On 6 February 2023, southeastern Türkiye experienced two Mw 7.7 and 7.6 earthquakes. The earthqua... more On 6 February 2023, southeastern Türkiye experienced two Mw 7.7 and 7.6 earthquakes. The earthquake sequence caused widespread damage and tens of thousands of casualties in Türkiye and Syria. We analyze mainshocks and aftershocks, combining complementary source characterization techniques, relying on local, regional, and teleseismic data. Backprojection analysis and finite source inversion for the mainshocks resolve coseismic slip, rupture length, and propagation mode along the main faults, whereas centroid moment tensor inversion for 221 aftershocks resolves details of the fault network. The first mainshock nucleated on a splay fault and activated the neighboring East Anatolian fault zone (EAFZ). It ruptured bilaterally along ∼500 km first toward northeast and later to south-southwest on multiple, previously partly dormant fault segments. The second mainshock ruptured the east–west-oriented Sürgü-Misis fault zone (SMFZ), reaching a slip of 7 m. The analysis of aftershocks with hete...
The 2021 volcanic eruption at La Palma, Canary Islands, was the island’s most voluminous historic... more The 2021 volcanic eruption at La Palma, Canary Islands, was the island’s most voluminous historical eruption. Little is known about this volcano’s feeding system. During the eruption, seismicity was distributed in two clusters at ~10-14 km and ~33-39 km depth, separated by an aseismic zone. This gap coincides with the location of weak seismic swarms in 2017-2021 and where petrological data have implied pre-eruptive magma storage. Here we use seismological methods to understand the seismic response to magma transfer, with 8,488 hypocentral relocations resolving small-scale seismogenic structures, and 156 moment tensors identifying stress heterogeneities and principal axes flips. Results suggest a long-lasting preparatory stage with the progressive destabilisation of an intermediate, mushy reservoir, and a co-eruptive stage with seismicity controlled by the drainage and interplay of two localised reservoirs. Our study provides new insights into the plumbing system that will improve th...
Reservoir-triggered seismicity has been observed near dams during construction, impoundment, and ... more Reservoir-triggered seismicity has been observed near dams during construction, impoundment, and cyclic filling in many parts of the earth. In Turkey, the number of dams has increased substantially over the last decade, with Atatürk Dam being the largest dam in Turkey with a total water capacity of 48.7 billion m3. After the construction of the dam, the monitoring network has improved. Considering earthquakes above the long-term completeness magnitude of MC= 3.5, the local seismicity rate has substantially increased after the filling of the reservoir. Recently, two damaging earthquakes of Mw5.5 and Mw5.1 occurred in the town of Samsat near the Atatürk Reservoir in 2017 and 2018, respectively. In this study, we analyze the spatio-temporal evolution of seismicity and its source properties in relation to the temporal water-level variations and the stresses resulting from surface loading and pore-pressure diffusion. We find that water-level and seismicity rate are anti-correlated, which...
An earthquake swarm affected the Bransfield Strait, Antarctica, a unique rift basin in transition... more An earthquake swarm affected the Bransfield Strait, Antarctica, a unique rift basin in transition from intra-arc rifting to ocean spreading. The swarm, counting~85,000 volcanotectonic earthquakes since August 2020, is located close to the Orca submarine volcano, previously considered inactive. Simultaneously, geodetic data reported up to~11 cm northwestward displacement over King George Island. We use a broad variety of geophysical data and methods to reveal the complex migration of seismicity, accompanying the intrusion of 0.26-0.56 km 3 of magma. Strike-slip earthquakes mark the intrusion at depth, while shallower normal faulting the~20 km long lateral growth of a dike. Seismicity abruptly decreased after a Mw 6.0 earthquake, suggesting the magmatic dike lost pressure with the slipping of a large fault. A seafloor eruption is likely, but not confirmed by sea surface temperature anomalies. The unrest documents episodic magmatic intrusion in the Bransfield Strait, providing unique insights into active continental rifting.
On October 27, 2017, an Mw4 earthquake occurred close to the municipality of Montesano sulla Marc... more On October 27, 2017, an Mw4 earthquake occurred close to the municipality of Montesano sulla Marcellana, less than 10 km external to the concession of the largest European onshore hydrocarbon reservoir—the Val d’Agri oilfield (Southern Italy). Being a weak event located outside the extended monitoring domain of the industrial concession, the relevance of this earthquake and the possible links with the hydrocarbon exploitation were not extensively discussed. Actually, the analysis of shallow seismic events close to subsurface exploitation domains plays a significant role in the definition of key parameters in order to discriminate between natural, triggered, and induced seismicity, especially in tectonically active regions. The study of weak-to-moderate earthquakes can improve the characterization of the potentially destructive seismic hazard of this particular area, already struck by M > 6.5 episodes in the past. In this work, we analyze the source parameters of this Mw4 earthqua...
