Seismic Velocity Structure Research Articles

Shallow Crustal Shear Velocity and Vp/Vs Across Southern California: Joint Inversion of Short‐Period Rayleigh Wave Ellipticity, Phase Velocity, and Teleseismic Receiver Functions

AbstractNear‐surface seismic velocity structure plays a critical role in ground motion amplification during large earthquakes. In particular, the local Vp/Vs ratio strongly influences the amplitude of Rayleigh waves. Previous studies have separately imaged 3D seismic velocity and Vp/Vs ratio at seismogenic depth, but lack regional coverage and/or fail to constrain the shallowest structure. Here, we combine three datasets with complementary sensitivity in a Bayesian joint inversion for shallow crustal shear velocity and near‐surface Vp/Vs ratio across Southern California. Receiver functions–including with an apparent delayed initial peak in sedimentary basins, and long considered a nuisance in receiver function imaging studies–highly correlate with short‐period Rayleigh wave ellipticity measurements and require the inclusion of a Vp/Vs parameter. The updated model includes near‐surface low shear velocity more in line with geotechnical layer estimates, and generally lower than expected Vp/Vs outside the basins suggesting widespread shallow fracturing and/or groundwater undersaturation.

Shallow Shear-Wave Velocity Structure beneath the West Lake Area in Hangzhou, China, from Ambient-Noise Tomography

Urban geophysical exploration plays an important role in the sustainable development of and the mitigation of geological hazards in metropolitan areas. However, it is not suitable to implement active seismic methods in densely populated urban areas. The rapidly developing ambient-noise tomography (ANT) method is a promising technique for imaging the near-surface seismic velocity structure. We selected the West Lake area of the city of Hangzhou as a case study to probe the shallow subsurface shear-wave velocity (Vs) structure using ANT. We conducted seismic interferometry on the ambient-noise data recorded by 28 seismograph stations during a time period of 17 days. Fundamental-mode Rayleigh-wave group- and phase-velocity dispersion data were measured from cross-correlation functions and then inverted for a 3D Vs model of the uppermost 1 km that covers an area of about 7 km × 8 km. The tomographic results reveal two prominent anomalies, with high velocities in the southwest and low velocities in the northeast. The fast anomaly corresponds to the presence of limestone and sandstone, whereas the slow anomaly is due to the relatively low-velocity rhyolite and volcanic tuff in the area. The boundary between the two anomalies lies to the NE of an NW–SE trending fault, indicating that the fault dips toward the NE. In addition, the pronounced low-velocity anomalies appear under the Baoshi mountain, likely due to the thick rhyolite and volcanic tuff beneath the extinct volcano. Our results correlate well with regional geological features and suggest that ANT could be a promising technique for facilitating the exploration of urban underground space.

Quaternary sodic and potassic intraplate volcanism in northeast China controlled by the underlying heterogeneous lithospheric structures

AbstractThe origin and mantle dynamics of the Quaternary intraplate sodic and potassic volcanism in northeast China have long been intensely debated. We present a high-resolution, three-dimensional (3-D) crust and upper-mantle S-wave velocity (Vs) model of northeast China by combining ambient noise and earthquake two-plane wave tomography based on unprecedented regional dense seismic arrays. Our seismic images highlight a strong correlation between the basalt geochemistry and upper-mantle seismic velocity structure: Sodic volcanoes are all characterized by prominent low seismic velocities in the uppermost mantle, while potassic volcanoes still possess a normal but thin upper-mantle “lid” depicted by high seismic velocities. Combined with previous petrological and geochemical research findings, we propose that the rarely erupted Quaternary potassic volcanism in northeast China results from the interaction between asthenospheric low-degree melts and the overlying subcontinental lithospheric mantle. In contrast, the more widespread Quaternary sodic volcanism in this region is predominantly sourced from the upwelling asthenosphere without significant overprinting from the subcontinental lithospheric mantle.

