We introduce a comprehensive strategy to monitor subsurface fault behavior and associated geophysical environment (e.g., micro-seismicity, stress, groundwater), using a deep borehole based monitoring system. This study provides an in-depth overview of the TELLUS (The Earth Login Leverage for Underground Signal) project, with its individual monitoring system tracking subsurface fault movements with high precision by deploying borehole seismometers, and gathering important data on various geophysical properties.

This paper details site selection process and characterizations, the operational framework on monitoring system installations, and the potential of deep borehole monitoring approach in advancing subsurface-related geophysical studies. We conducted extensive review of the distributions of major fault systems in the south-eastern part of South Korea. Subsequently, we strategically selected and arranged candidates for monitoring system installations. Total of six TELLUS deep borehole monitoring systems were installed in the vicinity of the major faults (Yangsan and Ulsan fault).

In the TELLUS observatories, preliminary monitoring data is being collected in real time, and this is establishing a foundation for a more precise understanding of the behavior of subsurface faults and the related geophysical environment. It would be expected that the comprehensive analysis of these datasets will further elucidate the intricate subsurface geophysics. This enhanced understanding promises to contribute substantially to our seismic risk assessment capabilities and to the broader field of geoscience research, offering new insights into earthquake prediction and geophysical phenomena.

How to cite: Jo, Y., Park, S., and Lee, C.: Deep borehole based subsurface geophysical monitoring network: TELLUS Project, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4975, https://doi.org/10.5194/egusphere-egu24-4975, 2024.

This study focuses on the marine area located between Çandarlı Bay and Lesbos Basin in the Northeastern Aegean Sea. The high quality multibeam bathymetry data, their processing and interpretation with the onshore structures and an integrated interpretation with seismic reflection profiles allowed to map the offshore active faults, to prepare the seismic stratigraphy and to evaluate the tectonic evolution of the marine area between Çandarlı Bay and Lesbos Basin. In addition, thickness maps were generated from seismic reflection profiles and seismic stratigraphic units were correlated with the Foça-1 well to reveal the characteristics of the deposited strata in this region. The seismic stratigraphic units were also compared with onshore geological units.

Five major seismic stratigraphic units were identified and all of which are compatible with each other from the Çandarlı Bay to the Gulf of Izmir. The findings suggest continuous sedimentation from the Burdigalian (Lower Miocene) to the present day. A predominantly volcaniclastic sequence deposited in the Burdigalian-Serravalian period rests on basement rocks. This unit is overlain by Tortonian clastics and carbonates interbedded with volcanic rocks. Tortonian sediments are followed by about 300-500 m thick clastics and anhydrites, which were deposited in an environment corresponding to the Messinian salinity crisis in the Mediterranean Sea. The post-Messinian unit is of Pliocene age and starts with upper Miocene limestones at the base and transitioning upwards into clastic rocks. The stratigraphy concludes in the upper part with a Quaternary unit, which is mainly composed of fine-grained clastics and has been influenced by sea-level changes.

The study area is dominated by both NW-SE and NNW-SSE-striking normal faults and two distinct tectonic phases have been identified. The first phase spans from the Miocene to the end of the upper Miocene and is characterized as a supra-detachment basin associated with the development of core complexes in the region. These faults do not extend to the surface in seismic sections and are indicative of an early-stage tectonic activity. The homogeneity of the sediment thickness suggests a slowdown in tectonic activity during the Tortonian-Messinian period. In the Plio-Quaternary period, the sediment thicknesses indicate uplift in the surrounding region. Additionally, the bathymetric traces of faults shaping the Lesbos Basin to the west of Çandarlı Bay indicate the presence of a new tectonic system.

How to cite: Elitez, İ., İslam, T., Gültekin, D. İ., and Yaltırak, C.: Miocene to Quaternary Seismic Stratigraphy and Tectonic Evolution of the Marine Area Between Çandarlı Bay and Lesbos Basin, Northeastern Aegean Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-608, https://doi.org/10.5194/egusphere-egu24-608, 2024.

