Wave Arrival Time Data Research Articles

P and S Wave Anisotropic Tomography of the Banda Subduction Zone

AbstractIn subduction zones with slab‐slab interactions, the pattern of mantle convection is very complex and still unclear. In this study, we jointly invert a large number of P and S wave arrival time data of local earthquakes for 3‐D isotropic and anisotropic velocity structures of the Banda subduction zone. Along the curved Banda arc, the subducting Indo‐Australian slab is detected clearly as a high‐velocity zone, and its azimuthal anisotropy changes along the arc strike, representing fossil anisotropy within the slab and modified anisotropy by the subduction processes. Around the northern edge of the Banda slab, a semi‐toroidal pattern of anisotropy appears in low‐velocity anomalies, representing mantle flow extruded from the Banda arc and escaped from a gap of the Banda‐Molucca slab toward the northeast. Our 3‐D anisotropic tomography uncovers the mantle convection pattern induced by the slab‐slab interactions, shedding new light on the complex dynamical processes in this curved subduction zone.

Subducting slabs, Hainan plume and intraplate volcanism in SE Asia: Insight from P-wave mantle tomography

We present a high-resolution 3-D model of P wave tomography of the crust and mantle down to 1600 km depth under Southeast Asia (SE Asia), which is determined by inverting more than 1.3 million P wave arrival time data of local, regional and teleseismic events recorded at 1241 seismic stations deployed in SE Asia and mainland China. Strong and weak smoothing inversions are conducted to reveal different characteristic scales of velocity anomalies in the mantle. Both seismic and aseismic portions of the subducted plates in different parts of the circular subduction system in SE Asia are clearly revealed as high-velocity bodies in our tomographic images. The intraplate volcanoes in Borneo are underlain by significant low-velocity anomalies in the upper mantle, whereas a clear high-velocity anomaly is visible in the mantle transition zone and the lower mantle. We deem that hot and wet upwelling flows in the big mantle wedge above the stagnant slab in the mantle transition zone may have caused the intraplate volcanism in Borneo. A large low-velocity anomaly consisting of a flat head and a twisted tail extending to the bottom of our model is imaged beneath the southeastern Asian basalt province. The low-velocity anomaly possibly represents a hot zone whose shape may have been modified by mantle wind associated with a plate rotation. The hot zone has a lower-mantle origin and probably contains some small mantle upwellings inside, with the Hainan plume being a major trunk.

Crustal flow and fluids affected the 2021 M7.4 Maduo earthquake in Northeast Tibet

• Fluids exist in the Maduo source zone and affected the rupture nucleation. • Depth-varying seismic anisotropy occurs and reflects lower-crustal flow. • The ductile flow in the mid-lower crust affected the seismotectonics. We present detailed 3-D tomographic images of P and S wave velocity (Vp, Vs), Poisson's ratio (ν) and Vp azimuthal anisotropy of the crust and uppermost mantle beneath the source area of the 21 May 2021 Maduo earthquake (M 7.4) in the NE Tibetan plateau. The images are obtained by inverting a large number of P and S wave arrival-time data of 11,235 local earthquakes and Maduo aftershocks recorded at 67 seismic stations. Our results show that the 2021 Maduo mainshock occurred in a low-Vs and high-ν anomaly, probably reflecting crustal fluids that affected the rupture nucleation. Our Vp anisotropy results show that at 40 km depth under the southern part of the study region, the fast-velocity direction (FVD) is NW-SE, which is mainly controlled by the India-Eurasia collision. At 60 km depth under the study region and at 40 km depth under the northern part of the region, the FVDs are NE-SW to N-S, reflecting lower crustal flow. The FVDs are roughly E-W at 60 km depth beneath the Qilian mountain range, which reflect the lower crustal flow that is blocked by a rigid terrane in the north. The lower crustal flow may lead to intra-crustal and crust-mantle decoupling and affect seismotectonics in NE Tibet.

Penggunaan Automated Phase Picking pada Estimasi Hiposenter Gempa Lokal: Studi Kasus Lengan Atas Sulawesi

Hypocenter estimation was carried out in several local earthquake events in the upper arm of Sulawesi Island. The process is carried out by utilizing P and S wave arrival time data obtained through automated phase picking. The Wadati diagram was used to test the feasibility of the data and HYPOINVERSE was used to estimate the hypocenter. Vp/Vs ratio shows varied results and RMS shows poor data picking. These results have an impact on RMS values which are quite high in the estimation of the hypocenter. There was an inverse comparison of the results when viewed through the RMS value between Wadati and HYPOINVERSE. Picking data improvements need to be done in order to get a better estimate.

