Sub-slab mantle anisotropy beneath south-central Chile

Knowledge of mantle flow in convergent margins is crucial to unravelling both the contemporary geodynamics and the past evolution of subduction zones. By analysing shear-wave splitting in both teleseismic and local arrivals, we can determine the relative contribution from different parts of the subduction zone to the total observed SKS splitting, providing us with a depth constraint on anisotropy. We use this methodology to determine the location, orientation and strength of seismic anisotropy in the south-central Chile subduction zone. Data come from the TIPTEQ network, deployed on the forearc during 2004–2005. We obtain 110 teleseismic SKS and 116 local good-quality shear-wave splitting measurements. SKS average delay times are 1.3 s and local S delay times are only 0.2 s. Weak shear-wave splitting from local phases is consistent with a shape preferred orientation (SPO) source in the upper crust. We infer that the bulk of shear-wave splitting is sourced either within or below the subducting Nazca slab. SKS splitting measurements exhibit an average north-easterly fast direction, with a strong degree of variation. Further investigation suggests a relationship between the measurement's fast direction and the incoming ray's back-azimuth. Finite-element geodynamic modelling is used to investigate the strain rate field and predicted LPO characteristics in the subduction zone. These models highlight a thick region of high strain rate and strong S-wave anisotropy, with plunging olivine a-axes, in the sub-slab asthenosphere. We forward model the sub-slab sourced splitting with a strongly anisotropic layer of thick asthenosphere, comprising an olivine a-axis oriented parallel to the direction of subduction. The subducting lithosphere is not thick enough to cause 1.2 s of splitting, therefore our results and subsequent models show that the Nazca slab is entraining the underlying asthenosphere; its flow causes it to be strongly anisotropic. Our observation has important implications for the controlling factors on sub-slab mantle flow and the movement of asthenospheric material within the Earth

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

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

Probing the rheology of continental faults:Decade of post-seismic InSAR time-series following the 1997 Manyi (Tibet) earthquake

The physical processes driving post-seismic deformation after large earthquakes are still debated. As in most cases relatively short observation time periods are being used, it is still challenging to distinguish between the different proposed mechanisms and therefore a longer observation time is needed. The 1997 MW 7.6 Manyi, Tibet, earthquake has an excellent InSAR data archive available to study the post-seismic deformation up to ~13 yr after the earthquake. The coseismic and early post-seismic phases of theManyi earthquakewere already investigated in detail by numerous studies with viscoelastic and afterslip models being used to explain the post-seismic deformation. We use SAR (Synthetic Aperture Radar) data obtained from the ERS and Envisat satellites covering the central part of the Manyi fault from 1997 to 2010 to significantly extend the observation period. We test different viscoelastic (uniform Maxwell, Standard linear solid and Burgers body rheology below an uppermost elastic layer) and afterslip models to assess the most suitable mechanism for post-seismic deformation. While a Maxwell rheology (misfit = 2.23 cm) is not able to explain the observed long timeseries, the standard linear solid (misfit=2.07 cm) and Burgers body models (misfit=2.16 cm) with two relaxation times, cannot reproduce sufficiently the localized deformation patterns. The afterslip model (misfit = 1.77 cm) has the lowest misfit and explains well the temporal and spatial pattern of observed deformation. A combined mechanism model that considers the effects of both afterslip and viscoelastic relaxation is also a feasible process, where the viscoelastic relaxation can slightly improve the fit to the data especially at larger distances from the fault. Themaximum average line-of-sight velocity is~4mmyr-1 during 2008-2010, suggesting that the post-seismic deformation of the Manyi earthquake might be vanishing and gradually stepping into an interseismic phase

Slip distribution of the 2015 Lefkada earthquake and its implications for fault segmentation

It is widely accepted that fault segmentation limits earthquake rupture propagations and therefore earthquake size. While along-strike segmentation of continental strike-slip faults is well observed, direct evidence for segmentation of off-shore strike-slip faults is rare. A comparison of rupture behaviours in multiple earthquakes might help reveal the characteristics of fault segmentation. In this work, we study the 2015 Lefkada earthquake, which ruptured a major active strike slip fault offshore Lefkada Island, Greece. We report ground deformation mainly on the Lefkada Island measured by interferometric synthetic radar (InSAR), and infer a coseismic distributed slip model. To investigate how the fault location affects the inferred displacement based on our InSAR observations, we conduct a suite of inversions by taking various fault location from different studies as a prior. The result of these test inversions suggests that the Lefkada fault trace is located just offshore Lefkada Island. Our preferred model shows that the 2015 earthquake main slip patches are confined to shallow depth (<10 km), with amaximum slip of~1.6 m. In comparison to the 2003 earthquake,which mainly ruptured the northern part of the Lefkada fault, we suggest that the 2015 earthquake closed the seismic gap, at least partially, left by the 2003 earthquake by rupturing the shallow part of the Lefkada fault. The spatial variation in slip distributions for the two earthquakes reveals segmentation along strike, and possibly downdip of the Lefkada fault. A comparison of aftershock locations and coseismic slip distribution shows that most aftershocks appear near the edge of main coseismic slip patches