Neotectonic movements pose significant hazards and hold crucial scientific and social relevance, notably in seismic hazard assessment and subsurface utilization. In regions like northern Germany, a presumed aseismic region, understanding these processes remains limited because many faults are buried under sediments. Despite confirmed neotectonic activity, the specifics of these processes and associated structures remain largely unknown.

Improving our understanding of neotectonic activity requires investigations of recently-active fault zones, such as the Osning Fault System (OFS) in North Rhine-Westphalia, Germany. Using near-surface geophysics becomes crucial in this endeavour, which so far were not used at the OFS.

The OFS stands out as a site of recent and historical seismic activity, experiencing several large earthquakes over the past four centuries. Notably, major earthquakes in 1612 and 1767 with intensities ranging from VI to VII on the MSK scale, emphasize the seismic significance of the OFS. Unlike other faults in the region, the faults of the OFS reach the basement and the fault zone dips north-eastward. Furthermore, the former iceload from Scandinavia influenced the fault system by facilitating glacial isostatic adjustment, which subsequently enabled fault reactivation. The complex nature of the fault system spans various geological phases, prompting a comprehensive investigation approach to understand its regional neotectonic evolution.

Our geophysical and geological approach integrates high-resolution 2D P- and SH-wave reflection seismics and retrodeformation of previously-published cross-sections. This is complemented by surface geological maps and limited drilling information. Our aim is to identify and interpret fault geometry and kinematics.

While P-wave seismic surveys used for imaging of deep structures often lack high-resolution in the shallow subsurface, the integration of SH-wave reflection seismics compensates for this limitation, offering enhanced resolution, especially at the near-surface. The survey involved three P-wave profiles employing a hydraulically-driven vibrator vehicle and four SH-wave profiles utilizing an electro-dynamic micro-vibrator with varying source point spacing.

These seismic profiles successfully delineate fault structures within the Cretaceous formations, revealing previously unidentified extensions of the OFS. Although the P-wave profiles inadequately image the Quaternary layers, there are indications that the faults extend into this formation. The SH-wave profiles, with their superior resolution in the near-surface due to lower wave velocities, confirm these assumptions, revealing further faulting and deformation features within the Quaternary sediments. Interpretation and fault imaging are further enhanced by full waveform inversion of P- and S-wave data, testing of different migration methods for the S-wave data, and seismic attribute analysis.

Retrodeformation and balancing of existing cross-sections and the interpreted seismic profiles allows the fault geometry and kinematics to be assessed. We determined which adjustments were necessary to make the profiles more geologically plausible. This combined geophysical and geological approach enabled a more comprehensive interpretation and understanding of the local fault geometry and the neotectonic evolution of the OFS.

How to cite: Wadas, S., Tanner, D., and Polom, U.: The structure and neotectonic evolution of the Osning Fault System in Germany derived from near-surface P- and SH-wave reflection seismics and retrodeformation modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15033, https://doi.org/10.5194/egusphere-egu24-15033, 2024.

    We deployed a dense array of 173 seismometers covering a 30 by 40 km2 area in the Tainan frontal thrust of southwestern Taiwan from February to June 2021. The seismic array with an inter-station distance of about 2 km, spanned from west to east, across four major tectonic regimes: Anping Plain, Tainan Tableland, Dawan Lowland, and Chungchou Tableland. Structurally, the Houchiali Fault separates the Tainan Tableland and Dawan Lowland, while the right-lateral Hsinhua Fault is located 5 kilometers northeast of the Houchiali Fault. For the Eikonal tomography analysis, we included data from ten BATS (Broadband Array in Taiwan for Seismology) stations, five stations from CWASN (Central Weather Administration Seismographic Network), and one station from TSMIP (Taiwan Strong Motion Instrumentation Program). These stations were strategically positioned at distances ranging from 40 to 80 kilometers away from the dense array, with azimuths between 45° to 140° and 270° to 360°. We then calculated the cross-correlation function (CCF) between 173 seismometers and these stations. These results were subsequently used by beamforming to measure the relative surface wave arrival times. To perform Eikonal tomography, we calculated the surface wave propagation of the ambient noise and the shallow velocity structure for each period between 4 and 10 seconds. Our result shows that the velocity on Tainan Tableland is almost uniform, which is probably due to the gently folded character of the underneath Tainan anticline. Meanwhile, a low-velocity zone of approximately 2.5 by 2.5 km2 with a 4s period was revealed northeast of the Houchiali Fault and southwest of the Hsinhua Fault, with a shear-wave velocity of approximately 0.5 km/s. Upon reaching the Hsinhua Fault to the northeast, the velocity increases five times to 2.5 km/s. For the 4s period, the average velocity in this region is approximately 1.5 km/s, however, the velocity distribution does not conform with the regional velocity models. This suggests the presence of potentially unexamined small-scale structures in this area. Furthermore, this area encountered strong shaking from three local or regional moderate earthquakes that occurred in 1946, 2010, and 2016 respectively. Coincidentally, the low-velocity zone aligns roughly with the soil liquefaction sites caused by these three events.