The Alpine mountains in central Europe are characterized by a heterogeneous crust accumulating di... more The Alpine mountains in central Europe are characterized by a heterogeneous crust accumulating different tectonic units and blocks in close proximity to sedimentary foreland basins. Centroid moment tensor inversion provides insight into the faulting mechanisms of earthquakes and related tectonic processes, but is significantly aggravated in such an environment. Thanks to the dense AlpArray seismic network and our flexible bootstrap-based inversion tool Grond we are able to test different setups with respect to the uncertainties of the obtained moment tensors and centroid locations. We evaluate the influence of frequency bands, azimuthal gaps, input data types and distance ranges and study the occurrence and reliability of non-DC components. We infer that for most earthquakes (Mw≥3.3) a combination of time domain full waveforms and frequency domain amplitude spectra in a frequency band of 0.02-0.07 Hz is suitable. Relying on the results of our methodological tests, we perform deviatoric MT inversions for events with Mw>3.0. We present here 75 solutions and analyse our results in the seismo-tectonic context of historic earthquakes, seismic activity of the last three decades and GNSS deformation data. We study regions of high seismic activity, namely the western Alps, the region around Lake Garda, the SE Alps, besides clusters further from the study region, in the northern Dinarides and the Apennines. Seismicity is particularly low in the eastern Alps and in parts of the central Alps. We apply a clustering algorithm to focal mechanisms, considering additional focal mechanisms from existing catalogs. Related to the NS compressional regime, E-W to ENE-WSW striking thrust faulting is mainly observed in the Friuli area in the SE Alps. Strike-slip faulting with a similarly oriented pressure axis is observed along the northern margin of the central Alps and in the northern Dinarides. NW-SE striking normal faulting is observed in the NW Alps showing a similar strike direction as normal faulting earthquakes in the Apennines. Both, our centroid depths as well as hypocentral depths in existing catalogs indicate that Alpine seismicity is predominantly very shallow; about 80% of the studied events have depths shallower than 10 km.
Reykjavik is almost entirely heated by geothermal energy. Yet, recent growth of the city signific... more Reykjavik is almost entirely heated by geothermal energy. Yet, recent growth of the city significantly increased the heat demand. Past experiences in Iceland's capital region showed that hydraulic stimulation of existing geothermal wells is suited to improve hydraulic performance and energy supply. However, fluid injection may also trigger felt or even damaging earthquakes, which are of concern in populated areas and pose a significant risk to stimulation operations. Consequently, soft stimulation concepts have been developed to increase geothermal well performance while minimizing environmental effects such as induced seismicity. In a demonstration project of hydraulic soft stimulation in October 2019, more than 20.000 m 3 of water were injected into well RV-43 in Reykjavik in multiple stages and with different injection schemes. The hydraulic performance of the well was improved without inducing felt seismicity. An a priori seismic risk assessment was conducted and for the first time the risk was continuously updated by an adaptive traffic light system supported by a sophisticated realtime microseismic monitoring. Our results confirm that it is possible to improve the performance of geothermal wells in Reykjavik and worldwide with acceptable technical, economic, and environmental risks. Here we provide an overview of the entire stimulation project including site description, stimulation design, zonal isolation, logging, seismic risk assessment and mitigation measures, realtime seismic, hydraulic and chemical monitoring, and stimulation results and challenges.
The 24 January 2020 Mw 6.77 Elazığ-Sivrice earthquake (Turkey), responsible for 42 casualties and... more The 24 January 2020 Mw 6.77 Elazığ-Sivrice earthquake (Turkey), responsible for 42 casualties and ~1600 injured people, is the largest earthquake affecting the East Anatolian Fault (EAF) since 1971. The earthquake partially ruptured a seismic gap. The mainshock was preceded by two foreshocks with Mw ≥ 4.9 and small seismicity clusters occurring in the previous months close to the nucleation point of the main rupture. The significant aftershock sequence comprises twelve earthquakes with Mw≥4.5 within 60 days. We jointly model quasi co-seismic static surface displacements from Interferometric Synthetic Aperture Radar (InSAR) and high-frequency co-seismic data from seismological networks at local, regional and teleseismic distances to retrieve source parameters of the mainshock. We reconstruct the rupture process using a Bayesian bootstrap based probabilistic joint inversion scheme to obtain source parameters and their uncertainties. Full moment tensor for 18 fore-/after-shocks with Mw≥4.3 are obtained based on the modeling of regional broadband data. The posterior mean model for the 2020 Elazığ-Sivrice mainshock shows that the earthquake, with a magnitude Mw 6.77, ruptured at shallow depth (5±2 km) with a left-lateral strike-slip focal mechanism, with a dip angle of 74°±2° and a causative fault plane strike of 242°±1°, which is compatible with the orientation of the EAF at the centroid location. The rupture nucleated in the vicinity of small foreshock clusters and slowly propagated towards WSW, with a rupture velocity of 2100±130 m s-1 and ~27 s rupture duration. The main rupture area, with a length of ~26±5 km, only covered 70% of the former seismic gap, leaving a smaller, unbroken segment of ~30 km length to the SE with positive stress change. The subsequent aftershock sequence extended over a broader region of ~70 km in length, spreading to both sides of the mainshock rupture patch into the regions experiencing a stress increase according to our Coulomb stress modeling. Our results support the hypothesis of a shallow locking depth of the Anatolian micro-plate, which has a possible implication to the seismic bursts along the EAF and alternating seismic activity on the North Anatolian and the East Anatolian faults. 1
SUMMARYA mid-aperture broad-band test array (OBS array DOCTAR) was deployed from June 2011 to Apr... more SUMMARYA mid-aperture broad-band test array (OBS array DOCTAR) was deployed from June 2011 to April 2012 about 100 km north of the Gloria fault in the Eastern North Atlantic in about 5000 m water depth. In addition arrays were installed on Madeira Island and in western Portugal mainland. For the first time in the Eastern North Atlantic, we recorded a large number of high frequency Po and So waves from local and regional small and moderate earthquakes (ML < 4). An incoherent beamforming method was adapted to scan continuous data for such Po and So arrivals applying a sliding window waveform migration and frequency–wavenumber technique. We identify about 320 Po and 1550 So arrivals and compare the phase onsets with the ISC catalogue (ISC 2015) for the same time span. Up to a distance of 6° to the DOCTAR stations all events listed in the ISC catalogue could be associated to Po and So phases. Arrivals from events in more than 10° distance could be identified only in some cases. Only ...
