Seismicity from february 2006 to september 2007 at the Rwenzori Mountains, East African Rift: earthquake distribution, magnitudes and source mechanisms

We have analysed the microseismic activity within the Rwenzori Mountains area in the western branch of the East African Rift. Seismogram recordings from a temporary array of up to 27 stations reveal approximately 800 events per month with local magnitudes ranging from –0.5 to 5.1. The earthquake distribution is highly heterogeneous. The majority of located events lie within faults zones to the east and west of the Rwenzoris with the highest seismic activity observed in the northeastern area, where the mountains are in contact with the rift shoulders. The hypocentral depth distribution exhibits a pronounced peak of seismic energy release at 15 km depth. The maximum extent of seismicity ranges from 20 to 32 km and correlates well with Moho depths that were derived from teleseismic receiver functions. We observe two general features: (i) beneath the rift shoulders, seismicity extends from the surface down to ca. 30 km depth; (ii) beneath the rift valley, seismicity is confined to depths greater than 10 km. From the observations there is no indication for a crustal root beneath the Rwenzori Mountains. The magnitude frequency distribution reveals a b-value of 1.1, which is consistent with the hypothesis that part of the seismicity is caused by magmatic processes within the crust. Fault plane solutions of 304 events were derived from P-polarities and SV/P amplitude ratios. More than 70 % of the source mechanisms exhibit pure or predominantly normal faulting. T-axis trends are highly uniform and oriented WNW–ESE, which is perpendicular to the rift axis and in good agreement with kinematic rift models. At the northernmost part of the region we observe a rotation of the T-axis trends to NEN–SWS, which may be indicative of a local perturbation of the regional stress field

Seismicity at the Rwenzori Mountains, East African Rift: earthquake distribution, magnitudes and source mechanisms

We have analysed the microseismic activity within the Rwenzori Mountains area in the western branch of the East African Rift. Seismogram recordings from a temporary array of up to 27 stations reveal approximately 800 events per month with local magnitudes ranging from –0.5 to 5.1. The earthquake distribution is highly heterogeneous. The majority of located events lie within faults zones to the East and West of the Rwenzoris with the highest seismic activity observed in the northeastern area, where the mountains are in contact with the rift shoulders. The hypocentral depth distribution exhibits a pronounced peak of seismic energy release at 15 km depth. The maximum extent of seismicity ranges from 20 to 32 km and correlates well with Moho depths that were derived from teleseismic receiver functions. We observe two general features: (i) beneath the rift shoulders seismicity extends from the surface down to ca. 30 km depth; (ii) beneath the rift valley seismicity is confined to depths greater than 10 km. From the observations there is no indication for a crustal root beneath the Rwenzori Mountains. The magnitude frequency distribution reveals a b-value of 1.1, which is consistent with the hypothesis that part of the seismicity is caused by magmatic processes within the crust. Fault plane solutions of 304 events were derived from P-polarities and SV/P amplitude ratios. More than 70 % of the source mechanisms exhibit pure or predominantly normal faulting. T-axis trends are highly uniform and oriented WNW-ESE, which is perpendicular to the rift axis and in good agreement with kinematic rift models. At the northernmost part of the region we observe a rotation of the T-axis trends to NEN-SWS, which may be indicative of a local perturbation of the regional stress field