How to cite: Yang, T.-C., Chen, Y.-N., and Rau, R.-J.: Shallow velocity structure of the Tainan frontal thrust based on Eikonal tomography, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7098, https://doi.org/10.5194/egusphere-egu24-7098, 2024.

Seismic travel time tomography is a widely used technique to image the Earth’s interior. Recently, there have been growing interests in employing deep learning for seismic tomography, and physics-informed neural networks (PINNs) are attractive for their integration of physical information into the networks and their greater stability compared to conventional neural networks. PINNs have been successfully used in 2D tomography, including cross-hole tomography (Waheed et al., 2021) and surface wave phase velocity tomography (Chen et al., 2022). However, 3D seismic tomography based on PINNs has not been developed. Here we propose a novel method for 3D travel time tomography based on PINNs and show its effectiveness using both synthetic and real data. The network consists of two branches, one taking in the 3D coordinates of a pair of source and receiver for fitting observed travel times and another taking in the receiver location for predicting velocities. The loss function also consists of two terms, the data fitting residual term and the Eikonal equation residual term. In this way, the two branches are connected using the Eikonal equation loss function term. After training, the network can simultaneously reconstruct the travel time fields and estimate the subsurface velocities. Our method is tested using synthetic and real travel time data for seismic network in Parkfield, California. Compared to traditional travel time tomography methods, this approach offers many advantages, including meshless modeling, no need for regularization and independent on the initial velocity models.

How to cite: Yang, S. and Zhang, H.: Physics-informed neural networks for 3D seismic travel time tomography, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7896, https://doi.org/10.5194/egusphere-egu24-7896, 2024.

The Longitudinal Valley in eastern Taiwan situated at the boundary between the Philippine Sea Plate and the Eurasian Plate, making it one of the significant seismogenic regions in Taiwan. In 2018, a magnitude 6.4 earthquake occurred offshore of Hualien, at the northern end of this valley. More than 4,000 aftershocks received over following two weeks beneath the Longitudinal Valley. Surprisingly, Hualien City and the Milun Fault, situated near the epicenter, did not experience notable aftershocks. However, they did display evident co-seismic deformation during the mainshock. Milun fault Drilling and All-inclusive Sensing (MiDAS) project aims to establish a comprehensive and long-term monitoring system, which involves three boreholes encompassing the fault for both surface and subsurface geoscientific observations. This study utilized a 700-meter DAS (Distributed Acoustic Sensing) setup within the borehole A of the MiDAS project to conduct VSP (Vertical Seismic Profiling) experiments. Additionally, electrical logging and surface reflection seismic data were gathered.  According to the 2D reflection seismic profile, the results depicted a gently sloping stratum on the northwest side arching towards the southwest. Most layers tapered beneath the Milun Terrace, and the fault-induced stratigraphic disturbance was not clearly discernible, making it challenging to determine if the layers were intersected by the fault. However, pronounced folding structures were evident beneath the terrace, correlating with areas of intense co-seismic deformation. From the VSP experiment, the data exhibited pronounced P-waves and Tube waves, suggesting the presence of fluids around the casing rather than rocks or cement. After data processing, the P-wave velocity correlated well with the downhole sonic logging data. Also, reflection signals with two-way travel times ranging from 0.27 to 0.45 seconds were observed in both seismic profiles. This suggests that the VSP effectively resolved reflection signals from depths of 340 to 612 meters. These signals displayed a high resemblance to lithology indicators such as Gamma ray and resistivity, confirming the authenticity of the separated reflection signals obtained from shallow depth, lower signal-to-noise ratio data.