P Wave Tomography Beneath Greenland and Surrounding Regions: 1. Crust and Upper Mantle

AbstractWe study the 3‐D P wave velocity (Vp) structure of the crust and upper mantle beneath Greenland and surrounding regions using the latest P wave arrival time data. The Greenland Ice Sheet Monitoring Network (GLISN), initiated in 2009, is an international project for seismic observation in these regions using 34 stations. We use a regional seismic tomography method to simultaneously invert P wave arrival times of local earthquakes and P wave relative traveltime residuals of teleseismic events. These data are extracted from the ISC‐EHB catalog; however, for the teleseismic data, we picked new arrival times from seismograms using a cross‐correlation method. Our tomographic results clearly reveal the Iceland plume, the Jan Mayen plume, and a newly discovered “Svalbard plume,” which merge together in the mantle transition zone. A high‐Vp body exists beneath the Greenland Sea, which might act as an obstacle against the rising Svalbard plume. Furthermore, our results reveal a remarkable low‐Vp anomaly elongated in the NW‐SE direction at depths ≤250 km beneath central Greenland, which is connected with the Iceland and Jan Mayen hotspots. Although previous studies have suggested a similar feature, our result is the first to show the low‐Vp zone existing at all depths in the Greenland lithosphere, and its spatial distribution coincides with a high heat flux region. These characteristics indicate that the low‐Vp zone reflects residual heat from the Iceland plume when the Greenland lithosphere passed over this plume at ~80–20 Ma. Our results also indicate the possible existence of residual heat from the Jan Mayen plume.

P Wave Tomography Beneath Greenland and Surrounding Regions: 2. Lower Mantle

AbstractWe study the 3‐D P wave velocity (Vp) structure of the whole mantle beneath Greenland and surrounding regions using the latest P wave arrival time data. We use a new method of global seismic tomography by setting 3‐D grid nodes densely in the study volume to enhance the resolution. We invert ~5.6 million arrival times of P, pP, and PP waves from 16,257 earthquakes extracted from the ISC‐EHB catalog, which were recorded at 12,549 seismic stations in the world. Our results reveal a hot plume rising from the core‐mantle boundary beneath central Greenland, which is called the Greenland plume. This plume has the main conduit and two branches in the lower mantle, and the main conduit rises to the mantle transition zone (MTZ) beneath eastern Greenland, so it is distinguishable from the Iceland plume that appears to rise from a midmantle depth (~1,500 km) beneath Iceland. The Iceland plume itself is a powerful plume, but it may also be fed by hot mantle materials from three joints with other plumes: a branch of the Greenland plume at ~1,500 km depth, a plume beneath Western Europe at ~1,000 km depth, and the main conduit of the Greenland plume at the MTZ, leading to many active volcanoes in Iceland. We deem that the main conduit of the Greenland plume mixes with the Iceland plume incompletely and splits mainly into the Jan Mayen and Svalbard plumes in the upper mantle, which supply magma to the Jan Mayen hotspot and a geothermal area in western Svalbard, respectively.

4D seismic tomography of Nevado del Ruiz Volcano, Colombia, 2000–2016

To track and analyze temporal changes in the volcanic system of Nevado del Ruiz, Colombia, repeated three-dimensional seismic tomography (4D seismic tomography) for P and S wave velocities (Vp and Vs, respectively) during the period 2000–2016 was performed. P and S wave arrival time data were classified on the basis of changes in volcanic activity into five time periods: 2000–2006, 2007–2009, 2010–2012, 2013–2014, and 2015–2016. Our results showed clear changes in 2013–2014 and 2015–2016, periods of heightened volcanic activity after an episode of unrest in 2010. Regions of low Vp, low Vs, and high Vp/Vs (>1.9) were found beneath and northwest of the active crater at depths of 2–3 km below sea level. The enhancement of those anomalies suggests that a new pulse of magma arrived in the volcanic system at that time. Regions with low Vp/Vs values (1.3–1.6) in the upper part of the volcano, at depths from −3 to −2 km, are interpreted as the result of vapor accumulation within highly fractured zones of the hydrothermal system. The ratios of fractional change of Vp and Vs in 2015–2016 suggest that partial melting zones were located beneath the active crater at depths of −2 to 0 km. Our results show that the volcanic system of Nevado del Ruiz has experienced changes associated with the input of new magma, which ascended and interacted with preexisting magma bodies and led to the recent emplacement of a small dome at the bottom of the active crater. We combined our results and evidence from previous studies to propose a model of the Nevado del Ruiz plumbing system from 1985 to 2016.

