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Quantifying Changes in the seismic velocity (dv/v) from repeating sources or ambient noise surveys is an established tool in seismology.dv/vhas been shown to correlate to a multitude of parameters such as stress variations, pore pressure changes, and modifications in the microscopic and macroscopic integrity of the sampled medium (i.e., “damage”). In turn, these parameters are impacted by an even greater number of mechanisms, for example, precipitation, temperature, surface loading, seismic activity, hydrology, volcanic activity, or tidal forces. However, their relationships todv/vare often non-linear, hard to quantify, and poorly understood. Additionally, for “field data”, it can be challenging to decipher the contribution of each individual parameter todv/v changes. In this work, we analyse continuous seismic data and interdisciplinary datasets from Mt St Helens (USA) and the Kamchatka peninsula (Russia) to illustrate the diversity of mechanisms impactingdv/vand discuss potential strategies to unravel the observations. We show that, for volcanic areas in particular, analysingdv/vis a complex task due to the addition of volcanic activity, high-relief topography, and strongly heterogeneous subsurface structures., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The ITU/WMO/UNESCO-IOC Joint Task Force for Science Monitoring and Reliable Telecommunications (JTF SMART) is working to integrate environmental sensors into trans-ocean commercial submarine telecommunications cables to construct a global deep ocean observing array. The telecommunication signal along cables is boosted by repeaters every 50-100 km; these will be equipped with pressure and temperature sensors and accelerometers. Real-time ocean-bottom observations will quantify changes in climate, ocean circulation, and sea level, enable life-saving tsunami and earthquake early warning, and yield new insights into tectonic and deep Earth processes. Thus, United Nations Sustainable Development Goals (i.e., SDG 9, 13, 14) are addressed. SMART Cables is an endorsed Project of the United Nations Decade of Ocean Science for Sustainable Development.We will review SMART systems and report progress toward realising that vision. Ocean modelling experiments are performed to quantify the benefits of SMART cables for monitoring Essential Ocean Variables and other characteristics. A wet demonstration system will be installed off Sicily in June 2023. Portugal has issued a request for tender for the 100% state-owned domestic CAM system connecting Continental Portugal, Azores, and Madeira in a 3700 km ring with 50 SMART repeaters to be installed in 2025. It is motivated by earthquake and tsunami disaster risk reduction, and improving understanding of the Atlantic Meridional Overturning Circulation. Further systems in various planning stages include Mediterranean, New Zealand-Chathams, Vanuatu-New Caledonia, Chile-Sydney, Chile-Antarctica, New Zealand-Antarctica, Indonesia, and Trans-Arctic Europe-Japan. Finally, the European Union has included SMART capability in its international submarine cable connectivity program (CEF2)., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

In last few years, a number of technologies to use fibre optic cables as sensing devices have been established, among them Distributed Acoustic Sensing (DAS) and State-of-Polarisation (SoP). The potential of these technologies for monitoring a range of Earth System parameters in submarine cables has been demonstrated through several pilot experiments. Yet, continuous access to sub-marine optical fibre scientific data has currently not been achieved anywhere, neither has full integration of the various techniques.The SUBMERSE project links Research and Education Networks (NRENs), universities, research institutes and industry to establish multi-method monitoring along submarine optical telecommunication cables at several key cable routes branching off from Portugal, Madeira, Svalbard and in the Aegean. Those pilot sites should serve as a blueprint for establishing continuous monitoring services along many more cables.The project comprises technical developments for integrating DAS and SoP measurements, for establishing differential SoP measurements between repeaters and for operating DAS in ‘lit’ fibres, i.e., fibres carrying telecommunications traffic. Furthermore, a range of geoscientific and marine biology use cases are included, which seek to establish code/services for monitoring earthquakes, tracking whales, measuring the sea state and other Earth System variables. Effective data management, dissemination and training through community specific services will be addressed in several tasks as they are crucial for the success of this project due to the large size of data, sensitivity of a subset of the recordings and current lack of established community standards. SUBMERSE clearly commits to open and FAIR data exchange., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The 22 December 2018 Anak-Krakatau volcanic flank collapse triggered a devastating tsunami along the Sunda Strait coast, highlighting the need for tsunami early warning systems to detect both volcanic collapse and landslide tsunami triggering from real-time seismic data. Seismically, landslides are deficient in high-frequency radiation compared to tectonic earthquakes. In this study, carried out in the framework of the TSUNAMI_RISK project, we implemented spectral analysis and back-projection to locate low-frequency events (40-70 s) along the Indonesian coast, specifically volcano-induced landslides. Using the Indonesian seismic network, we successfully detected and located the Anak-Krakatau volcanic flank collapse. The back-projection method allowed us to estimate the location about 100 seconds after the event initiation, with detection achieved with stations up to 1° from the source. To assess potential biases in our approach, we evaluated two weeks of continuous long-period seismic data (11-25 November 2019) from high-risk volcano tsunami areas in eastern Indonesia, such as Batu Tara, Iliwerung, Gamalama, and Sangeang Api. We conclude by discussing the expected false alarm rates for various warning thresholds based on the current network configuration in Indonesia and explore the routes to implementing this approach for operational tsunami early warning., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Understanding physical processes prior and during eruptions remains challenging, due to uncertainties about subsurface structures and undetected processes within the volcano. Here, the authors use a dedicated fibre-optic cable to obtain strain data and identify volcanic events and image hidden near-surface volcanic structural features at Etna volcano, Italy. In the paper Jousset et al. (2022), we detect and characterize strain signals associated with explosions, and we find evidences for non-linear grain interactions in a scoria layer of spatially variable thickness. We also demonstrate that wavefield separation allows us to incrementally investigate the ground response to various excitation mechanisms, and we identify very small volcanic events, which we relate to fluid migration and degassing. We recorded seismic signals from natural and man-made sources with 2-m spacing along a 1.5-km-long fibre-optic cable layout near the summit of actives craters of Etna volcano, Italy. Those results provide the basis for improved volcano monitoring and hazard assessment using DAS. This data publication contains the full data set used for the analysis. This data set comprises strain-rate data from 1 iDAS interrogator (~750 traces), velocity data from 15 geophones and 4 broadband seismometers, and infrasonic pressure data from infrasound sensors. For further explanation of the data and related processing steps, please refer to Jousset et al. (2022).