How to cite: Guan, Z.-K., Kuo-Chen, H., Chen, C.-R., and Ma, K.-F.: Retrieving Seismic Reflective Waves from DAS VSP: a Case Study from MiDAS Project, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13950, https://doi.org/10.5194/egusphere-egu24-13950, 2024.

In the Ivrea Verbano Zone (IVZ) Italy, which is characterized with lower-crustal rocks and fragments of upper mantle rocks, a high-resolution seismic survey is conducted across the Balmuccia Peridotite. This study is in preparation of a proposed deep scientific drilling project which focuses on targeting mantle rocks and understanding the region's complex geology. Specifically, we target characterizing structures within the peridotite body.

The seismic survey employs a fixed spread of 200 vertical geophones and 160 3C-sensors, spaced at ca. 10 m along three sub-parallel receiver lines spaced 40-80 m apart. Vibroseis source points are at 22 m stations along a 2.2 km line utilizing a 12-140 Hz 10 s linear sweep with 3 s listening time. The survey aims to explain the seismic characteristics of the peridotite body and its relation to the surrounding geological structures.

The P-wave traveltime tomography reveals a range of seismic velocities within the peridotite from 6 to 8 km/s, with a mean velocity of ca. 7 km/s. These variations reflect the heterogeneity of the peridotite, influenced by the presence of fractures and faults. Notably, the higher velocities observed are consistent with findings from laboratory studies on small-scale samples from the area. The reflection seismic analysis shows subvertical reflectors that coincide with the peridotite boundaries mapped at the surface. These reflectors come together at a depth of 0.175 km b.s.l., suggesting that the peridotite has a lens-like structure. In addition, several features within the peridotite suggest a highly fractured body. Nevertheless, limitations in the imaging process do not allow for a thorough interpretation of the area below the imaged lens-shaped body. A deep reflector is identified at approximately 1.3 km depth. This feature potentially marks the top of the Ivrea Geophysical Body (IGB), aligning with previous geophysical estimations.

How to cite: Pasiecznik, D., Greenwood, A., and Bleibinhaus, F.: Seismic insights into the structure of the Balmuccia Peridotite within the Ivrea Verbano Zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7510, https://doi.org/10.5194/egusphere-egu24-7510, 2024.

The Main Himalayan Thrust (MHT) demarcates the boundary between the underthrusting Indian Plate and the overriding Himalayan orogeny. Stress accumulation on the MHT due to the underthrusting of the Indian Plate leads to the occurrence of bigger earthquakes in the Himalayas. It is, therefore, imperative to understand the MHT's geometry in different Himalayan segments. Furthermore, how this geometry varies along the Himalayan arc while taking into account the uneven distribution of earthquakes offers a comprehensive insight into earthquake nucleation in the region. The central seismic gap is one of the most significant segments of the Himalayas, which is considered a potential region for the proposed great earthquake in the future. This study focuses on defining the MHT geometry by constructing a 3-D model within the central seismic gap using the seismic interferometry technique.  We analyzed the data sets obtained from 159 broadband stations spread across the area to construct a comprehensive three-dimensional geometry of MHT. The different arc normal cross-sections highlight variations in the MHT's geometry along the arc in the central seismic gap.

Keywords: Main Himalayan Thrust, seismic interferometry, central seismic gap, crustal imaging.

