Philippine Sea Research Articles

Low-temperature thermochronology of Ryoke belt granitoids, SW Japan: New insights into the recent cooling history from monazite fission-track dating

Traditional low-temperature thermochronology techniques such as apatite fission track (AFT) and apatite (U-Th-Sm)/He (AHe) have so far produced limited information on the neotectonic (late Miocene and younger) evolution of Japan. However, the development of monazite fission track (MFT) dating, which has an ultra-low temperature sensitivity (<∼25 – 46°C), provides a fresh opportunity to directly analyse recent denudation histories. Low-temperature thermochronology methods have been applied to granitoid samples from the Ryoke belt, located in eastern Yamaguchi and Nara Prefectures, SW Japan. Zircon (U-Th)/He (ZHe), AFT and AHe data and modelled thermal histories reveal Late Cretaceous - Pliocene cooling related to granitoid thermal relaxation combined with paleo-Izanagi and Pacific plate subduction along the eastern Eurasian continental margin. MFT dating reveals Plio – Pleistocene cooling ages, coincident with elastic loading caused by Philippine Sea plate subduction since the Middle - Late Miocene, along with the Quaternary collision of NE and SW Japan at the Itoigawa-Shizuoka Tectonic Line. Silica concentration, non-thermal “radiation assisted” annealing, and changing climatic conditions are proposed as possible caveats on MFT annealing kinetics since the samples are residing in an “open” system. Taking all these factors into consideration, the apparent MFT ages are interpreted as minimum cooling ages, being slightly younger than the thermal events that cooled the samples through the MFT nominal closure temperature. Estimated denudation rates based on MFT dating are higher in Nara relative to eastern Yamaguchi Prefecture, with the difference likely reflecting variations in the tectonic regime, timing of uplift and uplift mechanisms of the two regions.

Variability of the Deep Western Boundary Current in the Northern Philippine Sea

Abstract The deep western boundary current (DWBC) was studied based on a full-depth mooring east of Luzon Island in the Northern Philippine Sea deep basin during the period from January 2018 to May 2020. On average, the DWBC in the Philippine Sea flows southward with a velocity of approximately 1.18 cm s−1 at a depth of 3050 m. Significant intraseasonal and seasonal variations of the DWBC are identified. The intraseasonal variations have multiple spectral peaks in the range of 30–200 days, with the most obvious peak at approximately 120 days. On the seasonal time scale, the DWBC intensifies in summer/autumn and weakens in winter/spring, corresponding well with the seasonal variation of the ocean bottom pressure (OBP) from the Gravity Recovery and Climate Experiment. Both intraseasonal and seasonal variations have no significant correlation with the temporal variations in the upper and middle layers but have a certain correlation with transport through the Yap–Mariana Junction (YMJ). A set of experiments based on an inverted-reduced-gravity model and the OBP data reveal that the temporal variations originating from the YMJ could propagate counterclockwise along the boundary of the deep basin to the western boundary of the deep Philippine Sea, dominating the temporal variations of DWBC.

2D and 3D modelling of Molucca Sea double subduction zone based on primary waves travel time seismic tomography inversion

Molucca Sea is one of the areas with a high level of seismicity in Indonesia due to its location between three plates. The activities of them forming a quite unique subduction zone. The Eurasian Plate (Sangihe Microplate) move to the East and The Philippine Sea Plate (Halmahera Microplate) continues to Westward caused The Molucca Sea Microplate to be pressed, and subducts in two opposite directions modelat the same time forming an inverted “U”. This research used 3663 hypocenters and 9 BMKG stations in the last 5 years (2016 - 2021) with a magnitude of 3 - 8 Mw and a depth of 3 - 630 kilometers. Seismic inversion of travel time tomography is used to produce 2D models of the subduction zone around The Molucca Sea. The distribution of high ΔVp values is ranging from 3.5% to 4% is associated with a solid medium, like The Sangihe Plate, Halmahera Plate, and The Molucca Sea Plate subduction slab. While the low ΔVp is ranging from 1.8% to 2% is associated with the destruction zone and the presence of thermal fluids such as magma or partial melting (the presence of volcanoes). The 3D model is based on the Gaussian Process Regression principle using magnitude and depth data as initial model parameters. Plotting of the thrust faults at the front of the two arcs, namely The West Sangihe Thrust Fault and The East Halmahera Thrust Fault. It can be seen that the subduction under The Sangihe Microplate is deeper (627.2 kilometers) has the average subduction angle of 45.3° compared to the subduction under The Halmahera Microplate (280.0 kilometers) has the average subduction angle of 35.8°