Layered anisotropy and mantle flow beneath AlpArray from shear-wave splitting

Seismic anisotropy provides a unique link between directly observable surface structures and the more elusive dynamic processes in the mantle below. The ability to infer the vertically- and laterally-varying anisotropic structures is of great significance for the geodynamic interpretation of surface-recorded waveform effects. In the first part of this presentation, we assess the capabilities of different observables for the inversion of XKS phases to uniquely resolve the anisotropic structure of the upper mantle (Rümpker et al., 2023). For this purpose, we perform full-waveform calculations for relatively simple models of upper-mantle anisotropy. In addition to waveforms, we consider the effects on apparent splitting parameters and splitting intensity. The results show that it is not generally possible to fully constrain the anisotropic parameters of a given model, even if complete waveforms are considered. However, inversions of both waveforms and apparent splitting parameters lead to similar models that exhibit systematic variations of anisotropic parameters. These characteristics may be exploited to better constrain the inversions. The results also show that splitting intensity has some significant drawbacks: First, even from measurements over a wide range of back-azimuth, there is no characteristic signature that would indicate depth variations of anisotropy. Secondly, identical azimuthal variations of splitting intensity for different anisotropic structures do not imply that the corresponding split waveforms are also similar. Thus, fitting of observed and calculated splitting intensities could lead to anisotropic models that are incompatible with the observed waveforms. In the second half, we present the first comprehensive analysis of layered anisotropy for the complete Alpine range based on apparent splitting parameters determined at 591 seismic stations of the AlpArray experiment (Link & Rümpker, 2023). Our findings suggest a combination of asthenospheric and distinct lithospheric contributions to the splitting observations, which can be seen as a refinement of previously reported models of single-layer anisotropy. The enhanced vertical resolution exposes the impact of successive Mediterranean tectonic episodes, such as the opening of the Provencal-Ligurian and Tyrrhenian Basins alongside the Adriatic slab retreat, as well as the Pannonian Basin opening and the Aegean slab retreat, resulting in deformation of the lithosphere and flow in the asthenospheric mantle. The dominant role of the larger scale Mediterranean subduction systems on mantle dynamics becomes evident. The observations provide supporting evidence that the European slab has broken off at its boundaries and that the resulting gaps channel flow from the mantle beneath the Eurasian plate to the Adriatic and Aegean subduction systems. The results provide important constraints on geodynamic processes involved in forming the European Alps, as previous and ongoing tectonic episodes can be inferred from the anisotropic fabric of the lithosphere-asthenosphere system

Testing observables for teleseismic shear-wave splitting inversions: ambiguities of intensities, parameters, and waveforms

We assess the capabilities of different observables for the inversion of core-refracted shear waves (XKS phases) to uniquely resolve the anisotropic structure of the upper mantle. For this purpose, we perform full-waveform calculations for relatively simple, canonical models of upper-mantle anisotropy. The models are characterized by two and four domains of different anisotropic properties. Specifically, we assume hexagonal symmetry with arbitrarily chosen strength of the anisotropy and orientation of the horizontal fast axis. XKS waveforms, generated from plane-wave initial conditions, traverse through anisotropic models and are recorded at the surface by a single station (in case of vertical variations) and by a dense station profile across the laterally and vertically varying structure. In addition to waveforms, we consider the effects of anisotropic variations on apparent splitting parameters and splitting intensity. The results show that, generally, it is not possible to fully resolve the anisotropic parameters of a given model, even if complete waveforms (under noisefree conditions and for the complete azimuthal range) are considered. This is because waveforms for significantly different anisotropic models can be indistinguishable. However, inversions of both waveforms and apparent splitting parameters lead to similar models that exhibit systematic variations of anisotropic parameters. These characteristics may be exploited to better constrain the inversions. The results also show that splitting intensity holds some significant drawbacks: First, even from measurements over a wide range of back-azimuth, there is no characteristic signature that would indicate depth variations of anisotropy. Secondly, identical azimuthal variations of splitting intensity for different anisotropic structures do not imply that the corresponding split waveforms are also similar. Thus, fitting of observed and calculated splitting intensities could lead to anisotropic models that are incompatible with the observed waveforms. We conclude that (bandlimited) XKS-splitting inversions and related tomographic schemes, even if based on complete waveforms, are not sufficient to fully resolve the heterogeneous anisotropic structures of the upper mantle and that combinations with alternative methods, based on e.g., receiver-function splitting, P-wave travel-time deviations, or surface waves, are required

Identifying Main Lithospheric Structures in the Eastern Alpine Domain by Joint Inversion of Receiver Function and Surface Wave Measurements for Seismic Anisotropy