Seismic swarms close to volcanoes often signal the onset of unrest. Establishing whether magma is... more Seismic swarms close to volcanoes often signal the onset of unrest. Establishing whether magma is the culprit and the unrest can be flagged as magmatic may be challenging. Here we analyze the spatio-temporal pattern of a seismic swarm that occurred in November 2015-February 2016 around Jailolo volcano, a long-dormant and poorly studied volcano located on Halmahera island, North Moluccas, Indonesia. The swarm included four M w > 5 earthquakes and hundreds of events were felt by the population. We relocate the earthquakes using both the Indonesian Seismic Network and singlestation location techniques. We find that the earthquakes cluster in a narrow strip, stretching 5 km E-W and 20 km N-S, migrating southward away from Jailolo volcano at ∼10 km/d. We investigate the source mechanisms of the largest earthquakes via full moment tensor inversion. The non-double-couple component is around 50%, such that the earthquakes, besides normal faulting, included a relatively large opening component. After a thorough examination of the possible causes of the Jailolo swarm we conclude that a laterally propagating dike of tens of millions of cubic meters is the triggering mechanism for the seismicity. The swarm marks the first documented magmatic unrest at Jailolo. We find that there is a probability >0.1 that the unrest will last for more than 2 years. This magmatic unrest calls for the classification of Jailolo volcano as active, and for an urgent assessment of the associated volcanic hazard.
We develop an amplitude spectral ratio method for event couples from clustered earthquakes to est... more We develop an amplitude spectral ratio method for event couples from clustered earthquakes to estimate seismic wave attenuation (Q −1) in the source volume. The method allows to study attenuation within the source region of earthquake swarms or aftershocks at depth, independent of wave path and attenuation between source region and surface station. We exploit the high frequency slope of phase spectra using multitaper spectral estimates. The method is tested using simulated full wavefield seismograms affected by recorded noise and finite source rupture. The synthetic tests verify the approach and show that solutions are independent of focal mechanisms, but also show that seismic noise may broaden the scatter of results. We apply the event couple spectral ratio method to NorthWest Bohemia, Czech Republic, a region characterized by the persistent occurrence of earthquake swarms in a confined source region at mid-crustal depth. Our method indicates a strong anomaly of high attenuation in the source region of the swarm with an averaged attenuation factor of Q p < 100. The application to S phases fails due to scattered P phase energy interfering with S phases. The Q p anomaly supports the common hypothesis of highly fractured and fluid saturated rocks in the source region of the swarms in NorthWest Bohemia. However, high temperatures in a small volume around the swarms cannot be excluded to explain our observations. 1 Introduction The intrinsic and scattering attenuation of the amplitudes of seismic waves is described by the dimensionless factor Q. The mapping of spatio-temporal changes of Q is an important step in seismology, since Q is controlled by temperature, rock porosity, fluid saturation and rock composition (Toksöz et al., 1981). Hence, this factor may help to unravel the possible causes of fluid-induced earthquakes, or thermal anomalies in crustal regions affected by magmatic intrusions. For instance, NorthWest Bohemia is regularly affected by earthquake swarms lasting several days or weeks with thousands of recorded events with largest magnitudes up to Ml 4.4 (Fischer et al., 2014). The causes of the repeated earthquake swarms which occur in narrow focal zones remain under debate. Relative earthquake localizations are very precise because of the high waveform quality recorded with a dense local permanent network (Bouchaala et al., 2013). Different tomography studies revealed consistent fig-1 Copyright statement.
A crucial issue to characterize hydraulic fractures is the robust, accurate and automated detecti... more A crucial issue to characterize hydraulic fractures is the robust, accurate and automated detection and location of acoustic emissions (AE) associated with the fracture nucleation and growth process. Waveform stacking and coherence analysis techniques are here adapted using massive datasets with very high sampling (1 MHz) from a hydraulic fracturing experiment that took place 410 m below surface in the Äspö Hard Rock Laboratory (Sweden). We present the results obtained during the conventional, continuous water-injection experiment HF2 (Hydraulic Fracture 2). The resulting catalogue is composed of more than 4000 AEs. Frequency-magnitude distribution from AE magnitudes (M AE) reveals a high b-value of 2.4. The magnitude of completeness is also estimated approximately M AE 1.1 and we observe an interval range of M AE between 0.77 and 2.79. The hydraulic fractures growth is then characterized by mapping the spatiotemporal evolution of AE hypocentres. The AE activity is spatially clustered in a prolate ellipsoid, resembling the main activated fracture volume (~ 105 m 3), where the lengths of the principal axes (a = 10 m; b = 5 m; c = 4 m) define its size and its orientation can be estimated for a rupture plane (strike ~ 123°, dip ~ 60°). An asymmetric rupture process regarding to the fracturing borehole is clearly exhibited. AE events migrate upwards covering the depth interval between 404 and 414 m. After completing each injection and reinjection phase, the AE activity decreases and appears located in the same area of the initial fracture phase, suggesting a crackclosing effect.
Journal of Geophysical Research: Solid Earth, 2017
Receiver functions (RF) have been used for several decades to study structures beneath seismic st... more Receiver functions (RF) have been used for several decades to study structures beneath seismic stations. Although most available stations are deployed on shore, the number of ocean bottom station (OBS) experiments has increased in recent years. Almost all OBSs have to deal with higher noise levels and a limited deployment time (∼1 year), resulting in a small number of usable records of teleseismic earthquakes. Here we use OBSs deployed as midaperture array in the deep ocean (4.5-5.5 km water depth) of the eastern mid-Atlantic. We use evaluation criteria for OBS data and beamforming to enhance the quality of the RFs. Although some stations show reverberations caused by sedimentary cover, we are able to identify the Moho signal, indicating a normal thickness (5-8 km) of oceanic crust. Observations at single stations with thin sediments (300-400 m) indicate that a probable sharp lithosphere-asthenosphere boundary (LAB) might exist at a depth of ∼70-80 km which is in line with LAB depth estimates for similar lithospheric ages in the Pacific. The mantle discontinuities at ∼410 km and ∼660 km are clearly identifiable. Their delay times are in agreement with PREM. Overall the usage of beam-formed earthquake recordings for OBS RF analysis is an excellent way to increase the signal quality and the number of usable events.