Seafloor sediment thickness beneath the VoiLA broad-band ocean-bottom seismometer deployment in the Lesser Antilles from P-to-S delay times

Broad-band ocean-bottom seismometer (OBS) deployments present an opportunity to investigate the seafloor sediment thickness, which is important for constraining sediment deposition, and is also useful for subsequent seismological analyses. The Volatile Recycling in the Lesser Antilles (VoiLA) project deployed 34 OBSs over the island arc, fore- and backarc of the Lesser Antilles subduction zone for 15 months from 2016 to 2017. Using the amplitudes and delay times of P-to-S (Ps) scattered waves from the conversion of teleseismic earthquake Pwaves at the crust–sediment boundary and pre-existing relationships developed for Cascadia, we estimate sediment thickness beneath each OBS. The delay times of the Ps phases vary from 0.20 ± 0.06 to 3.55 ± 0.70 s, generally increasing from north to south. Using a single-sediment and single-crystalline crust earth model in each case, we satisfactorily model the observations of eight OBSs. At these stations we find sediment thicknesses range from 0.43 ± 0.45 to 5.49 ± 3.23 km. To match the observations of nine other OBSs, layered sediment and variable thickness crust is required in the earth model to account for wave interference effects on the observed arrivals. We perform an inversion with a two-layer sediment and a single-layer crystalline crust in these locations finding overall sediment thicknesses of 1.75 km (confidence region: 1.45–2.02 km) to 7.93 km (confidence region: 6.32–11.05 km), generally thinner than the initial estimates based on the pre-existing relationships. We find agreement between our modelled velocity structure and the velocity structure determined from the VoiLA active-source seismic refraction experiment at the three common locations. Using the Ps values and estimates from the VoiLA refraction experiment, we provide an adjusted relationship between delay time and sediment equations for the Lesser Antilles. Our new relationship is H=1.42dt$^{1.44}$ , where H is sediment thickness in kilometres and dt is mean observed Ps delay time in seconds, which may be of use in other subduction zone settings with thick seafloor sediments

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

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

Seafloor sediment thickness beneath the VoiLA broad-band ocean-bottom seismometer deployment in the Lesser Antilles from P-to-S delay times

Broad-band ocean-bottom seismometer (OBS) deployments present an opportunity to investigate the seafloor sediment thickness, which is important for constraining sediment deposition, and is also useful for subsequent seismological analyses. The Volatile Recycling in the Lesser Antilles (VoiLA) project deployed 34 OBSs over the island arc, fore- and backarc of the Lesser Antilles subduction zone for 15 months from 2016 to 2017. Using the amplitudes and delay times of P-to-S (Ps) scattered waves from the conversion of teleseismic earthquake Pwaves at the crust–sediment boundary and pre-existing relationships developed for Cascadia, we estimate sediment thickness beneath each OBS. The delay times of the Ps phases vary from 0.20 ± 0.06 to 3.55 ± 0.70 s, generally increasing from north to south. Using a single-sediment and single-crystalline crust earth model in each case, we satisfactorily model the observations of eight OBSs. At these stations we find sediment thicknesses range from 0.43 ± 0.45 to 5.49 ± 3.23 km. To match the observations of nine other OBSs, layered sediment and variable thickness crust is required in the earth model to account for wave interference effects on the observed arrivals. We perform an inversion with a two-layer sediment and a single-layer crystalline crust in these locations finding overall sediment thicknesses of 1.75 km (confidence region: 1.45–2.02 km) to 7.93 km (confidence region: 6.32–11.05 km), generally thinner than the initial estimates based on the pre-existing relationships. We find agreement between our modelled velocity structure and the velocity structure determined from the VoiLA active-source seismic refraction experiment at the three common locations. Using the Ps values and estimates from the VoiLA refraction experiment, we provide an adjusted relationship between delay time and sediment equations for the Lesser Antilles. Our new relationship is H=1.42dt$^{1.44}$ , where H is sediment thickness in kilometres and dt is mean observed Ps delay time in seconds, which may be of use in other subduction zone settings with thick seafloor sediments

Subsurface Insights of the Maricunga Gold Belt through Local Earthquake Tomography

With the advancement of the use of geophysical methods in mining exploration, the possibility of restudying known mineral deposits that could have greater potential than that previously estimated is opening up, as is the case in the Maricunga Belt (MB), which is a metallogenic belt located east of Copiapó, Chile, with a length of 200 km and oriented in the NNE-SSW direction. This belt hosts significant gold deposits classified as porphyry gold (-copper), epithermal gold (-silver) of a high sulphidation type, and transitional gold, in some districts. In this work we studied the characteristics of the MB through local earthquake tomography (LET), which revealed a clear spatial correlation between low Vp/Vs anomalies and the gold deposits, demonstrating that lithologic interpretation using Vp and Vs values of the seismic tomography makes sense for the most common rocks associated with the genesis of porphyry-type deposits. Furthermore, high Vp/Vs anomalies were correlated to the main regional faults around the study zone, which seem to have a robust structural control regarding the location of the deposits