We present a new 3D shear-wave velocity model and Moho map of Scandinavia, which is based on the inversion of the merged phase dispersion curves from ambient noise and earthquake-generated Rayleigh waves. A classic two step inversion scheme is used where first maps of phase velocities at different periods are derived, and then a 1D transdimensional Bayesian method is applied to determine the VSV-depth structure. We assess the question of what compensates for the unusual high Scandes mountains and aim to identify the different tectonic domains of the adjacent continental lithosphere (Baltic Shield). While the southern Scandes lacks a pronounced crustal root, we observe a crustal root below the northern Scandes that is decreasing towards the central Scandes. A ∼10 km thick high-density lower crustal layer is present below the northern Scandes and generally thickening to the east below the Baltic Shield. The lithosphere-asthenosphere boundary (LAB) below the Scandes is deepening as well from west to east with a sharp step and a strong VSV decrease with depth of 9% in the north and of 5.5% in the south. The LAB of the thinner lithosphere is at 150 km depth in the north and varies from 90 to 120 km depth in the south. Both LAB steps coincide with the mountain front. The central area shows rather smoothly varying structures (170 km LAB depth, −4% VSV with depth) towards the east and no clear spatial match with the front. We infer therefore distinct uplift mechanisms along the Scandes. The southern Scandes might sustain their topography due to dynamic support from the mantle, while the northern Scandes experience both crustal and mantle lithosphere isostasy. In both cases, we suspect a dynamic support from small-scale edge-driven convection that developed at the sharp lithospheric steps. Beneath the Archean Karelia craton in northern Finland, we find low-velocity areas below 150 km depth while a 250 km deep lithospheric keel is imaged below the Paleoproterozic southern Finland. The Norrbotten craton in northern Sweden can be identified at mantle depths as a unit different from the Karelia craton, Scandes and Paleoproterozic central Sweden.

We present a new seismic tomography model for the crust and upper-mantle beneath the Central Andes based on multi-scale full seismic waveform inversion, proceeding from long periods (40–80 s) over several steps down to 12–60 s. The spatial resolution and trade- offs among inversion parameters are estimated through the multi-parameter point-spread functions. P and S wave velocity structures with a spatial resolution of 30–40 km for the upper mantle and 20 km for the crust could be resolved in the central study region. In our study, the subducting Nazca slab is clearly imaged in the upper mantle, with dip-angle variations from the north to the south. Bands of low velocities in the crust and mantle wedge indicate intense crustal partial melting and hydration of the mantle wedge beneath the frontal volcanic arc, respectively and they are linked to the vigorous dehydration from the subducting Nazca plate and intermediate depth seismicity within the slab. These low velocity bands are interrupted at 19.8-21° S, both in the crust and uppermost mantle, hinting at the lower extent of crustal partial melting and hydration of the mantle wedge. The variation of lithospheric high velocity anomalies below the backarc from North to South allows insight into the evolutionary foundering stages of the Central Andean margin. A high velocity layer beneath the southern Altiplano suggests underthrusting of the leading edge of the Brazilian Shield. In contrast, a steeply westward dipping high velocity block and low velocity lithospheric uppermost mantle beneath the southern Puna plateau hint at the ongoing lithospheric delamination.

The data publication contains a dataset for fast assessment of earthquakes based on seismic waveforms. The dataset encompasses Northern Chile. Due to the large scale of the dataset, it is intended for use in machine learning. A similar dataset for chile has been published as Münchmeyer et al. (2020). A similar dataset for Japan can be obtained using the scripts at https://github.com/yetinam/TEAM The datasets are provided as a hdf5-file (Folk et al. 2011), a hierachical file format. Source code for reading and processing the data is available at https://github.com/yetinam/TEAM. The hdf5-file contains the two groups “metadata” and “data” that are described below. These groups are the hdf5-analog of folders in a file system.

The temporary seismic array of MySCOLAR in northern Myanmar consists of 30 broadband stations. The overall scientific goals are to understand the transition from continental collision to oceanic subduction, to quantify the partitioning of deformation in the accretionary prism, in the Burma Plate and along the strike-slip Sagaing fault system and to image the subducting Indian Plate beneath Myanmar and southwest China. The seismological analysis methods applied to this dataset will include location of local earthquakes and determining their focal mechanisms, surface wave tomography from ambient noise and earthquake data, body wave tomography from local and teleseismic earthquakes, full waveform inversion for Earth structure, receiver functions, and shear wave splitting. A subset of the stations was transmitting data in real time, and these stations contributed to real-time earthquake analysis by the Department of Meteorology and Hydrology (DMH) in Myanmar and the GEOFON earthquake monitoring service. Waveform data are available from the GEOFON data centre, under network code 6C.

This dataset contains a high resolution Moho map of the in the Eastern Alps focused on the SWATH-D network. The Moho map was produced by manually picking the Moho on narrow transects (CCP stacks) calculated with the receiver function method. These manual picks were then fit with a spline in 3-D. Three separate and sometimes overlapping maps are included corresponding to the European, Adriatic, and Pannonian Mohos. In addition to Moho depth, Ps travel time and crustal average Vp/Vs are also reported.

A new seismic model for crust and upper mantle of the south Central Andes is derived from full waveform inversion, covering the Pampean flat subduction and adjacent Payenia steep subduction segments. Focused crustal low‐velocity anomalies indicate partial melts in the Payenia segment along the volcanic arc, whereas weaker low‐velocity anomalies covering a wide zone in the Pampean segment are interpreted as remnant partial melts. Thinning and tearing of the flat Nazca slab is inferred from gaps in the slab along the inland projection of the Juan Fernandez Ridge. A high‐velocity anomaly in the mantle below the flat slab is interpreted as relic Nazca slab segment, which indicates an earlier slab break‐off triggered by the buoyancy of the Juan Fernandez Ridge during the flattening process. In Payenia, large‐scale low‐velocity anomalies atop and below the re‐steepened Nazca slab are associated with the re‐opening of the mantle wedge and sub‐slab asthenospheric flow, respectively.

We investigate the anisotropic nature of the mantle in a part of the Arabia-Eurasia collision zone where several of the constituent tectonic blocks of Iran come into contact. Our aim is to identify the dominant source of anisotropy in order to infer the pattern of mantle deformation as it responds to the forces of continental convergence. The broadband seismic data used in this study were recorded by 68 seismic stations in three temporary arrays in NW Iran (between 45º00'E - 51º00'E and 35º00'N - 39º00'N) installed and operated non-concurrently between 2008 and 2016 by the Institute for Advanced Studies in Basic Sciences (IASBS) and the University of Cambridge, and one permanent station of the Iranian National Seismic Network (INSN). The stations were repositioned during deployment and the recording span at individual stations varied between 4 and 31 months. The temporary arrays were mainly linear profiles that traversed the Talesh and western Alborz Mountains, NW Iran, and parts of the central Iran Plateau to the vicinity of the UDMA. Our dataset includes seismic records of S and SK(K)S phases. For the SK(K)S splitting measurements we used teleseismic earthquakes with magnitudes greater than 5.5 and epicentral distances between 90º and 130º, and for the direct S phase analysis we considered events larger than magnitude 5.5 in the distance range of 40º to 80º.

The Anillo is a dense temporary seismic and geodetic network extending approximately 200 km along the strike of the subduction zone in North Chile in order to investigate how earthquakes and aseismic slip scale over a broader spectrum of source sizes, to understand the complex relationships between seismic and aseismic deformation, and to identify possible structural controls. This experiment is embedded into a larger scale experimental effort carried out by institutions in Germany and Chile. Waveform data are available from the GEOFON data centre, under network code Y6.

