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The offshore part of the North Anatolian Fault (NAF) beneath the Marmara Sea is a well-known seismic gap for future M > 7 earthquakes in the sense that more than 250 yr have passed since the last major earthquake in the Central Marmara region. Although many studies discussed the seismic potential for the future large earthquake in this region on the basis of historical record, geodetic and geological observations, it is difficult to evaluate the actual situation on the seismic activity and structure along the NAF beneath the Marmara Sea due to the lack of ocean bottom seismic observations. Using ocean bottom seismometer observations, an assessment of the location of possible asperities that could host an expected large earthquake is undertaken based on heterogeneities in the microseismicity distribution and seismic velocity structure. Specifically, seismic tomography and precise hypocentre estimations are conducted using offshore seismic data whose recording period is 11 months. About five times more microearthquakes are detected with respect to events recorded in a land-based catalogue. A comparison with previously published results from offshore observation data suggests that the seismicity pattern had not changed from 2014 September to 2017 May. The location accuracy of microearthquakes is greatly improved from only the land-based earthquake catalogue, particularly for depth direction. There are several aseismic and inactive zones of microearthquake, and the largest one is detected using land-based seismic observation, whereas other zones are newly detected via offshore observations. The obtained velocity model shows a strong lateral contrast, with two changing points. The western changing point corresponds to a segmentation boundary, where the dip angle of the NAF segments changed. High-velocity zones from tomographic images are characterized by low seismicity eastward of the segment boundary. To the east of 28.50°E, the high-velocity zone becomes thicker in the depth direction and is characterized by low seismicity. Although the low seismic activity alone could be interpreted as both strong coupling and fully creeping, the high-velocity features at the same can be concluded that these zones are consist of brittle material and strong coupling. From comparison with other geodetic and seismic studies, we interpret these zones as locked zones that had been ruptured by the past large earthquakes and could be ruptured by future ones. These zones might accumulate strain since the main shock rupture associated with the 1766 May M s 7.3 earthquake, the latest major earthquake in this region. [ABSTRACT FROM AUTHOR]

In this study, the creep rate across the North Anatolian Fault was directly measured in the western Sea of Marmara using the seafloor acoustic ranging technique; the data reveal coupling conditions on the fault interface and stress accumulation with implications for regional seismic risk evaluation. Continuous measurements over a period of 3.5 years at a site in the Western High clearly indicate right‐lateral displacement at a rate of 10.7 ± 4.7 mm/year (95% confidence level); approximately half of the regional block motion at this location is released by this steady motion. A simple model of three elastic layers—a partially creeping sedimentary layer (8 km) at the top with the observed rate, a locked (3 km) and fully creeping layer in the middle, and a bottom layer—assumed from seismicity, reasonably explains onshore Global Navigation Satellite System data for the surrounding region. Plain Language Summary: The North Anatolian Fault (NAF) cuts across the east and west of the Turkish mainland and has repeatedly generated destructive earthquakes (magnitude ~7‐class) to release accumulated strain originating from regional plate motion. The NAF beneath the Sea of Marmara has remained as a "seismic gap" for more than a century and the stress condition of the fault plane is thought to be nearly matured. An earthquake in this region is expected to cause severe damage in Istanbul, the most populous city in Turkey. However, the submarine environment prevents investigation of the fault movement using the traditional geodetic techniques applicable to onshore surveys. We applied a seafloor geodetic technique using continuous acoustic ranging for 3.5 years and found that the NAF in the western Sea of Marmara is creeping at nearly half of the regional plate motion. Despite the partial creep, strain accumulation on the fault has already reached the equivalent of a M7‐class earthquake. Key Points: First direct evidence of creep in the submarine part of the North Anatolian Fault based on a seafloor geodetic surveyEstimated creep rate for the western part of the Sea of Marmara amounts to nearly half of the regional block motionA fault model incorporating the observed creep rate and the coupling condition estimated from microseismicity well explains GNSS data [ABSTRACT FROM AUTHOR]

