Ocean Bottom Seismometer Array Research Articles

A Reciprocity‐Based Efficient Method for Improved Source Parameter Estimation of Submarine Earthquakes With Hybrid 3‐D Teleseismic Green's Functions

AbstractAccurate source parameters of global submarine earthquakes are essential for understanding earthquake mechanics and tectonic dynamics. Previous studies have demonstrated that teleseismic P coda waveform complexities due to near‐source 3‐D structures are highly sensitive to source parameters of marine earthquakes. Leveraging these sensitivities, we can improve the accuracy of source parameter inversion compared to traditional 1‐D methods. However, modeling these intricate 3‐D effects poses significant computational challenges. To address this issue, we propose a novel reciprocity‐based hybrid method for computing 3‐D teleseismic Green's functions. Based on this method, we develop a grid‐search inversion workflow for determining reliable source parameters of moderate‐sized submarine earthquakes. The method is tested and proven on five Mw5+ earthquakes at the Blanco oceanic transform fault (OTF) with ground truth locations resolved by a local ocean bottom seismometer array, using ambient noise correlation and surface‐wave relocation techniques. Our results show that fitting P coda waveforms through 3‐D Green's functions can effectively improve the source location accuracy, especially for the centroid depth. Our improved centroid depths indicate that all the five Mw5+ earthquakes on the Blanco transform fault ruptured mainly above the depth of 600°C isotherm predicted by the half‐space cooling model. This finding aligns with the hypothesis that the rupture zone of large earthquakes at OTFs is confined by the 600°C isotherm. However, it is noted that the Blanco transform fault serves as a case study. Our 3‐D source inversion method offers a promising tool for systematically investigating global oceanic earthquakes using teleseismic waves.

Sequencing Seismic Noise Correlations for Improving Surface Wave Retrieval and Characterizing Noise Sources

Abstract Cross-correlating continuous seismic data is a commonly employed technique to extract coherent signals to image and monitor the subsurface. However, due largely to site effects and poorly characterized noise sources in oceanic environments, its application to ocean-bottom seismometer (OBS) recordings often requires additional processing. In this contribution, we propose a method to improve the quality of the retrieved surface waves from OBS data and characterize the noise sources. We first cluster the pre-stack noise cross-correlation functions (NCFs) based on a sequencing algorithm, followed by selectively stacking those consisting of coherent and stable signals that are consistent with predicted surface-wave arrival times. Synthetic tests show that the sequenced NCFs can be used to recover the spatial and temporal distribution of noise sources. Applying the method to an OBS array offshore California increases the signal-to-noise ratios of the obtained Rayleigh waves. In addition, we find that the annual temporal distribution of selected NCFs with frequencies ranging from 0.04 to 0.1 Hz is nearly homogeneous during the recording period. In contrast, many NCFs excluded for stacking are temporally clustered. This method has the potential to be applied to other OBS recordings or possibly onland deployments, thus helping to obtain high-quality surface waves and to analyze temporal noise source characteristics.

Mantle flow underneath the South China Sea revealed by seismic anisotropy.

It has long been established that plastic flow in the asthenosphere interacts constantly with the overlying lithosphere and plays a pivotal role in controlling the occurrence of geohazards such as earthquakes and volcanic eruptions. Unfortunately, accurately characterizing the direction and lateral extents of the mantle flow field is notoriously difficult, especially in oceanic areas where deployment of ocean bottom seismometers (OBSs) is expensive and thus rare. In this study, by applying shear wave splitting analyses to a dataset recorded by an OBS array that we deployed between mid-2019 and mid-2020 in the South China Sea (SCS), we show that the dominant mantle flow field has a NNW-SSE orientation, which can be attributed to mantle flow extruded from the Tibetan Plateau by the ongoing Indian-Eurasian collision. In addition, the results suggest that E-W oriented flow fields observed in South China and the Indochina Peninsula do not extend to the central SCS.