How to cite: Ali T S, S., Gupta, S., Kumar, S., Sivaram, K., and Rawat, V.: Imaging of Main Himalayan Thrust in Central Seismic Gap using seismic interferometry  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4302, https://doi.org/10.5194/egusphere-egu24-4302, 2024.

The Pyrenees constitute a natural laboratory where hundreds of geological and geophysical data have been acquired during the last decades. It represents a roughly E-W oriented doubly vergent orogen formed during the Alpine Orogeny. Deep seismic reflection data obtained during the 80s revealed its crustal architecture that resulted from the subduction of the Iberian plate under the European lithosphere at its central part.

In this work we applied seismic interferometry to the same passive dataset through two different techniques aiming to construct two independent images of the Central Pyrenean lithosphere, to enhance the current knowledge of the area. The main objectives are to compare them and correlate the obtained results with previous data. Data were acquired within the IMAGYN project along a NE-SW 70 km-long profile extending from the Southern Pyrenees (Pedraforca and Cadí Units, northern Iberia) to the northern part of the Axial Zone, close to Ax-les-Thermes (France). Data came from three to five months of continuous recording from an almost linear array of 43 seismic stations (being 17 and 26 broadband and short-period stations, respectively). The two applied techniques are (1) the global-phase seismic interferometry (GloPSI), using continuous recordings of teleseismic (30⁰ < epicentral distance < 95⁰) and global earthquakes (> 120⁰ epicentral distance), and (2) the use of continuous ambient seismic noise recordings through autocorrelation. Despite both methods rely on different energy sources, they are complementary and use static receivers. In the first method (GloPSI), we extracted global phases (PKP, PKiKP and PKIKP) and their reverberations within the lithosphere. The selected phases were autocorrelated and stacked to construct a high-resolution pseudo zero-offset reflection image. The second approach provided an approximation to the zero-offset reflection response of a single station. Results reveal features that can be correlated in both reflection images. The crust-mantle boundary is mapped as a relative flat interface at approximately 35-40 km depth. Crustal interfaces detected at 15 and 25 km depth can be related to the Conrad discontinuity and other compositional changes within the crust.

(This work is part of the project “High-resolution imaging of the crustal-scale structure of the Central Pyrenees and role of Variscan inheritance on its geodynamic evolution” (IMAGYN), PID2020-114273GB-C22 funded by MCIN/AEI/10.13039/501100011033)

How to cite: Soto, R., Andrés, J., Gabàs, A., Bellmunt, F., Macau, A., Clariana, P., Rey-Moral, C., Rubio, F., Izquierdo-Llavall, E., Mochales, T., Pueyo, E. L., and Ayala, C.: Imaging the crustal structure of the Central Pyrenees using Seismic Interferometry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12789, https://doi.org/10.5194/egusphere-egu24-12789, 2024.

   The Sichuan-Yunnan Region is located at the southeastern margin of the Tibetan plateau in southwest China. Determining the structure of southeaster margin of the Tibetan plateau is very important for understanding the eastward growth of the plateau. At present, most of seismic tomography studies are focused on resolving multiscale velocity anomalies of the crust and mantle. However, seismic attenuation structure of the crust and upper mantle in the Sichuan-Yunnan Region is less studied, which can be used to better understand the temperature regime and the distribution of partial melting. In this study, based on a high-resolution community velocity model of the crust and uppermost mantle in this area (Liu et al., 2021), we determined body wave attenuation structure in the Sichuan-Yunnan region using data from 350 stations and 9837 seismic events observed between 2010 and 2013 (Figure 1).

Figure 1. Distribution of seismic events (red circles) and stations (blue triangles) in the Sichuan-Yunnan region.