Due to the deep socioeconomic implications, induced seismicity is a timely and increasingly relev... more Due to the deep socioeconomic implications, induced seismicity is a timely and increasingly relevant topic of interest for the general public. Cases of induced seismicity have a global distribution and involve a large number of industrial operations, with many documented cases from as far back to the beginning of the twentieth century. However, the sparse and fragmented documentation available makes it difficult to have a clear picture on our understanding of the physical phenomenon and consequently in our ability to mitigate the risk associated with induced seismicity. This review presents a unified and concise summary of the still open questions related to monitoring, discrimination, and management of induced seismicity in the European context and, when possible, provides potential answers. We further discuss selected critical European cases of induced seismicity, which led to the suspension or reduction of the related industrial activities.
Large volcanic eruptions on Earth commonly occur with a collapse of the roof of a crustal magma r... more Large volcanic eruptions on Earth commonly occur with a collapse of the roof of a crustal magma reservoir, forming a caldera. Only a few such collapses occur per century, and the lack of detailed observations has obscured insight into the mechanical interplay between collapse and eruption. We use multiparameter geophysical and geochemical data to show that the 110-square-kilometer and 65-meter-deep collapse of Bárdarbunga caldera in 2014-2015 was initiated through withdrawal of magma, and lateral migration through a 48-kilometers-long dike, from a 12-kilometers deep reservoir. Interaction between the pressure exerted by the subsiding reservoir roof and the physical properties of the subsurface flow path explain the gradual, near-exponential decline of both collapse rate and the intensity of the 180-day-long eruption.
Journal of Geophysical Research: Solid Earth, 2015
Earthquakes occurring close to hydrocarbon fields under production are often under critical view ... more Earthquakes occurring close to hydrocarbon fields under production are often under critical view of being induced or triggered. However, clear and testable rules to discriminate the different events have rarely been developed and tested. The unresolved scientific problem may lead to lengthy public disputes with unpredictable impact on the local acceptance of the exploitation and field operations. We propose a quantitative approach to discriminate induced, triggered, and natural earthquakes, which is based on testable input parameters. Maxima of occurrence probabilities are compared for the cases under question, and a single probability of being triggered or induced is reported. The uncertainties of earthquake location and other input parameters are considered in terms of the integration over probability density functions. The probability that events have been human triggered/induced is derived from the modeling of Coulomb stress changes and a rate and state‐dependent seismicity mode...
A spatially localized seismic sequence originated few tens of kilometres offshore the Mediterrane... more A spatially localized seismic sequence originated few tens of kilometres offshore the Mediterranean coast of Spain, close to the Ebro river delta, starting on 2013 September 5, and lasting at least until 2013 October. The sequence culminated in a maximal moment magnitude M w 4.3 earthquake, on 2013 October 1. The most relevant seismogenic feature in the area is the Fosa de Amposta fault system, which includes different strands mapped at different distances to the coast, with a general NE-SW orientation, roughly parallel to the coastline. However, no significant known historical seismicity has involved this fault system in the past. The epicentral region is also located near the offshore platform of the Castor project, where gas is conducted through a pipeline from mainland and where it was recently injected in a depleted oil reservoir, at about 2 km depth. We analyse the temporal evolution of the seismic sequence and use full waveform techniques to derive absolute and relative locations, estimate depths and focal mechanisms for the largest events in the sequence (with magnitude mbLg larger than 3), and compare them to a previous event (2012 April 8, mbLg 3.3) taking place in the same region prior to the gas injection. Moment tensor inversion results show that the overall seismicity in this sequence is characterized by oblique mechanisms with a normal fault component, with a 30 • low-dip angle plane oriented NNE-SSW and a subvertical plane oriented NW-SE. The combined analysis of hypocentral location and focal mechanisms could indicate that the seismic sequence corresponds to rupture processes along shallow low-dip surfaces, which could have been triggered by the gas injection in the reservoir, and excludes the activation of the Amposta fault, as its known orientation is inconsistent with focal mechanism results. An alternative scenario includes the iterated triggering of a system of steep faults oriented NW-SE, which were identified by prior marine seismics investigations.
On 1 April 2014, Northern Chile was struck by a magnitude 8.1 earthquake following a protracted s... more On 1 April 2014, Northern Chile was struck by a magnitude 8.1 earthquake following a protracted series of foreshocks. The Integrated Plate Boundary Observatory Chile monitored the entire sequence of events, providing unprecedented resolution of the build-up to the main event and its rupture evolution. Here we show that the Iquique earthquake broke a central fraction of the so-called northern Chile seismic gap, the last major segment of the South American plate boundary that had not ruptured in the past century. Since July 2013 three seismic clusters, each lasting a few weeks, hit this part of the plate boundary with earthquakes of increasing peak magnitudes. Starting with the second cluster, geodetic observations show surface displacements that can be associated with slip on the plate interface. These seismic clusters and their slip transients occupied a part of the plate interface that was transitional between a fully locked and a creeping portion. Leading up to this earthquake, th...