Olivine Fabrics in the Oceanic Lithosphere Constrained by Pn Azimuthal Anisotropy

AbstractWe constrain olivine fabrics in the oceanic lithosphere using active and passive seismic observations of Pn azimuthal anisotropy. We first analyze active‐source data derived from a broadband ocean bottom seismometer array that was deployed in the Northwestern Pacific. We infer the azimuthal dependence of the Pn velocity, including the 2ϕ and 4ϕ terms of the sinusoidal functions, where ϕ is the back azimuth. We observe a skewed azimuthal dependence of the Pn velocity, with a large peak‐to‐peak amplitude of about 12%. Similar features are observed using an independent data set of teleseismic Pn waves. We constrain the direction of the crystallographic axes of olivine to explain the observed azimuthal dependence and identify A‐type olivine with a slightly dipping a‐axis and slightly tilting b‐axis being compatible with our observations. In contrast, we find that D‐type olivine with any direction of crystallographic axes cannot explain our observations. Secondary deformation and recrystallization in the older Pacific may be responsible for this strong and complex seismic anisotropy in the study region.

Characteristics of Background Noise in the Oldest-1 Array Deployed on the Oldest Part of the Pacific Plate

ABSTRACT We characterize the sources of background noise recorded by an ocean-bottom seismometer array deployed on the oldest oceanic lithosphere of the western Pacific plate. This igneous oceanic crust is blanketed up to 1000 m cover of sediments, making it a unique place to investigate correlations between sediment thickness and background noise levels in the marine environment. Although the data are limited, we found that the thicker sediments attenuated short-period noise levels and amplified long-period noise levels, in agreement with observations made for seismic arrivals in onshore sedimentary basins. Comparison of the noise levels from our Oldest-1 deployment with those (1) in the western Pacific basin, (2) near the Mariana trench, and (3) offshore New Zealand indicates that the Oldest-1 has the highest noise levels in the double-frequency (secondary) microseism band by up to 7 dB. This result was unexpected given the greater water depths (averaging 5800 m) for the Oldest-1 array. However, even at these great depths, we find a strong correlation between significant wave heights and background noise levels in the short-period microseism band (2–5 s). In addition, the Oldest-1 array was subjected to a great number of intense close typhoons than the other arrays and was proximal to a great number of taller seamounts than the other arrays. Finally, tracking noise source azimuths using station pairs led to the identification of a possible new source for the single-frequency microseism band in the coastal areas of Papua New Guinea and Australia.

Shallow Low‐Velocity Layer in the Hyuga‐Nada Accretionary Prism and Its Hydrological Implications: Insights From a Passive Seismic Array

AbstractShear wave velocity (Vs) estimations of accretionary prisms can pose unique constraints to the physical properties of rocks, which are hard to obtain from compressional velocities (Vp) alone. Thus, it would help better understand the fluid processes of the accretion system. This study investigates the Vs structure of the Hyuga‐nada accretionary prism using an array of ocean‐bottom seismometers (OBSs) with a 2 km radius. Teleseismic Green's functions and a surface wave dispersion curve are inverted to one‐dimensional Vs structures using transdimensional inversion. The results indicate the presence of a low‐velocity zone 3–4 km below the seafloor. The reduced Vs corresponds to a reduced Vp feature obtained in a previous seismic refraction survey, and the high Vp/Vs ratio suggests the presence of high pore fluid pressure. In addition, the resolved lithological boundary exhibits a sharp offset that extends laterally across the OBS array. We attribute this offset to a blind fault below while acknowledging other possibilities, such as due to mud diapirism or intense fracturing. The predicted fault is located at the Kyushu–Palau Ridge flank, oriented roughly parallel to the ridge axis, and thus likely caused by ridge subduction. This fault may act as a fluid conduit, contributing to the formation of a fluid reservoir beneath the compacted sediment layers.

