Philippine Sea Plate Research Articles

Tectonic evolution in the Early to Middle Pleistocene off the east coast of the Boso Peninsula, Japan

Abstract The Kanto Basin developed, starting c. 3 Ma, influenced by the subduction of the Philippine Sea plate and the Pacific plate. Sediments in this basin have influenced the geomorphology of the Kanto region. In the Boso Peninsula, located at the eastern edge of the Kanto Basin, uplift continues in what is called the Kashima–Boso uplift zone. Although the development of this uplift after the Late Pleistocene is well understood, there are few data from the Early to Middle Pleistocene. In this study, we investigated the offshore shelf area east of the Boso Peninsula using a high-resolution seismic reflection survey, and report new information on the geological structure and uplift processes in the area from the Early to Middle Pleistocene. We identified the Kujukuri-oki anticline and the Kujukuri-oki normal fault zone. The Kujukuri-oki anticline, more than 47 km long, is north–south striking and deforms the Kujukuri-oki Group. There are numerous normal faults with displacements of less than tens of metres spread widely in the survey area (Kujukuri-oki normal fault zone). These findings reveal that the Kujukuri-oki anticline uplifted during the end of the Early Pleistocene and attenuated during the Middle Pleistocene. This anticline comprised the axis of the Kashima–Boso uplift zone at the Boso Peninsula from the Early to Middle Pleistocene and the Boso Peninsula is located at the western limb of this anticline.

Sedimentary records from Hengchun accretionary prism turbidites on Taiwan Island: Implication on late Neogene migration rate of the luzon subduction system

Although plate motion models for the Philippine Sea Plate (PSP) are relatively well established, they remain to be confirmed by reliable geological records. Oblique convergence between Eurasia and the PSP has resulted in the uplift of Taiwan Island along the plate boundary. As the latest exposed portion of Taiwan, the Hengchun Peninsula features well-preserved deep-sea turbidites with rounded mafic conglomerate clasts. We apply an integrated source-to-sink analysis and broad field investigations to generate a reliable kinematic reconstruction linking the Hengchun turbidite outcrops with South China Sea (SCS) borehole sediments. Deposited in the lowermost turbidite layer, the Hengchun rounded gabbro and basalt clasts resemble the SCS oceanic crust based on our U–Pb geochronological (~23.6–25.4 Ma) and elemental geochemical results (normal mid-ocean ridge basalts), along with previous magnetic anomaly constraints. The multimodal U–Pb spectra of the matrix-supported sandstones suggest a mixed provenance including central Vietnam, the Pearl River tributaries and potential intrabasinal uplifts, which have not received prior consideration. During the late Miocene, abundant terrigenous sediments from the eastward-flowing “Kontum-Ying-Qiong River” and Pearl River were transported into the submarine environment. The extensive turbidity currents scoured and eroded the SCS mid-ocean ridges, which feature numerous low-angle detachment faults. These materials of various sources were transported eastwards for thousands of kilometers prior to their incorporation into the accretionary prism adjacent to the Manila Trench. From the late Miocene to Pleistocene, the PSP moved northwestward at estimated velocities of ~61–65 mm/yr, facilitating the final uplift and exposure of the accretionary prism.

Crustal Stress State in Taiwan: Moderately Strong Crust Supporting Gravitational and Flexural Loading

AbstractIn Taiwan and other contractional tectonic wedges, there is evidence that the basal detachment is very weak and the overlying crust is relatively strong. In this study, we compute the absolute deviatoric 3‐D stress field in Taiwan using focal mechanism data and coseismic stress changes for the 1999 M7.6 Chi‐Chi earthquake. We estimate a minimum coefficient of friction of 0.3 in the upper 15 km crust, assuming hydrostatic pore pressure, consistent with a moderately strong crust. The estimated stress state is consistent with primary tectonic structures of Taiwan. The maximum compressive stress in the fold and thrust belt and the Coastal Range is approximately aligned with relative motion of the Eurasian and the Philippine Sea Plates, but the minimum and intermediate principal stresses vary laterally and with depth. A reverse‐faulting stress regime wraps around the Peikang high in western Taiwan suggesting inherited basement structure influences crustal stress state. Vertical maximum compressive stress is inferred under the high topography of the Central Range mountains. We present a simple stress model for Taiwan assuming spatial variations in gravitational potential energy are supported elastically. The model stress state is broadly consistent with the stress inversion result. The model shows that vertical maximum compressive stress in the Central Range is a result of high topography and thick crustal root. Consistent with a moderately strong crust, the model deviatoric stresses in areas of high seismicity in Taiwan are of the order 50–100 MPa. Lower deviatoric stresses under the Central Range are consistent with low rates of observed seismicity.

