examples of volcanic eruptions at Kamchatka Peninsula recorded by the IMS network. Supplementary information obtained by other observing networks can be found in reports issued by Kamchatkan Volcanic Eruption Response Team (KVERT) or Tokyo Volcanic Ash Advisory Center (VAAC). Such information can be compared with events listed in IDC bulletins.

How to cite: Bittner, P., Gore, J., Applbaum, D., Jimenez, A., Villarroel, M., and Mialle, P.: IDC events related to volcanic activity at Kamchatka Peninsula , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9926, https://doi.org/10.5194/egusphere-egu2020-9926, 2020

The key objectives of a ground-based geophysical mapping during an On-site Inspection are to detect, locate and characterize the zones of rock damage associated with an underground nuclear explosion (UNE). The cavity, rubble zone and fracturized rock matrix are also common features in the close vicinity of a cave of karstic origin. The natural cavities are mainly developed within the weakest zones of the rock matrix. The connatural features with an UNE are important but the thermal and pressure effects are lacking in the case of natural origin. However, the similarities may justify the efforts to investigate the cavern and its surroundings by geophysical methods.

The oval shaped cavern with a diameter of 28 m located 70 m below the surface was discovered within a clay mine in N-Hungary. The deep basement is composed of Triassic limestone, the cavern is located in the overlying Oligocene sandstone formation. As a result of hydrothermal activity in the Pleistocene a cave formed in the limestone which may have collapsed over time. The opening of the deep part of the cave influenced the overlying sandstone formation but the collapse did not reach the surface.

As a consequence of a UNE the cracks and open fissures could provide a pathway for the radioactive gas to find its way near to the surface. The detection of these fracturized zones require the highest possible resolution of the seismic imaging of the subsurface. Therefore, we made a 2D survey above the cavern site and determined that the optimal method is to generate and detect horizontally polarized (SH) waves. The electro-mechanically driven vibrator has provided a bandwidth ranging from 5 to 200 Hz which can be extended up to 400 Hz. The use of the Lightning type vibrator has broadened the seismic bandwidth achieving the maximum penetration of 250 m with substantial increase of the resolution.

The joint interpretation of the seismic and geoelectric tomographic results with the SH- wave reflection section has provided a clear pattern of the tectonized rock matrix around the cavern.

How to cite: Labak, P., Kovacs, A., and Hegedus, E.: S-wave reflection imaging of a tectonically determined cavern by use of next generation electro-mechanic vibrator, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13316, https://doi.org/10.5194/egusphere-egu2020-13316, 2020

Nuclear explosions are banned by the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Obviously, the CTBT needs robust and comprehensive verification tools to make sure that no nuclear explosion goes undetected. The detection of underground cavity due to nuclear explosions is a primary task for an on-site inspection (OSI) and resonance seismometry. Recently we have developed the finite-frequency-range spectral-power method that makes it possible to use seismic ambient noise recorded at the free surface above an underground cavity for localizing it. In this contribution we present results of application of the method to data recorded at a site of the Great Cavern near Felsopeteny, Hungary.

CTBTO performed several active and passive seismic measurements at the free surface above the Great Cavern in September 2019. Seismic ambient noise was recorded one week continuously at almost 50 stations with interstation distance around 50 m covering area 400 x 400 m.

The oval shaped cavern with a diameter of 28 m located 70 m below the surface was discovered within a clay mine in N-Hungary. The deep basement is composed of Triassic limestone, the cavern is in the overlying Oligocene sandstone formation. As a result of hydrothermal activity in the Pleistocene a cave formed in the limestone which may have collapsed over time. The opening of the deep part of the cave influenced the overlying sandstone formation but the collapse did not reach the surface.

We present the procedure of pre-processing and identification of a position of the cavern based on the recorded seismic ambient noise. We checked robustness of the obtained results. The results demonstrate potential of our methodology for the OSI purposes.

How to cite: Kristekova, M., Kristek, J., Moczo, P., and Labak, P.: Detection of the deep cavern at the Felsopeteny, Hungary site using seismic ambient noise data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13214, https://doi.org/10.5194/egusphere-egu2020-13214, 2020

Characterization of seismic events as underground nuclear explosions is a challenging task. Geophysical methods such as seismic monitoring systems are used by CTBTO to link post-explosion phenomena to potential sources. The main challenges in seismic monitoring involve accurately locating of sources and separating underground variations in seismic properties due to the explosion from naturally occurring variations. Underground detonations result in an immense change in pressure and temperature concentrated around the source origin. This results in the formation of characteristic static and dynamic phenomena. This study highlights the potential of using time-lapse seismic to identify ground zero by monitoring post-explosion dynamic phenomena. Time-lapse seismic, also known as 4D seismic, is successfully employed in the oil and gas industry for petroleum production monitoring and management. It involves taking more than one 2D/3D seismic at different calendar times over the same reservoir and studying the difference in seismic attributes.

Following an underground explosion, dynamic changes in rock and fluid properties are observable for a prolonged period, even up to several decades. This is prominent near to source origin, and it is a result of the redistribution of residual energy, such as pressure, temperature, and saturation. Frequent seismic monitoring surveys (time-lapse seismic) enables one to monitor the changes in rock and fluid properties. This study presents the characteristics of the time-lapse seismic signature observed in a heterogeneous medium (or heterogeneous cavity). We will look into the impact of factors affecting land 4D repeatability on the 4D signature. The significance of identifying the 4D signature related to the explosion in a seismic section, and the feasibility of detecting it during the OSI with resource and time constraints in place will be discussed. We present a fast detection method using machine learning for the detection of explosion related time-lapse signature, which could be an identifier of the source location or ground zero.