Seismicity Properties of the Chain Transform Fault Inferred Using Data From the PI‐LAB Experiment

AbstractOceanic transform faults are intriguing in that they do not produce earthquakes as large as might be expected given their dimensions. We use 1‐year of local seismicity (370 events aboveMC = 2.3) recorded on an array of ocean bottom seismometers (OBSs) and geophysical data to study the seismotectonic properties of the Chain transform, located in the equatorial Mid‐Atlantic. We extend our analysis back in time by considering stronger earthquakes (MW ≥ 5.0) from global catalogs. We divide Chain into three areas (east, central, and west) based on historical event distribution, morphology, and multidimensional OBS seismicity cluster analysis. Seismic activity recorded by the OBS is the highest at the eastern area of Chain where there is a lozenge‐shaped topographic high, a negative rMBA gravity anomaly, and only a few historicalMW ≥ 5.5 events. OBS seismicity rates are lower in the western and central areas. However, these areas accommodate the majority of seismic moment release, as inferred from both OBS and historical data. Higherb‐values are significantly correlated with lower rMBA and with shallower bathymetry, potentially related to thickened crust. Our results suggest high lateral heterogeneity along Chain. Patches with moderate to low OBS seismicity rates that occasionally hostMW ≥ 6.0 earthquakes are interrupted by segments with abundant OBS activity but few historical events with 5.5 ≤ MW < 6.0. This segmentation is possibly due to variable fluid circulation and alteration, which may also change in time.

Surface wave phase velocity variations underneath the Indian Ocean geoid low

The Indian plate has undergone a long tectonic journey since its separation from the Gondwanaland. Its protracted rift-drift motion has rendered several enigmatic features in the Indian Ocean. One such feature is the large geoid deficit (−106 m), observed in the south of Sri Lanka that is known as the Indian Ocean geoid low (IOGL). A broad consensus about plausible explanations behind the IOGL anomaly still remains elusive. For the first time, 17 Ocean Bottom Seismometers (OBSs) were deployed in the IOGL region to characterize the subsurface structural variations. Here, we examine the inter-station Rayleigh wave phase velocity variations within the period range of 15–197 s. We construct 1-D velocity-depth sections down to c.a.380 km or less along with N-S and E-W profiles, using ≥5.2 magnitudes (Mw) earthquake data that occurred at a depth ≤ 100 km at regional/teleseismic distances. Our result demarcates the regional Moho depths varying from c.a.11.6 ± 0.8 km to c.a.19.8 ± 1.2 km along the OBS array from south to north respectively. A thickened oceanic crust (c.a.9–10 km) is attributed to magmatic underplating in the lower crust near the aseismic Comorin ridge. The lithospheric thickness ranges between c.a.47.6 ± 2.9 km and c.a. 75.1 ± 3.6 km along a south-north profile. We observe a c.a. 20–23 km thick lithosphere-asthenosphere boundary (LAB). The depth interval between c.a.87 km and c.a. 280 km is characterized by a distinct low-velocity zone (LVZ). A thick LVZ appears to be associated with a thin lithosphere, which implies a hot thermal regime in the upper mantle. We envisage a possible connection of this hot thermal regime with the African LLSVP (large low shear velocity province) and/or a vertically deep mantle upwelling directly beneath the IOGL region. Our findings have significant implications for deciphering the possible causes behind the source of mantle upwelling, the large geoid anomaly and extensive lithospheric deformation across this region.

CMT inversion for small-to-moderate earthquakes applying to dense short-period OBS array at off Ibaraki region

We conducted centroid moment tensor (CMT) inversion for small-to-moderate aftershocks in the off Ibaraki region of the 2011 off the Pacific coast of Tohoku earthquake. In this study, we used high-frequency (0.4–1.0 Hz) seismograms from a dense array of short-period ocean bottom seismometers (OBSs) and a reliable three-dimensional (3-D) seismic velocity model. Higher frequency analysis and dense OBS arrays offer CMT solutions with high spatial resolutions. Since our OBS array observed aftershocks occurring immediately following the 2011 off the Pacific coast of Tohoku earthquake, we determined 536 CMT solutions for small-to-moderate aftershocks in the off Ibaraki region (JMA-scale magnitudes of 2.5–4.0). According to our CMT solutions, characteristics of the aftershock activities in the off Ibaraki region are classified into 5 groups: (1) thrust earthquakes, which are considered as interplate earthquakes, separated by the large slip area of the 2011 Ibaraki-oki earthquake (the largest {mathrm{M}}_{mathrm{w}} 7.6 aftershock of the 2011 off the Pacific coast of Tohoku earthquake) and the tectonic tremors; (2) intraslab strike-slip earthquakes located at the north of the fault area of the 2011 Ibaraki-oki earthquake; (3) intraslab normal-fault earthquakes, which suggest a tensional stress field within the subducting Pacific Plate due to the plate bending by the overriding Philippine Sea Plate; (4) various earthquake focal mechanisms above a subducting seamount, which suggest the 3-D complex fractures; and (5) normal-fault earthquakes shallower than the interplate earthquakes, which were possibly caused by the heterogeneity of the subducting seamount.Graphical

