Philippine Sea Plate Research Articles

Ancient and Medieval Events and Recurrence Interval of Great Kanto Earthquakes along the Sagami Trough, Central Japan, as Inferred from Historiographical Seismology

Abstract To know the Ancient and Medieval events and the recurrence interval of the great Kanto earthquakes such as the 1923 and 1703 ones generated by the subduction of the Philippine Sea plate along the Sagami trough, central Japan, I examined the latest dataset of historical records. I used only rank-A (contemporary) materials in the Online Database of Historical Documents on Japanese Earthquakes and Eruptions in the Ancient and Medieval Ages. Among destructive earthquakes in the Kanto district before the sixteenth century, the 878 Gangyo earthquake, which has been suspected an inland event generated from the Isehara fault, is considered the oldest candidate of the Kanto earthquake based on the large-scale disaster and intense aftershock activity, though tsunami is not mentioned. The 1293 Showo (Einin) Kamakura earthquake can be regarded as a great Kanto earthquake, because of severe damage in the Kanto district and remarkable aftershock activity. During the fifteenth century, both of the 1433 Eikyo and the 1495 Meio earthquakes are regarded as candidates of the Kanto earthquake; the former caused severe damage around Kamakura on the northern coast of Sagami Bay with high-aftershock activity and presumably caused tsunami, and the latter seems to have caused large tsunamis at Kamakura and the west coast of Sagami Bay. Although further investigation is necessary to clarify which one is the Kanto earthquake, we can say that an interplate earthquake probably occurred in the fifteenth century. Intervals between successive events, in 878, 1293, 1433 or 1495, 1703, and 1923, range from 140 to 270 yr except for the first interval of 415 yr. It is very difficult to confirm whether a great interplate earthquake occurred or not during this interval by means of historiographical seismology, because the Kanto district was in the worst situation of poor historical records in those days.

Cenozoic sill intrusion in the central and southern East China Sea Shelf Basin

Cenozoic igneous rocks dominated by basalts are exposed sporadically onshore East China, and they provide important information about the Cenozoic evolution of the region. Importantly, their geometries and distribution are still not well constrained in the 200–600 km wide offshore continental shelf region. Based on seismic profiles, this study presents the geometric characteristics of 133 igneous sills that are widely distributed in the central and southern regions of the East China Sea Shelf Basin (ECSSB). Sills in high-amplitude, continuous seismic reflections mostly occur in a time interval of 1.1–3.3 s in multiple seismic profiles, corresponding to a depth range of 1.1–4.8 km. They can be classified into five types: saucer-shaped sills, strata-concordant sills, laccoliths, inclined sheets, and hybrid sills. Of these, the saucer-shaped sills predominate. Strata deformation associated with these igneous sills indicate that sill intrusion has become intense since the Miocene, and it potentially affected petroleum systems in the ECSSB. Forced folds induced by the sills are prospective petroleum traps. We infer that sill intrusion in the ECSSB was due to magma upwelling caused by dehydration and/or small-scale convection in a large mantle wedge above the stagnant Pacific slab. The subduction of the Philippine Sea plate resulted in tectonic inversion in the ECSSB and extension in the Okinawa Trough since the Early Miocene triggered more magmatism in the tectonically thinned ECSSB. Thick sedimentary strata over a thin crust in the ECSSB may have induced more intrusive than extrusive Cenozoic magmatism.

Centroid moment tensor inversions of offshore earthquakes using a three-dimensional velocity structure model: slip distributions on the plate boundary along the Nankai Trough

SUMMARYDue to complex 3-D heterogeneous structures, conventional 1-D analysis techniques using onshore seismograms can yield incorrect estimation of earthquake source parameters, especially dip angles and centroid depths of offshore earthquakes. Combining long-term onshore seismic observations and numerical simulations of seismic wave propagation in a 3-D model, we conducted centroid moment tensor (CMT) inversions of earthquakes along the Nankai Trough between April 2004 and August 2019 to evaluate decade-scale seismicity. Green's functions for CMT inversions of earthquakes with moment magnitudes of 4.3–6.5 were evaluated using finite-difference method simulations of seismic wave propagation in the regional 3-D velocity structure model. Significant differences of focal mechanisms and centroid depths between previous 1-D and our 3-D catalogues were found in the solutions of offshore earthquakes. By introducing the 3-D structures of the low-velocity accretionary prism and the Philippine Sea Plate, dip angles and centroid depths for offshore earthquakes were well-constrained. Teleseismic CMT also provides robust solutions, but our regional 3-D CMT could provide better constraints of dip angles. Our 3-D CMT catalogue and published slow earthquake catalogues depicted spatial distributions of slip behaviours on the plate boundary along the Nankai Trough. The regular and slow interplate earthquakes were separately distributed, with these distributions reflecting the heterogeneous distribution of effective strengths along the Nankai Trough plate boundary. By comparing the spatial distribution of seismic slip on the plate boundary with the slip-deficit rate distribution, regions with strong coupling were clearly identified.