What Is Fling Step? Its Theory, Simulation Method, and Applications to Strong Ground Motion near Surface Fault Ruptures

ABSTRACT We present the theory of the fling step and a theoretical method for simulating accurately the near-fault strong motions, and apply it to reproduce various strong-motion records near surface faults. Theoretically, the fling step is the contribution of the static Green’s function in the representation theorem (Hisada and Bielak, 2003), and we show that this theory holds for any seismic velocity structure. We first demonstrate the validity of this theory using theoretical solutions of a circular fault model in a homogeneous full-space. Next, we apply the theory to layered half-spaces, present a theoretical method based on the wavenumber integration method, and introduce various techniques to simulate the near-fault ground motions including fling steps with high accuracy. Finally, we demonstrate the effectiveness of the method by reproducing various strong-motion records near surface fault ruptures and discuss the characteristics of near-fault strong motions including the fling step and the forward directivity pulse. We made all of the software and data used in this article available on the internet.

Crustal P-wave velocity structure and earthquake distribution in the Jiaodong Peninsula, China

The Jiaodong Peninsula of eastern China is densely populated and prone to frequent moderate-strong earthquakes that are mostly distributed along a group of northeastern oriented Mesozoic-Cenozoic faults. An improved understanding of the seismic velocity structure in the area is of great significance for the roles that the faults played in the formation and evolution of the various tectonic features and for the assessment of future earthquake risks. By utilizing earthquake records of the Shandong Provincial Seismic Network for the period from January 2013 to January 2020, this study simultaneously re-locates earthquakes and determines the three-dimensional P-wave velocity structure beneath the Jiaodong Peninsula using regional P-wave travel times by applying the double-difference tomography method. After the relocation, the travel time residuals are reduced by an order of magnitude, and the epicenters are more concentrated and demonstrate a closer spatial relationship with known active faults. The resulting P-wave velocity structure at different horizon depths indicates lateral heterogeneities in the study area. Clear differences in the characteristics of velocity anomalies are observed between the Jiaobei Uplift, Jiaolai Basin and Sulu ultra-high pressure metamorphic belt. The anomalies are mostly NE oriented, which is consistent with the strike of the regional faults and may suggest structural control of the faults to the geological configurations. Earthquakes generally occurred along the edges of high or low velocity regions, and areas accommodating the intersections of the Penglai-Weihai, Penglai-Qixia and Mouping-Jimo fault zones have the greatest potential for future damaging earthquakes in the area.

Global Patterns in Cratonic Mid‐Lithospheric Discontinuities From Sp Receiver Functions

AbstractThe goal of this study is to constrain the origins of layering in the seismic velocity structure within the cratonic mantle lithosphere (i.e. mid‐lithospheric discontinuities [MLDs]). For long‐lived stations in cratons worldwide, we calculated S‐to‐P converted phase receiver function stacks using time domain deconvolution and a k‐means algorithm to select robust, consistent receiver functions. Negative MLDs appear in only 50% of the receiver function stacks, indicating that negative MLDs are common but intermittent. The negative MLDs correspond to shear velocity drops of 1%–4%, which could be caused by layers of minerals created by metasomatism, although vertical layering in seismic anisotropy cannot be ruled out. In craton interiors, negative MLDs have a lower amplitude (<3% velocity drops) and can be explained by metasomatism of the original Archean mantle. Negative MLD amplitudes increase with decreasing upper mantle shear velocity (toward the outer margins of the cratons), but do not depend on the age of the craton. Thus, negative MLD amplitudes are not dominated by age‐related variations in the cratonic mantle composition, and, instead, are more strongly correlated with proximity to tectonic and metasomatic activity that occurred long after craton formation. Negative MLDs are less numerous among stations that have Paleoproterozoic and Archean thermotectonic ages, consistent with the view that shallow release of slab‐derived fluids during early “warm” subduction was less favorable for negative MLD formation. We also observe velocity gradients below 150 km at stations in craton boundaries and interiors, indicating the presence of seismic velocity changes at the cratonic lithosphere‐asthenosphere boundary and/or Lehmann discontinuity.