Acknowledgments: Authors would like to thank EPSRC and AWE for funding this project.

How to cite: Mathew, S., MacBeth, C., Mangriotis, M.-D., and Stevanovic, J.: Detecting explosion-induced dynamic phenomena using time-lapse seismic surveying, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-839, https://doi.org/10.5194/egusphere-egu2020-839, 2019

In order to monitor nuclear tests on a global scale, it is one of the IMS’s fundamental tasks to maintain a network of seismic stations with high data reliability. Installation is often the most critical aspect of a successful seismic station. A poorly designed layout leads to the introduction of noise that can hugely impact data quality, therefore rendering expensive, high performance equipment inadequate.

To facilitate quality installations and encourage better practices in the global seismic monitoring community, Güralp has developed a system that will allow for observatory-grade data.

The system is a classic seismic station composed of an ultra-low noise broadband seismometer: a Güralp 3T (120s or 360s), a high-performance digitizer-datalogger: the Güralp Affinity, a sensor cable and an atmospheric pressure enclosure.

The custom-built pressure enclosure enhances performance in vault installations, protecting the sensor from minuscule fluctuations in temperature and pressure, hence considerably reducing noise levels.

Güralp also provide best practice guidelines to assist researchers in designing their station, from site selection, installation and training to data retrieval and analysis.

Since 2001, Güralp have been managing the Eskdalemuir seismic array (EKA), the United Kingdom’s auxiliary station for the International Monitoring System. These years of experience with CTBT related monitoring have taught us that good results do not come from the instrument alone. This is why Güralp endeavors to accompany operators through every step of the process with a team of specialist engineers, applying 35 years of expertise from project conception to data retrieval.

How to cite: Filippi, S., Mohr, S., Balon, M., Hill, P., Watkiss, N., and Goessen, S.: Güralp recommendations for the installation of high-performance, observatory-grade seismic stations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21345, https://doi.org/10.5194/egusphere-egu2020-21345, 2020

The Italian National Institute for Oceanography and Experimental Geophysics (Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS) in Trieste (Italy) is offering, in agreement with the Italian CTBTO National Authority, its Cludinico (CLUD) seismic station as a Cooperating National Facility (CNF) to the CTBTO. As outlined in Pesaresi and Horn (2015) the additional data from the Italian CNF improve the CTBTO location capabilities in the Europe/Middle East area of about 21%, which might be of interest given the actual situation in Iran that breached the nuclear Joint Comprehensive Plan of Action (JCPOA) (Reuters, 2019).

In this presentation, we will illustrate the technical details of the solutions adopted to incorporate the Italian CNF into the CTBTO International Monitoring System (IMS): evaluation of CTBTO certification readiness, CTBTO Standard Station Interface (SSI) hardware and software procurement, test and installation, UPS upgrade, implementation and test of CTBTO communication, security measures.

Reference:

Pesaresi, D., and Horn, N.: Improving CTBTO monitoring capabilities: the Italian proposal for a CNF, CTBT Science and Technology 2015, Vienna, Austria, 22-26 June 2015, T4.1-P31, doi:10.13140/RG.2.1.2862.1927, 2015.

Reuters: "Iran further breaches nuclear deal, says it can exceed 20% enrichment", https://www.reuters.com/article/us-iran-nuclear-idUSKCN1VS05B, last access: 14 January 2020, 2019.

How to cite: Pesaresi, D., Bertoni, M., Del Negro, E., Parolai, S., and Comelli, P.: The Italian CTBTO Cooperating National Facility (CNF): status of the art, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7776, https://doi.org/10.5194/egusphere-egu2020-7776, 2020

The application of airborne remote sensing techniques permitted by the Comprehensive Nuclear‑Test‑Ban Treaty (magnetic and gamma survey as well as optical imaging including infrared measurements) is done through the prism of inspection team functionality – a logic which applies equally to air and ground-based techniques. Work undertaken over recent years through modelling and practical testing has aimed to better understand the ability of airborne remote sensing techniques to detect relevant observables under different conditions. This has led to the compilation of a concept of operations document that provides guidance on the application of inspection activities during an On-Site Inspection. As well as highlighting the relative merits of each technique, the document also addresses the relative likelihood a particular airborne technique will return relevant information and will avoid the commitment of resources to missions with little likelihood of success.

The paper also addresses the approaches which have been taken to streamline the acquisition of airborne remotely sensed data through bespoke installations, the identification of optimal data processing routines to facilitate the production of reports and the fusion of airborne data products with other data gathered during an inspection.

How to cite: Rowlands, A., Labak, P., Chiappini, M., Gaya-Pique, L., Buckle, J., and Seywerd, H.: The application of airborne remote sensing during an On-Site Inspection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20511, https://doi.org/10.5194/egusphere-egu2020-20511, 2020

The detection of anomalous concentration of Xenon radiosotopes in the subsurface gases during an On Site Inspection (OSI) is a strong indicator of a suspicious underground nuclear explosion. This implies that the sampling methodology ensure the collection of a reliable representative subsurface gaseous sample, avoiding the mixing with atmospheric gases. Radioxenon sampling in shallow layers can provide reliable results for desert areas, but different local geological features could result in more complex migration of subsurface gases to the very near superficial layers affecting the representativeness of the sample.

