SM6.4

SM6 EDI
Challenges in seismology to understanding volcanic islands and magmatic unrest 

Volcanic islands are simultaneously some of the tallest and fastest-forming geological features on Earth and constitute the site of significant geohazards ranging from volcanic eruptions, earthquakes, landslides, and tsunamis. Ocean island volcanoes are also some of the most enigmatic features in our planet, as their genesis is still not satisfactorily explained by conventional plate tectonics. The scientific community faces several challenges in studying volcanic islands, particularly in what regards processes taking place at depth. There is still a need to densify seismic networks in volcanic islands, using both land- and seafloor-based stations, to record the signals associated with volcanic and tectonic processes and automatically or manually detect and classify those signals. 3D images from the shallow crust to the deep mantle are crucial to unravel the geodynamic processes behind the generation of volcanism. More accurate quantification of temporal changes in the volcanic systems will help in the forecasting of potential eruptions and the monitoring of existing ones. On top of that, the presence of geothermal systems and induced seismicity from industrial exploration are also critical challenges in volcanic islands due to the system's complexity.
Considering the enormous diversity of interactions in volcanic islands, we welcome contributions from a wide range of studies including: seismo-volcanic monitoring and tracking of magma movements; characterization and location of volcanic tremor; 3D and 4D seismic imaging, including attenuation tomography; seismic ambient noise monitoring; machine learning to detect and classify volcanic earthquakes; active source studies to characterize volcanic flanks and landslides; induced and triggered seismicity in geothermal systems; and seismic sources.

Co-organized by GMPV10/NH2
Convener: João FontielaECSECS | Co-conveners: Graça Silveira, Karin Sigloch, Ricardo Ramalho, Adriano Pimentel
Presentations
| Mon, 23 May, 08:30–09:45 (CEST)
 
Room D3

Presentations: Mon, 23 May | Room D3

Chairpersons: João Fontiela, Graça Silveira
08:30–08:35
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EGU22-12002
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On-site presentation
João Fontiela, Nuno Afonso Dias, Graça Silveira, Mário Moreira, Fernando Carrilho, and Luís Matias
The last eruption in the Azores archipelago occurred in 1998-2000 and took place offshore, broadly 10 km WNW of Terceira. Terceira Island comprises four central polygenetic active volcanoes, Santa Bárbara, Pico Alto, Cinco Picos-Serra do Cume, Caldeira Guilherme Moniz, and a Basaltic Fissural zone. 
To study the seismicity at Terceira Island, we installed a dense seismic network with an average inter-station distance of 5 km. The total number of instruments in use were 31: 12 short-period (2 Hz) and five very short periods (4.5 Hz), both from Institute Dom Luiz (IDL), eight broadband (30s) from the University of Evora (UEv). The very short period instruments were installed around the Pico Alto geothermal power plant to improve the detectability of the micro-seismicity of the zone. The temporary seismic network operated at full capacity for 11 months and later with instruments from UEv and IPMA until the end of 2020. The permanent stations operated by the Instituto Português do Mar e da Atmosfera (IPMA), namely two broadband (120s), two short period (5s) and two accelerometers, completed the temporary network. 
This work presents the preliminary results obtained with the seismic network. We detected some volcano-tectonic earthquakes in this period, mostly related to the Santa Bárbara Volcano and calculated the focal mechanism to the most energetic events. Behind the regular seismicity around the island, we observe an abnormal number of earthquakes in the stations installed in Pico Alto and central part regions.

Acknowledgments: This work is co-funded by national funds through FCT - Fundação para a Ciência e a Tecnologia, I.P., under projects Ref UIDB/04683/2020, UIDB/50019/2020 e UIDP/04683/2020

How to cite: Fontiela, J., Afonso Dias, N., Silveira, G., Moreira, M., Carrilho, F., and Matias, L.: Seismicity of the Terceira Island (Azores) recorded by a temporary seismic network  , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12002, https://doi.org/10.5194/egusphere-egu22-12002, 2022.