Crustal and Upper‐Mantle Structure Below Central and Southern Mexico

AbstractThe Mexican subduction zone is a complex convergent margin characterized by a slab with an unusual morphology, an abnormal location of the volcanic arc, and the existence of slow earthquakes (slow slip events and tectonic tremors). The number of seismic imaging studies of the Mexican subduction zone has increased over the past decade. These studies have revealed the seismic wave velocity structure beneath Central and Southern Mexico. However, the existing tomographic models are either confined to specific areas or have coarse resolution. As a consequence, they do not capture the lateral and finer‐scale variations in the mantle and the crust structure. Here, we fit frequency‐dependent traveltime differences between observed and synthetic seismograms in a three‐dimensional model of Central and Southern Mexico to constrain crustal and upper mantle seismic velocity structures jointly. We use ∼3,300 seismic records, filtered between 5 and 50 s, from 74 regional earthquakes. We perform point spread function tests to assess the resolution and trade‐offs between model parameters. Our tomographic model reveals several seismic wave velocity features. These features include the geometry of the Cocos slab, the partial melting zone beneath the Trans‐Mexican Volcanic Belt, and the Yucatan slab diving below Los Tuxtlas Volcanic Field that are consistent with previous studies. We also identify horizontal structures in the lower crust and the shallow upper mantle that were not previously identified. These structures include slow seismic wave velocity anomalies that may be correlated with ultraslow seismic velocity zones and high conductivity regions, where slow earthquakes have been identified.

Seismic Velocity Heterogeneity of the Hikurangi Subduction Margin, New Zealand: Elevated Pore Pressures in a Region With Repeating Slow Slip Events

AbstractWe investigated the seismic velocity structure of the Hikurangi margin in New Zealand to uncover the physical features of the subduction zone and explore the relationships between microearthquake seismicity, seismic velocity structure, and slow slip events. Using local earthquake tomography with data collected from both temporary ocean bottom seismometers and on‐land permanent seismic stations, we used the tomography code TomoFD to iteratively perform a damped least squared inversion of absolute P and S arrival times to obtain relocated hypocenters and generate 3D velocity models for Vp and Vp/Vs. The seismic tomography images show two high Vp/Vs anomalies, one offshore and adjacent to a subducted seamount and the other beneath the North Island of New Zealand. The ∼50‐km wide offshore anomaly extends ∼10 km beneath the plate interface and lies directly beneath the area that slipped at least 50 mm during the 2 week‐long 2014 slow slip event. High Vp/Vs values may be related to high pore fluid pressures from subducted sediments, and such increases in pore fluid pressures have been suggested to trigger the occurrence of slow slip events in active subduction zones. The second onshore high Vp/Vs anomaly is located in the overlying plate and subducting slab and correlates with areas suggested by other geophysical techniques to be rich in fluids. Our seismic imaging supports interpretations that subduction processes in the Hikurangi margin are highly dependent on physical features such as subducted seamounts and fluid‐rich sediments.

Delineation of Upper Crustal Structure Beneath the Island of Lombok, Indonesia, Using Ambient Seismic Noise Tomography