Rayleigh-wave phase velocity measurements from both earthquakes and ambient noise were combined to image the 3-D shear-wave velocity structure beneath the eastern Alps and in the transitions towards the Pannonian Basin and the Dinarides. This allows us to resolve crust and upper mantle structures down to 300 km including the Moho topography. Continuous waveforms were collected from 1254 stations within a 9° radius for the time period from 2006 to 2018. More than 164,464 inter-station Rayleigh wave phase-velocity curves were automatically extracted after applying a strict quality control. Using the combined dataset, a period and distance dependence correction was applied to account for the bias observed between phase velocities from both datasets that amounts to ~1 % and increases towards longer periods. 2-D anisotropic phase velocity maps were then constructed spanning periods from 5 s to 250 s. 33,981 local dispersion curves were extracted and inverted for a 3-D shear-wave velocity model (PanREA2023) encompassing crust and mantle using a non-linear stochastic particle swarm optimization. At shallower crustal depths, the horst and graben structure of the Pannonian Basin is imaged, characterized by two NE-SW trending horsts and three graben systems. A pronounced crustal low-velocity anomaly extending to the Moho is found beneath the surrounding Carpathian orogen. A shallow south-dipping Eurasian slab was imaged beneath the eastern Alps down to only 150 km depth. Adriatic lithosphere is near-vertically dipping beneath the northern Apennines and northern Dinarides. The Adriatic slab is short reaching depths of around 150 km. Seismic discontinuities down to the mantle transition zone are analysed using S-to-P converted phases from teleseismic earthquakes. We stack broadband teleseismic S waveform data to retrieve S-to-P converted signals from below the seismic stations. In order to avoid processing artefacts, no deconvolution or filtering is applied. The Moho signals are always seen very clearly. In addition, a negative velocity gradient below the Moho depth is evident in many regions. A Moho depression is visible along larger parts of the Alpine chain reaching its largest depth of 60 km beneath the Tauern Window. The Moho depression ends abruptly near about 13°E below the eastern Tauern Window. East of 13°E the Moho shallows all the way to the Pannonian Basin. A prominent along-strike change was also detected in the upper mantle structure at about 14°E. There, the lateral disappearance of a zone of negative S-wave velocity gradient in the uppermost mantle is interpreted to indicate that the S-dipping European slab laterally terminates east of the Tauern Window. Joint inversion of surface wave dispersion curves and Moho travel times inferred from S-to-P converted phases allows to determine shear-wave velocity models consistent with both measurements. The uncertainty of the Moho depth estimates decreases from about 5 to 10 km considerably to 2 to 5 km depending on the depth of the Moho. The joint inversion further enables the determination of the sharpness of the negative discontinuity associated with the lithosphere-asthenosphere boundary. It appears to be rather sharp in the northern Alpine foreland and the Pannonian Basin

UNIBRA / DSEBRA – the German seismological contribution to AlpArray

UNIBRA was a joint initiative of German universities to install and maintain 74 seismic broadband stations at the beginning of the international AlpArray project in 2015 when the proposal for the 100 station broadband array DSEBRA was not yet approved by DFG. In this way, full participation of German teams in the AlpArray project could be secured. Most of these stations were deployed in southern Germany and a few in Austria. After approval in 2017 and installation of DSEBRA in 2018, the UNIBRA stations were replaced and further DSEBRA stations were deployed east of the SWATH-D array and also in Hungary. At that time, DSEBRA made up about one third of AlpArray’s temporary stations. After deinstallation of SWATH-D in autumn 2019 DSEBRA stations were used to reoccupy some of SWATH0-D’s critical sites. In spring 2020, the Covid19 pandemic started in Europe and it became unfeasible to move the DSEBRA stations to new sites. Instead of deinstallation, DFG allowed us to use remaining investment funds to continue the operation of DSEBRA at the current sites. As collaboration partners from Austria, Czech Republic, Poland, Slovakia and Hungary had already relocated many of their AlpArray stations to new sites towards the north-east and east of the Alps before Covid19 started, DSEBRA became part of the PACASE deployment with 214 temporary stations operated by partners from these countries and University of Lausanne. In summer 2022, new funds from DFG could be acquired by RU Bochum and LMU München to move 42 DSEBRA stations to Greece and Northern Macedonia and further 19 stations to Albania, Kosovo and Montenegro as part of the new AdriaArray project. The remaining DSEBRA stations stayed in Austria and Hungary to form a major part of AdriaArray’s backbone circling the Adriatic plate. With little exceptions, the DSEBRA stations have been in the field now without interruption for nearly 6 years. They massively contributed to the collection of a unique, large-scale and long-term seismological dataset which has enabled investigations into the structure of the crust and mantle beneath the greater Alpine area using receiver functions, shear-wave splitting, teleseismic body and surface wave tomography, local earthquake tomography and teleseismic full waveform inversion. Moreover, they allowed new insights into the seismic activity and hazard of active faults. DSEBRA will continue to do so in the framework of AdriaArray as part of an even larger seismic network comprising about 1300 permanent and temporary stations and doubling the size of AlpArray. Noise at the DSEBRA stations on the vertical component stayed below the Peterson high noise model by 20 dB over the entire seismic frequency band. Noise on the horizontal components was partially higher, in particular at low frequencies below 1 Hz. Thanks to special measures to avoid failures of mobile communication and battery charging and efforts to keep the low-power data logger running as long as possible in case of power failures, data availability of the DSEBRA stations reached extremely high values of 98% to 100%. The data were archived and disseminated on the EIDA node at LMU München during the experiment and transferred to the GEOFON for long-term archiving