Coseismic surface rupture during the 2018 Mw 7.5 Palu earthquake, Sulawesi Island, Indonesia

Abstract Sulawesi Island is located at the triple junction between the converging Australian, Sunda, and Philippine plates. The magnitude (Mw) 7.5 Palu earthquake occurred on 28 September 2018 on Sulawesi Island and caused serious casualties. The causative fault of the Palu earthquake was the left-lateral, strike-slip Palu-Koro fault, which has a rapid slip rate. We experienced this earthquake in Palu City and conducted field investigations on coseismic surface ruptures 1 d after the earthquake. Field surveys revealed that the coseismic surface ruptures were characterized by left-lateral offset, en echelon tensional cracks, mole tracks within a narrow zone, and large areas of sand liquefaction that increased the damage and losses. We measured the coseismic displacements along surface ruptures and observed a maximum coseismic offset of ∼6.2 m. The rupture traces in the north Palu Basin near Palu City mark the previously unmapped Palu-Koro fault. Based on the field investigations, we determined the exact location of the Palu-Koro fault within the Palu Basin and found that the Palu-Koro fault zone can be divided into three branches: F1, F2, and F3, forming a typical flower structure.

In Situ Geochemical Compositions of the Minerals in Basaltic Rocks from the West Philippine Basin: Constraints on Source Lithology and Magmatic Processes

Abstract Since the early Cenozoic, the West Philippine Basin (WPB) and the whole Philippine Sea Plate (PSP) has undergone a complex geological evolution. In this study, we presented K-Ar ages, in situ trace element, and major element compositions of minerals of basalts collected from the Benham Rise and the Central Basin Fault (CBF) in the WPB, to constrain their magmatic process and regional geological evolution. Olivine phenocrysts and microlites in the alkali basalts (20.9 Ma) from the Benham Rise have forsterite (Fo) contents of 56.90%–76.10% and 53.13%-66.41%, respectively. The clinopyroxenes in the tholeiites (29.1 Ma) from the CBF is predominantly diopside and augite, and it is depleted in light rare earth elements (LREEs) (LaN/YbN=0.13–3.40) and large-ion lithophile elements (LILEs). The plagioclases in the basalts from both of the Benham Rise and the CBF are predominantly labradorite and andesine, with a minor amount of bytownite, and it is enriched in LREEs, Ba, Sr, and Pb and exhibits strong positive Eu anomalies. However, there exist obvious differences in plagioclase compositions between these two tectonic sites. The source lithology of the Benham Rise basaltic rocks could be garnet pyroxenite, and yet that of the CBF could be spinel-lherzolite. The calculated mantle potential temperature beneath the Benham Rise is 1439°C–1473°C, which is significantly higher than that beneath the CBF (1345°C–1381°C), suggesting there existed thermal anomaly beneath the Rise during basaltic magmatism. This study also calculated the temperature and pressure of the clinopyroxenes and plagioclases, which have been used to indicate magmatic processes. Finally, we suggest that the Benham Rise basaltic rocks may be related to a mantle plume (e.g., the Oki-Daito mantle plume), and the CBF was once located in a back-arc spreading center behind an active subduction zone. The extinction of the Oki-Daito mantle plume activity might be at about 20.9 Ma, and cessation of the back-arc spreading of WPB was at about 29.1 Ma or younger.

Feedback of Slab Distortion on Volcanic Arc Evolution: Geochemical Perspective From Late Cenozoic Volcanism in SW Japan