Radon is currently use as tracer to reveal the effective sampling of gases form the deep surface, so its measurement is coupled with the collection of radioxenon subsurface gases. The detection of radon anomalous concentration in subsurface gases could indicate different causes: high Radon content in subsurface indicate high radon concentration underground caused by the accumulation in an underground and confined cavity; on the other side, low radon detection in subsurface indicate low radon concentration underground that can be indicative of the absence of an underground cavity or the presence of rocks in the cavity absorbing radon. This lead to the consideration that radon is not a univocal tracer for Xe surface sampling in the OSI. A portable isotopic analyzer (that measures d13C and CO2) could be used to localize the faults and fracturing that could lead to a seeping of the subsurface gases. Therefore, this technique could be proposed as an auxiliary equipment for a preliminary activity during an OSI and a monitoring tool during subsurface gas sampling.

How to cite: Telloli, C., Ferrucci, B., Rizzo, A., Salvi, S., Ubaldini, A., and Vaccaro, C.: Radon and CO2 tracer for radioxenon subsurface sampling in the On Site Inspection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21039, https://doi.org/10.5194/egusphere-egu2020-21039, 2020

An on-site inspection (OSI) is conducted to clarify whether a nuclear weapon test explosion or any other nuclear explosion has been carried out in violation of the Treaty. The conduct of inspection activities requires an approach that takes into account the operational, technical and time constraints specified in the Treaty. A systematic approach was developed, namely, the information-led search logic for the inspection team (IT) to function effectively. The core of the search logic is inspection data acquired. The realization of the search logic is the Inspection Team Functionality (ITF) which its essential element is having the most updated inspection data readily available to inspectors to facilitate the planning, processing and reporting.

To facilitate the work of an IT, the Provisional Technical Secretariat launched a project to develop a map centric tool to support the IT. The Geospatial Information Management system for On-site inspections (GIMO), supports decision-making and facilitates the progress of an inspection and not hinder it in anyway. At its core is the facilitation of the ITF concept and chain of custody of samples and electronic media. It is a single tool for planning inspection activities, managing data collection in the field, integration of data generated by the implementation of OSI techniques and reporting. Information security, chain of custody and confidentiality requirements are applied in GIMO following the need-to-know principle. GIMO, 3D geospatially centric software, has no software dependencies outside the internal local area network as required by the Treaty. The modular nature of GIMO means that additional functionality can be embedded as and when needed.

How to cite: Haquin Gerade, G., Labak, P., Rowlands, A., Steric, N., Shabelnyk, O., Ahlander, M., and Lobo, A.: GIMO – a new geospatial tool for On-site Inspection data collection and techniques integration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21517, https://doi.org/10.5194/egusphere-egu2020-21517, 2020

On-site inspection (OSI) is an element of the verification regime of the Comprehensive Nuclear Test Ban Treaty (CTBT), with the sole purpose to clarify whether a nuclear weapon test explosion or any other nuclear explosion has been carried out in violation of the Treaty. An OSI could be launched in any environment where a triggering event occurred. A challenging environment may affect not only the signatures and observables of a nuclear explosion, but also the possibility to conduct the OSI. Harsh environmental conditions, such as extreme climate conditions, high vegetation coverage and complicated topographic characteristics, among others, could slow down the deployment of field missions, and affect the state-of-health of OSI equipment and even the performance of inspectors, thereby compromising the whole inspection. Thus, the operationalization of OSI in different environments is an important aspect in the development of OSI capability. In this respect, well defined OSI environment is an important step towards the development of comprehensive OSI capabilities. Based on the analysis of historical underground nuclear explosions data and knowledge on the environmental impact on observables, equipment and inspectors, a definition of OSI environment was developed. Climatic conditions were grouped into the main five groups of the Köppen-Geiger classification scheme. Vegetation coverage was re-grouped in four of the 16 classes of land coverage (not including water bodies) following the International Geosphere-Biosphere Programme. Complicated landforms grouped in topographic classification using a digital elevation model based on slope gradient, surface texture and local convexity within neighboring cells was used to classify topographic relief of four types of landforms for OSI. In this presentation, it is shown how these key environmental aspects will impact the conduct of an OSI.

How to cite: Kuang, F. and Haquin Gerade, G.: Key Aspects for the Definition of On-site Inspection Challenging Environments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21601, https://doi.org/10.5194/egusphere-egu2020-21601, 2020

For detection of non-compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT) the global International Monitoring System (IMS) is being built up and nearly complete. The IMS is designed to detect and identify nuclear explosions through their seismic, hydroacoustic, infrasound, and radionuclide signature. The IMS data are collected, processed to analysis products, and distributed to the signatory states by the International Data Centre (IDC) in Vienna. The member states themselves may operate National Data Centers (NDC) giving technical advice concerning CTBT verification to their government. NDC Preparedness Exercises (NPE) are regularly performed to practice the verification procedures for the detection of nuclear explosions in the framework of CTBT monitoring. The NPE 2019 scenario was developed in close cooperation between the Italian NDC-RN (ENEA) and the German NDC (BGR). The fictitious state RAETIA announced a reactor incident with release of unspecified radionuclides into the atmosphere. Simulated concentrations of particulate and noble gas isotopes at IMS stations were given to the participants. The task was to check the consistency with the announcement and to serach for waveform events in the potential source region of the radioisotopes. In a next step, the fictitious neighbour state EASTRIA provided further national (synthetic) measurements and requested assistance from IDC with so called Expert Technical Analysis (ETA) about the origin of those traces. The presentation shows aspects of scenario design, event selection, and forward amospheric transport modelling as well as radionuclide and seismological analyses.   