Abstract: Recently, the occurrence of slow earthquakes such as low-frequency earthquakes and very low-frequency earthquakes have been recognized at depths of about 30 km in southwest Japan and Cascadia. These slow earthquakes occur sometimes in isolation and sometimes break into chain-reaction, producing tremor that migrates at a speed of about 5–15 km/day and suggesting a strong interaction among nearby small asperities. In this study, we formulate a 3-D subduction plate boundary model with two types of small asperities chained along the trench at the depth of 30 km. Our simulation succeeds in representing various types of slow earthquakes including low-frequency earthquakes and rapid slip velocity in the same asperity, and indicates that interaction between asperities may cause the very low-frequency earthquakes. Our simulation also shows chain reaction along trench with propagation speed that can be made consistent with observations by adjusting model parameters, which suggests that the interactions also explain the observed migration of slow earthquakes. [Copyright &y& Elsevier]

The rupture areas of the large interplate thrust earthquakes along the Nankai trough, offshore southwestern Japan, are divided into several segments. The 1944 Tonankai earthquake ruptured one of the segments off of the Kii Peninsula. In 2005, we conducted an ocean-bottom seismograph experiment at the eastern end of the rupture area of the 1944 earthquake. Little seismic activity was observed on the plate interface; most earthquake activity was within the subducting and overriding plates. Aftershocks of the 2004 off Kii Peninsula earthquake, which was an intraplate earthquake in the subducting Philippine Sea plate, were mainly located in the subducting oceanic crust and uppermost mantle. However, several earthquakes at the eastern end of the rupture area of the 1944 earthquake were in the sedimentary wedge, with a focal mechanism indicating deformation by a subducting seamount. The earthquakes and faults in the sedimentary wedge show deformation related to the irregular topography of the subducting oceanic crust to the east of the rupture area of the 1944 Tonankai earthquake. In contrast, few earthquakes were observed in the sedimentary wedge in the rupture area of the 1944 earthquake. Difference in topography of the subducting oceanic crust in the two areas might have caused spatial variations of interplate coupling, which, in turn, caused the observed differences in the deformation of the sedimentary wedge. The spatial variation of interplate coupling may affect rupture propagation during large interplate earthquakes and cause the segmentation of large interplate earthquakes along the Nankai trough. [ABSTRACT FROM AUTHOR]

In the summer of 2000, an ocean bottom seismometer (OBS) array was deployed along the western end of the Nankai Trough, off Cape Muroto. The array crosses an area where a buried subducting seamount was identified under the Nankai accretionary prism sediments during a multichannel seismic (MCS) reflection survey. In this paper we report the first results of shear-wave splitting (SWS) analysis of the three-component OBS data. One interesting result is a peculiar anomaly in the behaviour of shear-wave polarizations observed near the subducting seamount. We attempt to explain the observed spatial pattern in shear-wave polarizations by complex structure of stress field in accretionary prism sediments around seamount. We conclude, that the spatial pattern of SWS consistent with expected stress perturbations around seamounts, but there are several other reasons that can result in a similar type of seismic anisotropy. Finally, we have a somewhat broader look at the spatial pattern of shear-wave polarizations observed with another OBS array near the active front of Nankai Trough. [ABSTRACT FROM AUTHOR]

Along the Nankai trough, southwestern Japan, the Philippine Sea plate (PSP) is subducting beneath the Eurasian plate, and large interplate earthquakes have occurred repeatedly with a recurrence interval of about 100–200 years. The most recent large thrust event in the eastern Nankai trough off Kii Peninsula was the 1944 Tonankai earthquake. In this region, current seismicity is very low and hypocenters are not determined accurately by the land seismic network. We conducted microseismicity observations around the rupture area of the 1944 Tonankai earthquake using ocean bottom seismographs (OBSs). Hypocenters were determined using a 2-D seismic velocity structure model based on an airgun-OBS seismic survey. Results obtained show that the seismicity was relatively active near the trough axis. These earthquakes may relate to deformation of the subducting Philippine Sea plate. On the other hand, microseismicity in the rupture area of the 1944 Tonankai earthquake was very low. This low-level seismic activity in the co-seismic rupture area of the 1944 Tonankai earthquake likely relates to a single large asperity off Kii Peninsula. [Copyright &y& Elsevier]