08:35–08:40
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EGU22-10733
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On-site presentation
Maria C. Neves, Ana Laura Dias, and Susana Custódio

The relationship between seismicity rates and water load variations has been recognized across the world at various spatial and temporal scales. In the oceans, one of the most notable such observations is that earthquakes at mid-ocean ridges tend to occur preferentially during low tide. In the region of the Azores triple junction, the analysis of a seismic catalogue from 2008 to 2018 revealed that earthquakes in the ocean present a genuine and statistically significant semi-annual seasonality, with more earthquakes occurring in the summer than in the winter. We have looked for mechanisms that could justify this observation. First, we assembled several geophysical time-series of regionally averaged variables that could constitute likely earth loading mechanisms, such as ocean bottom pressure anomalies, and performed a singular spectral analysis to identify and characterize their main modes of variability. Then, we computed the correlation between the possible loading mechanisms and the principal components of the seismicity rate. We found that the variable that best correlates with the seismicity rate (correlation coefficient of 0.9) is the sea level anomaly, which at the Azores latitude presents a marked seasonality related to the barotropic response to changes in wind stress. We therefore suggest that the seismicity peaks during low tide at mid-ocean ridges and the enhanced seismicity in the summer months in the Azores region share an analogous stress triggering mechanism. This work presents the results of Coulomb stress models that help to verify this hypothesis and better understand the relationship between the Earth's deformation and the annual ocean water load variations. The authors would like to acknowledge the financial support of FCT through project UIDB/50019/2020–IDL. This is a contribution to the RESTLESS project PTDC/CTA-GEF/6674/2020.

How to cite: Neves, M. C., Dias, A. L., and Custódio, S.: Investigating the seasonal triggering of earthquakes in the Azores, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10733, https://doi.org/10.5194/egusphere-egu22-10733, 2022.

08:40–08:45
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EGU22-10076
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On-site presentation
Graça Silveira, Joana Carvalho, Sergey Kiselev, Eleonore Stutzmann, and Martin Schimmel

Madeira and Canaries are two intraplate hotspots located in the Northeast Atlantic, west of the Moroccan coast.  Within project SIGHT (SeIsmic and Geochemical constraints on the Madeira HoTspot system) we propose to answer the following questions: a) Is Madeira´s volcanism fed by a deep-seated mantle plume? b) Do the Madeira and Canary hotspots have a common or distinct origin? and c) What is the lithospheric nature of the corridor between the Canaries and the Atlas-Gibraltar?

The recent work of Civiero et al. (2021), combining results from seismic tomography, shear-wave splitting and gravity along with plate reconstruction, revealed that differently evolved upwellings might exist below the volcanic Canary and Madeira islands, with the Madeira hotspot possibly fed by a later-stage plumelet. However, a clear picture of the crust and uppermost mantle is still missing, and questions about how thick the crust is and the eventual presence of crustal underplating still need to be answered. 

We performed an ambient noise tomography using data from 50 seismic stations that we selected carefully to obtain the best inter-station path coverage. We processed the data in the period band between 10 to 50 sec, which will allow us to get, for the first time, a crustal and uppermost mantle tomographic model for the study region. The daily traces were cross-correlated using the phase cross-correlation technique, followed by a time-frequency weighted stack methodology developed by Schimmel et al. (2011). After computing the Rayleigh-wave group-velocity measurements, we inverted them to obtain the 2D group-velocity maps for different periods. In the period band of 10 to
20 s, the velocity maps evince low velocities beneath Madeira and Canary Islands and the Gulf of Cadiz region. Higher velocities characterize the remaining oceanic area. When the period increases (36 s) , some of the Canary Islands show slightly higher velocities, whereas others still present lower velocities. As expected, the low-velocity anomaly beneath the Gulf of Cadiz becomes stronger while the ones beneath the islands become weaker. Even so, the islands still show low velocities.

To determine the depth structure beneath the study area, we extracted velocity values at the different points of the group-velocity maps at different periods. We will then invert them to build a 1D S-wave velocity profile for each grid point as a function of depth. We will discuss the obtained 3D shear-wave velocity maps in the area's geodynamic context.

This is a contribution to projects SIGHT (Ref. PTDC/CTA-GEF/30264/2017). The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL.