How to cite: Ross, J. O., Gestermann, N., Gaebler, P., and Ceranna, L.: The National Data Centre Preparedness Exercise NPE 2019 - Scenario design and expert technical analyses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16106, https://doi.org/10.5194/egusphere-egu2020-16106, 2020

The International Data Centre (IDC) of the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO) investigates the best method to add the utilisation of High-Resolution Atmospheric Transport Modelling (HRATM) in their operational and automatised pipeline. Supporting the decision process, the IDC accomplished a comparison study with different approaches for applying HRATM. An initial validation study with the HRATM Flexpart-WRF, which is a Lagrangian particle dispersion model (LPDM), showed a performance which is dependent on the scenario and delivered results comparable to the conventional Flexpart model. The approach uses the Weather Research and Forecasting model (WRF) to generate high-resolution meteorological input data for Flexpart-WRF and WRF was driven by the National Centers for Environmental Prediction (NCEP) data having a horizontal resolution of 0.5 degrees and time resolution of 1h. Based on this initial study, an extended study was conducted to compare the results to FLEXPART-WRF using input data from the European Centre for Medium-Range Weather Forecasts  (ECMWF) for WRF and to results from the conventional Flexpart model using high-resolution ECMWF input data. Furthermore, a sensitivity study was performed to optimize the physical and computational parameters of WRF to test possible meteorological improvements prior to the comparison study.

The performance of the different approaches is evaluated by using observational data and includes statistical metrics which were established during the first ATM challenge in 2016. Observational data of seven episodes of elevated Xe-133 concentrations were selected from the IMS (International Monitoring System) noble gas system DEX33 located in Germany. Each episode consists of 6 to 11 subsequent samples with each sample being taken over 24 hours. Both Flexpart models were using the source terms from a medical isotope production facility in Belgium to simulate the resulting concentration time series at the DEX33 station for different output resolutions. Backward simulations for each sample were conducted, and in the case of Flexpart-WRF nested input of increased resolution around the source and receptor was used.

The simulated concentrations, as well as the measurements, are also compared to the simulated results produced by the conventional Flexpart model to guide the decision-making process.

How to cite: Philipp, A., Schoeppner, M., Kusmierczyk-Michulec, J., Bourgouin, P., and Kalinowski, M.: Comparison study for different approaches in applying High-Resolution Atmospheric Transport Modelling based on validation with Xe-133 observations in Europe , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7891, https://doi.org/10.5194/egusphere-egu2020-7891, 2020

Global radioactivity monitoring for the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) includes the four xenon isotopes 131mXe, 133Xe, 133mXe and 135Xe. These four isotopes are serving as important indicators of nuclear explosions. The state-of-the-art radioxenon emission inventory uses generic release estimates for each known nuclear facility. However, the release amount can vary by several orders of magnitude from year to year. The year 2014 was selected for a single-year radioxenon emission inventory that avoids this uncertainty. Whenever 2014 emissions reported by the facility operator are available these are incorporated into the 2014 emission inventory. This presentation summarizes this new emission inventory. The overall emissions by facility type are compared with previous studies. The global radioxenon emission inventory for 2014 can be used for studies to estimate the contribution of this anthropogenic source to the observed ambient concentrations at IMS noble gas sensors to support CTBT monitoring activities, including calibration and performance assessment of the verification system as described in the Treaty as well as developing and validating methods for enhanced detection capabilities of signals that may indicate a nuclear test. One specific application will be the third ATM Challenge that was announced in December 2019.

How to cite: Kalinowski, M.: Global radioxenon emission inventory for 2014, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2442, https://doi.org/10.5194/egusphere-egu2020-2442, 2020

"Precision seismology'' encompasses a set of methods which use differential measurements of time-delays to estimate the relative locations of earthquakes and explosions.  Delay-times estimated from signal correlations often allow far more accurate estimates of one event location relative to another than is possible using classical hypocenter determination techniques.  Many different algorithms and software implementations have been developed and different assumptions and procedures can often result in significant variability between different relative event location estimates.  We present a Ground Truth (GT) database of 55 military surface explosions in northern Finland in 2007 that all took place within 300 meters of each other.  The explosions were recorded with a high signal-to-noise ratio to distances of about 2 degrees, and the exceptional waveform similarity between the signals from the different explosions allows for accurate correlation-based time-delay measurements.  With exact coordinates for the explosions, we can assess the fidelity of relative location estimates made using any location algorithm or implementation.  Applying double-difference calculations using two different 1-d velocity models for the region results in hypocenter-to-hypocenter distances which are too short and the wavefield leaving the source region is more complicated than predicted by the models.  Using the GT event coordinates, we can measure the slowness vectors associated with each outgoing ray from the source region. We demonstrate that, had such corrections been available, a significant improvement in the relative location estimates would have resulted.  In practice we would of course need to solve for event hypocenters and slowness corrections simultaneously, and significant work will be needed to upgrade relative location algorithms to accommodate uncertainty in the form of the outgoing wavefield.  We present this dataset, together with GT coordinates, raw waveforms for all events on six regional stations, and tables of time-delay measurements, as a reference benchmark by which relative location algorithms and software can be evaluated.