Recurrence and Long-Term Evaluation of Kanto Earthquakes

ABSTRACT The 1923 Taisho Kanto earthquake—an interplate event along the Sagami trough where the Philippine Sea plate subducts beneath the Kanto region—produced ground shaking, crustal deformation, landslides, and tsunamis, and caused the worst earthquake disaster in Japan. Based on seismological and geodetic data, many fault models have been proposed, extending ∼100 km from the epicenter, with a moment magnitude (Mw) of 7.8–8.2, and large slips of ∼8 m located near the epicenter and beneath the Miura Peninsula. The penultimate 1703 Genroku Kanto earthquake produced similar macroseismic effects around Sagami Bay and the Miura Peninsula, but larger coastal uplift and tsunami in the Boso Peninsula. The proposed fault models extend off the Boso Peninsula with Mw of 8.1–8.5. In 2004, the Earthquake Research Committee (ERC) classified the Kanto earthquakes as “Taisho type” and “Genroku type” with recurrence intervals of 200–400 yr and 2300 yr, respectively. In 2014, the ERC revised the long-term evaluation to a recurrence interval of 180–590 yr and a 30 yr probability of 0%–5% based on the Brownian passage time model. With the Cabinet Office, the ERC considered the source area of the maximum possible earthquake of Mw 8.6–8.7. The recent historiographical and paleoseismological studies have identified other candidates for the past Kanto earthquakes in 1495, 1433, 1293, and 878. Various combinations of these candidates give a mean recurrence interval of 210–315 yr, an aperiodicity parameter of 0.04–0.76, and a 30 yr probability of 0.0%–19%. The Cabinet Office has calculated the seismic intensity and tsunami heights of various types of Kanto earthquakes. National and local governments estimate the damage from these hazards. For the Tokyo metropolitan area, the estimated damage and occurrence probability are more significant for M ∼7 earthquakes with various types and depths, and most mitigation efforts are directed at such events.

Temporal Variations in QP−1 ${Q}_{P}^{-1}$ and QS−1 ${Q}_{S}^{-1}$ Above a Megathrust Following Episodic Slow‐Slip Events

AbstractRecent observations beneath central Japan have shown that periodic fluid drainage occurs during slow‐slip events (SSEs) based on temporal variations in above the megathrust boundary of the subducting Philippine Sea slab. However, no previous studies have estimated associated . A comparison of and can provide clues to the mechanism of seismic attenuation because of different propagation characteristics of the two waves. We estimate temporal variations in and via spectral analyses of waveform data from November 2009 to August 2021 period. The results indicate that both and exhibit temporal variations at about 1‐year periodicity and there are systematic differences between and that have smaller values, less insignificant variation, and weaker correlations with SSEs. Furthermore, increases concurrent with SSEs. These differences suggest that attenuation is caused by the wave‐induced fluid flow. could be an important parameter for detecting the presence of fluid.

Miocene lamprophyre and felsic volcanics in the outer zone of SW Japan: arc building through multiple sources in an active convergent margin