Subduction with arrested volcanism: Compressional regime in volcanic arc gap formation along east Mindanao, Philippines

The eastern portion of the Philippine island arc system, from the Bicol Peninsula through Leyte island to the Pacific Cordillera of east Mindanao, is the overriding plate underneath which the West Philippine Basin (part of the Philippine Sea Plate) and Palau Basin oceanic plates subduct along the Miocene-Pliocene to present-day Philippine Trench. Southward younging of the trench, coupled with the presence of the left-lateral Philippine Fault Zone parallel to it, is recognized. Young volcanic centers (<7Ma to present) are well-developed in the Bicol Peninsula and in Leyte island but not in the Pacific Cordillera. There are only two volcanic centers that are exposed in the Pacific Cordillera, the Paco volcanic center in the north and the Leonard Kniassef in the south. There is a 250 km volcanic arc gap distance between the two volcanic centers. Reported inland and offshore geologic structures, from onramp structures to thrust faults, and strain rate computations suggest a predominantly compressional regime in the Pacific Cordillera from the Miocene to the present. This is consistent with the hinge advance character of the Philippine Trench and the recognized E-W directed σ1 in this part of the island arc system. It is forwarded that the prevailing compressional regime inhibited the extrusion of lavas leading to the non-formation of surficial volcanic centers between the Paco and Leonard Kniassef volcanic centers. Furthermore, the compression can also partly account for the presence of porphyry copper deposits in this part of Mindanao.

Quaternary deformation at the southeastern margin of the Ordos Block, North China: Inferred from structural analysis of basin-bounding faults

The southeastern margin of the Ordos Block (SEMOB) is located at the juncture of the North China Block, the Ordos Block, and the Qinling Orogen. A series of faulted basins has developed in this area since the Cenozoic. The latest focal mechanism solution and stress field analyses indicate that the SEMOB is located at the adjacent regions of present different stress field, although the structural activity and internal mechanisms of this changing remain unclear. In this paper we describe the results of a study into the fault systems and Quaternary deformation between the Linfen and Weihe basins on the SEMOB. Based on the deformation sequences, kinematic analyses and activity timing, we found that the strain field in SEMOB underwent three evolutionary stages during the Quaternary: NW–SE extension in the early Pleistocene, NE–SW extension in the middle Pleistocene, and ca. N–S oblique extension from the late Pleistocene to the Holocene. This evolution of the strain field affected both the geomorphic patterns and the activity of paleo-earthquake events along the SEMOB during the Quaternary. Quaternary extensional deformation in the upper crust along the SEMOB possibly resulted from crust-mantle decoupling, which may be related to eastward expansion of the Tibetan Plateau and subduction of the Pacific and Philippine Sea plates.