Mapping the mantle transition zone beneath the Indian Ocean geoid low from Ps receiver functions

The most pronounced high-amplitude (−40 to −106 m) geoid deficit with respect to the Earth's reference shape straddles the Indian Ocean. The existing hypotheses suggest a possible linkage of low density and low-velocity anomalies in the mid-and upper mantle and high-velocity anomalies near the base of the mantle to the source behind the lowest geoid. We obtained P to S wave radial receiver functions to investigate these mantle anomalies from an extensive array of Ocean Bottom Seismometers (OBSs) positioned in the centre of the Indian Ocean Geoid Low (IOGL). Our modelling suggests that the mean depths for d410 and d660 range from 386.0 ± 13.6 to 459.7 ± 2.9 km and 643.1 ± 7.4 to 710.2 ± 5.4 km with an average of 432.6 km and 680.2 km, respectively. The average thickness of the transition zone varies from 199.6 to 289.8 km. New results confirm an extensive ~800 km wide depression at d410 and d660 towards the centre of the geoid. The depression potentially implies a rather hot mantle material, in which majorite garnet to perovskite transition may become dominant at 660 km depth. The excess temperature calculated for d410 and d660 topography ranges from 139.5 to 557.5 K and 206 to >1000 K, respectively. The shear velocity anomalies derived from the excess temperature range from −0.89 to −3.52 (%) at d410, and at dd60 range from −1.13 to −7.87 (%). The velocity anomalies are highly underestimated at d660 and could be explained by hydrous mantle upwelling at the lower mantle transition zone. The present study outcomes offer new avenues towards exploring a mantle upwelling in the Indian Ocean and its linkage with geoid undulations.

A Robust Workflow for Acquiring and Preprocessing Ambient Vibration Data from Small Aperture Ocean Bottom Seismometer Arrays to Extract Scholte and Love Waves Phase-Velocity Dispersion Curves

The phase-velocity dispersion curve (DC) is an important characteristic of the propagation of surface waves in sedimentary environments. Although the procedure for DC estimation in onshore environments using ambient vibration recordings is well established, the DC estimation in offshore environments using Ocean Bottom Seismometers (OBS) array recordings of ambient vibrations presents three additional challenges: (1) the localization of sensors, (2) the orientation of the OBS horizontal components, and (3) the clock error. Here, we address these challenges in an inherent preprocessing workflow to ultimately extract the Love and Scholte wave DC from small aperture OBS array measurements performed between 2018 and 2020 in Lake Lucerne (Switzerland). The arrays have a maximum aperture of 679 m and a maximum deployment water depth of 81 m. The challenges related to the OBS location on the lake floor are addressed by combining the multibeam bathymetry map and the backscatter image for the investigated site with the differential GPS coordinates of the OBS at recovery. The OBS measurements are complemented by airgun surveys. Airgun data are first used to estimate the misorientation of the horizontal components of the OBS and second to estimate the clock error. To assess the robustness of the preprocessing workflow, we use two array processing methods, namely the three-component high-resolution frequency-wavenumber and the interferometric multichannel analysis of surface waves, to estimate the dispersion characteristics of the propagating Scholte and Love waves for one of the OBS array sites. The results show the effectiveness of the preprocessing workflow. We observe the phase-velocity dispersion curve branches in the frequency range between 1.2 and 3.2 Hz for both array processing techniques.