We have successfully conducted the first ambient noise tomography on the island of Lombok, Indonesia using local waveform data observed at 20 temporary stations. Ambient noise tomography was used to delineate the seismic velocity structure in the upper crust. The waveform data were recorded from August 3rd to September 9th, 2018, using short-period and broadband sensors. There are 185 Rayleigh waves retrieved from cross-correlating the vertical components of the seismograms. We used frequency-time analysis (FTAN) to acquire the interstation group velocity from the dispersion curves. Group velocity was obtained for the period range of 1 s to 6 s. The group velocity maps were generated using the subspace inversion method and Fast Marching Method (FMM) to trace ray-paths of the surface waves through a heterogeneous medium. To extract the shear wave velocity (Vs) from the Rayleigh wave group velocity maps, we utilize the Neighborhood Algorithm (NA) method. The 2-D tomographic maps provide good resolution in the center and eastern parts of Lombok. The tomograms show prominent features with a low shear velocity that appears up to 4 km depth beneath Rinjani Volcano, Northern Lombok, and Eastern Lombok. We suggest these low velocity anomalies are associated with Quaternary volcanic products, including the Holocene pyroclastic deposits of Samalas Volcano (the ancient Rinjani Volcano) which erupted in 1257. The northeast of Rinjani Volcano is characterized by higher Vs, and we suggest this may be due to the presence of igneous intrusive rock at depth.

Electrical Resistivity Structure Around the Atotsugawa Fault, Central Japan, Revealed by a New 2‐D Inversion Method Combining Wideband‐MT and Network‐MT Data Sets

AbstractThe Atotsugawa fault is one of the most active faults in Japan, and the strain accumulation at the fault is considered to be caused by an aseismic shear zone in the fluid‐rich lower crust. To identify the shear zone and investigate the origin of the aqueous fluid in the lower crust, we deployed a Network‐MT survey in addition to a conventional wideband‐MT survey around the fault and performed an inversion combining both the MT data sets. In the inversion, by modifying a conventional inversion algorism, we accurately represented kilometer‐scale dipoles of the Network‐MT measurement to provide constraints on the electrical resistivity structure. In the lower crust under the study area, there are localized conductive anomalies below the Atotsugawa fault, the Ushikubi fault, and the Takayama‐Oppara fault zone. Comparing our electrical resistivity structure with the seismic velocity structure, we interpreted that the lower‐crustal conductors are localized ductile shear zones with highly connected fluid. We considered that the localized ductile shear zones are responsible for the strain accumulation along the respective active faults. In addition, in the mantle wedge above the subducting Philippine Sea slab and its downward extension, a highly conductive portion is detected, which may be attributed to the fluid dehydrated from the Philippine Sea slab and/or the Pacific slab. The existence of the large conductive area supports the suggestion of previous seismic and geochemical studies that the fluid of the lower crust around the Atotsugawa fault originated from subducting slabs.

Presence of interplate channel layer controls of slip during and after the 2011 Tohoku-Oki earthquake through the frictional characteristics

There are significant differences between the middle and southern segments of the Japan Trench in terms of the seismic and aseismic slips on the plate interface and seismic velocity structures. Although the large coseismic slip of the 2011 Tohoku-Oki earthquake was limited to the middle segment, the observed negative residual gravity anomaly area in the southern segment corresponds to the postseismic slip area of the Tohoku-Oki earthquake. A density distribution model can explain the different slip behaviours of the two segments by considering their structural differences. The model indicates that the plate interface in the south was covered with a thick channel layer, as indicated by seismic survey imaging, and this layer resulted in a residual gravity anomaly. Numerical simulations which assumed evident frictional heterogeneity caused by the layer in the south efficiently reproduced M9 earthquakes recurring only in the middle, followed by evident postseismic slips in the south. This study proposes that although the layer makes the megathrust less compliant to seismic slip, it promotes aseismic slips following the growth of seismic slips on the fault in an adjacent region.

A Possible Roll-Over Slab Geometry Under the Caroline Plate Imaged by Monte Carlo Finite-Frequency Traveltime Inversion of Teleseismic SS Phases

The shape of a subducting slab varies as a function of trench motion. Two end-members of subduction modes are geodynamically possible: roll-back mode underneath neighboring plates and roll-over mode underneath the plate itself. Whereas most of major slabs seem to roll back while the Pacific plate shows a slab piling behavior down to ∼1,000 km depth under the Mariana trench, no clear evidence of slab roll-over in nature has been reported so far. Here we show a possible roll-over slab beneath the Caroline microplate, revealed from its three-dimensional seismic velocity structure derived by analyzing teleseismic reverberating SS phases. We suggest that slab roll-over is driven by at least two factors: 1) the overall buoyancy and fragility of the Caroline microplate at the surface, induced by a thin hot mantle plume that rises from depths ≥800 km; and 2) the pushing force of the Pacific plate acting on the trailing edge of the Caroline plate.