The Ethiopian rift is a unique natural environment to study the different stages of evolution from initial continental rifting to embryonic sea-floor spreading. We study the crust and uppermost mantle of the Afar, Main Ethiopian Rift and the adjoining plateaus using hierarchical Bayesian ambient seismic noise tomography. A shear wave velocity model of the crust is produced based on the point-wise linearized inversion of the dispersion curves extracted from the group velocity maps. This dataset provides 3-D shear velocity results from Eshetu et al. (2021). The file “3dmod.dat” contains the shear wave velocity model for the Ethiopian rift, sampled onto a regular grid. Poorly imaged cells are set to “nan”, see the main text for details. Note that the model primarly resolves S wave structure (Vs). P wave velocity (Vp) is not independently constrained but, during the inversion, calculated from Vs using empirical relations (Brocher, 2005).

Coda-Q is used to estimate the attenuation and scattering properties of the Earth. So far focus has been on earthquake data at frequencies above 1 Hz, as the high noise level in the first and second microseismic peak, and possibly lower scattering coefficient, hinder stable measurements at lower frequencies. In this work, we measure and map coda-Q in the period bands 2.5-5 s, 5-10 s and 10-20 s in the greater Alpine region using noise cross-correlations between station pairs, based on data from permanent seismic stations and from the temporary AlpArray experiment. The observed coda-Q for short interstation distances is independent of azimuth so there is no indication of influence of the directivity of the incoming noise field on our measurements. In the 2.5-5 s and 5-10 s period bands, our measurements are self-consistent, and we observe stable geographic patterns of low and high coda-Q in the period bands 2.5-5 s and 5-10 s. In the period band 10-20 s, the dispersion of our measurements increases and geographic patterns become speculative. The coda-Q maps show that major features are observed with high resolution, with a very good geographical resolution of for example low coda-Q in the Po Plain. There is a sharp contrast between the Po Plain and the Alps and Apennines where coda-Q is high, with the exception a small area in the Swiss Alps which may be contaminated by the low coda-Q of the Po Plain. The coda of the correlations is too short to make independent measurements at different times within the coda, so we cannot distinguish between intrinsic and scattering Q. Measurements on more severely selected data sets and longer time-series result in identical geographical patterns but lower numerical values. Therefore, high coda-Q values may be overestimated, but the geographic distribution between high and low coda-Q areas is respected. Our results demonstrate that noise correlations are a promising tool for extending coda-Q measurements to frequencies lower than those analysed with earthquake data.

The data publication contains a dataset for fast assessment of earthquakes and early warning based on seismic waveforms. The dataset encompasses Italy and surrounding refions. Due to the large scale of the dataset, it is intended for use in machine learning. A similar dataset for Japan, with the same specifications as the one provided in this data publications, can be obtained using the scripts at https://github.com/yetinam/TEAM

We study the crustal structure of Sri Lanka by analyzing data from a temporary seismic network deployed in 2016-2017 (Seneviratne et al., 2016) to shed light on the amalgamation process from the geophysical perspective. Rayleigh wave phase dispersion from ambient noise cross-correlation and receiver functions were jointly inverted using a transdimensional Bayesian approach (Bodin et al., 2012, Dreiling et al., 2019).

Volcanic and seismic activities produce a variety of phenomena that put population at risk. In order to assess the seismic and volcanic hazard, many instruments are deployed around active volcanoes and seismic fault zones. Their records are useful to observe the activity of the volcanoes and faults in order to understand better their behaviour and issue warnings to authorities in charge of the public security. In this study, we used optical cables around Etna volcano to observe seismic and pressure signals associated with volcanic activity. The 2018 expedition consisted of three experiments. Firstly, we used a 4-km long telecom line in Zafferana and recorded all geodynamic and other activities for about 20 days. At this site, the fibre is known to cross active faults that are linked to the eastern volcano flank slowly sliding towards the sea. Secondly, we tested a 40 km-long cable from an internet provider at the western side of the volcano. Thirdly, we used a fibre cable deployed at the summit area of the volcano to test its ability to detect small volcanic events. In each of the three parts, a DAS system (iDASTM interrogator provided by Silixa) sent repeatedly coherent light pulses with a gauge length of 10 m and deduced the strain rate from interferometric measurements of the back-scattered light. We were able to measure every 2 metres the strain rate associated with several volcanic event types, earthquakes and many other signals from human activity. We validated the optical records with records from additional sensors such as seismic broadband stations, geophones and infrasound sensors. We present results from all experiments.

The present dataset is a high-resolution earthquake catalog for the region of the 2014 M8.1 Iquique earthquake sequence, in the Northern Chile subduction zone. Events in the catalog were obtained processing seismic waveforms from >100 permanent and temporary seismic stations. The list of waveform data sources used, the multistage automatic earthquake detection and location procedure implemented to build the catalog, as well as the patterns outlined by the seismicity are described in Soto et al. (2019).

The ocean is key to understanding societal threats including climate change, sea level rise, ocean warming, tsunamis, and earthquakes. Because the ocean is difficult and costly to monitor, we lack fundamental data needed to adequately model, understand, and address these threats. One solution is to integrate sensors into future undersea telecommunications cables. This is the mission of the SMART subsea cables initiative (Science Monitoring And Reliable Telecommunications). SMART sensors would "piggyback" on the power and communications infrastructure of a million kilometers of undersea fiber optic cable and thousands of repeaters, creating the potential for seafloor-based global ocean observing at a modest incremental cost. Initial sensors would measure temperature, pressure, and seismic acceleration. The resulting data would address two critical scientific and societal issues: the long-term need for sustained climate-quality data from the under-sampled ocean (e.g., deep ocean temperature, sea level, and circulation), and the near-term need for improvements to global tsunami warning networks. A Joint Task Force (JTF) led by three UN agencies (ITU/WMO/UNESCO-IOC) is working to bring this initiative to fruition. This paper explores the ocean science and early warning improvements available from SMART cable data, and the societal, technological, and financial elements of realizing such a global network. Simulations show that deep ocean temperature and pressure measurements can improve estimates of ocean circulation and heat content, and cable-based pressure and seismic-acceleration sensors can improve tsunami warning times and earthquake parameters. The technology of integrating these sensors into fiber optic cables is discussed, addressing sea and land-based elements plus delivery of real-time open data products to end users. The science and business case for SMART cables is evaluated. SMART cables have been endorsed by major ocean science organizations, and JTF is working with cable suppliers and sponsors, multilateral development banks and end users to incorporate SMART capabilities into future cable projects. By investing now, we can build up a global ocean network of long-lived SMART cable sensors, creating a transformative addition to the Global Ocean Observing System.

In Münchmeyer et al. 2019 magnitudes scales for Northern Chile have been derived with a focus on low uncertainties. The data set consists of three parts. First, a version of the IPOC catalog with the derived magnitude scales ML and MA and their uncertainties. Second, the attenuation functions for different waveform features. Third, the full matrix of features and the resulting single station magnitude predictions.The underlying IPOC catalog was obtained from Sippl et al. (2018). Detailed data description is provided in the README and in Münchmeyer et al. (2019) to which these data are supplementary material.