How to cite: Kvaerna, T., Gibbons, S. J., Tiira, T., and Kozlovskaya, E.: A Seismic Event Relative Location Benchmark Case Study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7504, https://doi.org/10.5194/egusphere-egu2020-7504, 2020

We apply a hybrid method that couples global Instaseis databases (van Driel et al., 2015) with a local finite-difference code WPP (Nilsson et al., 2007) to study the 1960s-1980s nuclear explosions located at the USSR Degelen mountain test site. Observed teleseismic P waves (up to 2 Hz) display strong near-source signatures, yet the relative importance of contributing factors – such as explosion depth and yield, scattering from near-source topography and geological heterogeneities, as well as non-linear effects – are not well understood. An analysis of teleseismic waveforms suggest that these features are dependent on the source location within the Degelen mountain range, while depths and yields do not show a consistent effect. We therefore propose that the change in signal characteristics on teleseismic waveforms is related to the mountainous topography in the source region and we turn to deterministic hybrid modelling to test the effect of Degelen topography at teleseismic distances. Despite simplistic modelling assumptions, we achieve an excellent fit with the observed waveforms. Amplitudes are in good agreement and many observed features are reproduced by synthetic seismograms at 2 Hz, highlighting the importance of near-source 3-D effects on long-range wave propagation. Hybrid modelling of more realistic high-frequency scenarios could ultimately lead to waveform-based constraints on explosion locations, for example via grid-search methods or more advanced learning algorithms, or even improve nuclear discrimination methods.

How to cite: Pienkowska, M., Nippress, S., Nissen-Meyer, T., and Bowers, D.: High-frequency hybrid modelling of near-source topographic effects at teleseismic distances: The Degelen mountain case study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9878, https://doi.org/10.5194/egusphere-egu2020-9878, 2020

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) calls for a verification regime which involves interactions between the International Data Centre (IDC) component of the Provisional Technical Secretariat (PTS) established in Vienna, Austria, and the National Data Centres (NDC) of Member States of the Treaty. The results of location estimates of the same event by the two organizations is obtained using similar methods and software but potentially involve different seismo-acoustic networks and therefore a direct comparison of the distances and time differences is not sufficient and the different error estimates for the event should be taken into account. Most methods of location are using iterative linear inversions and the probability distributions are Gaussian, using the covariance matrix resulting from the last step of the iterative inversion process as the parameters of the Gaussian distributions. We explored the statistical tools available to compare two multi-dimensional distributions and measure a distance between them in an objective manner, including the Hellinger distance, the Bhattacharyya distance, and the Mahalanobis distance and we will show examples of application to the seismo-acoustic location problem. 

How to cite: Le Bras, R. and Qorbani, E.: Comparing two location estimates of the same seismo-acoustic event. A survey of available statistical tools., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3340, https://doi.org/10.5194/egusphere-egu2020-3340, 2020

The Source Physics Experiments (SPE) provided new insights into explosion phenomenology. In particular, the data reveal a mechanism for generating shear energy in the near-source region which may explain why certain North Korean declared nuclear tests do not conform to explosion/earthquake discriminants based on relative body wave (m_b) and shear wave (M_S) magnitudes.

The SPE chemical explosive detonations in granite included three scaled depth of burial (SDOB) categories: 1) nominally buried defines the burial depths from which m_b:M_S discriminants were derived; 2) deeply overburied, or Green’s function depth; and 3) moderately overburied, or between the two end cases above. This last category is a general descriptor for the North Korean declared nuclear tests which fail the m_b:M_S discriminant.

Near-source three-axis borehole accelerometers indicate that the nominal and deeply buried SPE experiments created the expected spherical shock environment dominated by radial ground motion with insignificant tangential response.

The moderately overburied SPE experiments indicate a significant contrast. The tangential records in these experiments are quiescent with initial shock arrival and then exhibit a sudden, significant surge immediately following the peak radial component. At distant ranges where the shock wave amplitude has attenuated the environment becomes more consistent with a spherical shock with no significant tangential components.

We interpret a “shear release” mechanism on an obliquely loaded rock joint:

1. During incipient loading the normal shock component forces closure of the joint.
2. In cases of low explosive loading and/or high in situ stress the tangential component is insufficient to cause joint sliding and this load is stored as shearing strain.
3. As the ground shock peak passes the joint unloads and dilates, and the now open joint allows a sudden release of the stored shear strain resulting in sudden joint rupture and slippage.

Step 3 above is essential for identifying when this mechanism occurs. For large in situ stress accompanied by low explosive loading (i.e., deep burial, or high SDOB) the joint fails to open and rupture does not occur. For low in situ stress accompanied by high explosive loading (i.e., shallow burial, or nominal SDOB) there is insufficient resistance to tangential slippage and no shear energy is stored for later release.

The above provides a fully geodynamic definition for why certain explosive events in jointed rock will fall within the correct explosion population of a m_b:M_S discriminant while others may not. Moreover, we illustrate that these observations for the SPE results map directly to generally accepted yield and depth combinations for the six declared North Korean nuclear tests.