Internal Deformation of Lithosphere Beneath Central Tibet

AbstractWe use a large number of high‐quality P wave arrival time data recorded by the Hi‐CLIMB project to determine a 3‐D model of azimuthal anisotropy tomography beneath central Tibet. In the Himalayan block, variations of fast velocity orientation (FVO) are revealed from the crust to the upper mantle. In contrast, the FVO in the Lhasa block exhibits only a slight difference between the lower crust and upper mantle, reflecting a coherent deformation there. Different FVOs are revealed near the Bangong‐Nujiang suture, which may reflect anisotropies in different parts of the underthrusting Indian plate. In the upper mantle beneath the Qiangtang block, a strong anisotropy is revealed in the shallower part, whereas a weak anisotropy appears in the deeper part, suggesting that a two‐layer anisotropy model is applicable there. A layered lithosphere is detected in the eastern part of the Lhasa block, whereas a consistent FVO is revealed in its western part. Our results indicate that strong deformation has occurred in both the Indian and Eurasian lithospheres.

Depth-varying azimuthal anisotropy in the Tohoku subduction channel

We determine a detailed 3-D model of azimuthal anisotropy tomography of the Tohoku subduction zone from the Japan Trench outer-rise to the back-arc near the Japan Sea coast, using a large number of high-quality P and S wave arrival-time data of local earthquakes recorded by the dense seismic network on the Japan Islands. Depth-varying seismic azimuthal anisotropy is revealed in the Tohoku subduction channel. The shallow portion of the Tohoku megathrust zone (<30 km depth) generally exhibits trench-normal fast-velocity directions (FVDs) except for the source area of the 2011 Tohoku-oki earthquake (Mw 9.0) where the FVD is nearly trench-parallel, whereas the deeper portion of the megathrust zone (at depths of ∼30–50 km) mainly exhibits trench-parallel FVDs. Trench-normal FVDs are revealed in the mantle wedge beneath the volcanic front and the back-arc. The Pacific plate mainly exhibits trench-parallel FVDs, except for the top portion of the subducting Pacific slab where visible trench-normal FVDs are revealed. A qualitative tectonic model is proposed to interpret such anisotropic features, suggesting transposition of earlier fabrics in the oceanic lithosphere into subduction-induced new structures in the subduction channel.

Seismic velocity azimuthal anisotropy of the Japan subduction zone: Constraints from P and S wave traveltimes

AbstractWe present 3‐D images of azimuthal anisotropy tomography of the crust and upper mantle of the Japan subduction zone, which are determined using a large number of high‐quality P and S wave arrival time data of local earthquakes and teleseismic events. A tomographic method for P wave velocity azimuthal anisotropy is modified and extended to invert S wave traveltimes for 3‐D S wave velocity azimuthal anisotropy. A joint inversion of the P and S wave data is conducted to constrain the 3‐D azimuthal anisotropy of the Japan subduction zone. Our results show that the subducting Pacific and Philippine Sea (PHS) slabs exhibit mainly trench‐parallel fast‐velocity directions (FVDs), which may reflect frozen‐in lattice‐preferred orientation of aligned anisotropic minerals formed at the mid‐ocean ridge as well as shape‐preferred orientation such as normal faults produced at the outer‐rise area near the trench axis. Trench‐normal FVDs are generally revealed in the mantle wedge, which may reflect corner flows in the mantle wedge due to the plate subduction and dehydration. Trench‐normal FVDs are also visible in the subslab mantle, which may reflect the subducting asthenosphere underlying the slabs. Our results also reveal toroidal mantle flows in and around a window (hole) in the PHS slab beneath SW Japan, suggesting that the occurrence of the PHS slab window may have caused a complex flow pattern in the mantle wedge above the Pacific slab.

