Local earthquake tomography in the Eastern and eastern Southern Alps using Swath D data

The AlpArray collaborative project was a multinational, European initiative focused on the Alpine-Apennine-Carpathian-Dinarid orogenic system, established to improve our understanding of the Alpine orogeny and geology (see e.g., Hetényi et al., 2018). The backbone of this project was the large AlpArray seismic network (AASN) with more than 600 stations deployed in the greater Alpine area (Hetényi et al., 2018). In addition to this regional network, the temporary Swath D network was installed as part of the 4D-MB project (Heit et al., 2021). This network included 163 stations that were operated for two years. It was located along the tip of the eastern Adriatic indenter and covered for example part of the proposed Moho gap in the Eastern Alps and the slab gap between Central and Eastern Alps. With an average station spacing of 15 km it was significantly denser than the AASN (average spacing of 52 km) and thus enabled for high-resolution imaging of key areas within the Alpine region, particularly on the crustal and upper mantle scale as well as the precise location of local earthquakes. The high density of the Swath D network was particularly important for high resolution imaging considering that there is only a moderate rate of seismicity in the study region. The Swath D data was therefore ideal to be used in local earthquake tomography (LET), which is a mature, powerful inversion method to provide high-resolution images of the subsurface especially on the crustal scale. In a first step earthquake arrival times of P- and S-waves (observed at Swath D and selected AASN stations) were picked and inverted for velocity models, station corrections, earthquake hypocenters and origin times (Jozi Najafabadi et al., 2021). In this way the seismicity (mainly in the upper 20 km) of the Alpine frontal thrust, e.g., the Friuli-Venetia region, the Giudicarie–Lessini and Schio-Vicenza domains, the Austroalpine nappes, and the Inntal area was revealed. In a second step the arrival time data were inverted for the 3-D velocity structure (Jozi Najafabadi et al., 2022). Due to the irregular distribution of earthquakes, extensive resolution testing was necessary. The predominantly shallow earthquakes still pose a challenge for the inversion, particularly in terms of resolution of the lower crust and upper mantle. Nevertheless, the derived P-wave velocity model revealed a highly heterogeneous crustal structure in the target area with prominent intracrustal anomalies, Moho topography and a thickened lower crust South of the Periadriatic fault. The models were intensly used for further detailed geological and geophysical investigations (e.g., Verwater et al., 2021). In a third step the distribution of seismic attenuation of P-waves (1/Qp) was calculated (attenuation tomography). Focussing on the upper crust, several distinct anomalies can be observed. The highest attenuation (lowest QP) anomaly is found in the Friuli-Venetian region which is also characterized by low VP and increased VP/VS. This anomaly may be related to a high fault and fracture density and the presence of fluid-filled sediments of the Venetian-Friuli basin along the eastern part of the Southern Alpine deformation front. Recently, attempts to obtain vp and vs models from LET have been extended to the greater Alpine region (e.g. Braszus et al., 2023)

GIPP: Geophysical Instrument Pool Potsdam

The Geophysical Instrument Pool Potsdam (GIPP) consists of field instruments, sensors and equipment for temporary seismological studies (both controlled source and earthquake seismology) as well as for magnetotelluric (electromagnetic) experiments. These instruments are mainly mobile digital recorders, broadband seis­mometers and short period sensors, and they are used to reveal the subsurface structure and to investigate earth­quakes. Sensors for magnetotellurics include induction coil and fluxgate magnetometers and non-polarizing silver / silver-chloride electrodes. It is operated by the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences. The instru­ment facility is open to all academic applicants, both national and international. Instrument applications are evalu­ated and ranked by an external steering board. Currently, for seismological applications >850 geophysical recorders, >170 broadband seis­mo­meters and >1300 short period geophones are available (among others). Available for magnetotelluric experiments are > 50 real-time data-loggers, >150 induction coils, and >500 electrodes. User guidelines and data policy are in force and data archives are provided (standard exchange formats)

Seismotectonic study of the Fergana region (Southern Kyrgyzstan): distribution and kinematics of local seismicity

We present new seismicity and focal-mechanism data for the Fergana basin and surrounding mountain belts in western Kyrgyzstan from a temporary local seismic network. A total of 210 crustal earthquakes with hypocentral depths shallower than 25 km were observed during a 12-month period in 2009/2010. The hypocenter distribution indicates a complex net of seismically active structures. The seismicity derived in this study is mainly concentrated at the edges of the Fergana basin, whereas the observed rate of seismicity within the basin is low. The seismicity at the dominant tectonic feature of the region, the Talas-Fergana fault, is likewise low, so the fault seems to be inactive or locked. To estimate the uncertainties of earthquake locations derived in this study, a strong explosion with known origin time and location is used as a ground truth calibration event which suggests a horizontal and vertical accuracy of about 1 km for our relocations. We derived 35 focal mechanisms using first motion polarities and retrieved a set of nine moment tensor solutions for earthquakes with moment magnitude (Mw) ranging from 3.3 to 4.9 by waveform inversion. The solutions reveal both thrust and strike-slip mechanisms compatible with a NW-SE direction of compression for the Fergana region. Two previously unknown tectonic structures in the Fergana region could be identified, both featuring strike-slip kinematics. The combined analysis of the results derived in this study allowed a detailed insight into the currently active tectonic structures and their kinematics where little information had previously been available