AbstractSouthwest Japan is an island arc formed by subduction of the Philippine Sea (PHS) plate. The Quaternary magmatism in this region is characterized by eruptions of high‐Sr andesites and dacites, considered to have been derived by melting of the PHS plate. The loci of these volcanoes spatially coincide with seismic discontinuities of the subducted PHS plate. Thus, the magmatism is interpreted as the result of slab melting at the plate tears. However, the processes that promote slab tearing remain unclear. In this study, we applied geochronological and geochemical analyses to late Cenozoic volcanic rocks in southwest Japan as tracers of slab morphology. Two different magma types, ocean‐island basalt (OIB) and island‐arc basalt (IAB), have occurred over 12 million years (Myr). These two magmas are attributed to different integrations of melts extracted from an originally fertile mantle; the OIBs from high temperature melt (1,300–1,400°C) were extracted at a depth of 40–80 km, whereas the IABs were extracted from a shallower, lower temperature region (30–60 km, 1,200–1,350°C). Secular change in Sr enrichment of IAB likely arose due to a transition of slab‐derived fluids, incorporated into magmas as they formed, from water‐ to melt‐dominant one. Progressive shallowing of the subducted PHS plate is responsible for secular change in the properties of slab‐derived fluids as well as rollback of OIB volcanoes. Production of chemically variable magmas in the Chugoku district is the surface expression of distorting slab morphology by interaction between mantle and the subducting plate.

Geological characteristics of the Nankai Trough subduction zone and their tectonic significances

The Nankai Trough subduction zone is a typical subduction system characterized by subduction of multiple geological units of the Philippine Sea Plate (the Kyushu-Palau Ridge, the Shikoku Basin, the Kinan Seamount Chain, and the Izu-Bonin Arc) beneath the Eurasian Plate in the southwest of Japan. This study presents a geophysical and geochemical analysis of the Nankai Trough subduction zone in order to determine the features and subduction effects of each geological unit. The results show that the Nankai Trough is characterized by low-gravity anomalies (−20 mGal to −40 mGal) and high heat flow (60–200 mW/m2) in the middle part and low heat flow (20–80 mW/m2) in the western and eastern parts. The crust of the subducting plate is 5–20 km thick. The mantle composition of the subducting plate is progressively depleted from west to east. Subduction of aseismic ridges (e.g., the Kyushu-Palau Ridge, the Kinan Seamount Chain, and the Zenisu Ridge) is a common process that leads to a series of subduction effects at the Nankai Trough. Firstly, aseismic ridge or seamount chain subduction may deform the overriding plate, resulting in irregular concave topography along the front edge of the accretionary wedge. Secondly, it may have served as a seismic barrier inhibiting rupture propagation in the 1944 Mw 8.1 and 1946 Mw 8.3 earthquakes. In addition, subduction of the Kyushu-Palau Ridge and hot and young Shikoku Basin lithosphere may induce slab melting, resulting in adakitic magmatism and the provision of ore-forming metals for the formation of porphyry copper and gold deposits in the overriding Japan Arc. Based on comparisons of their geophysical and geochemical characteristics, we suggest that, although the Izu-Bonin Arc has already collided with the Japan Arc, the Kyushu-Palau Ridge, which represents a remnant arc of the Izu-Bonin Arc, is still at the subduction stage characterized by a single-vergence system and a topographic boundary with the Japan Arc.

Earthquake triggering model based on normal-stress-dependent Nagata law: application to the 2016 Mie offshore earthquake

We propose a normal-stress-dependent Nagata law. Nagata et al. (J Geophys Res 117:B02314, 2012) revised the rate- and state-dependent friction law by introducing the shear stress dependence. We further extended the Nagata law by incorporating the normal stress dependence obtained by Linker and Dieterich (J Geophys Res 97:4923–4940, 1992). We performed numerical simulations of earthquake triggering by assuming the extended Nagata law. In the case of repeated earthquakes, we applied dynamic Coulomb failure function (CFF) perturbation due to normal or shear stress changes. CFF perturbation increased the slip velocity after the cessation of perturbation, relative to that of the repeated events without triggering. This leads to dynamic earthquake triggering for certain perturbation amplitudes with time to instability of 0 to several tens of days. In addition, triggering potential of the static CFF jump (ΔCFFs) was investigated. Static stress perturbation has a higher triggering potential than dynamic stress perturbation for the same magnitude of CFF. The equivalent ΔCFFeq is evaluated for dynamic perturbation that results in a triggering potential approximately the same as in the case of static stress perturbation if ΔCFFs = ΔCFFeq. We calculated ΔCFFeq on the interface of the Philippine Sea plate for the Mie offshore earthquake, which occurred around the Nankai Trough on April 1, 2016, using OpenSWPC. The results shows that ΔCFFeq is large around the trough, where slow slip events followed the Mie earthquake, suggesting that a large ΔCFFeq may have triggered slow slip events.