On 6 February 2023, southeastern Türkiye experienced two Mw 7.7 and 7.6 earthquakes. The earthqua... more On 6 February 2023, southeastern Türkiye experienced two Mw 7.7 and 7.6 earthquakes. The earthquake sequence caused widespread damage and tens of thousands of casualties in Türkiye and Syria. We analyze mainshocks and aftershocks, combining complementary source characterization techniques, relying on local, regional, and teleseismic data. Backprojection analysis and finite source inversion for the mainshocks resolve coseismic slip, rupture length, and propagation mode along the main faults, whereas centroid moment tensor inversion for 221 aftershocks resolves details of the fault network. The first mainshock nucleated on a splay fault and activated the neighboring East Anatolian fault zone (EAFZ). It ruptured bilaterally along ∼500 km first toward northeast and later to south-southwest on multiple, previously partly dormant fault segments. The second mainshock ruptured the east–west-oriented Sürgü-Misis fault zone (SMFZ), reaching a slip of 7 m. The analysis of aftershocks with hete...
The 2021 volcanic eruption at La Palma, Canary Islands, was the island’s most voluminous historic... more The 2021 volcanic eruption at La Palma, Canary Islands, was the island’s most voluminous historical eruption. Little is known about this volcano’s feeding system. During the eruption, seismicity was distributed in two clusters at ~10-14 km and ~33-39 km depth, separated by an aseismic zone. This gap coincides with the location of weak seismic swarms in 2017-2021 and where petrological data have implied pre-eruptive magma storage. Here we use seismological methods to understand the seismic response to magma transfer, with 8,488 hypocentral relocations resolving small-scale seismogenic structures, and 156 moment tensors identifying stress heterogeneities and principal axes flips. Results suggest a long-lasting preparatory stage with the progressive destabilisation of an intermediate, mushy reservoir, and a co-eruptive stage with seismicity controlled by the drainage and interplay of two localised reservoirs. Our study provides new insights into the plumbing system that will improve th...
Reservoir-triggered seismicity has been observed near dams during construction, impoundment, and ... more Reservoir-triggered seismicity has been observed near dams during construction, impoundment, and cyclic filling in many parts of the earth. In Turkey, the number of dams has increased substantially over the last decade, with Atatürk Dam being the largest dam in Turkey with a total water capacity of 48.7 billion m3. After the construction of the dam, the monitoring network has improved. Considering earthquakes above the long-term completeness magnitude of MC= 3.5, the local seismicity rate has substantially increased after the filling of the reservoir. Recently, two damaging earthquakes of Mw5.5 and Mw5.1 occurred in the town of Samsat near the Atatürk Reservoir in 2017 and 2018, respectively. In this study, we analyze the spatio-temporal evolution of seismicity and its source properties in relation to the temporal water-level variations and the stresses resulting from surface loading and pore-pressure diffusion. We find that water-level and seismicity rate are anti-correlated, which...
An earthquake swarm affected the Bransfield Strait, Antarctica, a unique rift basin in transition... more An earthquake swarm affected the Bransfield Strait, Antarctica, a unique rift basin in transition from intra-arc rifting to ocean spreading. The swarm, counting~85,000 volcanotectonic earthquakes since August 2020, is located close to the Orca submarine volcano, previously considered inactive. Simultaneously, geodetic data reported up to~11 cm northwestward displacement over King George Island. We use a broad variety of geophysical data and methods to reveal the complex migration of seismicity, accompanying the intrusion of 0.26-0.56 km 3 of magma. Strike-slip earthquakes mark the intrusion at depth, while shallower normal faulting the~20 km long lateral growth of a dike. Seismicity abruptly decreased after a Mw 6.0 earthquake, suggesting the magmatic dike lost pressure with the slipping of a large fault. A seafloor eruption is likely, but not confirmed by sea surface temperature anomalies. The unrest documents episodic magmatic intrusion in the Bransfield Strait, providing unique insights into active continental rifting.
On October 27, 2017, an Mw4 earthquake occurred close to the municipality of Montesano sulla Marc... more On October 27, 2017, an Mw4 earthquake occurred close to the municipality of Montesano sulla Marcellana, less than 10 km external to the concession of the largest European onshore hydrocarbon reservoir—the Val d’Agri oilfield (Southern Italy). Being a weak event located outside the extended monitoring domain of the industrial concession, the relevance of this earthquake and the possible links with the hydrocarbon exploitation were not extensively discussed. Actually, the analysis of shallow seismic events close to subsurface exploitation domains plays a significant role in the definition of key parameters in order to discriminate between natural, triggered, and induced seismicity, especially in tectonically active regions. The study of weak-to-moderate earthquakes can improve the characterization of the potentially destructive seismic hazard of this particular area, already struck by M > 6.5 episodes in the past. In this work, we analyze the source parameters of this Mw4 earthqua...