DEPAS (Deutscher Geräte-Pool für amphibische Seismologie): German Instrument Pool for Amphibian Seismology

The German Instrument Pool for Amphibian Seismology (DEPAS) provides the infrastructure for onshore, marine and amphibian seismological experiments. It consists currently of approx. 80 ocean-bottom seismometers (OBS) and 95 onshore seismic stations. Broadband sensors and custom-built data loggers enable a broad range of short- and long-term deployments to study architecture and dynamics of the Earth’s interior. The OBS are operated by the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI); the onshore stations are managed by the Helmholtz Centre Portsdam GFZ German Research Centre for Geosciences. The DEPAS instruments are available upon request for researchers affiliated to German universities or German research institutes within national or international projects. Applications for stations are evaluated by an external steering committee. Data will be stored in national archives and made available to the public after a waiting period

A comprehensive high resolution 3D P- and S-wave velocity model for the Alpine mountain chain using local earthquake data: Constraining crustal structure, lithologies and mountain-building processes

Based on the unprecedented amount of densly recorded seismic waveform data and recent advances in machine learning techniques the main objective of this project was the computation of a comprehensive high resolution 3D P- and S-wave velocity model for the Alpine region including station correction terms. Additionally, event locations and associated uncertainties as well as the automatically determined seismic arrival times should be published. The 3D crustal model delivers travel time correction terms for teleseimic tomography studies and thus sharpen the image of subducted slabs in the upper mantle. We used "SeisBench - A toolbox for machine learning in seismology" to assess the performance of several deep-neural-network based seismic picking algorithms and find PhaseNet to be most suitable. In order to consistently remove outliers from the P- and S- phase pick catalog we developed a purely data-driven pre-inversion pick selection method. We relocated a subset of 384 events while simultaneously inverting for the 1D P- & S-wave velocity structure including station corrections using the established VELEST as well as the recently developed McMC algorithms. This model yields the first consistent travel time based 1D S-wave model of the Greater Alpine region facilitating computation of synthetic travel times and the inclusion of S-phases during the localization process. Furthermore, it yields the starting model for the final 3D velocity model which is based on records from more than 3000 events on more than 1100 seismic broadband stations. Comparing our hypocentres with event locations from other studies indicates a horizontal and vertical accuracy of ~2km and ~6km, respectively, when using a 1D velocity model and station correction terms for the Greater Alpine region. Large scale features of the resulting velocity model are in good agreement with previous studies. The Molasse and Po basin in the northern and southern foreland, respectively, are showing up as prominent low velocity zones in the uppermost crust. Generally, the velocity isolines in the lower crust are in rather good agreement with Moho maps from previous studies and ambient noise tomographies

Crustal Structure of Sri Lanka Derived From Joint Inversion of Surface Wave Dispersion and Receiver Functions Using a Bayesian Approach

We study the crustal structure of Sri Lanka by analyzing data from a temporary seismic network deployed in 2016-2017 to shed light on the amalgamation process from a geophysical perspective. Rayleigh wave phase dispersion curves from ambient noise cross correlation and receiver functions were jointly inverted using a transdimensional Bayesian approach. The Moho depths in Sri Lanka range between 30 and 40 km, with the thickest crust (38-40 km) beneath the central Highland Complex (HC). The thinnest crust (30-35 km) is found along the west coast, which experienced crustal thinning associated with the formation of the Mannar Basin. V-P/V-S ratios lie within a range of 1.60-1.82 and predominantly favor a felsic to intermediate bulk crustal composition with a significant silica content of the rocks. A major intracrustal (18-27 km), slightly westward dipping (similar to 4.3 degrees) interface with high V-S (similar to 4 km/s) underneath is prominent in the central HC, continuing into the western Vijayan Complex (VC). The discontinuity might have been part of the respective units prior to the collision and could be an indicator for the proposed tilting of the Wanni Complex/HC crustal sections. It might also be related to the deep crustal HC/VC thrust contact with the VC as an indenting promontory of high V-S. A low-velocity zone in the central HC could have been caused by fluid influx generated by the thrusting process

Wide-angle seismic transect reveals the crustal structure of(f) southern Sri Lanka