ABSTRACT Subduction-related magmatism is a significant contributor to arc building and continental growth. In the active convergent margin of the SW Japan arc, voluminous volcanic rocks and mafic dykes formed through subduction and back-arc rifting during Miocene. In this study, we investigate a suite of felsic volcanic rocks (rhyolite and dacite) and lamprophyres from central Shikoku Island, which occur in the north-central part of the Chichibu belt and Sanbagawa belt in the Outer zone of the SW Japan arc. Whole-rock geochemistry results show that the rhyolite and dacite samples are peraluminous, belong to the high-K calc-alkaline series, show LREE and LILE enrichment, HREE and HFSE depletion that suggest arc volcanic affinity. The lamprophyre samples belong to alkali lamprophyre, show LREE and HFSE enrichments, and display OIB (oceanic island basalt)-like geochemical features. The 206Pb/238U weighted mean ages of the zircons from the rhyolite and dacite samples range from 14.39 ± 0.21 Ma to 13.92 ± 0.30 Ma, whereas the lamprophyre shows 206Pb/238U weighted mean age of 12.21 ± 0.35 Ma. Zircon Lu-Hf isotope data suggest that the rhyolite and dacite were derived from the remelting of the arc crustal rocks and that the lamprophyre was derived from depleted mantle sources and was contaminated by crustal materials. The felsic volcanic rocks (rhyolite and dacite) can be correlated to Miocene (15.5 Ma to 13.5 Ma) magmatism in the Outer Zone of the SW Japan arc. The formation of lamprophyre dykes in the SW Japan arc may be related to the subhorizontal mantle flows beneath the SW Japan arc associated with the opening of the Japan Sea back-arc basin or asthenosphere upwelling through the slab-tear of the Philippine Sea Plate beneath the SW Japan arc. Our study shows that the SW Japan Arc was built by multiple magmatism from diverse sources in an active convergent margin.

Geophysical investigation of the Mado Megamullion oceanic core complex: implications for the end of back-arc spreading

Detachment faulting is one of the main styles of seafloor spreading at slow to intermediate mid-ocean ridges. However, we have limited insight into its role in back-arc basin formation. We surveyed a remnant back-arc spreading center in the Philippine Sea and determined the detailed features and formation processes of the Mado Megamullion (Mado MM) oceanic core complex (OCC). This was undertaken in the context of back-arc evolution, based on the shipborne bathymetry, magnetics, and gravity with radiometric age dating of the rock samples collected. The Mado MM OCC has a typical OCC morphology with prominent corrugations on the domed surface and positive gravity anomalies, suggesting that there has been an exposure of the lower crust and mantle materials by a detachment fault. The downdip side of the detachment continues to the relict axial rift valley, which has indicated that the Mado MM OCC was formed at the end of the back-arc basin opening. The spreading rate of the basin decreased once when the spreading direction changed after six million years of stable trench perpendicular spreading. The rate then further decreased immediately prior to the end of the spreading when the Mado MM OCC was formed. The existence of other OCC-like structures in the neighboring segment and the previously reported OCCs along the Parece Vela Rift have indicated that the melt-poor, tectonic-dominant spreading is a widespread phenomenon at the terminal phase of back-arc spreading. The decrease in spreading rate in the later stage is consistent with the previous numerical modeling because of the decrease in trench retreat. In the Izu–Bonin–Mariana arc trench system, the rotation of the spreading axis and the resultant axis segmentation have enhanced the lithosphere cooling and constrained mantle upwelling, which caused the tectonic-dominant spreading at the final phase of the basin evolution.

Abundance and Distribution of the Larvae of Family Carangidae in the Philippine Waters

A spatio-temporal study on the abundance of the Carangidae larvae in Philippine waters was conducted to minimize the lack of information on their spawning grounds. The study analyzed fish larvae data from M/V DA-BFAR cruises between 2006 and 2018, covering various areas in the Philippines. A total of 589 sampling stations were examined and grouped into seven zones: Batanes-Polillo, Catanduanes-Eastern Samar, Inland waters (Bohol Sea, Davao Gulf, Lagonoy Gulf, Leyte Gulf, Ragay Gulf, Sibuyan Sea, Samar Sea, Tayabas Bay, and the Visayan Sea), Mindanao-Sulawesi Sea, Sulu Sea, West Philippine Sea, and Davao Oriental-Surigao. The results indicated a patchy distribution of carangid larvae, with the highest concentration found in northern Palawan (Sulu Sea and West Philippine Sea areas). The total density of composite samples ranged from 1 to 865 larvae/1000m³. The abundance showed fluctuation over the years, with higher densities in 2016 and 2017 compared to other sampling years. The highest mean density was observed in September 2017 in the West Philippine Sea, while the lowest was recorded in May 2015 in Batanes-Polillo waters. The variations in carangid larvae abundance in Philippine waters are likely related to the species' spawning habits, as the timing of reproduction may vary by region based on ecological factors. Other factors influencing the results include sampling incidence and the sampling period of M/V DA-BFAR.