Estimating small-scale VP/VS variations at depth can be a powerful tool to infer lithology and hydration of a rock, with possible implications for frictional behavior. In principle, from the differential arrival times of P and S phases from a set of spatially clustered earthquakes, an estimate of the local VP/VS can be extracted, because the VP/VS is the scaling factor between the P and S differential times for each pair of earthquakes. We critically review the technique proposed by Lin and Shearer (2007), in which the mean value over all stations is subtracted from the differential arrival times of each pair of events in order to make the method independent of a priori information on origin times. The demeaned differential P and S arrival times are plotted on a plane, and the VP/VS ratio is estimated by fitting the points on this plane. We tested the method by both theoretical analysis and numerical simulations of P and S travel times in several velocity models. We found that the method returns exact values of VP/VS only in the case of a medium with homogeneous VP/VS , whereas, when a VP/VS gradient is present, the estimates are biased as an effect of systematic differences between P and S takeoff angles. We demonstrated that this bias arises from the demeaning of the arrival times over the stations. In layered models with VP/VS decreasing with depth, we found that VP/VS is overestimated or underestimated, respectively, for takeoff angles larger or smaller than 90°. In mosst realistic local earthquake monitoring settings, the take-off angles are not equally distributed but there will be a dominance of downward going rays, resulting in an overall bias. We calculated analytically the dependence of this bias on the takeoff angles. Additional simulations showed that the difference between the calculated and the expected VP/VS is reduced for simple horizontally layered velocity structures (

Estimating small-scale V-P/V-S variations at depth can be a powerful tool to infer lithology and hydration of a rock, with possible implications for frictional behavior. In principle, from the differential arrival times of P and S phases from a set of spatially clustered earthquakes, an estimate of the local V-P/V-S can be extracted, because the V-P/V-S is the scaling factor between the P and S differential times for each pair of earthquakes. We critically review the technique proposed by Lin and Shearer (2007), in which the mean value over all stations is subtracted from the differential arrival times of each pair of events in order to make the method independent of a priori information on origin times. The demeaned differential P and S arrival times are plotted on a plane, and the V-P/V-S ratio is estimated by fitting the points on this plane. We tested the method by both theoretical analysis and numerical simulations of P and S travel times in several velocity models. We found that the method returns exact values of V-P/V-S only in the case of a medium with homogeneous V-P/V-S, whereas, when a V-P/V-S gradient is present, the estimates are biased as an effect of systematic differences between P and S takeoff angles. We demonstrated that this bias arises from the demeaning of the arrival times over the stations. In layered models with V-P/V-S decreasing with depth, we found that V-P/V-S is overestimated or underestimated, respectively, for takeoff angles larger or smaller than 90 degrees. Moreover, we calculated analytically the dependence of this bias on the takeoff angles. Our simulations also showed that the difference between the calculated and the expected V-P/V-S is reduced for simple horizontally layered velocity structures (< 0.06), whereas it is 0.27 in a more realistic velocity model mimicking a subduction zone.

On September 16, 2015, the convergent Chilean margin again experienced a great subduction megathrust earthquake. The MW=8.2 Illapel earthquake occurred in the Metropolitan segment north of where the Juan-Fernandez ridge meets the Chile trench and subduction style and geometry change over a short distance. Combining GPS displacement measurements, InSAR interferograms, strong motion data, broadband seismological waveforms and backprojection of high frequency teleseismic signals we derive a comprehensive description of the coseismic rupture. Further, we determine accurate depths for the mostly offshore aftershock sequence by careful observations of teleseismic depth phases and derive moment tensors for the larger earthquakes from waveform modelling of bodyand surface waves. The rupture nucleated near the coast but then propagated to the north and updip. The resulting simple rupture geometry is approximately circular with a peak slip of 6 m, and a diameter of approximately 100 km, centered below the middle slope of the forearc. Forward modelling of tsunami propagation for this model successfully predicts approximate tsunami wave heights measured at 3 tide gauges along the North Central Chile coast, confirming that the rupture diminished towards the trench. Similar to previous observations, high frequency seismic radiation is mostly emitted downdip of the region of most intense slip, but unlike in most previous events, the high frequency emitters do not track the whole rupture along-strike but are confined to a small region within ∼ 50 km of the epicenter. The time evolution of high frequency seismic radiation also peaks earlier than the long period rupture evolution, indicating that the final phase of the rupture progressed smoothly. The aftershocks extend significantly beyond the limits of the main rupture in both north and south direction; their pattern of propagation suggests triggering by coseismic changes to the Coulomb failure stress. Plate interface events dominate the aftershock sequence but there are also some thrust events in the forearc crust and some shallow normal faulting events in the oceanic crust below the trench. In 1943, an earthquake of comparable along-strike extent occurred in the Illapel area. The similar extent of the aftershock zone and tsunami heights therefore make this part of the margin a candidate site for generating characteristic earthquakes, in particular as the 1943 event was itself preceded by an event in 1880, again with apparently the same part of the margin affected. The approximate match of peak slip and accumulated slip deficit in the 72 years since the 1943 event also support this interpretation. However, the 1943 Illapel event appears to have had a shorter source time function and probably a smaller magnitude than the 2015 event, pointing to differences in the detailed rupture evolution. The coupling is mostly close to fully locked in this area at least along the coast line but nevertheless the coseismic rupture is associated with a local peak in the locking pattern, whereas a distinct narrow partially interseismically creeping area is found just to the south of the main rupture. The northern transition to lower locking is more gradual but also here the rupture can be said to have terminated against a zone of reduced locking. Although locally the recent Illapel earthquake has relieved much of the accumulated stress, the segment immediately adjacent to the north remains unbroken since 1922, and presents a serious earthquake and tsunami hazard.

On September 16, 2015, the Chilean margin experienced a great subduction megathrust earthquake. The MW=8.2 Illapel earthquake occurred in the Metropolitan segment north of where the Juan-Fernandez ridge meets the Chile trench and subduction style and geometry change. Combining GPS displacement measurements, InSAR interferograms, strong motion and broadband seismological waveforms, and high frequency (HF) teleseismic backprojection we derive a comprehensive description of the coseismic rupture. We further determine moment tensors and obtain accurate depth estimates for mostly offshore aftershock sequence by careful observations of teleseismic depth phases. The rupture nucleated near the coast but then propagated to the north and updip, with a circular geometry of diameter 100 km and a peak slip of ~6 m below the forearc. HF seismic radiation is mostly emitted downdip of the region of most intense slip, but unlike in most previous events the high frequency emitters do not track the whole rupture but are confined to a small region. The time evolution of HF seismic radiation also arrests earlier than the long period rupture evolution, indicating that the final phase of the rupture progressed more smoothly. The aftershocks, mostly plate interface events, extend significantly beyond the limits of the main rupture in both directions. In 1943, a comparable event struck the Illapel area. The similar extent of the aftershock zone and tsunami heights therefore make this part of the margin a candidate site for generating characteristic earthquakes. The approximate match of peak slip and accumulated slip deficit in the 72 years since the 1943 event also support this interpretation. However, the 1943 event appears to have had a shorter source time function and probably a smaller magnitude, pointing to differences in the detailed rupture evolution. The coupling is mostly close to fully locked in this area at least along the coast line but nevertheless the coseismic rupture is associated with a local peak in the locking pattern, whereas a narrow partially creeping area is found just to the south of the main rupture. Although locally the recent Illaped earthquake has relieved much of the accumulated stress, the segment immediately adjacent to the north remains unbroken since 1922, and presents a serious earthquake and tsunami hazard.