Triple Seismic Zone Revisited

Subduction off Miyagi and Fukushima prefectures, northern Honshu, has been recognized as having a triple-planed structure of seismicity at the deep thrust zone in the 40-60 km depth range. This triple seismic zone is composed of thrust-type earthquakes, down-dip compressional and down-dip tensional earthquakes from top to bottom. At the time of the 2011 Tohoku-oki earthquake, peculiar phenomena such as radiation of short-period seismic waves and pre- and after- slow slips within the asperities of M7 class earthquakes were observed over this thrust zone west of the 2011 main rupture zone. Further to the south, where the Philippine Sea plate is subducting beneath Kanto, a triple seismic zone has also been recognized particularly under southwest Ibaraki prefecture. The thrust-type earthquakes at the top of the triple seismic zone off northern Honshu and beneath Kanto have been believed to be interplate events representing the relative motion between the overriding and subducting plates. I conclude that the thrust-type earthquakes beneath southwest Ibaraki prefecture are in fact within the crust of the subducting Philippine Sea slab, not at the surface because their slip vectors are different from the relative motion between the subducting and overriding plates. Therefore, there would be an aseismic plate boundary above the seismicity. I also show that the dip angles of the westerly dipping fault planes of the thrust-type earthquakes off Miyagi prefecture are smaller by 6° in average than the dip of the slab surface in this region, except for the six years prior to the Tohoku-oki earthquake, i.e. prior to 2006. Furthermore, the slip vectors coincide with the relative motion between the overriding and subducting plates only during this period. I infer that the topmost earthquakes of the triple seismic zone off Miyagi prefecture prior to 2006 are thus likely to be within the crust of the subducting plate. The slow slips before and after the Tohoku-oki earthquake would have occurred not within the asperities but along the aseismic plate interface, and the short-period seismic waves would have been radiated due to fractures within the crust associated with the overshooting rupture at the time of the Tohoku-oki earthquake. Many of the so-called repeating earthquakes at the topmost surface of the subducting plate would be in fact intra-crustal events within the slab. M9 earthquakes would interact with the triple seismic zone, not only mechanically, but also through fluid migration, because earthquakes in the triple seismic zone involve dehydration reaction. The irregularity of the occurrence of M9 earthquakes might be due to the inhomogeneous distribution of hydrated minerals in the incoming plate. The subduction zones having M9 earthquakes or under Kanto have a collisional character. I propose to term subduction having both a collisional character and a triple seismic zone as “super-subduction”. The relative motion between the plates is accommodated by the deformations of the crust of the subducting slab as a “plate boundary zone”. The viewpoint of “super-subduction” is necessary to understand earthquakes in subduction zones with a collisional character and dehydration reactions in the slab.

New hypothesis to explain Quaternary forearc deformation and the variety of plate boundary earthquakes along the Suruga\u2013Nankai Trough by oblique subduction of undulations on the Philippine Sea Plate

Plate-boundary earthquakes of magnitude 8 or greater along the Suruga–Nankai Trough subduction zone have repeated at intervals of 90–150 years, but with widely varying magnitudes and rupture areas. We propose, based on geologic data on crustal movements of the forearc wedge, that these earthquake variations are controlled by two separate locked zones in the deeper part of the plate boundary. Long-wavelength topographic undulations, composed of alternating zones of uplift and subsidence along the forearc wedge, are associated with 2000 to 3000 m of vertical relief that has accumulated during Quaternary time. We suggest that this crustal deformation in the forearc wedge is caused not by stress loading and release during earthquake cycles, but rather by vertical displacements of the plate boundary caused by the westward movement of undulations in the obliquely subducting slab of the Philippine Sea Plate as it subducts beneath Southwest Japan. Dating of emergent marine shell fossil assemblages shows that the Kii Mountains, an uplift zone at the midpoint of the trough, has undergone uplift events at intervals of 400–600 year, and the latest event of those occurred when the 1707 Hoei earthquake ruptured the entire plate boundary along the trough. We infer that the plate boundary under the Kii Mountains is a locked zone and that slips of this zone, which accompanied ruptures of the entire plate boundary, caused uplift of the mountains by decreasing the plate boundary depth. A similar locked zone is inferred under the uplift zone of the Akaishi Mountains, along the eastern margin of the trough, and the 1854 Ansei earthquake pair was presumably caused by the slip of this zone. During the two 1854 events, the locked zone under the Kii Mountains presumably restricted the rupture propagation to the eastern half of the trough, then a rupture of the western half of the trough followed within 32 h. These locked zones are inferred to slip independently every few hundred years and determine the major patterns of characteristic ruptures along the Suruga–Nankai Trough.