Decadal changes in rapid intensification of western North Pacific tropical cyclones modulated by the North Pacific Gyre Oscillation

This study investigates the modulation of western North Pacific (WNP) tropical cyclone rapid intensification (RI) by the North Pacific Gyre Oscillation (NPGO) on decadal timescales. There is a significant inverse relationship between basinwide RI numbers during July–November and the simultaneous NPGO index from 1970 to 2021. During the positive NPGO phase, suppressed RI occurs over almost the entire WNP, with a distinctly different spatial distribution compared to the Pacific Decadal Oscillation-driven pattern of RI modulation. RI occurrence is significantly reduced over the eastern Philippine Sea (10°–25°N, 140°–155°E). This is primarily caused by enhanced negative low-level vorticity, which can be linked to the horizontal extent of the anomalous low-level anticyclone. By comparison, over the western Philippine Sea (10°–25°N, 125°–140°E), there are only weak RI occurrence changes due to offsetting influences of increased mid-level humidity and decreased low-level vorticity.

The Effects of the Pandemic on the Selected Hotel Industry in Puerto Princesa City, Palawan: Basis for the Development of Recovery Strategies

Puerto Princesa City is one of the most popular tourist sites in the nation. The City encompasses the east coast, which faces the Sulu Sea, and the west coast, which faces the West Philippines Sea and is located in the province of Palawan's center. It occupies a vast tract of land characterized by a high forest cover set in a chain of mountain ranges home to diverse flora and fauna. Its imposing mountains and lush rainforests ensure adequate coordination from the Department of Trade in the industry in helping the hospitality industry watershed, where water overflows into life-giving rivers, waterfalls, and streams. The study enerally aimed to ascertain the Pandemic's effects on the chosen hotel sector in Puerto Princesa City, Palawan, and to develop a basis for developing recovery strategies. It was conducted in twenty hotels on the initiatives and interventions of the government during this covid-19 Pandemic. Henceforth, on the perceived recovery strategies by the hotel owners/managers, the hotel suggested adopting digital solutions, pricing tactics, and safety protocols that fit in this new normal.

Seasonal Variability of the Deep Western Boundary Current in the Philippine Sea

The deep western boundary current (DWBC) in the Philippine Sea has been expected to play a crucial role in transporting lower circumpolar deep water and to contribute to regional and global climate regulation. Two-year-long near-bottom current measurements reveal a southward-flowing DWBC with a mean velocity of 5 cm/s and seasonal variations—weaker in summer and stronger in winter. Seasonal variability in the DWBC is hypothesized to be induced by changes in the North Equatorial Current bifurcation latitude (NECBL) and upper pycnocline depth through potential vorticity conservation. Data-assimilated reanalysis model (GLORYS12V1) outputs, which reproduce the seasonal variability of DWBC similarly to the observation, are used for further analysis. During the seasonal period, the NECBL displays significant coherence (&gt;0.9) with the first-mode empirical orthogonal function principal component of the simulated along-slope DWBC. The upper pycnocline depth, varying seasonally within a range of approximately 27 m, induces seasonal variability in a deep anticyclonic eddy trapped by topography. In summer, the intensified deep anticyclonic eddy obstructs the adjacent southward-flowing DWBC, weakening its strength, whereas in winter, the southward flow of the DWBC is enhanced due to the weakening of the deep anticyclonic eddy.

Deep genesis of the high heat flow anomaly in the Okinawa Trough

The Okinawa Trough (OT) is characterized by high heat flow anomalies up to 10,000 mW/m2 (especially in the middle area), which is rarely seen in heat flow research all over the world. The genesis of the high heat flow anomaly is probably related to mantle upwelling. However, the deep genesis is yet to be studied. We used heat flow values collected from the Global Heat Flow Database to calibrate the locations of high heat flow anomalies. Gravity data were used to calculate the residual geoid and interlayer geoid anomalies, which can help in characterizing the morphology of mantle upwelling beneath the OT. Earthquake hypocenter distribution maps were presented to show the position of the Philippine Sea Plate beneath the OT. According to P-wave tomography profile and previous research, we discussed the deep genesis of the high heat flow anomaly and proposed a preliminary model to explain the process. We suggest that the deep mantle upwelling, probably from a depth of −1200 km, enters the upper mantle through the front of the subducted Pacific plate. The deep mantle upwelling converges with the molten mantle flow caused by the fluid from the dehydration of the Pacific Plate and the Philippine Sea Plate. Two mantle upwelling branches were revealed to feed the southern OT (SOT) and Changbai Volcano, and the mantle upwelling under the middle OT (MOT) was the second branch from the mantle upwelling branch under the SOT. The MOT should be more influenced by mantle upwelling from deep sources with extremely high temperatures.