LITHOS-CAPP is the German contribution to the international ScanArray experiment. ScanArray is an array of broadband seismometers with which we aim to study the lithosphere and upper mantle beneath the Scandinavian Mountains and the Baltic Shield. LITHOS-CAPP contributed 20 broadband recording stations from September 2014 to October 2016, 10 in Sweden and 10 in Finland, continuously recordings at 100 samples per second. The stations were deployed by the KIT Geophysical Institute and GFZ section 2.4 (seismology). They form part of the temporary network ScanArrayCore (FDSN network code 1G 2012-2017), Scientific Technical Report STR - Data; 17/02, GIPP Experiment and Data Archive

Scientific Technical Report STR - Data; 17/06, The island of Madagascar occupies a key region in both the assembly and the multi-stage breakup of Gondwanaland, itself part of the super-continent Pangaea. Madagascar consists of an amalgamation of continental material with the oldest rocks being of Archaean age. Its ancient fabric is characterised by several shear zones, some of them running oblique to the N-S trend, in particular in the south of the island. More recently during the Neogene, moderate volcanism has occurred in the Central and Northern part of the island, and there are indications of uplift throughout Eastern Madagascar over the last 10 Ma. Although Madagascar is now located within the interior of the African plate and far away from major plate boundaries (>1000 km from the East African rift system and even further from the Central and South-West Indian Ridges), its seismic activity indicates that some deformation is taking place, and present-day kinematic models based on geodetic data and earthquake moment tensors in the global catalogues identify a diffuse N-S-oriented minor boundary separating two microplates, which appears to pass through Madagascar. In spite of the presence of Archaean and Proterozoic rocks continent-wide scale studies indicate a thin lithosphere (

LITHOS-CAPP is the German contribution to the international ScanArray experiment. ScanArray is an array of broadband seismometers with which we aim to study the lithosphere and upper mantle beneath the Scandinavian Mountains and the Baltic Shield. LITHOS-CAPP contributed 20 broadband recording stations from September 2014 to October 2016, 10 in Sweden and 10 in Finland, continuously recordings at 100 samples per second. The stations were deployed by the KIT Geophysical Institute and GFZ section 2.4 (seismology). They form part of the temporary network ScanArrayCore (FDSN network code 1G 2012-2017). This data publication contains the original log-files of the recorders.

The SWATH-D experiment is dense deployment of 154 seismic stations in the Central and Eastern Alps between Italy and Austria, complementing the larger-scale sparser AlpArray Seismic Network (AASN). SWATH-D will provide high resolution images from the surface into the upper mantle, and allow observations of local seismicity. SWATH-D focuses on a key area of the Alps where the hypothesized flip in subduction polarity has been suggested, and where an earlier seismic profile (TRANSALP) has imaged a jump in the Moho. Where mains power is available (at ca. 80 sites) stations are providing realtime data via the cellphone network and are equipped with Güralp CMG-3EPSC (60s) seismometers and Earth Data Recorders EDR-210. The rest of the stations are offline and consist mainly of Nanometrics Trillium Compact (120s) and Güralp CMG-3EPSC (60s) seismometers equipped with either Omnirecs CUBE3 or PR6-24 Earth Data Loggers. All stations are equipped with external GPS antennas and the sampling rate is 100 Hz (Heit, et al., 2018). The network will operate for 2 years starting in July 2017. The Swath-D data will be used directly by 20 individual proposals of the MB-4D Priority Program (Mountain Building Processes in Four Dimensions, 2017) of the German Research Foundation (DFG) and data products derived from it will contribute to additional 13 proposals. SWATH-D is thus an important link between the MB-4D Priority Program and the international AlpArray communities and a scientific service to many of the proposals within the DFG Priority Program. Waveform data are available from the GEOFON data centre, under network code ZS, and are embargoed until August 2023. After the end of embargo, data will be openly available under CC-BY 4.0 license according to GIPP-rules.

Hundreds of submarine communication cables cross the world's oceans. Today, these cables are unaware of their environment. However, repeaters spaced at ~50 km intervals along them offer access to power and bandwidth, providing the opportunity to add sensor capability to future SMART cables (Science Monitoring And Reliable Telecommunications), a concept advanced by a Joint Task Force of the International Telecommunication Union, the World Meteorological Organization and the Intergovernmental Oceanographic Commission of UNESCO1. Two NASA workshops focused on applications in climate research and oceanography2. In the workshop described here, research scientists, practitioners from earthquake observatories and tsunami warning centers, and engineers discussed potential applications of SMART cables for earthquake and tsunami early warning and reviewed existing approaches and how they can benefit from SMART cables. They also considered what possibilities exist in research on Earth structure, the physics of earthquakes, and tsunami excitation and propagation. According to current planning, a first generation of SMART cables will be equipped with a simple instrumentation package containing accelerometers, pressure gauges and temperature sensors in order to make the sensor package simple and able to withstand the rough deployment conditions in standard cable-laying operations. Most destructive tsunamis are triggered by great earthquakes along the plate boundary faults in subduction zones. Their offshore location makes quick detection and assessment of their tsunamigenic potential a real challenge using land-based networks. The DART system of ocean bottom pressure detectors can detect ocean-crossing tsunamis but sensors are too sparse and too far from shore to be much help in local warning. Dedicated submarine cables present another real-time solution but come with a hefty price tag. Thus a comprehensive coverage of all endangered subduction zones is out of reach, particularly in the developing world. Already a few cables crossing the Pacific can reduce the time-to-detection of potentially tsunamigenic earthquakes along the Ring of Fire by ~20%, and the detection of the actual tsunami wave would be reduced by a similar fraction. With trench-parallel cables even larger improvements are possible. The continuous high sampling rates possible in a cable allow separation of tsunami and seismic wavefields, allowing reliable tsunami measurements in the near field. Wide science benefits are expected from the faithful recordings of offshore earthquakes as well as from the vastly improved coverage of the ocean basins from even a small number of SMART cables.

International audience; The Precambrian rocks of Madagascar were formed and/or modified during continental collision known as the Pan-African orogeny. Aborted Permo-Triassic Karoo rifting and the subsequent separation from Africa and India resulted in the formation of sedimentary basins in the west and volcanic activity predominantly along the margins. Many geological studies have documented the imprint of these processes, but little was known about the deeper structure. We therefore deployed seismic stations along an SE-NW trending profile spanning nearly all geological domains of southern Madagascar. Here we focus on the crustal structure, which we determined based on joint analysis of receiver functions and surface waves derived from ambient noise measurements. For the sedimentary basin we document a thinning of the underlying crystalline basement by up to ∼60% to 13 km. The crustal velocity structure demonstrates that the thinning was accomplished by removal or exhumation of the lower crust. Both the Proterozoic and Archean crust have a 10 km thick upper crust and 10–12 km thick midcrust. However, in contrast to the typical structure of Proterozoic and Archean aged crust, the Archean lower crust is thicker and faster than the Proterozoic one, indicating possible magmatic intrusions; an underplated layer of 2–8 km thickness is present only below the Archean crust. The Proterozoic mafic lower crust might have been lost during continental collision by delamination or subduction or thinned as a result of extensional collapse. Finally, the Cretaceous volcanics along the east coast are characterized by thin crust (30 km) and very large V P ∕V S ratios.