ISC-GEM: Global Instrumental Earthquake Catalogue (1900–2009), II. Location and seismicity patterns

We present the final results of a two-year project sponsored by the Global Earthquake Model (GEM) Foundation. The ISC-GEM global catalogue consists of some 19 thousand instrumentally recorded, moderate to large earthquakes, spanning 110years of seismicity. We relocated all events in the catalogue using a two-tier approach. The EHB location methodology (Engdahl et al., 1998) was applied first to obtain improved hypocentres with special focus on the depth determination. The locations were further refined in the next step by fixing the depths to those from the EHB analysis and applying the new International Seismological Centre (ISC) location algorithm (Bondár and Storchak, 2011) that reduces location bias by accounting for correlated travel-time prediction error structure. To facilitate the relocation effort, some one million seismic P and S wave arrival-time data were added to the ISC database for the period between 1904 and 1970, either from original station bulletins in the ISC archive or by digitizing the scanned images of the International Seismological Summary (ISS) bulletin (Villaseñor and Engdahl, 2005, 2007). Although no substantial amount of new phase data were acquired for the modern period (1964–2009), the number of phases used in the location has still increased by three millions, owing to fact that both the EHB and ISC locators use most well-recorded ak135 (Kennett et al., 1995) phases in the location.We show that the relocation effort yielded substantially improved locations, especially in the first half of the 20th century; we demonstrate significant improvements in focal depth estimates in subduction zones and other seismically active regions; and we show that the ISC-GEM catalogue provides an improved view of 110years of global seismicity of the Earth. The ISC-GEM Global Instrumental Earthquake Catalogue represents the final product of one of the ten global components in the GEM program, and is available to researchers at the ISC (www.isc.ac.uk) website.

A COMPARISON OF MODEL BASED AND DIRECT OPTIMIZATION BASED FILTERING ALGORITHMS FOR SHEARWAVE VELOCITY RECONSTRUCTION FOR ELECTRODE VIBRATION ELASTOGRAPHY.

Tissue stiffness estimation plays an important role in cancer detection and treatment. The presence of stiffer regions in healthy tissue can be used as an indicator for the possibility of pathological changes. Electrode vibration elastography involves tracking of a mechanical shear wave in tissue using radio-frequency ultrasound echoes. Based on appropriate assumptions on tissue elasticity, this approach provides a direct way of measuring tissue stiffness from shear wave velocity, and enabling visualization in the form of tissue stiffness maps. In this study, two algorithms for shear wave velocity reconstruction in an electrode vibration setup are presented. The first method models the wave arrival time data using a hidden Markov model whose hidden states are local wave velocities that are estimated using a particle filter implementation. This is compared to a direct optimization-based function fitting approach that uses sequential quadratic programming to estimate the unknown velocities and locations of interfaces. The mean shear wave velocities obtained using the two algorithms are within 10%of each other. Moreover, the Young's modulus estimates obtained from an incompressibility assumption are within 15 kPa of those obtained from the true stiffness data obtained from mechanical testing. Based on visual inspection of the two filtering algorithms, the particle filtering method produces smoother velocity maps.

Incorporating fault zone head wave and direct wave secondary arrival times into seismic tomography: Application at Parkfield, California

We present a three‐dimensional (3D) P wave velocity (Vp) model of the Parkfield region that utilizes existing P wave arrival time data, including fault zone head waves (FZHWs), and data from direct wave secondary arrivals (DWSAs). The first‐arrival and DWSA travel times are obtained as the global‐ and local‐minimum travel time paths, respectively. The inclusion of FZHWs and DWSAs results in as much as a 5% and a 10% increase in the across‐fault velocity contrast, respectively, for the Vp model at Parkfield relative to that of Thurber et al. [2006]. Viewed along strike, three pronounced velocity contrast regions are observed: a pair of strong positive velocity contrasts (SW fast), one NW of the 1966 Parkfield earthquake hypocenter and the other SE of the 2004 Parkfield earthquake hypocenter, and a strong negative velocity contrast (NE fast) between the two hypocenters. The negative velocity contrast partially to entirely encompasses peak coseismic slip estimated in several slip models for the 2004 earthquake, suggesting that the negative velocity contrast played a part in defining the rupture patch of the 2004 Parkfield earthquake. Following Ampuero and Ben‐Zion (2008), the pattern of velocity contrasts is consistent with the observed bilateral rupture propagation for the 2004 Parkfield earthquake. Although the velocity contrasts also suggest bilateral rupture propagation for the 1966 Parkfield earthquake, the fault is creeping to the NW here, i.e., exhibiting velocity‐strengthening behavior. Thus, it is not surprising that rupture propagated only SE during this event.