Damage Prediction Observation for Existing Buildings in Sabah under Moderate Risk Earthquakes

Seismic design in building construction is still new in Malaysia. Sabah, a Malaysian state, is situated southeast of the Eurasian Plate, between the highly active Philippine Sea Plate and Indo-Australian Plate, and has a history of earthquakes, with the largest measuring a magnitude of 6.3 (2015). Although small earthquakes occur annually, most old buildings in Sabah were built pre-code and designed without considering earthquake loadings. This study aimed to analyze the potential damage to buildings in Sabah based on their vulnerability to moderate earthquakes. More than 500 buildings in seven districts were evaluated using a quantitative method based on score assignment, within 100 kilometers of the epicenters. According to the findings, more than 160 buildings in the Kota Kinabalu and Kudat districts were assessed as vulnerable to Grade 4 damage. In Ranau, Kota Marudu, Tawau, Semporna, and Lahad Datu, most buildings had a Grade 3 damage potential, with some at Grade 2 or 4. This study’s findings provide a summary of the damage risk for structures in Sabah and offer a starting point for planning and developing safer buildings that can withstand local seismic conditions. The resulting building-grade damage map can be used as a reference for future damage mitigation measures.

Influence of the Antarctic oscillation on summer precipitation over East Asia

As the strongest atmospheric mode in the extratropical Southern Hemisphere, the Antarctic oscillation (AAO) can affect regional and even global climate through zonal asymmetric and symmetric dynamic mechanisms. Observational analysis indicates that AAO can affect the zonal-mean vertical velocity of the Northern Hemisphere through eddy-induced meridional circulation in boreal summer, corresponding to the out-of-phase zonal precipitation anomaly at approximately 20°N and the in-phase zonal precipitation anomalies at approximately 30°N and 50°N. In particular, the precipitation anomalies related to AAO in East Asia are much stronger than those in the other regions at the same latitude, and the AAO-related precipitation anomaly agrees well with the precipitation leading mode over East Asia. The strong meridional circulation anomaly in the East Asia sector associated with the asymmetry of AAO results in strong subsidence over the Philippine Sea, and the tropical cooling triggers a Pacific-Japan-like (PJ-like) pattern anomaly, which is conducive to moisture transport over East Asia. In addition, the transient eddy forcing related to AAO causes abnormal circulation over East Asia, which further strengthens the PJ-like pattern. The zonal precipitation anomalies in the Northern Hemisphere are superimposed on the moisture transport and precipitation anomalies related to the PJ-like pattern, hence promoting positive precipitation anomalies in the Yangtze River valley, northeastern China, South Korea and southern Japan. In contrast, negative precipitation anomalies occur in southern China and North China. Our results indicate that AAO is a potential indicator for the East Asian summer precipitation anomaly.

The Philippine springtime (February–April) sub-seasonal rainfall extremes and extended-range forecast skill assessment using the S2S database

During the first half month of April 2022, the Philippines experienced severe disasters associated with the weak but deadly tropical storm Megi that caused 214 deaths and two sunken ships. This prompted us to investigate the extended-range prediction skill of the springtime Philippine sub-seasonal scale rainfall extremes in the subseasonal-to-seasonal (S2S) prediction database. The results suggest that the S2S models can well predict the extremity of the 2022 springtime sub-seasonal peak rainfall event (SPRE) ten days ahead. In addition to the La Niña sea surface temperature anomalies, this prolonged rainfall event, from March 26 - April 14, 2022, was associated with an anomalous cyclonic circulation straddled over the South China Sea (SCS) and the Philippine Sea and persisted for two weeks. The strong relationship between the El Niño and Southern Oscillation (ENSO) and the springtime (February–April) rainfall variability in the Philippines is clearly revealed in the analysis of 25 years of observational and hindcast data. The extremely wet SPREs tend to occur during the La Niña springs, while the extremely dry SPREs tend to occur during the El Niño springs. The Madden-Julian oscillation (MJO) and equatorial Rossby (ER) waves that are capable of modulating the sub-seasonal rainfall extremes were weak when the deadly SPRE occurred in April 2022. Thus, the extended-range forecast skill of this example can be interpreted as the baseline skill of the current S2S prediction revealed in the multi-model database. The findings suggest that the SPRE is a useful item to be included in the operational forecast as potential opportunities to harness the benefits of S2S prediction and applications.