Meso‐Cenozoic rheological structure of the Linqing Depression: Implications for cratonic destruction

The evolution of the lithospheric rheological structure can reveal geodynamic processes of cratonic destruction. Using data of palaeo‐heat flow, crustal structure, and rheological parameters, we studied the rheological structure of the Linqing Depression. The results show that the lithosphere of the Linqing Depression experienced two stages of weakening, one in the Cretaceous and the other in the Palaeogene, with lithospheric strengths of approximately 2.09 and 1.67 × 1012 N/m and effective elastic thicknesses (Te) of approximately 24 and 20 km, respectively. The Meso‐Cenozoic rheological structures correspond to the ‘jelly sandwich’, ‘crème brûlée’, and ‘banana split’ models. The transformation of these models is closely related to the thermal regime, stress environment, and mantle water content of the Linqing Depression. The rheological structure evolution of the Linqing Depression is correlated with the cratonic destruction of the North China Craton. The geodynamic mechanism of lithospheric weakening in the late Early Cretaceous is related to the subduction and retreat of the Pacific Plate, and the lithospheric weakening in the late Early Palaeogene was a combined effect of the Indo‐Asian collision and subductions of the Pacific and Philippine Sea plates.

Stepwise accelerations in the rate of sea-level rise in the area north of Tokyo Bay during the Early Holocene

Tokyo Bay is situated near the triple junction of the North America, Pacific, and Philippine Sea plates, where relative sea-level changes since the Last Glacial Maximum (LGM) have been strongly affected by subduction tectonics. In this study, 55 sea-level index points were extracted from the post-LGM incised valley fills beneath the lowlands north of Tokyo Bay and used to construct a detailed relative sea-level curve for the Early Holocene. Sea-level jumps were detected at 10.1–10.0, 9.2–9.1, and 8.9–8.7 ka, each of 2–7 m and each followed by an acceleration in the rate of sea-level rise. These paired phenomena are attributed to coseismic subsidence and an interseismic decrease in the rate of uplift, respectively. The Early Holocene sea-level changes in Tokyo Bay are characterized by remarkably short sea-level jumps of <100 years and accelerations in the rates of sea-level rise with relatively long durations of 200–800 years. After separating these tectonic effects from relative sea-level changes, sea-level jumps that have been hypothesized to be eustatic events, such as Meltwater Pulses 1C and 1D and the pre-8.2-ka event, were not identified based on a sea-level detection limit of 3 m, but a decrease in the rate of sea-level rise at 8.2 ka, which can be regarded as a global phenomenon, was verified in Tokyo Bay.

Contrasting volcano spacing along SW Japan arc caused by difference in age of subducting lithosphere

The SW Japan arc built by subduction of the Philippine Sea (PHS) plate exhibits uneven distribution of volcanoes: thirteen Quaternary composite volcanoes form in the western half of this arc, Kyushu Island, while only two in the eastern half, Chugoku district. Reconstruction of the PHS plate back to 14 Ma, together with examinations based on thermal structure models constrained by high-density heat flow data and a petrological model for dehydration reactions suggest that fluids are discharged actively at depths of 90–100 km in the hydrous layer at the top of the old (> 50 Ma), hence, cold lithosphere sinking beneath Kyushu Island. In contrast, the young (15–25 Ma) oceanic crust downgoing beneath Chugoku district releases fluids largely at shallower depths, i.e. beneath the non-volcanic forearc, to cause characteristic tectonic tremors and low-frequency earthquakes (LFEs) and be the source of specific brine springs. Much larger amounts of fluids supplied to the magma source region in the western SW Japan arc could build more densely-distributed volcanoes.

Seismological evidence of a dehydration reaction in the subducting oceanic crust beneath western Shikoku in southwest Japan