The Alpine mountains in central Europe are characterized by a heterogeneous crust accumulating di... more The Alpine mountains in central Europe are characterized by a heterogeneous crust accumulating different tectonic units and blocks in close proximity to sedimentary foreland basins. Centroid moment tensor inversion provides insight into the faulting mechanisms of earthquakes and related tectonic processes, but is significantly aggravated in such an environment. Thanks to the dense AlpArray seismic network and our flexible bootstrap-based inversion tool Grond we are able to test different setups with respect to the uncertainties of the obtained moment tensors and centroid locations. We evaluate the influence of frequency bands, azimuthal gaps, input data types and distance ranges and study the occurrence and reliability of non-DC components. We infer that for most earthquakes (Mw≥3.3) a combination of time domain full waveforms and frequency domain amplitude spectra in a frequency band of 0.02-0.07 Hz is suitable. Relying on the results of our methodological tests, we perform deviatoric MT inversions for events with Mw>3.0. We present here 75 solutions and analyse our results in the seismo-tectonic context of historic earthquakes, seismic activity of the last three decades and GNSS deformation data. We study regions of high seismic activity, namely the western Alps, the region around Lake Garda, the SE Alps, besides clusters further from the study region, in the northern Dinarides and the Apennines. Seismicity is particularly low in the eastern Alps and in parts of the central Alps. We apply a clustering algorithm to focal mechanisms, considering additional focal mechanisms from existing catalogs. Related to the NS compressional regime, E-W to ENE-WSW striking thrust faulting is mainly observed in the Friuli area in the SE Alps. Strike-slip faulting with a similarly oriented pressure axis is observed along the northern margin of the central Alps and in the northern Dinarides. NW-SE striking normal faulting is observed in the NW Alps showing a similar strike direction as normal faulting earthquakes in the Apennines. Both, our centroid depths as well as hypocentral depths in existing catalogs indicate that Alpine seismicity is predominantly very shallow; about 80% of the studied events have depths shallower than 10 km.
Reykjavik is almost entirely heated by geothermal energy. Yet, recent growth of the city signific... more Reykjavik is almost entirely heated by geothermal energy. Yet, recent growth of the city significantly increased the heat demand. Past experiences in Iceland's capital region showed that hydraulic stimulation of existing geothermal wells is suited to improve hydraulic performance and energy supply. However, fluid injection may also trigger felt or even damaging earthquakes, which are of concern in populated areas and pose a significant risk to stimulation operations. Consequently, soft stimulation concepts have been developed to increase geothermal well performance while minimizing environmental effects such as induced seismicity. In a demonstration project of hydraulic soft stimulation in October 2019, more than 20.000 m 3 of water were injected into well RV-43 in Reykjavik in multiple stages and with different injection schemes. The hydraulic performance of the well was improved without inducing felt seismicity. An a priori seismic risk assessment was conducted and for the first time the risk was continuously updated by an adaptive traffic light system supported by a sophisticated realtime microseismic monitoring. Our results confirm that it is possible to improve the performance of geothermal wells in Reykjavik and worldwide with acceptable technical, economic, and environmental risks. Here we provide an overview of the entire stimulation project including site description, stimulation design, zonal isolation, logging, seismic risk assessment and mitigation measures, realtime seismic, hydraulic and chemical monitoring, and stimulation results and challenges.
The 24 January 2020 Mw 6.77 Elazığ-Sivrice earthquake (Turkey), responsible for 42 casualties and... more The 24 January 2020 Mw 6.77 Elazığ-Sivrice earthquake (Turkey), responsible for 42 casualties and ~1600 injured people, is the largest earthquake affecting the East Anatolian Fault (EAF) since 1971. The earthquake partially ruptured a seismic gap. The mainshock was preceded by two foreshocks with Mw ≥ 4.9 and small seismicity clusters occurring in the previous months close to the nucleation point of the main rupture. The significant aftershock sequence comprises twelve earthquakes with Mw≥4.5 within 60 days. We jointly model quasi co-seismic static surface displacements from Interferometric Synthetic Aperture Radar (InSAR) and high-frequency co-seismic data from seismological networks at local, regional and teleseismic distances to retrieve source parameters of the mainshock. We reconstruct the rupture process using a Bayesian bootstrap based probabilistic joint inversion scheme to obtain source parameters and their uncertainties. Full moment tensor for 18 fore-/after-shocks with Mw≥4.3 are obtained based on the modeling of regional broadband data. The posterior mean model for the 2020 Elazığ-Sivrice mainshock shows that the earthquake, with a magnitude Mw 6.77, ruptured at shallow depth (5±2 km) with a left-lateral strike-slip focal mechanism, with a dip angle of 74°±2° and a causative fault plane strike of 242°±1°, which is compatible with the orientation of the EAF at the centroid location. The rupture nucleated in the vicinity of small foreshock clusters and slowly propagated towards WSW, with a rupture velocity of 2100±130 m s-1 and ~27 s rupture duration. The main rupture area, with a length of ~26±5 km, only covered 70% of the former seismic gap, leaving a smaller, unbroken segment of ~30 km length to the SE with positive stress change. The subsequent aftershock sequence extended over a broader region of ~70 km in length, spreading to both sides of the mainshock rupture patch into the regions experiencing a stress increase according to our Coulomb stress modeling. Our results support the hypothesis of a shallow locking depth of the Anatolian micro-plate, which has a possible implication to the seismic bursts along the EAF and alternating seismic activity on the North Anatolian and the East Anatolian faults. 1
SUMMARYA mid-aperture broad-band test array (OBS array DOCTAR) was deployed from June 2011 to Apr... more SUMMARYA mid-aperture broad-band test array (OBS array DOCTAR) was deployed from June 2011 to April 2012 about 100 km north of the Gloria fault in the Eastern North Atlantic in about 5000 m water depth. In addition arrays were installed on Madeira Island and in western Portugal mainland. For the first time in the Eastern North Atlantic, we recorded a large number of high frequency Po and So waves from local and regional small and moderate earthquakes (ML < 4). An incoherent beamforming method was adapted to scan continuous data for such Po and So arrivals applying a sliding window waveform migration and frequency–wavenumber technique. We identify about 320 Po and 1550 So arrivals and compare the phase onsets with the ISC catalogue (ISC 2015) for the same time span. Up to a distance of 6° to the DOCTAR stations all events listed in the ISC catalogue could be associated to Po and So phases. Arrivals from events in more than 10° distance could be identified only in some cases. Only ...