    Q value is a reduction to a dimensionless form of the more usual measures of attenuation (Knopoff, 1964. Body wave attenuation tomography typically needs to extract a parameter called t* from the displacement or velocity spectrum in the frequency domain to calculate Q using the following equation:

    Here the influence of frequency on Q was not considered. Using the measured absolute t* values, the attenuation structure was then determined with the following relationship between t* and Q:

    For the Sichuan-Yunnan region, we constructed three-dimensional Qp and Qs models with a horizontal spatial grid interval of 0.4° × 0.4°. Our attenuation models exhibit a high consistency with large-scale features in previous researches. In the shallow depths (<20 km), inside the Chuan-Dian diamond block, it exhibits high Q anomalies, which may be related to the high density and low porosity characteristics of the Emeishan Large Igneous Province (ELIP). Previous studies suggest the presence of high Vp and Vs anomalies in the same zone (Liu et al., 2021). In comparison, most of the fault zones show low Q anomalies, indicating that fractures and fluids in the crust can increase the attenuation of seismic waves. At deeper depths (20-40 km), the ELIP still maintains a high Q anomaly, and separates two clear stripes of low Q anomalies along the Lijiang-Xiaojinhe Fault and Xiaojiang Fault. This suggests the existence of partial melting along the two fault zones that could be caused by upwelling of hot mantle materials. In addition, high Q values are observed in the Yangtze craton and Sichuan Basin, corresponding to stable tectonic block and weak tectonic activity.

How to cite: Wang, J., Zhang, H., Hu, J., and Liu, Y.: Body wave seismic attenuation tomography of the crust in the Sichuan-Yunnan Region, China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8875, https://doi.org/10.5194/egusphere-egu24-8875, 2024.

Fault imaging and characterization of crustal structures, along with source parameter estimation, strongly depend on reliable direct seismic wave arrival times. To lower the detection threshold of small earthquakes and improve the quality of fault imaging, dense arrays of seismic stations are being installed more and more frequently in different parts of the world. Given the rising number of new, denser seismic networks and the growing demand for high-quality seismic catalogs, the use of automatic techniques is crucial to efficiently process and analyze the vast amount of seismic data, contributing to the advancement of seismic research and monitoring capabilities.

The Irpinia fault system (Southern Italy) hosted in 1980 the M 6.9 earthquake and is currently monitored by the Irpinia seismic network (ISNet), which is composed of 31 seismic stations covering an area of about 100x70 km² along the Campania-Lucania Apennine chain. ISNet was integrated by 200 seismic stations grouped in small-aperture arrays of 10 stations during the 1-year DETECT project (DEnse mulTi-paramEtriC observations and 4D high resoluTion imaging). DETECT focused on the acquisition of a unique multiparametric dataset and aimed to monitor and image the fault system during the inter-seismic phase fostering at the same time the collaboration among various institutions.

The use of seismic arrays, in a region characterized by high seismic hazard, presents a unique opportunity to introduce a novel technique that accurately reveals the first arrivals of seismic phases of small earthquakes. We examined 226 micro-earthquakes, with magnitude ranging from -0.27 to 2.28, detected by the seismic arrays during the first six months of the DETECT project (September 2021 - February 2022). We present a novel approach that utilizes high-order statistics (HOS), computed on the vertical components of waveforms, to identify P-wave arrival times and provide reliability measurements for our predictions. The advantage of the arrays’ geometry enabled the integration of the HOS technique with a criterion designed to select the optimal onsets. The proposed methodology incorporated over 3,300 additional P-wave arrival times into the existing catalog. Furthermore, semblance measurements among traces recorded at the same array highlighted the superior quality of the selected picks compared to the existing ones.

This work is partially supported by project TOGETHER - Sustainable geothermal energy for two Southern Italy regions: geophysical resource evaluation and public awareness financed by European Union – Next Generation EU (PRIN-PNRR 2022, CUP D53D23022850001).

How to cite: Messuti, G., Palo, M., Scarpetta, S., Napolitano, F., Scotto di Uccio, F., Capuano, P., and Amoroso, O.: Accurate seismic phase picking using High-Order Statistics: the case study of the Irpinia Seismic Array in Southern Italy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6400, https://doi.org/10.5194/egusphere-egu24-6400, 2024.

Detailed imaging of the elastic and anelastic properties of the crustal structures of geothermal regions is crucial from a scientific and industrial standpoint. The Hengill volcano region is the most productive high-temperature geothermal region in Iceland, located in the southwestern region of Iceland (30 km east of Reykjavík). To the north of the Hengill volcano is the Nesjavellir geothermal subfield, which lies within the Hengill fissure swarm trending N30°E.