Strain Partitioning and Exhumation in Oblique Taiwan Collision: Role of Rift Architecture and Plate Kinematics

AbstractTaiwan is an archetypal example of continental accretionary wedges. Yet the generally poor knowledge of three‐dimensional strain distribution over time and role of architecture of the rifted margin shed doubt on the cylindrical two‐dimensional kinematic models of Taiwan collision. Here we provide new field‐based constraints on strain distribution, new Raman Spectroscopy on Carbonaceous Materials temperatures and apply mica‐chlorite multiequilibrium approach to determine pressure‐temperature in the Central Range of Taiwan. We identify three distinct structural domains that define zones of orthogonal shortening in the western Backbone Range and left‐lateral ductile shearing overprinted by left‐lateral transtensional brittle deformation in eastern Central Range. Field surveys show the lack of nappe stacking in the Backbone Range. Combining new temperature estimates with existing thermochronological constraints we emphasize that western Taiwan mostly inherited preorogenic thermal history. We show that metamorphic peak conditions of 5–6 kbar and 330–400 °C in the eastern Backbone Range and HP rocks of the Yuli Belt exhumed along the P‐T paths related to transcurrent deformation. We propose a three‐dimensional kinematic model of Taiwan accounting for the oblique motion of the Philippine Sea Plate relative to the plate boundary and the reactivation of a NS striking transform fault in the South China Sea rifted margin. Recent and ongoing strain partitioning in the Taiwan accretionary wedge is reflected by the coexistence of brittle left‐lateral shear, oblique extension, and contraction. Our results have impact on orogen‐based plate kinematic reconstructions that consider two‐dimensional kinematic evolution of orogens.

Kibi Plateau: A stable-coherent tectonic unit in the active Japanese Islands

The Kibi Plateau in the active Japanese Islands consists of mainly Permian to Cretaceous rocks that have been deeply weathered into a red soil, comprising a peneplain with U-shaped valley. Systematic geological analyses of the Eocene fluvial deposits revealed the paleo-rivers that existed in the eastern Asian continent and streamed out to the paleo-Pacific Ocean. Each paleo-river is traced in a flow line shape without any significant vertical and horizontal displacement. The Eocene shallow marine sediments in a possible coastal region have no relevant inclination. These geological data strongly suggest that the Kibi Plateau has been a stable-coherent tectonic unit since the Eocene through the opening of the Japan Sea and the associated quick rotation of SW Japan in the Middle Miocene. The Kibi Plateau region with a thick crust over 30 km existed as a stable eastern segment of the Asian continent in the Eocene. The Kibi Plateau tectonic unit drifted to the south without any destruction due to the peripheral successive tectonic events such as the Philippine Sea plate subduction and the reactivation of Median Tectonic Line. No subduction related arc volcanism since the Eocene has also influenced to preserve the stable tectonic unit.

Study of Medical Demand-Supply Balance for the Nankai Trough Earthquake

The Nankai Trough, which marks the boundary between the Eurasian and Philippine Sea plates, is forecasted to create a catastrophic earthquake and tsunami within 30 years. The Japanese government believes that the number of casualties would be huge. However, the exact number of severely injured (SI) people who would need emergency and intensive care has not been identified. This study, therefore, aimed to clarify the gap between medical supplies and forecasted demand. The official data estimating the number of injured people were collected, together with the number of intensive care unit (ICU) and high care unit (HCU) beds from each prefecture throughout Japan. The number of SI cases was recalculated based on official data. The number of hospital beds was then compared with the number of SI people. The total number of hospitals in Japan is 8,493 with 893,970 beds, including 6,556 ICU and 5,248 HCU beds. When the Nankai Trough earthquake occurs, 187 of the 723 disaster base hospitals (DBHs) would be located in the areas with a seismic intensity of an upper six on the Japanese Seismic Intensity Scale (JSIS) of seven, and 79 DBHs would be located in the tsunami inundation area. The estimated total number of injured people would be 661,604, including 26,857 severe, 290,065 moderate, and 344,682 minor cases. Even if all ICU and HCU beds were available for severe patients, an additional 15,053 beds would be needed. If 80% of beds were used in non-disaster times, the available ICU and HCU beds would be only 2,361. The Cabinet Office of Japan (Chiyoda City, Tokyo, Japan) assumes that 60% of hospital beds would be unavailable in an area with an upper six on the JSIS. The number of ICU and HCU beds that would be usable during a disaster would thus further decrease. The beds needed for severe patients, therefore, would be significantly lacking when the Nankai Trough earthquake occurs. It would be necessary to start the treatment of those severe patients who are "more likely to be saved."