How to cite: Silveira, G., Carvalho, J., Kiselev, S., Stutzmann, E., and Schimmel, M.: 3D Imaging of the crust and uppermost mantle of the Northeast Atlantic, from Madeira and Canaries to the Atlas-Gibraltar zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10076, https://doi.org/10.5194/egusphere-egu22-10076, 2022.

08:45–08:50
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EGU22-10184
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On-site presentation
Luciana Bonatto, David Schlaphorst, Graça Silveira, João Mata, Chiara Civiero, Claudia Piromallo, and Martin Schimmel

The Canary and Madeira archipelagos are two hotspots in the Eastern Atlantic (27º to 33º N) that are close (430 km) to each other. Their volcanism is thought to be caused by distinct mantle upwellings. Recent high resolution regional P-wave and S-SKS wave tomography images of the Ibero-western Maghrebian region show subvertical low velocity anomalies under the Canaries, the Atlas ranges and the Gibraltar Arc extending across all the upper mantle to the surface. The anomaly below the Canary archipelago and the Atlas are rooted beneath the mantle transition zone (MTZ) and appear to be connected to a broad and strong low-velocity anomaly in the lower mantle. Beneath Madeira, the slow anomaly has a blob-like shape and is only observed down to ~ 300 km depth, suggesting differences in the development stages of the upwellings at the origin of the two hotspots.

The  globally observed 410 and 660 upper-mantle seismic discontinuities are primarily linked to mineral phase transitions in olivine and the study of their local depth variations constrains the intra-mantle heat and mass transfer processes. The presence of discontinuities that are not globally observed may indicate the presence of compositional heterogeneities. For example, a sharp discontinuity has been detected at a depth of around 300 km (named the X discontinuity) beneath several hotspots (including the Canaries one) that could prove that the dominant peridotitic mantle mantle is locally enriched in basalt compositions. 

Here, we investigate the fine structure of the upper mantle beneath the Canary and Madeira volcanic provinces by means of P-to-S conversions at mantle discontinuities from teleseismic events recorded at 42 seismic stations (24 in the Canaries and 18 in Madeira). We compute 1304 high quality receiver functions (984 in the Canaries and 320 in Madeira) and stack them in the relative time-slowness domain to identify discontinuities in the 200-800 km depth range. Receiver functions are computed in different frequency bands to investigate the sharpness of the observed discontinuities. From the analysis of stacked receiver functions, we obtain robust and clear converted phases from the globally detected 410 and 660 discontinuities beneath both volcanic provinces. However, a reflector at ~300 km depth is only observed beneath the Canaries. For the Canary’s dataset we also detect multiples (Ppds, where d is the discontinuity depth) from the reflector at 300 km and from the 410 discontinuity while for the Madeira’s one, we only detect multiples from the 410. This study allows for a detailed comparison between the two archipelagos. The analysis of arrival times and amplitude of detected phases helps constraining the depth, width, and velocity jump of the observed discontinuities. These parameters and their interpretation based on mineral physics will add new constraints to the understanding of the geodynamical context of the Canary Island and Madeira hotspots. 

This is a contribution to project SIGHT (SeIsmic and Geochemical constraints on the Madeira HoTspot; Ref. PTDC/CTA-GEF/30264/2017). The authors would like to acknowledge the financial support of FCT through project UIDB/50019/2020 – IDL.

How to cite: Bonatto, L., Schlaphorst, D., Silveira, G., Mata, J., Civiero, C., Piromallo, C., and Schimmel, M.: Unveiling the heterogeneous structure of the upper-mantle beneath the Canary and Madeira volcanic provinces, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10184, https://doi.org/10.5194/egusphere-egu22-10184, 2022.