Abstract: The Nankai and Hyuga-nada seismogenic segments, in the western part of the Nankai subduction zone off southwestern Japan, have sometimes ruptured separately and sometimes simultaneously. To investigate the relationships among heterogeneities of seismic structure, spatial variation of the incoming plate, and the seismogenic segments, we carried out seismic observations in the western Nankai subduction zone and modeled the area with 3D seismic tomography using both onshore and offshore seismic data. Our seismic observations suggested that the pattern of seismicity is related to heterogeneities within the subducted plate rather than the seismogenic segments. The up-dip depth limit of seismicity along the plate boundary and in the oceanic crust is typically around 15 km, corresponding to the depth of dehydration of the oceanic crust. In addition, the seaward-extended seismicity observed where the subducted plate was considered to have rough internal structures. In the resulting velocity model, the up-dip limit of the area where the P-wave velocity just above the plate boundary exceeds 6 km/s corresponds to the up-dip limit of coseismic slip in the 1968 Hyuga-nada and 1946 Nankai earthquakes. Between the two coseismic rupture zones is an area of lower P-wave velocity about 40 km wide that is evidence of lateral heterogeneities in the upper plate along the trough-parallel direction. Structural heterogeneities in the upper plate may explain the variety of coseismic slip patterns in this region. [Copyright &y& Elsevier]

Abstract: A multi-purpose seismic experiment named the 2002 integrated seismic experiment Southwest Japan was conducted in 2002 along a more-than-240-km-long line across southwest Japan from the Pacific coast to the Japan Sea coast. Its profile provides the first crustal-scale cross section across the Japanese island arc, which highlights a number of significant points related to the structural development of the arc. Major outstanding points are that the Japanese island arc is composed of two completely different crusts juxtaposed by the Median Tectonic Line (MTL), and that the MTL started its activity associated with lower crustal thinning and formation of an upper crustal-scale half-graben in Late Cretaceous. [Copyright &y& Elsevier]

Abstract: Many destructive earthquakes have occurred in the northern Japan trench region owing to the subduction of the Pacific plate. Two plate bending points, where the dip of the subducting plate changes markedly, have been found, and correlations between the bending points and the updip limits of large earthquake rupture zones have been previously found in the regions off Aomori ( N) and off Miyagi ( N). In this study, we investigate whether sharp plate bending is also present in the region off Iwate ( N), located approximately halfway between the off-Aomori and off-Miyagi regions. We analysed the existing off-Iwate wide-angle seismic experimental data by applying a new method, in which observed reflection phases are directly projected onto the depth–distance domain from the time–distance domain. As a result, we found a plate bending point in the region off Iwate; this is the first time such a point has been found there. Distances between the three bending points and the trench axis are almost uniform and sharp plate bending is a common feature in the northern Japan trench subduction zone. The bending axis, which is defined by connecting bending points, approximately coincides with the updip limit of the rupture zones of large interplate earthquakes. Seismic swarm activities have been observed inside the subducting oceanic crust along the bending axis, and we infer that some of these activities are caused by bending of the subducting plate. In addition, P-wave velocities of the upper plate vary across the bending axis, implying that the position of the bending axis is closely related to physical properties of the upper plate and to seismic activities. [Copyright &y& Elsevier]