How to cite: Steedman, D. and Bradley, C.: Identification a of shear mechanism in moderately overburied chemical explosive experiments and relation to the DPRK declared nuclear tests, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1633, https://doi.org/10.5194/egusphere-egu2020-1633, 2019

At regional distances (<~1700 km) the phases Pn, Pg and Lg are generally the most prominent arrivals from a crustal seismic source.  Amplitude ratios of Pn or Pg to Lg have been investigated by several authors (e.g. Hartse et al. 1997 Bull. Seism. Soc. Am.) as earthquake/explosion discriminators.  Theory and observation show that explosions generate shear phases less efficiently than earthquakes, hence the amplitude ratio of Pn and Pg to Lg is expected to be higher for explosions, especially at frequencies above ~2 Hz.  Walter et al. (2108 Seismol. Res. Lett. DOI 10.1785/0220180128) showed that amplitude ratios Pg/Lg and Pn/Lg at 2-4 Hz were clear discriminants between the six announced nuclear tests of the Democratic People's Republic of Korea (DPRK) and a population of earthquakes.  We investigate regional-phase amplitudes for stations MDJ (distance ~376 km) and USRK (~406 km). Walter et al. found a weak dependence of Pg/Lg in the 2-4 Hz band at MDJ on the magnitude Mw of the explosion. We find this dependence at USRK also.  We also explore the regional amplitude behaviour at a range of frequencies, and dependence on different magnitude measures, such as network body-wave and surface-wave magnitudes.

© British Crown Owned Copyright 2019/AWE

This work is distributed under the Creative Commons Attribution 4.0 License. This licence does not affect the Crown copyright work, which is re-usable under the Open Government Licence (OGL). The Creative Commons Attribution 4.0 License and the OGL are interoperable and do not conflict with, reduce or limit each other.

How to cite: Peacock, S., Bowers, D., and Selby, N.: Magnitude Scaling of Regional-phase Amplitudes from the DPRK Announced Nuclear Tests, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2203, https://doi.org/10.5194/egusphere-egu2020-2203, 2020

Previous studies have shown that explosion sources produce fewer aftershocks and that they are generally smaller in magnitude compared to aftershocks of similarly sized earthquake sources (Jarpe et al., 1994, Ford and Walter, 2010). It has also been suggested that the explosion-induced aftershocks have smaller Gutenberg-Richter b-values (Ryall and Savage, 1969, Ford and Labak, 2016) and that their rates decay faster than a typical Omori-like sequence (Gross, 1996). Recent chemical explosion experiments at the Nevada National Security Site (NNSS) were observed to generate vigorous aftershock activity and allow for further comparison between earthquake- and explosion-triggered aftershocks. Of the four recent chemical explosion experiments conducted between July 2018 and June 2019, the two largest explosions (i.e. 10-ton and 50-ton) generated hundreds to thousands of aftershocks. Preliminary analysis indicates that these aftershock sequences have similar statistical characteristics to traditional tectonically driven aftershocks in the region.

The physical mechanisms that contribute to differences in aftershock behavior following earthquake and explosion sources are poorly understood. Possible mechanisms may be related to weak material properties in the shallow subsurface that do not give rise to stress concentrations large enough to support brittle failure. Additionally, minimal changes in the shear component of the stress tensor for explosion sources may also contribute to differences in aftershock distributions. Here, we compare aftershock statistics and productivity of the explosion-related aftershocks at the NNSS site to synthetic catalogs of aftershocks triggered by explosion sources. These synthetic catalogs are built by coupling strains that result from modeling the explosion source process with the SW4 wave propagation code with the 3D physics-based earthquake simulation code, RSQSim. We compare statistical properties of the aftershock sequence (e.g. productivity, maximum aftershock magnitude, Omori decay rate) and the spatiotemporal relationship between stress changes and event locations of the synthetic and observed aftershocks to understand the primary mechanisms that control them.

Prepared by LLNL under Contract DE-AC52-07NA27344.

How to cite: Kroll, K., Ichinose, G., Ford, S., Pitarka, A., Walter, W., and Dodge, D.: Understanding explosion-related aftershocks using field experiments and physics-based simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6035, https://doi.org/10.5194/egusphere-egu2020-6035, 2020

Seismic and acoustic recordings have long been used for the forensic analysis of various natural and anthropogenic events, especially in the realm of nuclear treaty monitoring. More recently, multi-phenomenological analysis has been applied to these signals with great success, providing unique constraints for studying a broad range of source events, including man-made noise, earthquakes and explosions. In particular, the fusion of seismic and infrasonic data has proven valuable for the analysis of explosive yield, significantly improving on the yield estimates obtained from either seismic or acoustic analysis alone.

Unfortunately, the seismo-acoustic analysis of local explosions is complicated by the fact that the two phenomena are potentially co-dependent. Large seismic waves displace the earth like a piston, potentially inducing acoustic waves into the atmosphere as they pass. Similarly, large acoustic waves can couple into the earth, inducing ground motion along their path. This co-dependence can be problematic, particularly when the passing acoustic shockwave couples into the earth coincident with a seismic phase arrival, thereby corrupting the signal.

To address this problem, we present a method for isolating the shockwave response of a seismic sensor, such that any underlying seismic phase arrivals can be recovered. This is accomplished by employing the adaptive noise cancellation model, where a co-located infrasound sensor is used as a reference measurement for the shockwave. In this model, the adaptive filter learns the transform between the relative atmospheric pressure (as recorded by the infrasound sensor), and the resulting ground motion (as recorded by the seismometer). In this way, the filtered infrasound recording approximates the seismic shockwave response, and can be subtracted from the seismograph to recover the phase arrivals.