08:50–08:55
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EGU22-9335
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ECS
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Virtual presentation
Eleanor Dunn, Chris Bean, and Andy Bell

Dynamic stress perturbations have triggered earthquakes thousands of kilometres away from the source. This process, known as dynamic triggering, occurs due to dynamic excitation from both local and regional earthquakes which trigger volcanic seismicity and can yield additional information about both the pre-eruptive state of volcanic systems and about material behaviour. Earthquakes are more likely to be triggered on faults already close to failure so dynamic triggering also offers a means to investigate the stress state of the subsurface. However, the mechanisms underpinning dynamic triggering remain enigmatic. Current understanding is confined to statistical studies of the response to many triggered earthquakes in many different crustal volumes with seismicity rates being used as a proxy for the state of stress. Generally, the background stress state does not change significantly during the window of seismic observation. This makes it difficult to study the same seismically active region over an extended period at different stress states. Volcanoes are ideal natural laboratories for studying the factors that influence dynamic triggering as they experience rapid, high-amplitude changes in stress due to magma accumulation and withdrawal. 
 One such example is Sierra Negra, Galápagos Islands, and utilising the current understanding of dynamic triggering observed prior to the 2018 eruption, Sierra Negra, this project aims to resolve some unanswered questions. These include: 1) What new evidence of dynamic triggering is there at Sierra Negra, post-2018 eruption? 2) Is there a critical stress which is reached when Sierra Negra is being reinflated, post-eruption, which leads to subsequent triggering? 3) Are there non-linear wave effects at work? 4) Is there the possibility to compare Sierra Negra to a volcano which may also be demonstrating signs of dynamic triggering e.g., Hekla, Iceland? A collection of seismic data from locations such as Sierra Negra and Hekla will be supported by numerical simulations of dynamic excitation. This project aims to better understand the role that the interplay between ground motion and the properties of a volcanic edifice play in a volcano’s pathway to eruption. This project is part of the Seismological Parameters and INstrumentation Innovative Training Network (SPIN-ITN) funded by the European Commission. The overarching goal of SPIN is to advance seismic observation, theory, and hazard assessment. SPIN is divided into 4 work packages (WP) with each WP consisting of 3-4 PhD projects, hosted at different beneficiary institutes. The majority of the SPIN projects began in September-October 2021.  

How to cite: Dunn, E., Bean, C., and Bell, A.: Ground motion and unrest triggering on volcanoes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9335, https://doi.org/10.5194/egusphere-egu22-9335, 2022.

08:55–09:00
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EGU22-9750
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Presentation form not yet defined
Seismic velocity changes prior to the 2018 eruption of Sierra Negra volcano, Galapagos Island, using coda wave interferometry.
(withdrawn)
Mariantonietta Longobardi, Christopher J Bean, Andrew Forbes Bell, Mario Calixto Ruiz, James Grannell, and Stephen Hernandez
09:00–09:05
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EGU22-10215
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ECS
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Virtual presentation
Joana Carvalho, Graça Silveira, Sergey Kiselev, Susana Custódio, Ricardo Ramalho, Eleonore Stutzmann, and Martin Schimmel

Using seismic data from 38 broadband seismic stations deployed across the volcanic islands of Cape Verde, we construct the first 3D-model of Sv-wave velocities for the uppermost 30 km of the region. We computed phase cross-correlations for vertical component recordings for all possible inter-island stations followed by a time-frequency phase-weighted stack to obtain robust Rayleigh wave group velocity dispersion curves in the period band between 10 s and 24 s. Next, the dispersion curves were inverted, through the Fast Marching Surface Tomography package (FMST), in order to obtain the 2D group velocity-maps. We then inverted the group-velocity maps for the 3D shear-wave velocity structure of the crust and uppermost mantle beneath Cape Verde. As major features we considered the following: 1) low-velocity anomalies beneath and in the vicinities of the islands of Brava and Fogo, which we attribute to the predominance of melting pockets in these islands. Furthermore, the local seismicity also suggests the occurrence of ongoing intrusive processes beneath Fogo and Brava, which translates into a hotter, melt-rich upper crust and uppermost mantle 2) high-velocity anomalies in the northern islands, especially strong in the area surrounding the island of São Nicolau, that can reflect non-altered crust or remnants of magma chambers or solidified basaltic intrusions, which fed the ancient volcanism in these islands. The observed features are also distributed in three domains, according to the island volcanism age and latest major shield-building stages. If this is more than a coincidence, it can reflect different states of thermal maturity of the crust and uppermost mantle as a result of modification by magmatism and as a function of time. Our study, which allowed to image the crustal and uppermost mantle structure beneath Cape Verde, complements earlier deeper structure studies of the region and may also contribute to the characterization of the local seismicity by providing a new velocity model for structure.