Shallow seismic investigations of the accretionary complex offshore Central Chile

Thrust ridges are accretionary structures often associated with local uplift along splay faults and cold seep activity. We study the influence of a NS-trending thrust ridge system on the transition between the accretionary prism and the continental framework (shelf break) offshore the Maule Region (central Chile at 35°–36°S) by examining its 2-D and 3-D seismic velocity structure. The experiment comprises five densely spaced seismic refraction lines running subparallel to the trench and recorded at nine OBH/S (ocean bottom hydrophone/seismometers) deployed along the central line. Results show a narrow margin-parallel volume (approximately 6 × 50 × 5 km3) whose velocity distribution is consistent with sedimentary rocks. The shallow sedimentary unit is characterized by the presence of very low velocity hydrate-bearing sediments (<1.7 km/s), which are interpreted as highly porous sedimentary rocks (>50% porosity) within the Gas Hydrate Stability Zone (GHSZ) suggesting low hydrate content. These zones spatially correlate with fluid activity in the vicinity of the NS trending thrust ridges based on local high heat flow values (>40 mWm‐−2) and seepage mapping. On the other hand, the splay faults that crop out on the flanks of the thrust ridge structures might be responsible for tectonically induced vertical fluid migration.

Crustal attenuation beneath western North America: Implications for slab subduction, terrane accretion and arc magmatism of the Cascades

The western margin of the North American continent is a composite of diverse crustal domains featuring ancestral cratons, the Cordilleran terranes and the subducting Cascadia slab. Their existence and spatial extent have been largely inferred from seismic imaging, which is highly sensitive to variations in both crustal seismic velocity and attenuation structures. In this study, we improve upon existing, yet relatively limited, knowledge of crustal attenuation by developing a new Lg-wave attenuation (Q0) model. Benefiting from a comprehensive data set from dense seismic networks in Canada and the United States, our model reveals a clear transition from the low-attenuation (Q0>450) Canadian Shield to the highly-attenuated (Q0<150) crust near the Cascade arc. In the former region, the low-attenuation structures agree well with the distribution of thick, high-velocity crust in the Cordilleran foreland, suggesting dominant effects of elastic properties such as scattering and geometry of the crustal waveguide on the attenuation characteristics of stable cratons. On the other hand, we observe strong crustal attenuations in magmatically active areas including the Cascade arc and the Yellowstone-Snake River Plain volcanic province. The excellent spatial correlation between the high-attenuation Cascades and the surface projection of the subducting slabs further suggests a possible role of slab dehydration in increasing the fluid content of the overlying continental crust and modifying seismic attenuation. The sharp termination of the highly-attenuated crust forms a structural boundary in the Cascade back-arc that likely marks the eastern subsurface limit of the Siletzia terrane. The close spatial affinity of this boundary to the isotopic ratio lineament of 87Sr/86Sr = 0.706, which is the suggested ancestral continental margin, highlights the distinctive crustal types and/or heterogeneous structures between young accreted terranes and ancient cratons.