The experimental data comes from a set of three low-yield near-surface chemical explosions conducted by LLNL as part of a field experiment, known as FE2. The explosions were recorded at eight stations, located at varying distances from the source (between 64m and 2km), with each station consisting of a co-located three-component seismic velocity transducer and differential infrasound sensor. The adaptive technique is demonstrated for recovering seismic arrivals in both the vertical and horizontal channels across all eight stations, and evaluated using leave-one-out cross-validation across the three explosions.

How to cite: Dickey, J., Pasyanos, M., Martin, R., and Peña, R.: Seismo-Acoustic Shockwave Isolation for Low-Yield Local Explosions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9799, https://doi.org/10.5194/egusphere-egu2020-9799, 2020

We have performed 3D simulations of underground chemical explosions conducted recently in granitic outcrop as part of the Source Physics Experiment (SPE) campaign. The main goal of these simulations is to understand the nature of the shear motions recorded in the near field considering uncertainties in a) the geological characterization of the joints, such as density, orientation and persistency and b) the geomechanical material properties, such as, friction angle, bulk sonic speed, poroelasticity etc. The approach is probabilistic; joints are depicted using a Boolean stochastic representation of inclusions conditional to observations and their probability density functions inferred from borehole data. Then, using a novel continuum approach, joints and faults are painted into the continuum host material, granite. To ensure the fidelity of the painted joints we have conducted a sensitivity study of continuum vs. discrete representation of joints. Simulating wave propagation in heterogeneous discontinuous rock mass is a highly non-linear problem and uncertainty propagation via intrusive methods is practically forbidden. Therefore, using a series of nested Monte Carlo simulations, we have explored and propagated both the geological and the geomechanical uncertainty parameters. We have probabilistically shown that significant shear motions can be generated by sliding on the joints caused by spherical wave propagation. Polarity of the shear motion may change during unloading when the stress state may favor joint sliding on a different joint set. Although this study focuses on understanding shear wave generation in the near field, the overall goal of our investigation is to understand the far field seismic signatures associated with shear waves generated in the immediate vicinity of an underground explosion. Therefore, we have abstracted the near field behavior into a probabilistic source-zone model which is used in the far field wave propagation.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

How to cite: Ezzedine, S., Vorobiev, O., Antoun, T., and Walter, W.: Uncertainty Propagation and Stochastic Interpretation of Shear Motion Generation due to Underground Chemical Explosions in Jointed Rock, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10441, https://doi.org/10.5194/egusphere-egu2020-10441, 2020

Underground blasting within an extensive tunnel complex occurs as part of regular operations at Redmond Salt Mine, located in central Utah, United States. During the period of October 2017 – July 2019, we monitored these explosions using seismic and infrasound sensors. The experiment recorded approximately 1000 mining-related blasts as well as several hundred small earthquakes that naturally occur in the monitoring region at source to receiver offsets of 3-25 km. The data collected early in the experiment allow us to explore the characteristics of infrasound signals generated in subterranean tunnels, which show a variety of interesting characteristics, including components related to the structure of the underground tunnel complex, and a time-varying propagation efficiency. We present analyses that attempt to explain these properties. In addition, the data collected during the experiment allow us to test location algorithms at local distances by comparing computed locations with those taken from ground-truth logs. Finally, comparison of the tectonic and explosion signals allows us to examine possible discrimination methods that will effectively differentiate explosions from earthquakes at local distances.

How to cite: Downey, N., Albert, S., and Bowman, D.: Seismoacoustic Monitoring of Underground Explosions at Redmond Salt Mine, Utah, United States, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22299, https://doi.org/10.5194/egusphere-egu2020-22299, 2020

Two principal performance measures of the International Monitoring System (IMS) stations detection capability are the rate of automatic detections associated with events in the Reviewed Event Bulletin (REB) and the rate of detections manually added to the REB. These two metrics roughly correspond to the precision (which is the complement of the false-discovery rate) and miss rate or false-negative rate statistical measures of a binary classification test, respectively. The false-discovery and miss rates are clearly significantly influenced by the number of phases detected by the detection algorithm, which in turn depends on prespecified slowness-, frequency- and azimuth- dependent threshold values used in the short-term average over long-term average ratio detection scheme of the IMS stations. In particular, the lower the threshold, the more the detections and therefore the lower the miss rate but the higher the false discovery rate; the higher the threshold, the less the detections and therefore the higher the miss rate but also the lower the false discovery rate. In that sense decreasing both the false-discovery rate and the miss rate are conflicting goals that need to be balanced. On one hand, it is essential that the miss rate is as low as possible since no nuclear explosion should go unnoticed by the IMS. On the other hand, a high false-discovery rate compromises the quality of the automatically generated event lists and adds heavy and unnecessary workload to the seismic analysts during the interactive processing stage.

A previous study concluded that a way to decrease both the miss and false-discovery rates as well as the analyst workload is to increase the retiming interval, i.e., the maximum allowable time that an analyst is allowed to move an arrival pick without having to declare a new arrival. Indeed, when a detection needs to be moved by an interval larger than the retiming interval, not only is this a much more time-consuming task for the analyst than just retiming it, but it also affects negatively both the associated rate (the automatic detection is deleted and therefore not associated to an event) and the added rate (a new arrival has to be added to arrival list). The International Data Centre has increased the retiming interval from 4 s to 10 s since October 2018. We show how this change affected the associated-detections and added-detections rates and how the values of these metrics can be further improved by tuning the detection threshold levels.