Abstract: The Japan Trench subduction zone, located east of NE Japan, has regional variation in seismicity. Many large earthquakes occurred in the northern part of Japan Trench, but few in the southern part. Off Miyagi region is in the middle of the Japan Trench, where the large earthquakes (M >7) with thrust mechanisms have occurred at an interval of about 40 years in two parts: inner trench slope and near land. A seismic experiment using 36 ocean bottom seismographs (OBS) and a 12,000 cu. in. airgun array was conducted to determine a detailed, 2D velocity structure in the forearc region off Miyagi. The depth to the Moho is 21 km, at 115 km from the trench axis, and becomes progressively deeper landward. The P-wave velocity of the mantle wedge is 7.9–8.1 km/s, which is typical velocity for uppermost mantle without large serpentinization. The dip angle of oceanic crust is increased from 5–6° near the trench axis to 23° 150 km landward from the trench axis. The P-wave velocity of the oceanic uppermost mantle is as small as 7.7 km/s. This low-velocity oceanic mantle seems to be caused by not a lateral anisotropy but some subduction process. By comparison with the seismicity off Miyagi, the subduction zone can be divided into four parts: 1) Seaward of the trench axis, the seismicity is low and normal fault-type earthquakes occur associated with the destruction of oceanic lithosphere. 2) Beneath the deformed zone landward of the trench axis, the plate boundary is characterized as a stable sliding fault plain. In case of earthquakes, this zone may be tsunamigenic. 3) Below forearc crust where P-wave velocity is almost 6 km/s and larger: this zone is the seismogenic zone below inner trench slope, which is a plate boundary between the forearc and oceanic crusts. 4) Below mantle wedge: the rupture zones of thrust large earthquakes near land (e.g. 1978 off Miyagi earthquake) are located beneath the mantle wedge. The depth of the rupture zones is 30–50 km below sea level. From the comparison, the rupture zones of large earthquakes off Miyagi are limited in two parts: plate boundary between the forearc and oceanic crusts and below mantle wedge. This limitation is a rare case for subduction zone. Although the seismogenic process beneath the mantle wedge is not fully clarified, our observation suggests the two possibilities: earthquake generation at the plate boundary overridden by the mantle wedge without serpentinization or that in the subducting slab. [Copyright &y& Elsevier]

A seismic experiment with six explosive sources and 391 seismic stations was conducted in August 2001 in the central Japan region. The crustal velocity structure for the central part of Japan and configuration of the subducting Philippine Sea plate were revealed. A large lateral variation of the thickness of the sedimentary layer was observed, and the P-wave velocity values below the sedimentary layer obtained were 5.3–5.8 km/s. P-wave velocity values for the lower part of upper crust and lower crust were estimated to be 6.0–6.4 and 6.6–6.8 km/s, respectively. The reflected wave from the upper boundary of the subducting Philippine Sea plate was observed on the record sections of several shots. The configuration of the subducting Philippine Sea slab was revealed for depths of 20–35 km. The dip angle of the Philippine Sea plate was estimated to be 26° for a depth range of about 20–26 km. Below this depth, the upper boundary of the subducting Philippine Sea plate is distorted over a depth range of 26–33 km. A large variation of the reflected-wave amplitude with depth along the subducting plate was observed. At a depth of about 20–26 km, the amplitude of the reflected wave is not large, and is explained by the reflected wave at the upper boundary of the subducting oceanic crust. However, the reflected wave from reflection points deeper than 26 km showed a large amplitude that cannot be explained by several reliable velocity models. Some unique seismic structures have to be considered to explain the observed data. Such unique structures will provide important information to know the mechanism of inter-plate earthquakes. [Copyright &y& Elsevier]

Silent-slip events have been detected at several subduction zones, but the cause of these events is unknown. Using seismic imaging, we detected a cause of the Tokai silent slip, which occurred at a presumed fault zone of a great earthquake. The seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down-dip of a subducted ridge. We propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip. [ABSTRACT FROM AUTHOR]

Wide-angle seismic surveys performed in the last decade have clarified the 3-D crustal structure along the Nankai Trough. The geometry and velocity structure of the southwestern Japan subduction zone are now well constrained. Comparing these observations with the rupture distribution of historic great thrust earthquakes, it appears that the coseismic rupture occurred along plate boundaries deeper than the wedge/backstop boundary (the boundary between the Neogene–Quaternary accretionary wedge and the crust forming the backstop). From the view of spatial relationship, both rupture distributions of the last two large events and the crust forming the backstop are considerably retreated from the trough axis in the west and east off the Kii Peninsula. In both areas, seamount or ridge subduction is apparent in seismic results, geomorphological data and geomagnetic data. The landward indentation of the deformable backstop, which corresponds to the crustal block of old accreted sediments, may be formed by seamount subduction according to published results of sandbox modeling. In particular, the subducted seamount may be a structural factor affecting the recession of the crustal block forming the backstop. [Copyright &y& Elsevier]

• A subduction geometry model beneath the northern Ryukyu forearc is established. • Slow earthquakes and regular interplate earthquakes are discretely located. • A seismically quiet, potential M8 seismogenic/tsunamigenic zone exists. • Intraslab seismicity indicates heterogeneities of slab age or hydration degree. 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. [ABSTRACT FROM AUTHOR]