SUMMARYThe Philippine Sea plate (PHS) is subducting beneath southwest Japan from the Nankai Trough. In this region, deep low-frequency earthquakes/tremors (LFEs) are active and their epicentres are distributed along the downdip of the source region of M8-class earthquakes that have occurred every 100–150 yr. The LFE activity may potentially be strongly related to the occurrence of great earthquakes between tectonic plates. To investigate the structural features around the LFEs, we applied teleseismic receiver function (RF) analysis to the seismograms observed at permanent and temporal seismograph stations in western Shikoku and we detected seismological evidence of a slab dehydration reaction linked to LFE activity. Based on the first-order harmonic decomposition of RFs, we first estimated the average plunge azimuth of the PHS beneath western Shikoku. Considering the backazimuth dependence of converted phase amplitude, we constructed the cross-section of the radial component RFs, excluding the incoming waveform data from the updip directions of the dipping slab. In this profile, the parallel negative and positive P-to-S converted phase alignment within a distance of 10 km were imaged. These phase alignments corresponded to the top of the slab and the slab Moho discontinuity, respectively. At the northern side of the profile, the landward (continental) Moho was also detected. In the region where LFEs were actively distributed, both the landward Moho and slab surface were unclear. The second-order harmonic decomposition of RFs for several kilometres above the slab Moho indicated that the anisotropic symmetry axes suddenly changed at the southern limit of the LFE active region; the fast axes in the region were normal to the trench though axes in the southern area were parallel. According to the thermal and pressure condition, a phase transition with a dehydration reaction can occur in the oceanic crust near the southern edge of the LFE active region. Once the dehydration process advances, released water causes the serpentinization of the mantle material near the slab surface and the corresponding seismic velocity decreases. The impedance contrast decreases at the boundary between the lower crust and the mantle wedge, as well as that between the mantle wedge and oceanic crust; therefore, the amplitudes of the converted phases at these boundaries also become small. Considering that serpentinite ordinarily has strong anisotropy with a seismic fast axis direct to the shear direction, all features observed are evidence of the dehydration process in the flat slab.

Tsunamis and submarine landslides in Suruga Bay, central Japan, caused by Nankai–Suruga Trough megathrust earthquakes during the last 5000 years

Magnitude 8 megathrust earthquakes that have generated large tsunamis with wave heights of >5 m in coastal areas of southwestern Japan have occurred every 100–150 years along the Nankai–Suruga Trough, which marks the convergent boundary between the Eurasian and Philippine Sea plates. Using sedimentological, geochemical, and paleontological analyses, as well as 14C dating, we examined tsunami deposits in 12 sediment cores collected from the Hamatome lowland, Yaizu Plain, Shizuoka, which faces the Suruga Trough. Four sandy tsunami deposits (Sands 1–4) are identified in sediments deposited during the last 5000 years. Sand 1 was deposited between 805 and 405 BC, suggesting that only one tsunami left a sedimentological imprint in the Hamatome lowland lagoon during the period of ∼4000 years between 3090 and 2913 BC and the AD 1096 Eicho-Tokai earthquake. This very low frequency of tsunami inundation may suggest that this tsunami was larger than those typically associated with megathrust earthquakes. Sands 2–4 are correlated with tsunamis associated with the Eicho-Tokai, AD 1361 Shohei-Koan, and AD 1498 Meio-Tokai earthquakes, respectively. Our data indicate that a sand spit, which had previously protected the lagoon from oceanic waves, was abruptly destroyed following the AD 1096 Eicho-Tokai earthquake and did not re-form at the same location. This disappearance of the sand spit is inferred to have involved an earthquake-induced submarine landslide. The data presented in this paper, together with information obtained from historical documents, indicates that M8 megathrust earthquakes in AD 1096 and AD 1498 caused submarine landslides in the study area, resulting in coastal subsidence and damage to coastal areas additional to that caused by the tsunamis.

A New Ground-Motion Prediction Equation of Japanese Instrumental Seismic Intensities Reflecting Source Type Characteristics in Japan

ABSTRACT A new equation for predicting Japanese instrumental seismic intensities at arbitrary surface sites in Japan for Mw 5.4–8.7 and distances ranging from 10 to 1000 km was derived from approximately 30,000 observed intensities for various types of earthquakes. The equation incorporates the differences in the subsurface characteristics immediately beneath each site using VS30. The equation can also predict the abnormal intensities (which are indispensable in Japan) due to subducting slabs using the depth of the slab surface beneath each site from the Crustal Activity Modeling Program standard plate model. The prediction equation can be applied to five source types: Pacific Ocean plate (PAC) interplate, PAC intraplate, very shallow crustal, shallow (≤50 km) Philippine Sea plate (PHS) intraplate, and intermediate-depth (&amp;gt;50 km) PHS intraplate earthquakes. Although the equation is applicable at various magnitudes and distances, the standard deviations (σ) are 0.5–0.6, which are smaller than those of other equations with narrower distance ranges. Smaller σ values were achieved by the inversion of 29,837 Japanese instrumental seismic intensities from 68 selected earthquakes of five source types with a common site effect at each station. A deep Mw 7.9 earthquake that occurred at a depth of 680 km in 2015 near the Ogasawara Islands and subjected all of Japan to long-duration shaking due to waves propagating through the mantle was effectively employed to constrain the VS30 term of the equation. The equations for PAC interplate and very shallow earthquakes were validated by seven earthquakes that were not used for the inversion; the standard deviations for these earthquakes fell in the range of 0.41–0.53. The formula for very shallow crustal earthquakes is also able to predict the intensities of PHS interplate earthquakes. Hence, this equation is useful not only for engineering applications but also for historical seismology to distinguish the source types of ancient earthquakes.