Plate geometry model and seismicity in the northern Ryukyu subduction zone, Japan, deduced from amphibious seismic observations

In this study, the geometry of the subducted Philippine Sea Plate interface beneath the northern Ryukyu Trench subduction zone was estimated based on seismic tomography and focal mechanism estimations using passive data obtained from a temporary amphibious seismic network and permanent land stations. Using relocated low-angle thrust-type earthquakes and information obtained from previous studies, we constrained the plate geometry from the trench axis to a depth of 60 km with uncertainties of <2 km between the depths of 15 and 30 km. The estimated plate geometry model appears to be smooth; there is no clear evidence of the existence of large subducted seamounts or ridges in the forearc of the study area. The active earthquake region near the plate boundary was primarily limited to the depth range of 15–30 km. These upper and lower limits may be controlled by the temperature along the plate boundary and the existence of serpentinized mantle in the hanging-wall plate, respectively. Moreover, we determined that the active region of interplate earthquakes is clearly separated from the area dominated by short-term slow slip events and low-frequency tremors, suggesting that suitable frictional conditions along the plate boundary for regular and slow earthquakes are different. There are two areas that were nearly seismically quiet, indicating either full interplate coupling or full creeping conditions. On comparing the historical record in the seismic catalog, tsunami records, and the very-low frequency earthquake distribution, one of the areas has a plausible tsunamigenic potential for an M∼8 event. The spatial pattern of seismicity shows obvious variations between the northern and southern halves of the study area; intraslab earthquakes were dominant in the northern half, whereas interplate earthquakes were dominant in the southern half. Furthermore, the lower limit of the seismogenic zone in the northern half was ∼5–10 km deeper than that in the southern half. These regionalities indicate that differences in age and/or temperature or degree of hydration in the north–south direction exist in the slab.

Mineral Analysis and Physical Characteristics of Beach Sands in Latuhalat and Suli Village to Support the Beach Geotourism of the Maluku Province

From the aspect of the earth, the Maluku islands are a unique formation. Because Maluku is at the confluence of four world plates (Indo-Australian plate, Pacific plate, Philippine sea plate and Eurasian plate) so it has an interesting geological structure. One of unique formation is having thousands of islands with charming white sand beaches. White sand beaches are part of the geotourism diversity in Maluku, so research is conducted at Latuhalat (Santai and Namalatu beaches) and Suli (Natsepa beach) which aim to identify the shape and size of beach sand, analyze mineral content using the X-RD and X-RF methods. The results showed the shape of sand grains in the Santai and Namalatu beaches of Latuhalat Village were identified as oval-shaped irregularly with an average size of Santai sand grains of 0.12 mm and Namalatu sand of 0.21 mm. While the shape of sand grains in Natsepa beach, Suli Village was identified as spherical with an average grain size of 0.10 mm. Beaches in the three locations are classified as types of sandy beaches (sand sediments) with sandstone rock types. Sand on the Namalatu beach, Santai beach, and Natsepa beach has very fine to medium grain size andcontains Aragonite (CaCO3) and Quartz (SiO2) minerals that are dominant. The shape and size of the grains of sand and its constituent minerals are very good to support the Namalatu and Santai beach geotourism in Latuhalat Village and Natsepa beach in Suli Village.

Upper Plate Stress Controls the Distribution of Mariana Arc Volcanoes

AbstractWe present a spatial analysis of volcano distribution and morphology in the young, intraoceanic Mariana Arc. Both the quality of fit to idealized models and the divergence from those ideals indicate that Mariana Arc volcanoes are arranged into five great circle segments, rather than a single small circle or multiple small circles. The alignment of magmatic centers suggests that magma transport is controlled by the stress regime in the deep crust and/or lithospheric mantle of the Philippine Sea Plate, into which the arc is emplaced, and that arc‐normal tension is the dominant process operating in the deep lithosphere along the whole arc. Volcano morphologies indicate that the stress regime in the shallow crust varies between arc‐normal tension and compression, which also implies that the stress field can vary with depth in the arc lithosphere. We show that this horizontal and vertical stress partitioning can be related to the changing dip of the subducting plate and the breadth of the zone where it is coupled with the overriding plate. The variation in stress regime is consistent with both the distribution of seismicity in the Philippine Sea Plate and with the structural fabrics of the nonvolcanic part of the plate margin to the south. Our analysis suggests that the upper plate exerts the principal control on the distribution of volcanoes in the Mariana Arc. Where tension in the deeper parts of arc lithosphere is sufficiently concentrated, then a distinct volcanic front is produced.