How to cite: Saragiotis, C. and Kitov, I.: Tuning IMS station processing parameters and detection thresholds to increase detection precision and decrease detection miss rate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8949, https://doi.org/10.5194/egusphere-egu2020-8949, 2020

Autonomous algorithms can improve the processing of aftershock sequences, for example by reducing the analyst workload. We present a system for automatic detection and location of aftershocks in a specific region following a large earthquake. The system seeks to identify all signals generated by seismic events in the target region, while passing over signals generated by sources in all other regions. For a given station, we can generate a sensitive empirical matched field (EMF) detector for the target region using only an empirical template from the mainshock signal. These EMF detectors perform much better on seismic arrays than on 3-component stations. For each selected station in the network, a multivariate detector combines the EMF detector with an optimized continuous AR-AIC detector to generate a target-optimized detection list. For arrays, an additional continuous calibrated f-k process reliably screens out likely signals from other sources. A region-specific phase association algorithm takes the screened detection lists from each station and generates a preliminary aftershock bulletin. We have processed aftershock sequences from four major earthquakes: the Tohoku event in 2011 (Japan), the Illapel event in 2015 (Chile), the Papua New Guinea event in 2018 and the Gorkha event in 2015 (Nepal).

We evaluate the results in detail by comparing the automatically generated origins and corresponding phase arrival times with matching events and associated arrivals in the analyst reviewed (REB) and automatic (SEL3) bulletins issued by the CTBTO Preparatory Commission. Between 40% and 65% of all events in the REB are found to closely match the locations and origin times of the events found by our EMF-based procedure. The resulting discrepancies are assessed with respect to signal-to-noise ratio, number of defining stations, and epicentral distance. Furthermore, the REB events not detected by the EMF method are analyzed and a few phase misidentifications (i.e., P vs. pP) are assessed to better understand the limitations of the autonomous procedure. In general, we find that our EMF solutions are closer to the matching REB events than the corresponding SEL3 events. The analyst is helped both by the improved location estimates and a lower number of qualitatively incorrect event hypotheses. A key factor in the performance is the number of contributing seismic arrays. Aftershock sequences in the southern hemisphere performed the worst given the poorer array coverage.

How to cite: Köhler, A., Kværna, T., and Gibbons, S. J.: Assessment of the empirical matched field processing algorithm for autonomous tracking of aftershock sequences, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9396, https://doi.org/10.5194/egusphere-egu2020-9396, 2020

The Municipality of Zapopan, Jalisco, is located west of the Guadalajara Metropolitan Zone at the intersection of three rift zones: Tepic-Zacoalco, Chapala-Tula, and Colima. The importance of this region lies in the recent population growth that it has experienced in a few years. This growth has been supported by the development in commercial and service activities, and mainly in industry and technology, being ranked as the second-most populous city in Mexico, behind the federal capital.

The western region of the Guadalajara Metropolitan Zone (GMZ) has numerous fault systems where, historically, there have been significant earthquakes and seismic swarms such as those that occurred in 1685-1687, 1875, 1932, 1995 and 2002, showing similar characteristics. Besides, it is in this region where the Caldera de la Primavera is located, a rhyolitic volcanic caldera that continues presenting seismic and geothermal activity.

Recently, in the years 2015 and 2016, new seismic swarms occurred and were recorded instrumentally for the first time by the Jalisco Seismic and Accelerometric Network (RESAJ). The two seismic sequences took place in two alignments in the same direction as the Colima rift. These epicenters suggest the existence of two almost parallel normal faults, and that would be forming the Graben of Zapopan. Due to the length of these faults, 16 km for the east fault, and 28 km for the west fault, earthquakes of magnitudes 6.2 - 6.5 could be generated.

In the framework of the CeMIEGeo P-24 project (SENER-CONACyT), we continue studying the seismicity of this region with the deployment of 25 seismic stations in the vicinity of La Caldera de la Primavera. This study revealed the high seismicity that was taking place in the area of ​​Zapopan, Tesistán Valley, and La Caldera de la Primavera.

Based on these new studies and the knowledge of the seismic history of the region, a collaboration agreement has been established between the Research Group UDG-CA-276 SisVOc and Civil Protection of the Municipality of Zapopan for the installation of a local seismic network that will allow to define tectonic and structurally the fault systems of the region and mitigate the possible effects of the local seismicity in the population. Since May 2019, three Obsidian 8X seismic stations with Lennartz 1Hz LE3D and Episensor sensors and two accelerometers installed in the city have been operating, constituting the Zapopan Seismic and Accelerometric Network (RESAZ). The RESAZ operates together with the nearest stations of the RESAJ. In this work, we present the first results of the seismicity analysis recorded in Zapopan.

How to cite: Núñez, D., Núñez-Cornú, F. J., Alarcón, E., Quinteros-Cartaya, C. B. M., Suárez-Plascencia, C., and Ramírez, S.: The Seismic Network of Zapopan: Evaluating the local seismicity of the western Guadalajara Metropolitan Zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12105, https://doi.org/10.5194/egusphere-egu2020-12105, 2020