We have inferred the Vp/Vs ratio along the segment of the Peru-Chile subduction margin corresponding to the rupture area of the Tocopilla earthquake (Mw 7.7, 14/11/2007). This event nucleated in Northern Chile and broke the southern ~100 km of the ~500 km Northern Chile Southern Peru seismic gap, which at the time had not seen an earthquake of this magnitude since the M~9 event of 1877. Tocopilla event activated two main co-seismic slip patches: one around the epicenter and another north-east of the Mejillones Peninsula. We have applied the Lin and Shearer (2007) approach to the aftershock sequence of the Tocopilla event. In this approach, the relative time shift between the S phases of a pair of nearby events at one station are plotted as function of the time shifts between the P phases of the same pair. The process is repeated for a cluster of events. If the events are close enough to assume a locally uniform Vp/Vs ratio and the P-reciprocal wavefront can be approximated as planar, the points lay on a line, whose slope is an estimation of the local Vp/Vs. The technique is extended to a set of stations demeaning the time shifts from each pair of events. The time shifts are inferred maximizing the cross-correlation function between the event pairs. The technique has been applied to clusters of events sharing similar waveforms and spatially clustered hypocentres. We have adopted a robust linear regression and have assigned a statistical error to the best fit. Clusters have been identified along the whole profile of the subducting slab, although most clusters falls within a sub-vertical branch of the subduction interface hosting a major aftershock (Michilla earthquake, 16/12/2007, Ml 6.8) and its aftershocks. This branch falls inside the subducted Nazca Plate at depths of 40-50 km, north-east of the Mejillones Peninsula, and shows Vp/Vs mostly in the range 1.7-1.8. Clusters of the plate interface shallower than about 30 km show Vp/Vs around 1.9, while at intermediate depths (30-40 km) Vp/Vs is ~1.8. We speculate on the existence of hydrated crust producing the highest Vp/Vs (~1.9 or larger) observed at shallow depths.

We have inferred the Vp/Vs ratio along the segment of the Peru-Chile subduction margin corresponding to the rupture area of the Tocopilla earthquake (TE, Mw 7.7, 14/11/2007). This event nucleated in Northern Chile and broke the southern ~100 km of the ~500 km Northern Chile Southern Peru seismic gap, which had not seen an earthquake of this magnitude since the M~9 event of 1877. TE activated two main co-seismic slip patches: one around the epicenter and another north-east of the Mejillones Peninsula. We have applied the Lin and Shearer approach [1] to the aftershock sequence of TE. In this approach, the relative time shift between the S phases of a pair of nearby events at one station are plotted as function of the time shift between the P phases of the same pair. The process is repeated for a set of events. If the events are close enough to assume a uniform local Vp/Vs and the P-reciprocal wavefront can be approximated as planar, the points lay on a line, whose slope is an estimation of the local Vp/Vs. The technique is extended to a set of stations demeaning the time shifts from each pair of events. The time shifts are inferred maximizing the cross-correlation function between the event pairs. The technique has been applied to clusters of events sharing similar waveforms and spatially clustered hypocentres. We have adopted a robust linear L2 regression and have assigned a statistical error to the best fit. Most clusters are identified within a sub-vertical branch of the subduction interface hosting a major aftershock (Michilla earthquake, 16/12/2007, Ml 6.8) and its aftershocks. This branch falls inside the subducted Nazca Plate at depths of 40-50 km, north-east of the Mejillones Peninsula, and shows Vp/Vs mostly in the range 1.8-1.9. Clusters close to the Mejillones Peninsula and to the epicenter displays Vp/Vs around 1.7 and 1.8, respectively. References [1] - Lin, G., & Shearer, P. (2007). Estimating local Vp/Vs ratios within similar earthquake clusters. Bulletin of the Seismological Society of America, 97 (2), 379-388.

The Maule earthquake (2010 February 27, Mw 8.8, Chile) broke the subduction megathrust along a previously locked segment. Based on an international aftershock deployment, catalogues of precisely located aftershocks have become available. Using 23 well-located aftershocks, we calibrate the classic teleseismic backprojection procedure to map the high-frequency seismic radiation emitted during the earthquake. The calibration corrects traveltimes in a standard earth model both with a static term specific to each station, and a ‘dynamic’ term specific to each combination of grid point and station. The second term has been interpolated over the whole slipping area by kriging, and is about an order of magnitude smaller than the static term. This procedure ensures that the teleseismic images of rupture development are properly located with respect to aftershocks recorded with local networks and does not depend on accurate hypocentre location of the main shock. We track a bilateral rupture propagation lasting ∼160 s, with its dominant branch rupturing northeastwards at about 3 km s−1. The area of maximum energy emission is offset from the maximum coseismic slip but matches the zone where most plate interface aftershocks occur. Along dip, energy is preferentially released from two disconnected interface belts, and a distinct jump from the shallower belt to the deeper one is visible after about 20 s from the onset. However, both belts keep on being active until the end of the rupture. These belts approximately match the position of the interface aftershocks, which are split into two clusters of events at different depths, thus suggesting the existence of a repeated transition from stick-slip to creeping frictional regime.

Northern Chile has recently been struck by the Mw 8.1 Pisagua earthquake, which occurred on 01/04/2014 and partially filled the Iquique seismic gap. The Pisagua earthquake has been preceded by intense foreshock activity which started in July 2013 and culminated in a cluster of events in March 2014. We have inferred the rupture dynamics of the mainshock and of its largest (Iquique) aftershock (03/04/2014, Mw 7.6) by backprojecting the high-frequency seismic radiation released during the events and recorded by 310 stations of USArray. The time-evolution of the high-frequency (1-4 Hz) energy radiated during the mainshock shows that the rupture lasted about 80 s, with most of the energy released between 25 s and 50 s from the onset. The cumulative energy emitted during the whole rupture process mainly originated downdip the epicenter just off the coast line, approximately in the latitude range 19.5°-20°S. This region falls at the down-dip side of the co-seismic slip area, similarly to the case of the Maule earthquake (South Chile, 27/02/2010, Mw 8.8) and of other large earthquakes. Differently from the Maule case, most aftershocks not located in the area of large seismic radiation. The time-evolution of the coherent seismic radiation displays an initial low-energy phase (lasting about 20s) during which the source starts to migrate from the nucleation point at the epicenter towards the south-east, activating deeper parts of the subduction interface. After reaching points close to the coast line (after ~30s from the onset), the source moves back towards the epicenter mainly activating in sequence two patches of the interface located around 20°S,70.5°W (F1) and 19.7°S,70.2°W (F2), shortly followed by the activation of points close to the area of the largest co-seismic slip (F3). In the last ~15s of the fracturing (F4), the re-activation of the area releasing energy during F2 is observed. Thus, despite the simple bullseye co-seismic slip pattern, the history of energy radiation is quite scattered, suggesting peculiar and sharply site-dependent frictional properties along this segment of the Chilean subduction interface. The Mw7.6 aftershock displays a similar time-evolution of the radiating source, with an initial low-energy stage, during which the rupture front migrates from the epicenter towards deeper zones. Most energy is released about 25s from the onset, when the rupture front reaches points around 20.5°S,70.0°W.