GO_3D_OBS: the multi-parameter benchmark geomodel for seismic imaging method assessment and next-generation 3D survey design (version 1.0)

Abstract. Detailed reconstruction of deep crustal targets by seismic methods remains a long-standing challenge. One key to address this challenge is the joint development of new seismic acquisition systems and leading-edge processing techniques. In marine environments, controlled-source seismic surveys at a regional scale are typically carried out with sparse arrays of ocean bottom seismometers (OBSs), which provide incomplete and down-sampled subsurface illumination. To assess and minimize the acquisition footprint in high-resolution imaging process such as full waveform inversion, realistic crustal-scale benchmark models are clearly required. The deficiency of such models prompts us to build one and release it freely to the geophysical community. Here, we introduce GO_3D_OBS – a 3D high-resolution geomodel representing a subduction zone, inspired by the geology of the Nankai Trough. The 175km×100km×30km model integrates complex geological structures with a viscoelastic isotropic parameterization. It is defined in the form of a uniform Cartesian grid containing ∼33.6e9 degrees of freedom for a grid interval of 25 m. The size of the model raises significant high-performance computing challenges to tackle large-scale forward propagation simulations and related inverse problems. We describe the workflow designed to implement all the model ingredients including 2D structural segments, their projection into the third dimension, stochastic components, and physical parameterization. Various wavefield simulations that we present clearly reflect in the seismograms the structural complexity of the model and the footprint of different physical approximations. This benchmark model is intended to help to optimize the design of next-generation 3D academic surveys – in particular, but not only, long-offset OBS experiments – to mitigate the acquisition footprint during high-resolution imaging of the deep crust.

An OBS Array to Investigate Offshore Seismicity during the 2018 Kīlauea Eruption

AbstractOn 3 May 2018, Kīlauea Volcano, one of the most active volcanoes in the world, entered a new eruptive phase because of a dike intrusion in the East Rift zone. One day later, an Mw 6.9 earthquake, which was likely trigged by the dike intrusion, occurred in the submarine south flank of Kīlauea Volcano. In mid-July, an ocean-bottom seismometer (OBS) array consisting of 12 stations was deployed on the submarine south flank of Kīlauea Volcano to monitor the aftershocks and lava–water interaction near the ocean entry. Eleven OBSs were recovered in mid-September. Preliminary evaluation of the data reveals a large number of seismic and acoustic events, which provide a valuable dataset for understanding flank deformation and stability as well as lava–water interaction. Here, we introduce this dataset and document notable instrument malfunctions along with some initial seismic and acoustic observations.

Methoni Mw 6.8 rupture and aftershocks distribution from a dense array of OBS and land seismometers, offshore SW Hellenic subduction

Along the south-western offshore Hellenic subduction zone, the overriding Aegean upper plate above the Mediterranean oceanic lithosphere generates uncommon large earthquakes on the offshore megathrust fault. The largest subduction thrust event, for half a century, has been the 14 February 2008 Methoni earthquake (Mw = 6.8) that occurred offshore of the southwest coast of Peloponnesus. We conducted micro-seismicity experiments around the rupture area and forearc domain -between Peloponnesus and Crete- using ocean bottom seismometers (OBS) jointly with land-based seismological stations. Our first experiment in 2006, had revealed an association of the Matapan Trough, a 400-km-long forearc basin, with local seismicity clustering and a possible gap in activity over the later Methoni rupture area. Here we present new data of post-Methoni seismic activity, recorded during a time-span of 11 months, beginning in October 2008 within the period of proposed afterslip on the megathrust, by an extended and dense seismic array consisting of up to 33 OBS. A minimum 1D velocity model was constructed for the region to provide better constraints on absolute locations and double-difference relocation was applied to produce an enhanced image of the spatial distribution of hypocenters. The high resolution earthquake locations confirm correlation of the Matapan Trough with local seismicity as a regional feature, also filling up the previously observed gap. Over the Methoni rupture area, we constrain seismicity to be located mainly within the upper plate. Hypocenters are also resolved above the updip and downdip edges of the rupture area, respectively. Seismic activity provides hints of upper plate structures which were activated in response to post-seismic deformation spreading within the forearc crust. Our findings highlight the characteristics of a megathrust domain which is related with a highly deformable overriding plate and controlled by a segmented lower plate topography.