Geophysical properties and seismotectonics of the Tohoku forearc region

Abstract We investigated the detailed three-dimensional (3-D) isotropic and anisotropic structures of the crust and upper mantle under the NE Japan forearc region using a large number of P and S wave arrival-time data from onshore and offshore earthquakes. The suboceanic earthquakes used in this study are well relocated using the sP depth phases. We also determined the 3-D distribution of Poisson’s ratio, crack density and saturation rate using the 3-D P and S wave velocity model obtained by this study. The relatively complex anisotropic structures in the megathrust zone may reflect the complex geological structures, lithological variations and fluids in the accretional prism under the forearc region. The tomographic images reflect strong lateral heterogeneities in the megathrust zone under the Tohoku forearc. Areas with low velocity, high Poisson’s ratio, high crack density and high saturation rate may be due to entrapment of fluid-filled, unsolidated sediments on the plate interface close to the Japan Trench. Most of the large megathrust earthquakes since 1900 (M ⩾ 6.0) and the large 2011 Tohoku-oki earthquakes (M 6.0–9.0) are located in areas with high velocity, high Poisson’s ratio, low crack density and high saturation rate, which may represent strongly-coupled asperities in the megathrust zone resulting from the subducted oceanic ridges and/or seamounts. In contrast, the areas with high Poisson’s ratio may indicate that the fluids have infiltrated into the strongly coupled patches. We think that the great Tohoku-oki earthquakes were caused by not only the stress concentration but also the in situ structural heterogeneities in the megathrust zone.

Crustal heterogeneities beneath the 2011 Talala, Saurashtra earthquake, Gujarat, India source zone: Seismological evidence for neo-tectonics

During the 1st decade of the 21st century, the study area of Talala, Saurashtra of western India witnessed three damaging earthquakes of moderate magnitude, year 2007 [Mw 5.0; Mw 4.8] and in the year 2011 [Mw 5.1] that generated public panic in the region. The last damaging moderate earthquake of the 20th October 2011 in Talala region (21.09°N;70.45°E), located at about 200km south to the devastating 2001 Bhuj (23.412°N, 70.232°E) mainshock (Mw 7.6), jolted the entire Saurashtra region of Gujarat. A long series of aftershocks followed hereafter, recorded at nine seismograph/accelerograph stations. Hypocenters of aftershocks were relocated accurately using absolute and relative travel time (double-difference) method. In this study, we, for the first time, determined 3-D tomographic images of the upper crust beneath the 2011 Talala earthquake source zone by inverting about 1135 P and 1125 S wave arrival time data. Estimates of seismic velocities (Vp, Vs) and Poisson’s ratio (σ) structures offer a reliable interpretation of crustal heterogeneities and their bearing on geneses of moderate earthquakes and their aftershock sequences beneath the source zone. It is found that the 2011 Talala mainshock hypocenter depth (6km) is located near the boundary of the low and high velocity (Vp, Vs) and the source zone is associated with low-σ anomalies guarded by the prominent high-σ anomalies along the active fault zone having strike-slip motion beneath the earthquake source zone. The pattern of distribution of (Vp, Vs, σ) and its association with occurrences of aftershocks provide seismological evidence for the neo-tectonics in the region having left lateral strike-slip motion of the fault.

PandSwave upper mantle seismic velocity structure beneath the northern Apennines: New evidence for the end of subduction

Orogeny in the geologically young northern Apennines is explained by the eastward retreat of a convergence zone in which Adriatic lithosphere subducts to the west. It is unclear, however, whether all of the lithosphere or only the lower portion has subducted since 15 Ma, when the convergence zone lay near Corsica. We combine teleseismic P and S wave arrival time data from the RETREAT seismic network (2003–2006) and surrounding permanent stations to estimate tomographic images of the upper mantle structure beneath the northern Apennines. We image a vertically oriented slab that that extends to only ∼300 km depth. Our slab termination is shallower than previous studies but is confirmed by resolution tests. Furthermore, our images resolve the southern edge of the northern Apennines slab at ∼43°N, with no deep continuity with any slab segment to the south, as earlier proposed. Our results suggest that only a 300–400 km strip of lithosphere has subducted since 15 Ma beneath the northern Tyrrhenian Sea, a length roughly equivalent to the distance from the present-day west coast of Corsica to the crest of the northern Apennines. Although not a definitive indicator, the largely aseismic vertical slab configuration and its limited extent, coupled with other volcanic and geophysical indicators, suggests the delamination scenario of Bird (1979), in which only the lower portion of the continental lithosphere subducts.

