Repeating earthquake activities associated with the Philippine Sea plate subduction in the Kanto district, central Japan: A new plate configuration revealed by interplate aseismic slips

We detect repeating earthquakes associated with the Philippine Sea plate subduction to reveal the plate configuration. In the Kanto district, we find 140 repeating earthquake groups with 428 events by waveform similarity analysis. Most repeating earthquakes in the eastern part of the Kanto district occur with a regular time interval. They have thrust-type focal mechanisms and are distributed near the upper surface of the Philippine Sea plate. These observations indicate that the repeating earthquakes there occur as a repetition of ruptures on the isolated patches distributed on the plate boundary owing to the concentration of stress caused by aseismic slips in the surrounding areas. This shows that the distributions of repeating earthquakes suggest the aseismic slips in the surrounding areas of small patches. We determine spatial distributions of repeating earthquakes in the eastern part of the Kanto district and find that they correspond to the upper boundary of the Philippine Sea plate, that is, the upper boundary of the oceanic crust layer of the Philippine Sea plate. The plate geometry around Choshi is newly constrained by repeating earthquake data and a rather flat geometry in the eastern part of the Kanto district is revealed. The obtained geometry suggests uplift of the Philippine Sea plate due to the collision with the Pacific plate beneath Choshi.Repeating earthquakes in the western part of the Kanto district have extremely shorter recurrence times, and their focal mechanisms are not of the thrust types. These repeating earthquakes are classified as “burst type” activity and likely to occur on the preexistent fault planes which are distributed around the “collision zone” between the Philippine Sea plate and the inland plate. The variation among the repeating earthquake activities in the Kanto district indicates that regular repetition of repeating earthquakes is possible only on the plate boundary with a smooth and simple geometry.     

华南、巽他和掸邦地块之间边界断裂的上新世—第四纪大规模构造反转

印度- 亚洲碰撞后，大陆板块沿着大型的左行走滑断裂挤出。规模最大的哀牢山- 红河剪切带(ASRR)将印支地块(巽他地块)与华南地块分隔开来。长约1000 km的红河活动断裂(RRF)沿哀牢山的北侧延伸，目前呈现出右行走滑兼正断的活动性质。本文在讨论了红河断裂系及其周缘的第三纪和现今变形特征(滑移性质反转、渐新世/第四纪位错、全新世滑动速率、GPS测量、地震机制等)基础之上，重点论述了其在上新世—第四纪从云南东南部到越南西北部、北部湾西部、再到更南的沙巴地区的断裂分布和运动学特征。新的数据证实，跨过三联点和越南西北部的奠边府断层之后，华南地块与巽他地块之间的现今大部分右行走滑主要沿着Da River断裂向Day Nui Con Voi(或称大象山)东南方向延伸。Da River断裂与RRF大致平行，是2020年M w 5. 0莫州地震的发震断层。进一步研究表明，Da River断裂很可能沿着渐新世—中新世莺歌海/宋红盆地西缘和越南东南海岸(归仁剪切带)向南延伸得更远，至少延伸到“Ile des Cendres”火山群，并可能继续延伸到沙巴- 文莱逆冲带的西端，靠近婆罗洲北部的近海活动边缘。最后，我们讨论了在菲律宾群岛、台湾岛和巽他群岛之间南海大部分地区大规模构造反转的运动学效应。     

Seismological monitoring on the 2003 Tokachi-oki earthquake, derived from off Kushiro permanent cabled OBSs and land-based observations