Seismic swarms close to volcanoes often signal the onset of unrest. Establishing whether magma is... more Seismic swarms close to volcanoes often signal the onset of unrest. Establishing whether magma is the culprit and the unrest can be flagged as magmatic may be challenging. Here we analyze the spatio-temporal pattern of a seismic swarm that occurred in November 2015-February 2016 around Jailolo volcano, a long-dormant and poorly studied volcano located on Halmahera island, North Moluccas, Indonesia. The swarm included four M w > 5 earthquakes and hundreds of events were felt by the population. We relocate the earthquakes using both the Indonesian Seismic Network and singlestation location techniques. We find that the earthquakes cluster in a narrow strip, stretching 5 km E-W and 20 km N-S, migrating southward away from Jailolo volcano at ∼10 km/d. We investigate the source mechanisms of the largest earthquakes via full moment tensor inversion. The non-double-couple component is around 50%, such that the earthquakes, besides normal faulting, included a relatively large opening component. After a thorough examination of the possible causes of the Jailolo swarm we conclude that a laterally propagating dike of tens of millions of cubic meters is the triggering mechanism for the seismicity. The swarm marks the first documented magmatic unrest at Jailolo. We find that there is a probability >0.1 that the unrest will last for more than 2 years. This magmatic unrest calls for the classification of Jailolo volcano as active, and for an urgent assessment of the associated volcanic hazard.
We develop an amplitude spectral ratio method for event couples from clustered earthquakes to est... more We develop an amplitude spectral ratio method for event couples from clustered earthquakes to estimate seismic wave attenuation (Q −1) in the source volume. The method allows to study attenuation within the source region of earthquake swarms or aftershocks at depth, independent of wave path and attenuation between source region and surface station. We exploit the high frequency slope of phase spectra using multitaper spectral estimates. The method is tested using simulated full wavefield seismograms affected by recorded noise and finite source rupture. The synthetic tests verify the approach and show that solutions are independent of focal mechanisms, but also show that seismic noise may broaden the scatter of results. We apply the event couple spectral ratio method to NorthWest Bohemia, Czech Republic, a region characterized by the persistent occurrence of earthquake swarms in a confined source region at mid-crustal depth. Our method indicates a strong anomaly of high attenuation in the source region of the swarm with an averaged attenuation factor of Q p < 100. The application to S phases fails due to scattered P phase energy interfering with S phases. The Q p anomaly supports the common hypothesis of highly fractured and fluid saturated rocks in the source region of the swarms in NorthWest Bohemia. However, high temperatures in a small volume around the swarms cannot be excluded to explain our observations. 1 Introduction The intrinsic and scattering attenuation of the amplitudes of seismic waves is described by the dimensionless factor Q. The mapping of spatio-temporal changes of Q is an important step in seismology, since Q is controlled by temperature, rock porosity, fluid saturation and rock composition (Toksöz et al., 1981). Hence, this factor may help to unravel the possible causes of fluid-induced earthquakes, or thermal anomalies in crustal regions affected by magmatic intrusions. For instance, NorthWest Bohemia is regularly affected by earthquake swarms lasting several days or weeks with thousands of recorded events with largest magnitudes up to Ml 4.4 (Fischer et al., 2014). The causes of the repeated earthquake swarms which occur in narrow focal zones remain under debate. Relative earthquake localizations are very precise because of the high waveform quality recorded with a dense local permanent network (Bouchaala et al., 2013). Different tomography studies revealed consistent fig-1 Copyright statement.
A crucial issue to characterize hydraulic fractures is the robust, accurate and automated detecti... more A crucial issue to characterize hydraulic fractures is the robust, accurate and automated detection and location of acoustic emissions (AE) associated with the fracture nucleation and growth process. Waveform stacking and coherence analysis techniques are here adapted using massive datasets with very high sampling (1 MHz) from a hydraulic fracturing experiment that took place 410 m below surface in the Äspö Hard Rock Laboratory (Sweden). We present the results obtained during the conventional, continuous water-injection experiment HF2 (Hydraulic Fracture 2). The resulting catalogue is composed of more than 4000 AEs. Frequency-magnitude distribution from AE magnitudes (M AE) reveals a high b-value of 2.4. The magnitude of completeness is also estimated approximately M AE 1.1 and we observe an interval range of M AE between 0.77 and 2.79. The hydraulic fractures growth is then characterized by mapping the spatiotemporal evolution of AE hypocentres. The AE activity is spatially clustered in a prolate ellipsoid, resembling the main activated fracture volume (~ 105 m 3), where the lengths of the principal axes (a = 10 m; b = 5 m; c = 4 m) define its size and its orientation can be estimated for a rupture plane (strike ~ 123°, dip ~ 60°). An asymmetric rupture process regarding to the fracturing borehole is clearly exhibited. AE events migrate upwards covering the depth interval between 404 and 414 m. After completing each injection and reinjection phase, the AE activity decreases and appears located in the same area of the initial fracture phase, suggesting a crackclosing effect.
Journal of Geophysical Research: Solid Earth, 2017
Receiver functions (RF) have been used for several decades to study structures beneath seismic st... more Receiver functions (RF) have been used for several decades to study structures beneath seismic stations. Although most available stations are deployed on shore, the number of ocean bottom station (OBS) experiments has increased in recent years. Almost all OBSs have to deal with higher noise levels and a limited deployment time (∼1 year), resulting in a small number of usable records of teleseismic earthquakes. Here we use OBSs deployed as midaperture array in the deep ocean (4.5-5.5 km water depth) of the eastern mid-Atlantic. We use evaluation criteria for OBS data and beamforming to enhance the quality of the RFs. Although some stations show reverberations caused by sedimentary cover, we are able to identify the Moho signal, indicating a normal thickness (5-8 km) of oceanic crust. Observations at single stations with thin sediments (300-400 m) indicate that a probable sharp lithosphere-asthenosphere boundary (LAB) might exist at a depth of ∼70-80 km which is in line with LAB depth estimates for similar lithospheric ages in the Pacific. The mantle discontinuities at ∼410 km and ∼660 km are clearly identifiable. Their delay times are in agreement with PREM. Overall the usage of beam-formed earthquake recordings for OBS RF analysis is an excellent way to increase the signal quality and the number of usable events.