Seismology is often associated with the investigation of earthquake processes and indeed this is a very important target of the science. However, seismology is also the primary means by which we can obtain images of the internal structure of the Earth on all scales, ranging from the deep interior, i.e. the Earth’s core and lower mantle over the mantle lithosphere and crust, the scale at which plate tectonics ‘happens’ all the way to the unconsolidated material in the near-surface. Seismology uses earthquakes and artificial sources as signal generators but also the ambient background wavefield, previously considered to be merely noise. Methodological and instrumental innovations have allowed ever more detailed investigations of the earthquake process and the seismic structure, and opened up new targets such as as the monitoring of geomorphological events and investigations of the time-dependency of seismic structure. Finally seismology is not restricted to waveform analysis and the instrumental period but draws on historical data and, in the field of paleoseismology, geological markers to constrain seismic activity in the pre-instrumental past. Direct societal benefits from seismology accrue for example from improved seismic hazard estimates and early-warning technologies in the area of natural hazards, from improved recovery and usage of natural resources, including geothermal energy as a sustainable energy source and much more, e.g. the possibility to monitor and thus enforce compliance with the nuclear test ban treaty.

Madagascar occupies a key position in the assembly and breakup of the supercontinent Gondwana. It has been used in numerous geological studies to reconstruct its original position within Gondwana and to derive plate kinematics. Seismological observations in Madagascar to date have been sparse. Using a temporary, dense seismic profile across southern Madagascar, we present the first published study of seismic anisotropy from shear wave splitting analyses of teleseismic phases. The splitting parameters obtained show significant small-scale variation of fast polarization directions and delay times across the profile, with fast polarization rotating from NW in the center to NE in the east and west of the profile. The delay times range between 0.4 and 1.5 s. A joint inversion of waveforms at each station is applied to derive hypothetical one-layer splitting parameters. We use finite-difference, full-waveform modeling to test several hypotheses about the origin and extent of seismic anisotropy. Our observations can be explained by asthenospheric anisotropy with a fast polarization direction of 50°, approximately parallel to the absolute plate motion direction, in combination with blocks of crustal anisotropy. Predictions of seismic anisotropy as inferred from global mantle flow models or global anisotropic surface wave tomography are not in agreement with the observations. Small-scale variations of splitting parameters require significant crustal anisotropy. Considering the complex geology of Madagascar, we interpret the change in fast-axis directions as a ~150 km wide zone of ductile deformation in the crust as a result of the intense reworking of lithospheric material during the Pan-African orogeny. This fossil anisotropic pattern is underlain by asthenospheric anisotropy induced by plate motion.

In news reports we are accustomed to see earthquakes symbolised with a dot or star on a map and associated with a magnitude, the bigger the scarier. For at least moderately sized earthquakes seismologists additionally have been routinely determining the type of rupture just from observing the pattern of radiated seismic energy. In reality earthquakes do not occur as a point but rupture a fault plane. For small earthquakes this distinction can be neglected but for the largest earthquakes the rupture plane can extend for hundreds of kilometres, and the actual rupture propagation begins to have a strong influence on the hazard that the earthquake presents – whether the rupture proceeds to the north or the south and how deep and shallow it reaches determines which cities will be hit the hardest, whether shaking is moderate or intense, and whether a sizeable tsunami is triggered. The explosion of the availability of ground-, ocean- and space-based observation technologies in the last decade has allowed seismologists to map the rupture process in unprecedented detail even for challenging subduction zone earthquakes. The same technology can be used to observe potential precursory processes and the postseismic relaxation by which the earth regains its equilibrium following the disturbance that a great earthquake represents. Focussing on the Mw 8.1 Iquique earthquake in northern Chile on April 1, 2014, we will discuss the state-of-the-art in monitoring great earthquakes and their aftermath.

We back-project the seismic radiation released by Maule earthquake (Chile, 27/02/2010, Mw 8.8) in three frequency bands: 0.4-3 Hz, 1-4 Hz, 2-8 Hz. We measure the coherence of the seismic traces at 557 stations of US array by semblance. Travel times are estimated starting from a 1D global velocity model (ak135) corrected by two terms: a static correction and a dynamic correction. Static corrections are the mean time corrections to the 1D velocity model, and dynamic corrections are finer time shifts depending on the source-receiver path. Both terms are extracted from the time shifts between different receivers of P-phases of 23 high-magnitude calibration aftershocks, most of which have high precision locations based on the temporary deployment following the Maule earthquake (IMAD). The dynamic corrections are extended over a fine source grid by kriging interpolation. This procedure makes the backprojection results independent of the main shock catalog hypocentre and allows coherent imaging to higher frequencies. During the first 20 seconds of the rupture process, the source is stable nearby the nucleation point, which is close to epicentre proposed by Vigny et al (Science, 2011) based on high rate GPS motion. Afterwards, it moves bilaterally, with the northern front moving with an average velocity of ∼3 km/s. Most of the energy is emitted from the northern patch of the bi-lateral rupture (∼70%), with sporadic emissions from the southern patch. The maximum of stacked energy is located about 150 km north-eastwards from the epicenter and a relative maximum appears south of Arauco peninsula. In the dip direction, energy is mostly emitted from the down-dip edge of the co-seismic area, roughly matching the aftershock distribution. Specifically, we find that coherent radiation is emitted from two distinct belts nearly parallel to the trench. The position of these belts is in good agreement with the location of the aftershocks, which also are arranged in two disconnected zones of the subduction interface at different depths, the deeper of which is characterised by a large number of repeating event clusters (Rietbrock, Jenkins et al., this session). Thus, our backprojection analysis in combination with the aftershock distribution demonstrates the existence of a peculiar doubled downdip transition from seismogenic behaviour to stable sliding. We suspect fluids released from the downgoing plate to be the cause of the transitions in frictional behaviour because of (1) the co-location of high Vp/Vs ratios with the deep interface seismicity, (2) systematic decrease of depth of onset of deeper seismicity with younging incoming plate age, (3) patchy occurrence along-strike of deeper seismicity.