The history of Cenozoic magmatism and collision in NW New Guinea – New insights into the tectonic evolution of the northernmost margin of the Australian Plate

Evidence of Cenozoic magmatism is found along the length of New Guinea. However, the petrogenetic and tectonic setting for this magmatism is poorly understood. This study presents new field, petrographic, U–Pb zircon, and geochemical data from NW New Guinea. These data have been used to identify six units of Cenozoic igneous rocks which record episodes of magmatism during the Oligocene, Miocene, and Pliocene. These episodes occurred in response to the ongoing interaction between the Australian and Philippine Sea plates. During the Eocene, the Australian Plate began to obliquely subduct beneath the Philippine Sea Plate forming the Philippine–Caroline Arc. Magmatism in this arc is recorded in the Dore, Mandi, and Arfak volcanics of NW New Guinea where calc-alkaline and tholeiitic rocks formed within subduction-related fore-arc and extension-related back-arc settings from 32 to 27 Ma. Collision along this plate boundary in the Oligocene–Miocene jammed the subduction zone and caused a reversal in subduction polarity from north-dipping to south-dipping. Following this, subduction of the Philippine Sea Plate beneath the Australian Plate produced magmatism throughout western New Guinea. In NW New Guinea this is recorded by the middle Miocene (18–12 Ma) Moon Volcanics, which include an early period of high-K to shoshonitic igneous activity. These earlier magmatic rocks are associated with the subduction zone polarity reversal and an initially steeply dipping slab. The magmatic products later changed to more calc-alkaline compositions and were emplaced as volcanic rocks in the fore-arc section of a primitive continental arc. Finally, following terminal arc–continent collision in the late Miocene–Pliocene, mantle derived magmas (including the Berangan Andesite) migrated up large strike-slip faults becoming crustally contaminated prior to their eruption during the Plio–Pleistocene. This study of the Cenozoic magmatic history of NW New Guinea provides new data and insights into the tectonic evolution of the northern margin of the Australian Plate.

The Present-Day Crustal 3D Movement and Geodynamic Mechanisms in Hainan Island

The characteristics of the present-day crustal three dimensional (3D) movement with GNSS data during 1999-2018 and the precise leveling data during 1970s-2018 in Hainan Island and its adjacent area are analyzed. Based on the data and the horizontal movement field in the region of Southeast Asia, we discuss how the horizontal velocities in Hainan and the continental margin of the South China block (SCB) are affected by the Eurasian plate, the India-Australia plate and the Philippine Sea plate. The results show the movement of Hainan Island is the same as SCB. The horizontal velocities of the continental margin in South China are different in the east and the west along the boundary of the Northeastern Coast block and Yangtze Block. In Hainan Island, the 3D movement is different in two regions divided by Baisha fault. The movement of SCB is affected by Indo-European collision and extrusion. The movement of the continental margin is affected by both the Pacific Ocean plate, the Philippine Sea plate, the expansion of the South China Sea (SCS), the hot materials underplating and subsequent lithospheric extension. The effect on the regional movement by the Australia plate is smaller, but it can cause co-seismic step to SCB when there had a giant earthquake in Sumatra which is caused by the convergence of the Australia with the Sunda Plate in the NE direction.

The Yuli Belt in Taiwan: Part of the suture zone separating Eurasian and Philippine Sea plates

The Yuli Belt in Taiwan: Part of the suture zone separating Eurasian and Philippine Sea plates

A Major Hydrothermal Reservoir Underneath the Tatun Volcano Group of Taiwan: Clues from a Dense Linear Geophone Array

A dense linear geophone array is deployed across the Tatun volcano group (TVG) at the northern tip of Taiwan, where more than 7 million residents live in the Taipei metropolis. The array is composed of 50 geophones with a station spacing of ~ 200 m in average, and it is designed for striking in the NW–SE direction to record the many earthquakes in eastern Taiwan, where the Philippine Sea plate subducted beneath the Eurasia plate. The detailed examination of felt earthquakes shows consistent P-wave delays are recorded at particular stations of the array. The further forward modeling indicates there is a low-velocity zone (LVZ) at depths between ~ 0.5 and ~ 2.5 km beneath the major fumarole sites. Combining this preliminary result with previous studies including clustering seismicity, volcanic earthquakes, low-resistivity zone, strong degassing processes and shallow velocity structures, we suggest that the LVZ might be associated with the major hydrothermal reservoir at the TVG. The identification of the hydrothermal reservoir by the LVZ not only implies a potential volcanic threat, such as phreatic eruptions, in the future, but also provides the possibility of sustainable geothermal resources for replacing traditional nuclear and fossil fuel power plants. Detailed images of the LVZ and other volcanic structures will be obtained soon when dense geophone arrays with more than 600 geophones are deployed from 2020 to 2022.