Due to the deep socioeconomic implications, induced seismicity is a timely and increasingly relev... more Due to the deep socioeconomic implications, induced seismicity is a timely and increasingly relevant topic of interest for the general public. Cases of induced seismicity have a global distribution and involve a large number of industrial operations, with many documented cases from as far back to the beginning of the twentieth century. However, the sparse and fragmented documentation available makes it difficult to have a clear picture on our understanding of the physical phenomenon and consequently in our ability to mitigate the risk associated with induced seismicity. This review presents a unified and concise summary of the still open questions related to monitoring, discrimination, and management of induced seismicity in the European context and, when possible, provides potential answers. We further discuss selected critical European cases of induced seismicity, which led to the suspension or reduction of the related industrial activities.
Large volcanic eruptions on Earth commonly occur with a collapse of the roof of a crustal magma r... more Large volcanic eruptions on Earth commonly occur with a collapse of the roof of a crustal magma reservoir, forming a caldera. Only a few such collapses occur per century, and the lack of detailed observations has obscured insight into the mechanical interplay between collapse and eruption. We use multiparameter geophysical and geochemical data to show that the 110-square-kilometer and 65-meter-deep collapse of Bárdarbunga caldera in 2014-2015 was initiated through withdrawal of magma, and lateral migration through a 48-kilometers-long dike, from a 12-kilometers deep reservoir. Interaction between the pressure exerted by the subsiding reservoir roof and the physical properties of the subsurface flow path explain the gradual, near-exponential decline of both collapse rate and the intensity of the 180-day-long eruption.
Journal of Geophysical Research: Solid Earth, 2015
Earthquakes occurring close to hydrocarbon fields under production are often under critical view ... more Earthquakes occurring close to hydrocarbon fields under production are often under critical view of being induced or triggered. However, clear and testable rules to discriminate the different events have rarely been developed and tested. The unresolved scientific problem may lead to lengthy public disputes with unpredictable impact on the local acceptance of the exploitation and field operations. We propose a quantitative approach to discriminate induced, triggered, and natural earthquakes, which is based on testable input parameters. Maxima of occurrence probabilities are compared for the cases under question, and a single probability of being triggered or induced is reported. The uncertainties of earthquake location and other input parameters are considered in terms of the integration over probability density functions. The probability that events have been human triggered/induced is derived from the modeling of Coulomb stress changes and a rate and state‐dependent seismicity mode...
A spatially localized seismic sequence originated few tens of kilometres offshore the Mediterrane... more A spatially localized seismic sequence originated few tens of kilometres offshore the Mediterranean coast of Spain, close to the Ebro river delta, starting on 2013 September 5, and lasting at least until 2013 October. The sequence culminated in a maximal moment magnitude M w 4.3 earthquake, on 2013 October 1. The most relevant seismogenic feature in the area is the Fosa de Amposta fault system, which includes different strands mapped at different distances to the coast, with a general NE-SW orientation, roughly parallel to the coastline. However, no significant known historical seismicity has involved this fault system in the past. The epicentral region is also located near the offshore platform of the Castor project, where gas is conducted through a pipeline from mainland and where it was recently injected in a depleted oil reservoir, at about 2 km depth. We analyse the temporal evolution of the seismic sequence and use full waveform techniques to derive absolute and relative locations, estimate depths and focal mechanisms for the largest events in the sequence (with magnitude mbLg larger than 3), and compare them to a previous event (2012 April 8, mbLg 3.3) taking place in the same region prior to the gas injection. Moment tensor inversion results show that the overall seismicity in this sequence is characterized by oblique mechanisms with a normal fault component, with a 30 • low-dip angle plane oriented NNE-SSW and a subvertical plane oriented NW-SE. The combined analysis of hypocentral location and focal mechanisms could indicate that the seismic sequence corresponds to rupture processes along shallow low-dip surfaces, which could have been triggered by the gas injection in the reservoir, and excludes the activation of the Amposta fault, as its known orientation is inconsistent with focal mechanism results. An alternative scenario includes the iterated triggering of a system of steep faults oriented NW-SE, which were identified by prior marine seismics investigations.
On 1 April 2014, Northern Chile was struck by a magnitude 8.1 earthquake following a protracted s... more On 1 April 2014, Northern Chile was struck by a magnitude 8.1 earthquake following a protracted series of foreshocks. The Integrated Plate Boundary Observatory Chile monitored the entire sequence of events, providing unprecedented resolution of the build-up to the main event and its rupture evolution. Here we show that the Iquique earthquake broke a central fraction of the so-called northern Chile seismic gap, the last major segment of the South American plate boundary that had not ruptured in the past century. Since July 2013 three seismic clusters, each lasting a few weeks, hit this part of the plate boundary with earthquakes of increasing peak magnitudes. Starting with the second cluster, geodetic observations show surface displacements that can be associated with slip on the plate interface. These seismic clusters and their slip transients occupied a part of the plate interface that was transitional between a fully locked and a creeping portion. Leading up to this earthquake, th...
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Papers by Torsten Dahm