Slab Models Beneath Central Myanmar Revealed by a Joint Inversion of Regional and Teleseismic Traveltime Data

AbstractThe intermediate‐depth (50–180 km) seismicity beneath Myanmar provides direct evidence of the subducting Indian slab. However, the historic lack of regional seismic observations leads to previous low‐resolution models that show large variations in slab geometry beneath Myanmar. The depth extent and morphology of the slab are still poorly known. In this study, we conduct a joint inversion of regional and teleseismic P‐wave traveltimes from recently installed networks to image seismic velocity structures beneath central Myanmar by adopting an eikonal equation‐based traveltime tomography method. The observations contain a total of 6,069 regional first P‐wave arrivals and 29,787 teleseismic P‐wave differential traveltimes. We find a high P‐wave velocity anomaly beneath central Myanmar, which starts from ∼50 km depth and extends continuously to the mantle transition zone (MTZ) and is interpreted as the subducting Indian slab. Below 100 km depth, the dip angle of the slab in the south is ∼15° larger than that of the slab in the north, suggesting a possible slab tearing. Based on our tomographic results and previous studies, the slab in the north is inferred to have a deep stagnant segment lying above the 660‐km discontinuity in the MTZ, but whether it is connected with the shallow dipping slab cannot be confirmed. Meanwhile, the slab in the south may just stay in the upper mantle (above 410 km), but it may also have penetrated the 410‐km discontinuity. Taking into account all the scenarios, we propose four possible models of the Indian Plate subduction system beneath central Myanmar.

Seismic crustal shear velocity structure across NW Himalaya and Ladakh-Karakoram using receiver function modelling: Evidence of the Main Himalayan Thrust

To understand the nature of the underthrusting Indian Moho and Main Himalayan Thrust across the NW Himalaya and Ladakh-Karakoram (southern Tibet), we calculate the shear velocity models by inverting teleseismic receiver functions at 15 seismic stations. The crustal shear wave velocity models show that the Indian crust underthrusts gently. The depth of Indian Moho increases from ~46 km below the Gangetic Plain to ~78 km at the southern flank of the Karakoram Fault. The crustal velocity models also show a continuous intra-crustal low-velocity layer across the NW Himalaya and Ladakh-Karakoram. We combine our results with earlier results of magnetotelluric and seismological studies in this area. We infer that the observed intra-crustal low-velocity layer from the NW Himalaya to the southern flank of the Karakoram Fault is a possible expression of the Main Himalayan Thrust (MHT) in this region. Moreover, we observe higher shear velocities in the lowermost crust beneath the Higher Himalaya and Ladakh; and interpret them as the possible presence of the (partially) eclogitesed rocks in the lowermost crust.

Seismic velocity structure of the upper inner core in the north polar region

The Earth's upper inner core has a quasi-hemispherical velocity structure. One of the two quasi-hemispheres has a high-velocity region centered beneath Southeast Asia and is referred to as “East”. The remaining low-velocity region is referred to as “West”. The boundary positions between the quasi-hemispheres are rather scattered but well estimated at low- and mid-latitudes, whereas discrepancies between seismological hemisphere models increase at high latitudes. However, the boundary position at high latitudes is important when considering the origin of the quasi hemisphericity. Recent deployment of a temporary broadband seismic array in Thailand (Thai Seismic Array; TSAR) provides a means to analyze the inner core structure in the north polar region because rays from Central American earthquakes to the network are transmitted through the inner core and have their turning points approximately 150–400 km beneath the north polar region of the inner core boundary. We measured the differential traveltimes of earthquake waveforms between a seismic phase transmitted through the inner core (PKIKP, DF) and a phase that turns in the lower outer core (PKPbc, BC). The observed differential times are approximately 0.2–0.3 s smaller than theoretical estimates predicted by the Preliminary Reference Earth Model (PREM) over the observed distance range and 0.3–0.5 s smaller than those of AK135 at larger distances, which suggests lower velocities in the analyzed region of the upper inner core. Our analyses show that models with velocities ~0.03 km/s lower than those of PREM in the upper 300 km of the inner core, with a velocity about midway between PREM and AK135 in the F layer, sufficiently fit the observed differential times. This low-velocity feature in the upper inner core is characteristic of the “West”. The observations are also largely explained by the model obtained for the structure beneath the northeast Pacific in mid-latitudes classified as “West”. Our previous study also shows that the south polar region of the upper inner core has “West” characteristics. Our results thus suggest that the north and south polar regions of the upper inner core have a “West” structure and that the Earth's upper inner core has an eyeball-shaped high-velocity anomaly called the “East”. The shape of the hemispheric boundary is possibly an ellipse shortened in the north-south direction rather than a circle.