How to cite: Carvalho, J., Silveira, G., Kiselev, S., Custódio, S., Ramalho, R., Stutzmann, E., and Schimmel, M.: Crustal and uppermost mantle structure of Cape Verde from ambient noise tomography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10215, https://doi.org/10.5194/egusphere-egu22-10215, 2022.

09:05–09:10
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EGU22-10470
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Virtual presentation
Stéphanie Dumont, Joana Carvalho, Graça Silveira, and Ricardo S. Ramalho

Fogo is a volcanic island located in the Cape Verde Archipelago and is one of the most active volcanoes on Earth, with numerous historical eruptions. Fogo has been widely studied from different perspectives, yet detailed characterization of its seismic structure is still missing, since previous seismic studies chiefly focused on regional features or on magmatic-induced seismicity.

Seismic tomography has proven to be a powerful tool to determine the velocity structure in volcanic environments. The energy necessary to perform such studies can be obtained from the seismicity in volcano's vicinity or from ambient seismic noise. At short periods, it is challenging to get good surface wave dispersion measurements on waveforms resultant from earthquakes due to attenuation and scattering; waveforms retrieved from ambient noise cross-correlations are, however, especially useful to image crustal structure.

In this study we used 14 seismic stations from three different networks deployed on Fogo. Ambient noise cross-correlations were computed for all possible inter-station pairs among the same network, through the phase cross-correlation technique. The empirical Green’s functions (EGF) were then obtained through the time-frequency phase-weighted stack. To decompose the EGFs in the time-frequency domain and thus obtain the dispersion curves of the Rayleigh waves, we applied the multiple filtering analysis (MFA). The Rayleigh wave fundamental mode group velocity curves were then picked manually and visually inspected for periods between 1 to 10 s. Tomographic inversions of the previously obtained group-velocity measurements were performed using the Fast Marching Surface Tomography package (FMST). To obtain the depth structure beneath Fogo, we extracted the values of velocity, from the set of 2D group-velocity maps, for 608 points of the grid, which are, in practical terms, local dispersion curves. The further inversion of these curves enables the construction of 1D S-wave velocity profiles for each node as a function of depth. The resulting 3D shear-wave velocity model shows two clear high-velocity anomalies: a stronger, well-defined tabular anomaly located between ~5 and 9 km of depth and beneath the entire island footprint, and a weaker but distinct anomaly located at 3–4 km of depth and only extending beneath the southwestern island sector, being absent in the northeast where the lowest velocities are attained. We interpret these positive anomalies as the result of intrusions of denser, now cooled sills, pervasively below the island edifice (whose base is located at ~5 km) and within the underlying seafloor sediments and crust (where rheological, density and thermal contrasts favor the emplacement of such intrusions), and higher up within the island edifice, beneath the southwestern sector. This latter positive anomaly is consistent with surface deformation represented by the NW-SE Galinheiros normal fault, which cuts across the island and exhibits ~150 m of vertical displacement, with the southwestern block being elevated relatively to the northeastern one. This study presents the first 3D shear-wave velocity model for Fogo, providing new and better insights into the local volcano-tectonic structure.

This is a contribution to project SIGHT (PTDC/CTA-GEF/30264/2017), RESTLESS (PTDC/CTA-GEF/6674/2020)  and UIDB/50019/2020 – IDL, both funded by FCT.

How to cite: Dumont, S., Carvalho, J., Silveira, G., and Ramalho, R. S.: 3D-ambient noise Rayleigh wave tomography of Fogo volcano, Cape Verde, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10470, https://doi.org/10.5194/egusphere-egu22-10470, 2022.