We present results derived from a seismic refraction experiment and gravity measurements about the upper mantle and crustal structure of southern Sri Lanka and the adjacent Indian Ocean. A P-wave velocity model was derived using forward modelling of the observed travel times along a 509 km long, N-S trending profile at 81°E longitude. Our results show that the continental crust below southern Sri Lanka is up to 38 km thick. A ~65 km wide transition zone, which thins seaward to ~7 km thickness, divides stretched continental from oceanic crust. The adjacent, 4.7 to 7 km thick normal oceanic crust is covered by up to 4 km thick sediments. The oceanic crust is characterized by intra-crustal reflections and displays P-wave velocity variations, especially in oceanic layer 2, along our profile. In the central part of the profile, the uppermost mantle layer is characterized by normal P-wave mantle velocities of 8.0 -8.1 km/s. At the southern end of the profile, unusual low upper mantle seismic velocities, ranging from 7.5 to 7.6 km/s only, characterize the uppermost mantle layer. These low upper mantle velocities are probably caused by serpentinized upper mantle. At even greater depths the upper mantle layer is characterized by velocities of 8.3 km/s on average. The type of margin along our profile is difficult to identify, since it is characterized by features typical for different types of margins

Constraints on Crustal Structure in the Eastern and eastern Southern Alps

In the course of this study, an extensive seismological dataset from both the temporary SWATH-D network (Heit et al., 2021) and selected stations of the AlpArray Seismic Network (Hetényi et al., 2018) was analyzed. The primary aim of this endeavor was to gain comprehensive insights into the crustal structure of the southern and eastern Alps. The small inter-station spacing (average of ∼15 km within the SWATH-D network) allowed for depicting crustal structure at unprecedented resolution across a key part of the Alps. The methodological approach employed in this study entailed a sequential series of analyses to unveil the underlying features. The preliminary step encompassed the determination of the arrival times of both P and S seismic waves. Subsequently, a Markov chain Monte Carlo inversion technique was deployed to simultaneously calculate robust hypocenters, a 1-D velocity model, and station corrections (Jozi Najafabadi et al., 2021). This data was then utilized for calculation of 3-D VP and VP/VS models (Jozi Najafabadi et al., 2022). In addition, the path-averaged attenuation values were obtained by a spectral inversion of the waveform data of selected earthquakes. The attenuation structure (1/QP model) is then calculated using damped least square inversion of the path-averaged attenuation values (Jozi Najafabadi et al., 2023). These analyses resulted in a multidimensional depiction of the subsurface. The derived models for QP, VP and VP/VS indicate subsurface anomalies that can be attributed to rock’s physical parameters, presence of fluids within rocks and their motion in pores and fractures, temperature, and partial melting. The findings reflect head-on convergence of the Adriatic indenter (the part of the Adriatic Plate that has modified the Alpine orogenic edifice) with the Alpine orogenic crust. Furthermore, a highly heterogeneous crustal structure within the study area was unveiled. The velocity model illuminated decoupling of the lower crust from both its mantle substratum and upper crust. The Moho, taken to be the iso-velocity contour of Vp = 7.25 km/s, provided insights into the southward subduction of the European lithosphere, a phenomenon previously investigated in the Eastern and eastern Southern Alps (e.g., Kummerow et al., 2004 and Diehl et al., 2009). The most pronounced high-attenuation (low QP) anomaly is found to be closely correlated with the high density of faults and fractures in the Friuli-Venetian region, as well as the presence of fluid-filled sediments within the Venetian-Friuli Basin. Furthermore, the northwestern edge of the Dolomites Sub-Indenter (NWDI) corresponds to a low attenuation (high QP) anomaly which is interpreted as a reflection of the NWDI's stronger rocks compared to the surrounding areas

The 2010<i>M</i>_w8.8 Maule, Chile earthquake: Nucleation and rupture propagation controlled by a subducted topographic high

Knowledge of seismic properties in an earthquake rupture zone is essential for understanding the factors controlling rupture dynamics. We use data from aftershocks following the Maule earthquake to derive a three-dimensional seismic velocity model of the central Chile forearc. At 36°S, we find a highvp (&gt;7.0 km/s) and high vp/vs(?1.89) anomaly lying along the megathrust at 25 km depth, which coincides with a strong forearc Bouguer gravity signal. We interpret this as a subducted topographic high, possibly a former seamount on the Nazca slab. The Maule earthquake nucleated at the anomaly's updip boundary; yet high co-seismic slip occurred where the megathrust is overlain by lower seismic velocities. Sparse aftershock seismicity occurs within this structure, suggesting that it disrupts normal interface seismogenesis. These findings imply that subducted structures can be conducive to the nucleation of large megathrust earthquakes, even if they subsequently hinder co-seismic slip and aftershock activity