Japan Marine Science and Technology Center installed a cabled geophysical observatory system off Kushiro, Hokkaido Island in July 1999. This observatory system comprises three ocean bottom seismographs (OBSs), two tsunami gauges, and a geophysical/geochemical monitoring system. 4 years and 2 months after the installation, a megathrust earthquake (the 2003 Tokachi-Oki earthquake, 26th September in Japan Standard Time (JST), M_JMA 8.0) occurred along a plate boundary underneath a forearc basin where the system is located. The system recorded clear unsaturated seismograms just at 28.6 km from the epicenter. This paper demonstrates advantages brought by the cabled observatory to record the megathrust earthquake showing how earthquake detectability is improved dramatically combining permanent OBS and land-based observations around the region, and importance of the in situ monitoring on the seismogenic zone. In the present study, processing OBSs and land-based network together, and comparing magnitudes of common observed earthquakes with national authorized network, event detection level improved down to M 1.5, which is much lower than the previously designed as down to M  2. Comparing detection level before and after installing OBSs, we found dramatic improvement of the earthquake detection level in the interesting region. Real-time continuous observations of microearthquakes since 1999 have brought us tremendous findings. First, a seismic quiescence started about 10 days before the 2003 Tokachi-Oki earthquake. Second, aftershock distribution is not uniform over the focal area and can be divided into several sub-regions, which might indicate an existence of several asperities. We think that the geophysical observations helped to understand the initiation process of the rupture of the 2003 Tokachi-Oki earthquake and that observations including seismological, geodynamic, hydrogeological, and the other multidisciplinary observations would provide a clue to future understanding of seismogenic processes at subduction zones.     

Paleoseismological evidence for non-characteristic behavior of surface rupture associated with the 2004 Mid-Niigata Prefecture earthquake, central Japan

The 2004 Mid-Niigata Prefecture earthquake sequence (mainshock magnitude, M_JMA 6.8), which occurred in an active fold-and-thrust belt in northern central Japan, generated a small thrust surface rupture (< 20 cm of vertical displacement) along a previously unmapped northern extension of the active Muikamachi–Bonchi–Seien fault zone, on the eastern margin of the epicentral region. To better understand past seismic behavior of the rupture, we conducted a paleoseismic trenching study across the 10-cm-high west-side-up surface rupture at the foot of a pre-existing 1.8-m-high east-facing scarp, which probably resulted from past earthquake(s). A well-defined west-dipping thrust fault zone accompanied by drag folding and displacing the upper Pliocene to lower Pleistocene strata and the unconformably overlying upper Pleistocene (?) to Holocene strata was exposed. The principal fault zone is connected directly to the 2004 surface rupture. From the deformational characteristics of the strata and radiocarbon dating, we inferred that two large paleoseismic events occurred during the past 9000 years prior to the 2004 event. These two pre-2004 events have a nearly identical fault slip (at minimum, 1.5 m), which is ≥ 15 times that of the 2004 event (∼ 10 cm). These paleoseismic data, coupled with the geological and geomorphological features, suggest that the 2004 event represented non-characteristic behavior of the fault, which can potentially generate a more destructive earthquake accompanied by meter-scale surface displacement. This study provides insight into the interpretation of past faulting events and increases our understanding of rupture behavior.     

Formation of pockmarks and submarine canyons associated with dissociation of gas hydrates on the Joetsu Knoll,eastern margin of the Sea of Japan

This study, based on 3.5 kHz SBP, 3D seismic data and long piston cores obtained during MD179 cruise, elucidated the timing and causes of pockmark and submarine canyon formation on the Joetsu Knoll in the eastern margin of the Sea of Japan. Gas hydrate mounds and pockmarks aligned parallel to the axis on the top of the Joetsu Knoll are associated with gas chimneys, pull-up structures, faults, and multiple bottom-simulating reflectors (BSRs), suggesting that thermogenic gas migrated upward through gas chimneys and faults from deep hydrocarbon sources and reservoirs. Seismic and core data suggest that submarine canyons on the western slope of the Joetsu Knoll were formed by turbidity currents generated by sand and mud ejection from pockmarks on the knoll. The pockmark and canyon formation probably commenced during the sea-level fall, lasting until transgression stages. Subsequently, hydropressure release during the sea level lowering might have instigated dissociation of the gas hydrate around the base of the gas hydrate, leading to generation and migration of large volumes of methane gas to the seafloor. Accumulation of hydrate caps below mounds eventually caused the collapse of the mounds and the formation of large depressions (pockmarks) along with ejection of sand and mud out of the pockmarks, thereby generating turbidity currents. Prolonged pockmark and submarine canyon activities might have persisted until the transgression stage because of time lags from gas hydrate dissociation around the base of the gas hydrate until upward migration to the seafloor. This study revealed the possibility that submarine canyons were formed by pockmark activities. If that process occurred, it would present important implications for reconstructing the long-term history of shallow gas hydrate activity based on submarine canyon development.