Three‐Dimensional Seismic Attenuation Structure of Central Japan and Deep Sources of Arc Magmatism

AbstractWe estimate the 3‐D P wave attenuation (Qp−1) structure of central Japan to investigate arc magmatism. We first determine the corner frequencies of the earthquakes and then conduct a joint inversion to determine attenuation terms. The 3‐D Qp−1 structure is finally obtained via a tomographic inversion. High‐attenuation areas are located in the uppermost mantle along the volcanic front, which are interpreted as zones of partial melting at 30‐ to 100‐km depth due to the supply of slab‐derived fluids. The subducted Philippine Sea plate acts as a barrier to the upward migration of melt, resulting in the volcanic gap. The distribution of volcanoes is controlled primarily by the geometry of the subducted Philippine Sea plate. The attenuation structures beneath Mount Fuji and Mount Hakone show contrasting melt transport pathways, which are separated at ≤50‐km depth, suggesting that the independent melt supply system results in the distinct activities of these volcanoes.

Total Variation Regularization of Geodetically Constrained Block Models in Southwest Taiwan

AbstractRapid convergence between the Eurasian plate and the Philippine Sea Plate makes Taiwan one of the most active tectonic regions in the world, and this strain is accommodated on a complex network of thrust and strike‐slip faulting. Here, we use ascending and descending Interferometric Synthetic Aperture Radar (InSAR) data integrated with continuous GPS measurements to monitor interseismic deformation in southwest Taiwan. Geodetic observations show rapid (20–40 mm/year) southwestward motion in the Pingtung plain in south Taiwan and more than 10 mm/year uplift in the southern part of the Western Foothills. We use combined InSAR and CGPS measurements from 2004 to 2010 to constrain a block model of horizontal interseismic crustal deformation. Total variation regularization (TVR) serves as a tool for block model selection to algorithmically assess the block model geometry based on geodetic observations, simultaneously estimating block rotations and fault slip rates on block‐bounding faults. The block model results suggest that the margins of the Western Foothills accommodate most of the interseismic deformation in southwestern Taiwan. Thirty‐five independent blocks are required to explain interseismic crustal deformation with a mean residual velocity of 3.6 mm/year. Based on this horizontal model, we then forward predict vertical displacement and find good consistency between the predicted uplift and the geodetic observations. Our approach shows an efficient and objective way to estimate fault activities and seismic hazards in the interseismic period based on dense geodetic measurements and their uncertainties.

Fractures and Subsidiary Faults Developed in the Active Strike‐Slip Nojima Fault Zone, Japan, and Tectonic Implications

AbstractThe Nojima Fault Zone, on which the 1995 Mw 6.9 Kobe (Japan) earthquake occurred, is characterized by pulverized cataclastic rocks with numerous fractures and subsidiary faults. The fractures and subsidiary faults were observed in trench walls and using borehole televiewer (BHTV) images from boreholes NFD‐1 (~1,000‐m depth) and AFD‐1 (~700‐m depth), which were drilled through the Nojima Fault and its branch, the Asano Fault, respectively. Measurements show that the orientations of fractures and subsidiary faults in the two boreholes are concentrated at N10–70°E, averaging N40°E. In contrast, the orientations of fractures and subsidiary faults in the walls of a trench across the main surface trace of the Asano Fault are constrained to N0–40°E, averaging N20°E, parallel to the general trend of the Asano Fault but different with that observed in the deep holes at the Ogura site. Structural analysis shows that the fractures and subsidiary faults are mainly concentrated in Riedel Y and R shears of the right‐lateral strike‐slip Nojima Fault and its branch Asano Fault. Our findings demonstrate that the fractures and subsidiary faults within the Nojima Fault Zone were formed by dextral strike‐slip shearing under the current ENE‐WSW to E‐W compressive regional tectonic stress field, related to ongoing subduction of the Pacific and Philippine Sea plates beneath the Japanese Islands.