Seismic velocity structure of the Queen Charlotte terrace off western Canada in the region of the 2012 Haida Gwaii Mw 7.8 thrust earthquake

AbstractA well-recorded Mw 7.8 megathrust earthquake occurred on 27 October 2012 under the Queen Charlotte terrace off the west coast of Haida Gwaii, western Canada. In this study, we supplement limited earlier seismic refraction work on the offshore velocity structure off Haida Gwaii with data from ocean bottom seismometers (OBS) operating between 6 December 2012 and 5 January 2013. The OBS recorded a portion of the aftershock sequence, and an active-source seismic survey was conducted in January 2013 to acquire seismic refraction data in the region of the Haida Gwaii earthquake across the Queen Charlotte terrace. P-wave velocity analyses using first-arrival tomography showed relatively shallow (2.0–3.0 km below seafloor) high-velocity material with values up to 4.0 km/s beneath the terrace. At the one OBS station seaward of the deformation front on the abyssal plain, refraction velocities of ∼4.5 km/s indicated the top of the oceanic plate at ∼1–2 km below the seafloor. At several OBS stations, converted shear-wave velocities were determined within the sediment cover using reflected arrivals. The S-wave velocities ranged from 0.5 to 1.5 km/s, and the corresponding P/S velocity ratio was between 3.0 and 4.2. The new refraction data confirm earlier interpretations of high-velocity material in the shallow terrace that may indicate fractured oceanic crustal material lies significantly above the location where a subducted slab is thought to occur under the terrace. Transpressive deformation of the Pacific plate may explain these observations.

Seismic Velocity Structure in the Area of the 2007, Mw 8.0, Pisco-Peru Earthquake: Implications for the Mechanics of Subduction in the Vicinity of the Nazca Ridge

In this study, we present a velocity model for the area of the 2007 Pisco-Peru earthquake (Mw=8.0) obtained using a double-difference tomography algorithm that considers aftershocks acquired for 6 months. The studied area is particularly interesting because it lies on the northern edge of the Nazca Ridge, in which the subduction of a large bathymetric structure is the origin of geomorphological features of the central coast of Peru. Relocated seismicity is used to infer the geometry of the subduction slab on the northern flank of the Nazca Ridge. The results prove that the geometry is continuous but convex because of the subduction of the ridge, thereby explaining the high uplift rates observed in this area. Our inferred distribution of seismicity agrees with both the coseismic and postseismic slip distributions.

3-D travel-time tomography of southernmost Baja California Peninsula

A three-dimensional P and S-wave travel-time tomography, as well as Vp/Vs ratio images, were obtained for the southernmost tip of the Baja California Peninsula in Mexico. The area analyzed include Los Cabos block and part of the Gulf coast. The tomography obtained reveals in detail the heterogeneities of the crustal seismic velocity structure of the Los Cabos block. Results show an overall low velocity structure for both P- and S-waves under the Baja California peninsula down to a depth of ~25 km around the peninsular axis, particularly under Sierra La Laguna and near the Gulf of California (GoC) coast. Low P- and S-wave velocities are also present under the Gulf near the coast whereas fast velocities are observed under the Gulf away from the shoreline. Low and high Vp/Vs ratios are observed to alternate throughout the study area. The low seismic velocities may be explained because the crust in Los Cabos block is highly fractured and fluid-saturated, and has a high heat flux which is manifested as geothermal activity (e.g. hot springs observed all along the block). These low body wave velocities coincide with the existence of an asthenospheric mantle sandwiched by the continental crust of the Los Cabos block and a stalled subducted slab reported by Di Luccio et al. (2014). This mantle strip may be chemically depleted or dry inhibiting the development of active post-subduction volcanism.