Persistent Long‐Period Signals Recorded by an OBS Array in the Western‐Central Pacific: Activity of Ambrym Volcano in Vanuatu

AbstractStrong long‐period seismic signals at periods around 25 and 18 s appear in the ambient noise cross‐correlation functions recorded by an array of ocean bottom seismometers (OBSs) in the western‐central Pacific. The signal amplitude varies from time to time, and the apparent travel times of the signals are typically smaller than those expected for the Rayleigh waves propagating along the great circle connecting the station pairs. From the cross‐correlation functions, the signal sources are located in the Vanuatu Arc. Local data analysis suggests the signals originate from two different sources possibly located at depths of ~0–1 km below the sea level beneath the active cones of Ambrym volcano.

Comparing Orientation Analysis Methods for a Shallow-Water Ocean-Bottom Seismometer Array in the Bohai Sea, China

ABSTRACT The horizontal orientation estimation of an ocean-bottom seismometer (OBS) is critical for providing reliable data for seismological studies. In this article, we applied three independent polarization analysis methods to estimate the absolute horizontal orientation of a pilot shallow-water OBS array in Bohai Sea, China. The OBS array was deployed in the autumn of 2018 and comprised 32 broadband seismometers lasting around 1 yr, aimed at investigating the geodynamics of North China craton and adjacent areas. The timings of the data were corrected through the correlation analysis of regional and teleseismic earthquake events, before the polarization analysis. Polarization analysis using P wave and Rayleigh wave, from both natural earthquakes and noise cross correlations, was performed. In total, 28 out of 29 stations obtained the effective estimates combining the three methods, of which 11 stations showed relatively robust results with variations well below 10° among the three methods. However, the superiority of one method over the others is not obvious. As there is a trade-off between the accepted final estimates and the number of qualified measurements defined by parameters such as the correlation coefficient threshold, window length, and filtering options, we investigated these effects using 15 different groups of parameters for the three methods, and the deviation statistics showed a distribution with the root mean square deviation of 9.2° for the whole array.

Spatial relationship between shallow very low frequency earthquakes and the subducted Kyushu-Palau Ridge in the Hyuga-nada region of the Nankai subduction zone

SUMMARYShallow very low frequency earthquakes (sVLFEs) have occurred recurrently at the shallow plate interface of the Hyuga-nada region of the western Nankai subduction zone, Japan. Although the locations of sVLFE epicentres have been determined using land-based seismic records with moderate accuracy, it is necessary to determine their locations more precisely to explore the relationship between sVLFEs and other shallow slow earthquakes and examine the structural factors that may control sVLFE activity. Here, we identified sVLFE epicentres using seismic records obtained from temporarily deployed ocean bottom seismometers (OBSs) in the Hyuga-nada region. Seismic observations involved the deployment of 5–13 OBSs for approximately 1 yr, with deployments conducted three times during 2014–2016 each time with changing OBS numbers and array distribution. As a result, one sVLFE episode, containing successive Rayleigh wave pulses with slow velocities due to marine sediments, could be detected at a frequency band of 0.1–0.15 Hz per observation, resulting in a total of three episodes. Rayleigh wave amplitudes of ordinary earthquakes in the continuous records were suppressed using earthquake catalogues. We estimated the dispersion curve for the Rayleigh wave group velocity for each array, which represented the averaged group velocity within the array, using coda interferometry, and applied an envelope correlation method (ECM) using the group velocities to continuous records. These processing provided sVLFE epicentres with horizontal distance errors of <5 km. Our results showed that sVLFEs depths, which were inferred from the contour line of the top of the Phillipine Sea Plate, had increased from <10 km to 10–15 km in the region of the subducted Kyusyu-Palau Ridge (KPR). It was also apparent that migration of sVLFE epicentres occurred in 2015 from a depth of 15 km to shallower depths along the northern margin of the subducted KPR. These results identified the subducted KPR as a structural factor controlling the excitation conditions of sVLFE activities.