Scattering and absorption measurements have proven reliable proxies for the spatial extension of faults, thrusts and fluid reservoirs across tectonic, volcanic and hydrothermal settings. Scattering marks tectonic interactions and lithological contrasts due to wave-trapping mechanisms that increase energy across the earthquake coda. Fluid content is, instead, the primary controller of seismic absorption. Rock physics studies and numerical simulations have proven the sensitivity of this parameter to strain rate and pore space topology.

The present work aims to provide the first 3D images of seismic scattering and absorption accross the Nesjavellir geothermal area at different frequency bands, measured through peak delay mapping and coda-attenuation tomography, respectively. Manually picked seismic events that occurred between November 2017 and December 2022, recorded by three permanent and temporary seismic networks, have been used to provide the first attenuation imaging of this geothermal area.

The preliminary results show, firstly, the stability of the peak delay and coda attenuation results as the analysis parameters change. The 3D scattering and absorption imaging show that the well-resolved areas of the Hengill region are characterized by high scattering, coinciding with highly-fragmented fissures at the surface. High absorption anomalies mark the Nesjavellir geothermal sub-field, mainly between 4 and 6 km depth, where seismic tomographies highlight high Vp/Vs.

The work is supported by project TOGETHER - Sustainable geothermal energy for two Southern Italy regions: geophysical resource evaluation and public awareness financed by European Union – Next Generation EU ( PRIN-PNRR 2022, CUP D53D23022850001).

How to cite: Capuano, P., Napolitano, F., De Siena, L., Ágústsdóttir, T., Hjörleifsdóttir, V., Palo, M., and Amoroso, O.: Scattering and absorption imaging of the Nesjavellir (Iceland) geothermal area, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15578, https://doi.org/10.5194/egusphere-egu24-15578, 2024.

In the northern Los Angeles area, the interaction of complex tectonics and sedimentary structures has a significant influence on the attenuation characteristics of the crust. The region is also characterized by partially fluid-saturated crust and seismic sequences that promote intense fracturing. Modelling high-frequency seismic attenuation to image fine-scale crustal features and gain insight into the driving mechanisms of the seismicity is a powerful tool for seismic hazard assessment across this densely populated area.

We develop the first high-resolution 3D seismic attenuation model across the Chino and San Bernardino basins using 5,300 three-component seismograms from local earthquakes (M<3.6). The events were recorded by 410 nodal stations deployed along eight linear arrays during the 2017-2020 Basin Amplification Seismic Investigation experiment and 10 Southern California Seismic Network broadband stations. We present peak delay and coda-attenuation tomography in 6-12 and 12-24 Hz frequency bands (with horizontal and vertical grid spacings of 3 km and 1 km) as proxies of seismic scattering and absorption, respectively.

The attenuation models show distinct scattering contrasts in the uppermost 10 km of the crust across two major faults in the northern edge and in the middle of the Chino basin, suggesting variations in fracture intensity in the basement. Low scattering values characterize the crustal block bounded by these two faults, while high scattering coincides with zones of seismicity indicating highly fractured fault-rocks, such as the areas across the Cucamonga, Fontana, and San Andreas faults. The N-S pattern of high absorption and seismicity migration associated with the 2019 Fontana seismic sequence suggests potential groundwater movement across the fault into a buried intensely fractured zone in the basement that we interpret was once an elevated part of the Perris Block. Low scattering values beneath the Chino basin in the source region of the seismic sequence may confirm the presence of fluid-saturated rocks and increased pore pressure. The attenuation results allow the small-scale characterization of fractured basement rocks and fluid migration pathways, and show a heterogenous pattern of seismic wave amplification beneath the region.

How to cite: Nardoni, C. and Persaud, P.: Imaging Fracture Networks beneath the Los Angeles Metropolitan Area using High-Frequency Seismic Attenuation Tomography, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9648, https://doi.org/10.5194/egusphere-egu24-9648, 2024.