09:10–09:15
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EGU22-276
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ECS
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Highlight
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On-site presentation
Helena Seivane, Rosa Martín, Javier Almendros, William Wilcock, and Dax Soule

The temporary seismic network deployed from January 2019 to February 2020 in the Bransfield Strait as part of the BRAVOSEIS project has enabled the development of an earthquake catalogue for Orca submarine volcano. A STA/LTA algorithm, manual picking, and the HYPO71 location algorithm with a 1-D model based on previous studies was used to create a catalogue of 4988 earthquakes. The seismicity was characterized by low magnitude events (-1<ML<2.7) occurring mainly in the upper five-kilometers around Orca caldera. Declustering using the Gardner and Knopoff method, reduced the catalogue size by nearly 90%. The declustered catalogue is complete above a magnitude ML of 0.9 and the estimated b-value for the whole period studied is 1.03 +/- 0.18. Because of the noisy the oceanic environments, building the catalogue became an arduous task to perform manually even with a STA/LTA algorithm. Having catalogued such a numerous microseismic events and with the goal of enhancing the catalogue, we apply a super-efficient cross-correlation (SEC-C) method on the continuous network dataset. The effectiveness of SEC-C is soon corroborated by analysing the output of this template matching-based detector. A volcano-tectonic swarm previously catalogued manually between July and August 2019 is clearly identified by the preliminary results of the SEC-C method. The thresholds we imposed for the cross-correlation values and signal-to-noise ratios considered for the workflow from event detection to location were chosen to make the method as ‘blind’ as possible. More than six hundred events have been incorporated after the template matching procedure, considerably augmenting the catalogue.

How to cite: Seivane, H., Martín, R., Almendros, J., Wilcock, W., and Soule, D.: Application of Template Matching to OBS array observation in Orca Volcano (Bransfield Strait, Antarctica), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-276, https://doi.org/10.5194/egusphere-egu22-276, 2022.

09:15–09:25
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EGU22-12084
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ECS
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solicited
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Highlight
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Virtual presentation
Sergio Díaz, Valerie Maupin, Adriano Mazzini, Riccardo Minetto, Matteo Lupi, and Karyono Karyono

Mount Bromo is an andesitic stratovolcano in East Java, Indonesia, that entered into unrest between November 2015 and January 2016. The seismic activity was captured by the permanent seismic stations of the Indonesian seismological service (BMKG) and by a temporary (GIPP-provided) network deployed in the framework of the LusiLab Erc project. The goal of the temporary network deployed was to study the seismic signature of the newborn sediment-hosted geothermal system nicknamed LUSI. A preliminar inspection of the dataset showed that the activity of Bromo may have been recorded by stations of the temporary network. To investigate this further, we attempt an automatic detection and location of the impulsive and emergent signals recorded during Bromo’s eruption. We use the Recursive STA/LTA on each component of the stations and apply a coincidence trigger to adjust the pickings aside with a first-arrival validation through a polarization analysis. A total of 32.787 events were detected, and some of these are consistent with variations in the eruptive activity observed at Mt. Bromo. The accepted locations (RMS ≤ 1; 3.965 events) revealed multiple superficial sources, concentrated between 0 and 5 Km depth, originating from Mt. Bromo and 4 other main volcanic structures located in the surrounding region. Other sources were localized at greater depth, between 10 to 50 Km, and are attributed mainly to interactions between the magmatic chambers of the volcanoes, and movements in pre-existing sutures zones (faults) from overpressure of magmatic activity. Chronologically, a peak preceding the main eruption was found, characterized by an increase in Volcano-Tectonic-type (VT) signals beneath Mt. Bromo. This is consistent with other cases observed at similar strombolian-type volcanoes prior to eruptions. After an assessment of the automatic processing procedure used, we suggest improvements for future works by: 1) applying an association method based on the same principle as the coincidence trigger used in the detection step, and 2) using the polarization analysis in a sliding window along the event signal to re-pick the first-arrivals.

How to cite: Díaz, S., Maupin, V., Mazzini, A., Minetto, R., Lupi, M., and Karyono, K.: Automatic Detection and Location method of Tremor signals: A case study from East Java, Indonesia., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12084, https://doi.org/10.5194/egusphere-egu22-12084, 2022.

09:25–09:35
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EGU22-9225
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solicited
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Highlight
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On-site presentation
David Schlaphorst, Graça Silveira, Ricardo S. Ramalho, Pablo J. González, and Resurrección Antón

The Canary Islands, in the eastern North Atlantic, result from volcanism that is thought to be driven by an underlying mantle upwelling. Due to the movement of islands relative to the hotspot, these get progressively younger from east to west, with La Palma and El Hierro, situated in the north- and south-west of the archipelago being the most recent ones. In addition, those islands have experienced the most recent volcanism in the area (El Hierro: 2011/2012; La Palma: 2021), which was accompanied by large clusters of local seismicity. In the years since the eruption, further seismic clusters could be detected on El Hierro. A better understanding of crustal stress changes can help to monitor ongoing subsurface processes associated with future volcanism.