Deep structure and seismogenesis of the north‐south seismic zone in southwest China

To understand the deep structure and seismogenesis in the north‐south seismic zone (NSSZ) in southwest China, we collected a large number of P and S wave arrival time data from both local earthquakes and teleseismic events to invert simultaneously for the three‐dimensional P and S wave velocity (Vp, Vs) structures in the crust and upper mantle. The obtained Vp and Vs models are generally coherent with each other from the crust to the mantle transition zone. The NSSZ is characterized by strong structural heterogeneities from the mountainous areas of southeastern Tibet to the Yangtze Platform. Low‐Vp and low‐Vs anomalies are generally imaged in the upper crust under the Sichuan basin, showing good agreement with the surface geological features that the foreland basin contains primarily Mesozoic and Paleozoic sedimentary rocks with a thickness of several kilometers. The deep part of the Yangtze Platform is characterized by a cratonic lithospheric body dipping southwestward down to 400 km depth beneath the southeastern margin of the Tibetan Plateau, in sharp contrast to low‐velocity anomalies in the upper mantle beneath the Songpan‐Ganzi Fold System and the Northwest Qiangtang Block. At depths of the lower crust and uppermost mantle, a layer with low Vp and low Vs (1%–2%) is revealed under a region west of the Yangtze Platform, which may reflect the ductile flow in the lower crust. Our results indicate that the seismotectonics in the NSSZ is affected significantly by the ductile flow in the lower crust and strong heterogeneities in the upper mantle under southwest China in addition to the dominant role of the India‐Asia collision.

Time-dependent seismic tomography

SUMMARY Of methods for measuring temporal changes in seismic-wave speeds in the Earth, seismic tomography is among those that offer the highest spatial resolution. 3-D tomographic methods are commonly applied in this context by inverting seismic wave arrival time data sets from different epochs independently and assuming that differences in the derived structures represent real temporal variations. This assumption is dangerous because the results of independent inversions would differ even if the structure in the Earth did not change, due to observational errors and differences in the seismic ray distributions. The latter effect may be especially severe when data sets include earthquake swarms or aftershock sequences, and may produce the appearance of correlation between structural changes and seismicity when the wave speeds are actually temporally invariant. A better approach, which makes it possible to assess what changes are truly required by the data, is to invert multiple data sets simultaneously, minimizing the difference between models for different epochs as well as the rms arrival-time residuals. This problem leads, in the case of two epochs, to a system of normal equations whose order is twice as great as for a single epoch. The direct solution of this system would require twice as much memory and four times as much computational effort as would independent inversions. We present an algorithm, tomo4d, that takes advantage of the structure and sparseness of the system to obtain the solution with essentially no more effort than independent inversions require.

3D Velocity Structure around the Source Area of the Armenia Earthquake: 25 January 1999, Mw=6.2 (Colombia)

Selected P wave arrival time data of 518 events (mainly aftershocks) of the 25 January 1999 Armenia Earthquake (Mw=6.2, Colombia), recorded by 23 temporary seismic stations during February, 1999 to July, 2001, have been inverted simultaneously for both hypocenter locations and three-dimensional Vp structure. The surface geology of this area suggests a complex disposition of tectonic flakes composed by diverse rocks and crossed by the Romeral Fault System (RFS). However, there is a good correlation between high-velocity zones and old oceanic rocks affected by plutonic rocks. Metamorphic belts that wrap these rocks are well correlated with low-velocity zones. The disposition of tectonic flakes in depth is solved with a flower structure where the Cordoba fault slipped in a contact between high- and low-density rocks during the Armenia earthquake. The low-velocity zones would correspond to the older rocks that constitute the nucleus of the Central range. We infer that the source volume of the Armenia earthquake sequence lies within 75.64o - 75.72o W, 4.38o - 4.52oN and a depth of 5 - 21 km; the source volume is approximately 2 200 km3. Most of the well-located aftershocks occurred above the hypocenter of Armenia earthquake.

Three-dimensional P and S wave velocity structures beneath the Hokkaido corner, Japan-Kurile arc-arc junction

Abstract We applied an inverse method developed by Zhao et al. (J. Geophys. Res., 97, 19909–19928, 1992) to 42,834 P and 18,263 S wave arrival time data observed at 152 seismographic stations for 1143 local earthquakes at depths between 0 and 200 km in order to estimate three-dimensional P and S wave velocity structures beneath the Hokkaido corner, Japan-Kurile arc-arc junction. High- and low-velocity zones were clearly imaged in the Hidaka Mountain Range at depths shallower than 35 km. The low-velocity anomalies of P and S waves were found to be distributed in the mantle wedge at depths between 35 and 100 km beneath the volcanic front, as also observed in the Tohoku region. Another low-velocity zone was found to exist in the fore-arc region at depths of 50–70 km above the plate boundary; this zone was not detected in Tohoku, suggesting that the dehydration process in the fore-arc region is different from that in the Tohoku region.