Shear wave splitting across the Iceland hot spot: Results from the ICEMELT experiment

We report on observations of upper mantle anisotropy from the splitting of teleseismic shear waves (SKS, SKKS, and PKS) recorded by the ICEMELT broadband seismometer network in Iceland. In a ridge-centered hot spot locale, mantle anisotropy may be generated by flow-induced lattice-preferred orientation of olivine grains or the anisotropic distribution of magma. Splitting measurements of teleseismic shear waves may thus provide diagnostic information on upper mantle flow and/or the distribution of retained melt associated with the Iceland mantle plume. In eastern Iceland, fast polarization directions lie between N10°W and N45°W and average N24°W; delay times between the fast and slow shear waves are generally 0.7–1.35 s. In western Iceland, in contrast, the fast polarization directions, while less well constrained, yield an average value of N23°E and delay times are smaller (0.2–0.95 s). We propose that splitting in eastern Iceland is caused by a 100- to 200-km-thick anisotropic layer in the upper mantle. The observed fast directions in eastern Iceland, however, do not correspond either to the plate spreading direction or to a pattern of radial mantle flow from the center of the Iceland hot spot. We suggest that the relatively uniform direction and magnitude of splitting in eastern Iceland, situated on the Eurasian plate, may therefore reflect the large-scale flow field of the North Atlantic upper mantle. We hypothesize that the different pattern of anisotropy beneath western Iceland, part of the North American plate, is due to the different absolute motions of the two plates. By this view, splitting in eastern and western Iceland is the consequence of shear by North American and Eurasian plate motion relative to the background mantle flow. From absolute plate motion models, in which the Eurasian plate is approximately stationary and the North American plate is moving approximately westward, the splitting observations in both eastern and western Iceland can be satisfied by a background upper mantle flow in the direction N34°W and a velocity of 3 cm/yr in a hot spot reference frame. This inference can be used to test mantle flow models. In particular, it is inconsistent with kinematic flow models, which predict southward flow, or models where flow is dominated by subduction-related sources of mantle buoyancy, which predict westward flow. Our observations are more compatible with the flow field predicted from global seismic tomography models, which in particular include the influence of the large-scale lower mantle upwelling beneath southern Africa. While the hypothesized association between our observations and this upwelling is presently speculative, it makes a very specific and testable prediction about the flow field and hence anisotropy beneath the rest of the Atlantic basin.This work was supported by the National Science Foundation under grants EAR-9316137, OCE-9402991, and EAR-9707193.Peer Reviewe

Crustal and uppermost mantle shear-wave velocity structure beneath the Middle East from surface-wave tomography

We have constructed a 3-D shear-wave velocity (Vs) model for the crust and uppermost mantle beneath the Middle East using Rayleigh wave records obtained from ambient-noise cross-correlations and regional earthquakes. We combined one decade of data collected from 852 permanent and temporary broadband stations in the region to calculate group-velocity dispersion curves. A compilation of > 54000 ray paths provides reliable group-velocity measurements for periods between 2 and 150 s. Path-averaged group velocities calculated at different periods were inverted for 2-D group-velocity maps. To overcome the problem of heterogeneous ray coverage, we used an adaptive grid parametrization for the group-velocity tomographic inversion. We then sample the period-dependent group-velocity field at each cell of a predefined grid to generate 1-D group-velocity dispersion curves, which are subsequently inverted for 1-D Vs models beneath each cell and combined to approximate the 3-D Vs structure of the area. The Vs model shows low velocities at shallow depths (5–10 km) beneath the Mesopotamian foredeep, South Caspian Basin, eastern Mediterranean and the Black Sea, in coincidence with deep sedimentary basins. Shallow high-velocity anomalies are observed in regions such as the Arabian Shield, Anatolian Plateau and Central Iran, which are dominated by widespread magmatic exposures. In the 10–20 km depth range, we find evidence for a band of high velocities (> 4.0 km/s) along the southern Red Sea and Arabian Shield, indicating the presence of upper mantle rocks. Our 3-D velocity model exhibits high velocities in the depth range of 30–50 km beneath western Arabia, eastern Mediterranean, Central Iranian Block, South Caspian Basin and the Black Sea, possibly indicating a relatively thin crust. In contrast, the Zagros mountain range, the Sanandaj-Sirjan metamorphic zone in western central Iran, the easternmost Anatolian plateau and Lesser Caucasus are characterized by low velocities at these depths. Some of these anomalies may be related to thick crustal roots that support the high topography of these regions. In the upper mantle depth range, high-velocity anomalies are obtained beneath the Arabian Platform, southern Zagros, Persian Gulf and the eastern Mediterranean, in contrast to low velocities beneath the Red Sea, Arabian Shield, Afar depression, eastern Turkey and Lut Block in eastern Iran. Our Vs model may be used as a new reference crustal model for the Middle East in a broad range of future studies