Subduction-collisional processes between the Eurasian and Philippine Sea plates: Constraints from thermal-age paths of the Taiwan Orogen

Records of the thermal-age path of orogenic rocks are crucial in understanding subduction and collisional processes between a continent and an arc. Tectonic events and thermochronological data of the Taiwan Orogen have been extensively investigated. We use the zircon UPb youngest ages as the (initial) subduction age, the Raman Spectroscopy of Carbonaceous Material, 40Ar/39Ar age of mica porphyroblasts, whole-rock LuHf isochron age and thermal results as prograde peak metamorphic condition during subduction and basal accretion, and the apatite fission-track data for cooling age of the Taiwan Orogen to illuminate the subduction and collisional processes between the Eurasian Plate and the Luzon Arc. The Hsuehshan Range Nappe in the west of the Taiwan Orogen, whose subduction was initiated at 12 Ma, reached peak metamorphism with a temperature of ~250 °C to 490 °C at a depth of 8 km during the interval 6–2.5 Ma, mainly by basal superimposed accretion. The isothermal dwelling during peak metamorphism condition of the Hsuehshan Range suggests that tectonic underplating through basal superimposed accretion contribute significantly to the formation of the Taiwan Orogen, and led to a sudden increase of subduction and décollement depths, and slow downward thickening of the orogenic wedge and mountain root growth. Located in the east of the Taiwan Orogen, the Yuli Beltsubduction was initiated at 15.4–16 Ma, and attained peak metamorphism at a depth of 40 km, with a temperature of 550–560 °C and a pressure of 10.5–13 kbar during the interval 12.2–4.4 Ma due to basal superimposed accretion. The exhumation of the Hsuehshan Range commenced at ~2.5 Ma, which is later than that of the Yuli Belt at ~3.4 Ma. Such distinct and different thermal-age paths of the Hsuehshan Range and the Tananao Complex indicate that different tectonic wedges in the Taiwan Orogen are formed through different subduction and accretional processes.

Active deformation of Sagami Bay triggered by approach of the Izu island arc

Sagami Bay in central Japan is known as the seismogenic and tsunamigenic zone of the 1703 and 1923 Kanto earthquakes that caused devastating damage in the Tokyo metropolitan area. However, the tsunamigenic structure within the frontal wedges of the subduction zone remains unresolved after the last disastrous events. In the present study, the detailed structural features of the shallowest structure of the Sagami Tectonic Line in Sagami Bay were revealed by seismic reflection, a deep-towed high-resolution sub-bottom profiler, and swath bathymetry data. Based on our data, the middle segments of the Sagami Tectonic Line, which is distributed along the southwestern rim of the Miura Knoll, are considered highly likely to be an active fault that has been a repeatedly active feature. The trough-fill sediments of the south Sagami Bay are currently thought to be actively deformed by the compressive stress accompanying northwestward subduction of the tip of Izu-Ogasawara (Bonin) Arc crust on the Philippine Sea plate.

Upper Mantle Shear Wave Velocity Structure of Southeastern China: Seismic Evidence for Magma Activities in the Late Mesozoic to the Cenozoic