Special Sea‐Level and Circulation Anomalies in the Philippine Sea During the 2006/2007 and 2009/2010 El Niño Events

AbstractThe Philippine Sea (PS) tends to show sea‐level falling and cyclonic upper‐layer circulation anomalies during the developing stage (spring and summer) of El Niño, with notable influences on the circulation, climate, and biosystems of downstream regions. These changes show diversity across individual events, partly associated with El Niño‐Southern Oscillation complexity and bringing uncertainties to scientific attribution and prediction. Here, we focus on the positive sea‐level anomalies (SLAs) and anticyclonic circulation anomalies in the boreal spring and summer of 2006 and 2009, which are opposite in sign to the changes observed during other El Niño events. Correspondingly, exceptional changes were seen in downstream regions, such as the enhanced Indonesian throughflow in the Makassar Strait. Our analysis highlights the persistent equatorial easterly wind anomalies during the preceding winter, which were likely favorable for these special changes in 2006 and 2009. This is confirmed by sensitivity experiments of a simplified ocean model. The results show that the negative off‐equatorial wind stress curls in the western and central North Pacific play the key role in causing positive SLAs in the PS through evoking downwelling Rossby waves, and equatorial winds played a secondary role. Further analysis indicates that such special changes of the PS tend to take place in late‐onset weak El Niño events following the La Niña conditions.

Co-occurrence of a marine heatwave and a reported tomato jellyfish (&lt;i&gt;Crambione mastigophora&lt;/i&gt; Maas, 1903) bloom in March 2020 at El Nido, Palawan, Philippines

Globally, observations on marine species during marine heatwaves (MHWs) help outline the scope of the MHW’s possible biological effects. In line with this effort, this paper presents a 2020 MHW that coincided with a reported ‘tomato jellyfish’ (Crambione mastigophora Maas, 1903) bloom on 23 March 2020 in the Corong-Corong Bay of Palawan, Philippines. Detecting a moderate MHW from 21 March to 04 April 2020, the analysis of sea surface temperatures revealed that most areas surrounding the bloom site attained their peak positive anomalies on the same day as the reported bloom. Certain physical mechanisms present in the first quarter of 2020 may have played a role in the occurrence of both events: the presence of cyclonic eddies and parallel monsoonal winds alongshore can induce upwelling which promotes biological productivity in surface waters, while the observed weakening of winds have been associated with anomalous warming of the sea surface. Further studies are still highly recommended to determine the exact causes of the jellyfish bloom and what conditions make it more likely to happen during MHWs. However, if the C. mastigophora is hypothetically able to continually bloom amidst warming temperatures, the increasing trend of MHW frequency and intensity in the West Philippine Sea (where the reported bloom site is situated) may consequently yield more future co-occurrences. This paper aims to hopefully contribute to the existing knowledge of possible biological impacts associated with extreme marine events, especially in the Philippine context where both jellyfish blooms and MHWs are understudied.

Deep subduction of the Philippine Sea slab and formation of slab window beneath central Japan