In this study we present a detailed investigation of crustal seismic anisotropy using shear-wave splitting of local events to estimate splitting parameters and investigate features such as crustal structure or stress due to aligned cracks. The study of anisotropy through shear-wave splitting is a commonly used method to observe dynamic subsurface processes and their influence on the regional stress field. The abundance of data over the last decade allows for a detailed study of temporal variation. Accordingly, using 5 broadband three-component seismic island stations of the IGN network (Instituto Geográfico Nacional) we were able to collect over 200 high quality measurements from 2010 to 2019, the majority of which correspond to syn-eruptive events. Still, nearly half of the events were recorded after 2012, revealing ongoing dynamic crustal processes.

Over the decade, results derived from event clusters show variation of distinct locations around the island. Whereas before and during the eruption results were focused on the northern part of the island, newer clusters were observed on- and offshore to the south of the island. Furthermore, we observe significantly varying fast shear-wave polarisation direction, which in a volcanic environment can be attributed to stress changes due to magma influx as it alters local stress in the crust, or a fabric induced by the lateral intrusion of sills at crustal level and/or beneath the island edifice.

This is a contribution to project SIGHT (SeIsmic and Geochemical constraints on the Madeira HoTspot; Ref. PTDC/CTA-GEF/30264/2017). The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL.

How to cite: Schlaphorst, D., Silveira, G., Ramalho, R. S., González, P. J., and Antón, R.: Temporal variations in fast shear-wave polarisation direction observed during and after the 2011-2012 El Hierro eruption from local shear-wave splitting, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9225, https://doi.org/10.5194/egusphere-egu22-9225, 2022.

09:35–09:45
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EGU22-12158
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ECS
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solicited
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Highlight
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On-site presentation
Ana Laura Lordi Dias, Maria C. Neves, Susana Custódio, and Stephanie Dumont

This work provides an assessment of cyclical variations in seismicity and their relationship with hydrological disturbances in the Azores Triple Junction, looking in particular for seasonal and inter-annual modulations of the earthquake occurrence rate caused by sea-level anomaly and total wave height variations. The work involves the manipulation and the statistical analysis of the Azores seismic catalogue (considering only oceanic events), from 2008 to 2018. We analyzed the seasonal variations of the ocean seismicity by computing the ratio of Winter/Spring (JFMA) events and Summer/Fall (JASO) events, following demonstrated methodologies applied in previous studies in continental areas such as the New Madrid seismic zone and the Himalayan mountains. The seismicity rates in the Azores are higher during Summer/fall (JASO) and lower during Winter/Spring (JFMA), with a ratio JFMA/JASO significantly lower than 1. Different months were also considered for the Winter/Summer ratio (NDJF/MJJA) to observe if the seasonal pattern is still present and statistically significant. The results show that the seasonal variations are better captured when considering the NDJF/MJJA ratio and regions with higher number of events, such as between the Mid-Atlantic Ridge and Faial and Pico islands. Monte Carlo simulations and the Jack-knife approach confirmed that the probability of observing such a seasonality by chance is less than 1% mainly for magnitudes from M3.2 to M5.0, and is not the consequence of extreme deviations. The connection between the seasonal modulation and the hydrological loads was investigated using the Singular Spectrum Analysis. The principal components of the ocean seismicity rate present a strong correlation with the total wave height, and mainly with the sea-level anomaly, which might be possible triggers of the ocean seismicity rate in the Azores region. The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL. This is a contribution to the RESTLESS project PTDC/CTA-GEF/6674/2020.

How to cite: Lordi Dias, A. L., Neves, M. C., Custódio, S., and Dumont, S.: Evidence of seasonal modulation of seismic sequences in the Azores , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12158, https://doi.org/10.5194/egusphere-egu22-12158, 2022.