We investigate repeating earthquakes (REs) on the Chile subduction zone, in the first 9 months following the Maule 2010 M 8.8 earthquake. Using the aftershock catalogue of approximately 30,000 events (Rietbrock et al., 2012) and the data from the International Maule Aftershock Dataset (IMAD), we identify 1550 clusters of small magnitude (Mw ~1.5-3) events showing similar waveforms (cross-correlation coefficients>0.9). Clusters are found from the surface to depths of ~60km, indicating the generation of RE on pre-existing crustal faults and slab interface. A particularly dense band of clustered seismicity runs NE-SW along the length of Chile at 37-47km depth on the slab/continent interface, apparently defining the limit of plate coupling (Rietbrock et al, 2012; Lange et al., 2012). Relocation of deep clusters, via the double difference method (hypoDD), reveal that they lie within a region of increased fluid content (interpreted from high Vp/Vs ratio (Hicks et al., 2012)), and define streaks of seismicity orientated down-dip. Moment tensor analysis of selected aftershocks shows that larger events M 4-5 are located at the interface or deeper in the slab (5-8km beneath the slab interface) and show thrust motion along the direction of the plate interface. REs, in contrast, show predominantly strike-slip motion and are located close to the interface. Temporal analysis also shows non-constant recurrence times of events within clusters, which we interpret as an indication that the seismicity in the deep clusters are driven by pulses of after-slip from the Maule 2010 event together with episodic fluid migration. We introduce a model of Mixed Mode Fault Slip (MMFS) to explain our observations, where aseismic sliding of trench sediments in the subduction channel build up stress on fragments of ocean crust, causing them to repeatedly fracture and generate repeating earthquakes.

Using direct shear waves in addition to SKS waves for splitting measurement would be advantageous in splitting measurements since they sample possible anisotropic structures in the upper mantle with an increased range of incidence angles and back azimuths. However, source-side contamination of direct S-waves makes splitting measurements complicated although all shear waves theoretically suitable for splitting analyses as long as the angle of incidence at surface is smaller than 35_. Here we introduce the reference station technique, a direct S wave-based method in the estimation of shear-wave splitting parameters. The method depends on maximizing the correlation between seismic traces with direct-S wave signal at reference and target stations of a station pair after correcting the reference station for the receiver side anisotropy effect using SKS splitting parameters. The procedure effectively assumes the same source side anisotropy affecting the two stations for the same seismic event. Only a few stations with well-constrained SKS splitting results are needed as seeds to determine the splitting parameters of a large array in an iterative manner. Various synthetic tests show that (1) reference station technique is not critically sensitive to varying crustal thicknesses between reference and target stations (2) the use of events with increased angle of incidence will cause deviations in split time delays (STD) with respect to the true value of the STD. Applying the reference station technique to the real data obtained from the INDEPTH IV and ASCENT seismic experiments at the northern margin of Tibet generally resulted in a good agreement between SKS- and direct S-derived splitting parameters. Where differences exist, the resolved shear waves fast polarization directions (FPD) indicate a higher degree of internal consistency for closely spaced stations where we do not expect clear lateral variation. This is probably due to the much larger number of S waves available for splitting measurements compared to SKS. Compared to SKS phases that are strongly restricted to the signal quality and a narrow range of angle of incidences, the use of direct S-waves increases the number of analyzed events by a factor of _4.

An important tool for understanding deformation occurring within a subduction zone is the measurement of seismic anisotropy through observations of shear wave splitting (SWS). In Sumatra, two temporary seismic networks were deployed between December 2007 and February 2009, covering the fore arc between the fore-arc islands to the back arc. We use SKS and local SWS measurements to determine the type, amount, and location of anisotropy. Local SWS measurements from the fore-arc islands exhibit trench-parallel fast directions which can be attributed to shape preferred orientation of cracks/fractures in the overriding sediments. In the Sumatran Fault region, the predominant fast direction is fault/ trench parallel, while in the back-arc region it is trench perpendicular. The trench-perpendicular measurements exhibit a positive correlation between delay time and raypath length in the mantle wedge, while the fault-parallel measurements are similar to the fault-parallel fast directions observed for two crustal events at the Sumatran Fault. This suggests that there are two layers of anisotropy: one due to entrained flow within the mantle wedge and a second layer within the overriding crust due to the shear strain caused by the Sumatran Fault. SKS splitting results show a NNW-SSE fast direction with delay times of 0.8–3.0 s. The fast directions are approximately parallel to the absolute plate motion of the subducting Indo-Australian Plate. The small delay times exhibited by the local SWS (0.05–0.45 s), in combination with the large SKS delay times, suggest that the anisotropy generating the teleseismic SWS is dominated by entrained flow in the asthenosphere below the slab.

Based on joint consideration of S receiver functions and surface-wave anisotropy we present evidence for the existence of a thick and layered lithosphere beneath the Kalahari Craton. Our results show that frozen-in anisotropy and compositional changes can generate sharp Mid-Lithospheric Discontinuities (MLD) at depths of 85 and 150–200 km, respectively. We found that a 50 km thick anisotropic layer, containing 3% S wave anisotropy and with a fast-velocity axis different from that in the layer beneath, can account for the first MLD at about 85 km depth. Significant correlation between the depths of an apparent boundary separating the depleted and metasomatised lithosphere, as inferred from chemical tomography, and those of our second MLD led us to characterize it as a compositional boundary, most likely due to the modification of the cratonic mantle lithosphere by magma infiltration. The deepening of this boundary from 150 to 200 km is spatially correlated with the surficial expression of the Thabazimbi-Murchison Lineament (TML), implying that the TML isolates the lithosphere of the Limpopo terrane from that of the ancient Kaapvaal terrane. The largest velocity contrast (3.6–4.7%) is observed at a boundary located at depths of 260–280 km beneath the Archean domains and the older Proterozoic belt. This boundary most likely represents the lithosphereasthenosphere boundary, which shallows to about 200 km beneath the younger Proterozoic belt. Thus, the Kalahari lithosphere may have survived multiple episodes of intense magmatism and collisional rifting during the billions of years of its history, which left their imprint in its internal layering.

The territory of Lithuania and adjacent areas of the East European Craton have always been considered a region of low seismicity. Two recent earthquakes with magnitudes of more than 5 in the Kaliningrad District (Russian Federation) on 21 September 2004 motivated re-evaluation of the seismic hazard in Lithuania and adjacent territories. A new opportunity to study seismicity in the region is provided by the PASSEQ (Pasive Seismic Experiment) project that aimed to study the lithosphere–asthenosphere structure around the Trans-European Suture Zone. Twenty-six seismic stations of the PASSEQ temporary seismic array were installed in the territory of Lithuania. The stations recorded a number of local and regional seismic events originating from Lithuania and adjacent areas. This data can be used to answer the question of whether there exist seismically active tectonic zones in Lithuania that could be potentially hazardous for critical industrial facilities. Therefore, the aim of this paper is to find any natural tectonic seismic events in Lithuania and to obtain more general view of seismicity in the region. In order to do this, we make a manual review of the continuous data recorded by the PASSEQ seismic stations in Lithuania. From the good quality data, we select and relocate 45 local seismic events using the well-known LocSAT and VELEST location algortithms. In order to discriminate between possible natural events, underwater explosions and on-shore blasts, we analyse spatial distribution of epicenters and temporal distribution of origin times and perform both visual analysis of waveforms and spectral analysis of recordings. We show that the relocated seismic events can be grouped into five clusters (groups) according to their epicenter coordinates and origin and that several seismic events might be of tectonic origin. We also show that several events from the off-shore region in the Baltic Sea (at the coasts of the Kaliningrad District of the Russian Federation) are non-volcanic tremors, although the origin of these tremor-type events is not clear.