Precision Seismic Monitoring and Analysis at Axial Seamount Using a Real‐Time Double‐Difference System

AbstractSeven three‐component ocean bottom seismometers (OBS) of the Ocean Observatories Initiative (OOI) Cabled Array on top of Axial Seamount are continuously streaming data in real time to the Incorporated Research Institutions for Seismology (IRIS). The OBS array records earthquakes from the submarine volcano which last erupted on 24 April 2015, about 4 months after the array came online. The OBS data have proven crucial in providing insight into the volcano structure and dynamics (Wilcock et al., 2016, https://doi.org/10.1126/science.aah5563). We implemented a real‐time double‐difference (RT‐DD) monitoring system that automatically computes high‐precision (tens of meters) locations of new earthquakes. The system's underlying double‐difference base catalog includes nearly 100,000 earthquakes and was computed using kurtosis phase onset picks, cross‐correlation phase delay times, and 3‐D P and S velocity models to predict the data. The relocations reveal the fine‐scale structures of long‐lived, narrow (<200 m wide), outward dipping, convex faults on the east and west walls of the caldera that appear to form a figure 8‐shaped ring fault system. These faults accommodate stresses caused by the inflation of magma prior to and deflation during eruptions. The east fault is segmented and pulled apart in east‐west direction due to its interaction with the Juan de Fuca Ridge, which at this location forms an overlapping spreading center. The RT‐DD system enables the monitoring and rapid analysis of variations in fine‐scale seismic and fault properties and has the potential to improve prediction of timing and location of the next Axial eruption expected to occur in the 2022–2023 time frame.

Focal Parameters of Earthquakes Offshore Cape St. Vincent Using an Amphibious Network

Earthquakes with submarine foci are generally located with high uncertainties, and their focal mechanisms are poorly resolved, due to the confinement of the monitoring network onshore and the consequent poor azimuthal coverage. The use of amphibious networks, combining ocean-bottom seismometers (OBSs) and land stations, helps to reduce the epicentral distance and the azimuth gap, thus better constraining the hypocentral locations and decreasing the focal mechanism uncertainties. A second important factor in improving the location accuracy for offshore seismicity is the use of a suitable velocity model. The objective of this paper is to study how the combination of an amphibious network with 3D crustal models can improve offshore earthquake hypocenter locations and focal mechanisms. The study area is SW Iberia near Cape St. Vincent, which generated some of the most striking earthquakes and tsunamis in Europe in past centuries, such as the 1755 Lisbon earthquake (Imax = X). We deployed an array of six broadband OBSs 200 km offshore Cape St. Vincent to study the seismicity of the region for a period of 8 months. During this period, we detected 52 earthquakes, the largest with magnitude M (mbLg) ≈ 5. Thirty-eight earthquakes are relocated using land stations in Iberia and North Africa and the OBS array with different velocity models. Focal mechanisms and moment tensors are computed for a data set of seven earthquakes based on first-motion polarities, body-wave amplitude spectra and waveform cross-correlation. We show that if we include offshore OBS data and consider accurate 3D velocity models to locate earthquakes, the focal parameter uncertainties decrease substantially, thus improving the depth constraint. We also show that the locations and focal mechanisms obtained using the amphibious network agree with the regional stress pattern in the SW Iberia region.

Low-velocity layers in the northwestern margin of the South China Sea: Evidence from receiver functions of ocean-bottom seismometer data

The receiver function method was applied to teleseismic recordings of an active-source ocean-bottom seismometer array in the northwestern margin of the South China Sea. Forward modeling of the receiver functions was used to derive velocity interfaces beneath the stations. Two main groups of low-velocity layers were imaged. The first group in the reef area, with a thickness of 7–10 km, exhibits a seaward uptrend of the buried depth but poor continuity. The second group, with a thickness of 3–4 km, is more continuous along the Zhongsha Trough; the buried depth decreases asymmetrically from the flanks to the rift center. A high shear wave velocity layer of ~4.0 km/s is imaged at the crust bottom beneath the Zhongsha Trough, with thickness of ~3 km. The average Vp/Vs in the Cenozoic sediment is 2.0–3.0 and was affected by high fluid activity induced by intensive faults. The Vp/Vs of the crust varies between 1.77 and 1.88, suggesting an intermediate to basic composition under the continental extension. Together, these results suggest a two-stage evolution model of the continental crust. In the first stage, the crust deformed in a ductile manner and thus created considerable shearing structures with high anisotropy that caused the obvious decrease of the shear wave velocity. In the second stage, the shearing structures connected asymmetrically, driven by detachment faults in the upper crust and upwelling of the lower crust. The Zhongsha Trough then formed, probably with magmatic underplating due to lithospheric extension or rising of hot mantle materials.