Mantle-flow diversion beneath the Iranian plateau induced by Zagros’ lithospheric keel

Previous investigation of seismic anisotropy indicates the presence of a simple mantle flow regime beneath the Turkish-Anatolian Plateau and Arabian Plate. Numerical modeling suggests that this simple flow is a component of a large-scale global mantle flow associated with the African superplume, which plays a key role in the geodynamic framework of the Arabia-Eurasia continental collision zone. However, the extent and impact of the flow pattern farther east beneath the Iranian Plateau and Zagros remains unclear. While the relatively smoothly varying lithospheric thickness beneath the Anatolian Plateau and Arabian Plate allows progress of the simple mantle flow, the variable lithospheric thickness across the Iranian Plateau is expected to impose additional boundary conditions on the mantle flow field. In this study, for the first time, we use an unprecedented data set of seismic waveforms from a network of 245 seismic stations to examine the mantle flow pattern and lithospheric deformation over the entire region of the Iranian Plateau and Zagros by investigation of seismic anisotropy. We also examine the correlation between the pattern of seismic anisotropy, plate motion using GPS velocities and surface strain fields. Our study reveals a complex pattern of seismic anisotropy that implies a similarly complex mantle flow field. The pattern of seismic anisotropy suggests that the regional simple mantle flow beneath the Arabian Platform and eastern Turkey deflects as a circular flow around the thick Zagros lithosphere. This circular flow merges into a toroidal component beneath the NW Zagros that is likely an indicator of a lateral discontinuity in the lithosphere. Our examination also suggests that the main lithospheric deformation in the Zagros occurs as an axial shortening across the belt, whereas in the eastern Alborz and Kopeh-Dagh a belt-parallel horizontal lithospheric deformation plays a major role

Mantle-flow diversion beneath the Iranian plateau induced by Zagros' lithospheric keel.

Funder: German Research Foundation (DFG)Funder: Projekt DEALPrevious investigation of seismic anisotropy indicates the presence of a simple mantle flow regime beneath the Turkish-Anatolian Plateau and Arabian Plate. Numerical modeling suggests that this simple flow is a component of a large-scale global mantle flow associated with the African superplume, which plays a key role in the geodynamic framework of the Arabia-Eurasia continental collision zone. However, the extent and impact of the flow pattern farther east beneath the Iranian Plateau and Zagros remains unclear. While the relatively smoothly varying lithospheric thickness beneath the Anatolian Plateau and Arabian Plate allows progress of the simple mantle flow, the variable lithospheric thickness across the Iranian Plateau is expected to impose additional boundary conditions on the mantle flow field. In this study, for the first time, we use an unprecedented data set of seismic waveforms from a network of 245 seismic stations to examine the mantle flow pattern and lithospheric deformation over the entire region of the Iranian Plateau and Zagros by investigation of seismic anisotropy. We also examine the correlation between the pattern of seismic anisotropy, plate motion using GPS velocities and surface strain fields. Our study reveals a complex pattern of seismic anisotropy that implies a similarly complex mantle flow field. The pattern of seismic anisotropy suggests that the regional simple mantle flow beneath the Arabian Platform and eastern Turkey deflects as a circular flow around the thick Zagros lithosphere. This circular flow merges into a toroidal component beneath the NW Zagros that is likely an indicator of a lateral discontinuity in the lithosphere. Our examination also suggests that the main lithospheric deformation in the Zagros occurs as an axial shortening across the belt, whereas in the eastern Alborz and Kopeh-Dagh a belt-parallel horizontal lithospheric deformation plays a major role