Seismic tomography is a useful tool to obtain the velocity structure of the Earth's interior. Compared with the Vp and Vs models, the Vp/Vs model is of great significance for studying the properties of subsurface structure, such as fluid saturation and porosity.  However, due to different data quality and quantity for P- and S-wave, Vp and Vs models generally have different resolutions and uncertainties, leading to some artifacts in the Vp/Vs model. Tryggvason and Linde (2007) proposed to use the structural similarity in Vp and Vs models to better constrain the Vp/Vs model. However, the clustering relationship between Vp and Vp/Vs models is not optimized, which limits the further geological interpretations based on velocity models. In this study, we aim at developing a new seismic tomography method based on the variation of information for Vp and Vp/Vs models. This method follows joint inversion of magnetotelluric and gravity data based on the variation of information (Moorkamp, 2022), which can improve the clustering relationship between electrical resistivity and density.
 The 2021 Ms6.4 Yangbi earthquake is located at the intersection of the Red river fault and the nearly north-south trending Lijiang-Dali fault system on the southwestern boundary of the Sichuan-Yunnan block. This earthquake has the classic characteristic of a "foreshock-mainshock-aftershock" sequence. In this study, we have developed a new seismic tomography method based on the variation of information to couple the Vp/Vs model with the Vp model to obtain more reliable Vp, Vs, and Vp/Vs models in the source region of the Yangbi earthquake. Our results show that the foreshocks occur in structures with low Vp, high Vs and low Vp/Vs, while the main shock occurs in the area with high Vp, high Vs and low Vp/Vs. Based on the cross-plot analysis and petrophysical experimental data, we suggest that long-term stress accumulation causes shearing in areas with high quartz content at a depth of 10km. 

How to cite: Huang, Y., Zhang, H., Hu, J., liu, Y., Gao, J., and Moorkamp, M.: Seismogenic structure of the 2021 Ms6.4 Yangbi earthquake by seismic tomography based on the variation of information , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9088, https://doi.org/10.5194/egusphere-egu24-9088, 2024.

The intricate tectonics of Central-Southern Italy, characterized by its complex fault network and sparse seismicity distribution, have posed a significant challenge to understanding the region's seismic hazard, its three-dimensional structural assessment, and the role of fluids in the seismic release. Conventional geophysical techniques, often limited by low seismicity rates, have struggled to provide a comprehensive picture of the crustal structures, and a coherent geophysical model of the area is still absent. Leveraging the last decade’s expanded detection capabilities of the Italian seismic network, we were able to make up for this lack and employed seismic attenuation and scattering tomography methods to produce complete 3D attenuation models of the crust.

By analyzing the energy loss of seismic waves as their propagation through the crust, the study revealed a pervasive pattern of high attenuation zones that extend along the entire Apenninic Chain, particularly concentrated in Southern Italy. The distribution of these anomalies aligns closely with the regional fault structures suggesting a strict relationship with the fracture level due to the tectonic processes. In contrast to the bigger anomalies, the study also identified prominent low attenuation and scattering volumes corresponding to the Fucino and Morrone-Porrara fault systems. These are likely regions of accumulated stress where the locked seismic energy release contributes to the high seismic hazard. Furthermore, the study identified a previously undetected high-attenuation region beneath the Matese extensional system, indicating a potential source of both deep and shallow circulation of fluid. Another anomaly was detected near the L'Aquila 2009 seismogenic area, suggesting a regional distribution of fluid-rich areas.

The findings of this study provide unprecedented insights into the tectonic interactions and fluid sources of Central-Southern Italy, with significant implications for seismic hazard assessment, fluid exploration, and the development of effective mitigation strategies for this geologically active region.

How to cite: Talone, D., De Siena, L., Lavecchia, G., and de Nardis, R.: The Attenuation and Scattering Signature of Fluid Reservoirs and Tectonic Interactions in the Central-Southern Apennines (Italy) , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16491, https://doi.org/10.5194/egusphere-egu24-16491, 2024.