AbstractThe Fujian‐Taiwan Strait‐Taiwan region, as the active eastern margin of Eurasia since the Mesozoic, has experienced the subduction and retreat of the Paleo‐Pacific plate from the Middle Mesozoic, the breakup of the continental margin in the early Cenozoic, and then the collision with the Philippine Sea plate in the late Cenozoic. To better understand the tectonic evolution process, a high‐resolution 3‐D shear wave velocity model of the crust and upper mantle was obtained from surface wave tomography through a direct inversion method. Intermediate‐ to long‐period Rayleigh wave dispersion curves, extracted from teleseismic surface wave with the traditional two‐station method, were combined with the short‐period dispersion curves obtained from ambient noise cross correlation to obtain broader band surface wave data. Two groups of large‐scale low‐velocity bodies are found to be located in the upper mantle of Fujian and the Taiwan Strait and are inferred to be closely related to the magmatic activities caused by the Mesozoic subduction and Cenozoic extension, respectively. The low‐velocity anomalies in Fujian are concentrated in the west Cathaysia block in deep regions and split into small bodies in the uppermost mantle, while the low‐velocity body in the strait is divided into northern and southern parts, corresponding to the two‐stage Cenozoic extension. The velocity gradient was also calculated to estimate the lithospheric thickness, which is only 60–80 km, and the lithosphere‐asthenosphere boundary also features strongly horizontal variations and discontinuities. Our results provide important seismological constraints to investigate the geological and tectonic processes of this region.

Geological mapping of coastal and offshore Japan (by GSJ-AIST): collecting and utilizing the geological information

Abstract Devastating earthquakes and tsunamis capable of causing catastrophic damage to human societies and economies have frequently occurred on and around the Japanese islands. Because Japan is a long and narrow island arc in the Pacific Ocean located at the junction of four plates (the Eurasian and Philippine Sea Plates in the SW and the North America and Pacific Plates in the NE), it has probably suffered the highest number of earthquake and tsunami events anywhere in the world. Hence, geological and geoinformation investigations are supremely important, not only for understanding the geological development of Japan but also for forecasting the risks associated with geohazards and securing the safety of human lives and infrastructures. For these reasons, the Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (GSJ-AIST) has been conducting marine geological surveys since 1974 and, as of the end of 2019, produced a Marine Geology Map Series consisting of 90 geological maps. In addition, a coastal zone research project that aims to connect marine, coastal and land area geoinformation seamlessly was launched in 2008 to survey areas that have not yet been investigated, and six maps of geologically distinct areas have already been published based on that collected data.

Mapping of PGA Value Using PSA Method in West Halmahera Nort Maluku

The earthquake that occurred in the West Halmahera region was very detrimental, even though the human casualties were not very significant. But it will affect the stability and capacity of a region in terms of regional development. The mapping of earthquake-prone areas is carried out by a probabilistic seismic hazard analysis (PSHA) method to analyze soil movement parameters, namely Peak Ground Acceleration so that it can determine earthquake-prone areas in West Halmahera. The results of seismic hazard analysis show that the West Halmahera area is an area that is relatively prone to earthquake hazards because it is still strongly influenced by subduction (megathrust) earthquakes from the Philippine plate, Maluku sea and Sangihe. This is indicated by the value of earthquake acceleration on the Peak Ground Acceleration for the 500 year return period of around 0.38 - 3.69 g and 0.30 - 3.69 g for the 2500 year return period.

Stress loading and the occurrence of normal-type earthquakes under Boso Peninsula, Japan

Boso Peninsula, Japan, was formed by the interaction of the Philippine Sea, Eurasian and Pacific plates around the trench–trench–trench Boso triple junction. Normal-type earthquakes are persistently observed in the subducting Philippine Sea slab under the peninsula at a depth of ~ 30 km, including a recent (2019) Mw 4.9 earthquake which caused shaking throughout the Kanto region (greater Tokyo). Such shallow intraplate earthquakes are potentially hazardous to this heavily populated region, yet their mechanism is poorly understood, especially in the context of a three-plate system. Here, we calculate stress rates in the Philippine Sea slab and the surrounding area, using a subduction model constructed in a previous study, to explain the generation of the regional stress field and its effect on earthquake occurrence. In general, the calculated stress rates under Boso Peninsula are horizontally extensional both above and below the Eurasian–Philippine Sea plate interface. We apply our calculated stress rates to the nodal planes of the observed earthquakes to calculate the Coulomb failure function (ΔCFF). These calculated ΔCFFs are generally positive on normal-type earthquakes under Boso. The ΔCFFs are also consistent with earthquakes in adjacent areas that are seismically active, for example, in the Philippine Sea plate to the south, in the collision zone around Izu Peninsula, and in the cluster in the Eurasian plate northeast of Boso Peninsula, which further supports our stress loading model. Calculation of the individual contributions of Philippine Sea plate and Pacific plate subduction shows that the development of the stress field around Boso is dependent upon contributions from both subducting plates. In contrast, the arc–arc collision at Izu Peninsula has little influence.