The geometry of the Philippine Sea slab (PHS) subducting beneath the Japanese islands has been imaged to 400 km depth beneath the Kyushu and Chugoku regions, whereas the PHS slab geometry beneath the Hokuriku region has only been determined to ~ 140 km depth, thereby indicating a large east–west asymmetry in the slab subduction. However, geologic evidence suggests that there was symmetrical east–west seafloor spreading along the axis of the Kinan seamount chain when the Shikoku basin was an active spreading center in the PHS plate. This inconsistency suggests that the PHS slab should be present beneath the Hokuriku region. Here we perform P-wave travel-time tomography across central Japan and conduct a two-dimensional plate subduction numerical simulation that reproduces the dual subduction of the PHS and Pacific (PAC) plates to elucidate the PHS slab geometry beneath central Japan. The tomography results reveal a high-velocity anomaly at ~ 150–250 km depth that extends from Wakasa bay to Noto peninsula and a slab window beneath the Hokuriku region. The numerical simulation results suggest that the PHS slab may have torn when it collided with the PAC slab, with the once leading edge of the PHS slab now present along the upper surface of the PAC slab beneath Noto peninsula. These results indicate that the PHS slab exists at ~ 250 km depth beneath the Hokuriku region, although it has been torn owing to its collision with the PAC slab, with this tear propagating westward to form a triangular slab window beneath the Hokuriku region.Graphical

Multichannel seismic evidence for tectonic migration in the Mariana subduction system

AbstractThe Mariana subduction system is an ideal place for ocean–ocean convergence boundary research. As a typical example of an extensional subduction model, the subduction process, subduction mechanism and subduction effect of this system have long been popular research topics. The seismic reflection information of oceanic sedimentary layers shown on multichannel seismic profiles is an intuitive record of small‐ to medium‐scale structural activity and sedimentary sequence evolution and is an important source for studying plate‐coupling processes and tectonic dynamic action. This paper is based on the raw multichannel seismic data of MGL1204 in the Mariana arc forearc region; these data were obtained by using linear interference filter technique noise attenuation technology and multiple velocity iterations to accurately image the oceanic sedimentary layer, and the imagery is used to study the Cenozoic fracture activity and residual strata thickness distribution characteristics of the middle part of the Mariana forearc region and to explore the tectonic migration characteristics of the Cenozoic sedimentary basins. We believe that due to the influence of regional stress field changes caused by the movement directions of the Pacific Plate and Philippine Sea Plate towards the Eurasian Plate, the development of secondary structures in the western Pacific subduction zone shows a characteristic west‐to‐east migration, and the structural characteristics of the Mariana forearc Cenozoic sedimentary basin are consistent with the regional tectonic features. The faults, stratigraphy and depocentres of the oceanic sedimentary layer show a migration pattern from west to east and from north to south overall; 25 and 13 Ma are identified as two key tectonic migration periods, and the tectonic migration characteristics and time records of the basin also provide direct evidence for the plate reconstruction model of trench retreat and Philippine Sea plate rotation.

Ocean Bottom Seismometers in Suruga Bay reveal a shear zone inside the Philippine Sea plate slab

Suruga Bay lies along the Tokai segment of the Nankai-trough, which last ruptured in the 1854 M8.4 Ansei-Tokai Earthquake, making it a potential locus for the next major earthquake on the Nankai-trough. Moderate size earthquakes have occurred in the Suruga Trough axial region (M6.5 in 2009 and M6.2 in 2011) that resulted in a prominent excitation of seismicity. Unfortunately, our ability to accurately locate these events is hampered by a sparse routine seismic observation network around the bay. In order to improve location accuracy, a total of 2 sets of 7 OBSs were deployed inside Suruga Bay, 6 from the spring of 2017, and an additional OBS from spring 2019. By combining the data from our OBS stations with that of existing land-based stations surrounding Suruga Bay, we were able to accurately relocate the observed seismic activity inside the Philippine Sea plate (PHS). Our combined processing revealed a 25° northwest-dipping 2–5 km wide zone on which the regional seismic activity occurs. Overall, seismic activity is observed to be 5–10 km shallower than previously determined from routine network data alone. A striking feature of the analyzed activity is the presence of a prominent strike-slip component in the focal mechanisms, which exhibit P-axes almost exclusively oriented towards the north. Furthermore, taking into consideration the apparent difference in the direction of the P-axis of the stress field to the calculated sense of movement of Izu peninsula by GNSS data, and the ongoing internal deformation observed by geodetic and geological data, we propose the existence of a shear zone inside the subducting PHS starting from the southern tip of Izu Peninsula and extending across the bay under Shizuoka city. This zone of strain partitioning acts as a buffer between the northward-colliding Izu Peninsula and the northwest-directed subduction of the PHS observed by GNSS.