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Seismic fault surfaces have complex geometries over a broad-scale range. The Hurst exponent (H) is an important parameter characterizing the complexity of fault geometries. This exponent of seismic faults has only been estimated at outcrops and from surface traces of large earthquakes. This is because geometry data of subsurface faults usually have large uncertainties and many gaps. This study examined the applicability of the lifting scheme, which is one of the wavelet transform methods, to estimate H of irregularly sampled geometry of subsurface faults. We analysed the surface geometry of the subducting oceanic plate at the Nankai trough, Japan, which is part of the fault plane of interplate earthquakes. The geometries of the subducting plate were estimated along six survey lines by integrating seismic refraction and reflection surveys. Two-way traveltimes of reflected waves from the plate surface, which were measured from the reflection survey data, were converted to depth using the velocity structure estimated by refraction survey. The intervals between sampled points were irregular because the reflected waves were obscured or invisible in some places. The Hurst exponents were estimated from the scale dependence of the wavelet coefficients that were derived by the lifting scheme without interpolation. Analyses of the synthetic data simulating the irregularly sampled plate geometries indicated that the lifting scheme yields stable but largely biased estimates of H , especially for small H (<0.5). We introduced the empirical bias correction to achieve an unbiased estimation of the exponent. The analysis of plate geometry at the Nankai trough was conducted at narrow-scale ranges with consideration for the accuracy of velocity structures. We may conclude that H of five survey lines across the trough axis is > 0.8, and that of a line along trough is > 0.7. These estimated exponents had large uncertainties due to analyses at narrow-scale ranges, but were close to the estimates in studies of the surface traces of large earthquakes. [ABSTRACT FROM AUTHOR]

Shallow very low frequency earthquakes (sVLFEs) have occurred recurrently at the shallow plate interface of the Hyuga-nada region of the western Nankai subduction zone, Japan. Although the locations of sVLFE epicentres have been determined using land-based seismic records with moderate accuracy, it is necessary to determine their locations more precisely to explore the relationship between sVLFEs and other shallow slow earthquakes and examine the structural factors that may control sVLFE activity. Here, we identified sVLFE epicentres using seismic records obtained from temporarily deployed ocean bottom seismometers (OBSs) in the Hyuga-nada region. Seismic observations involved the deployment of 5–13 OBSs for approximately 1 yr, with deployments conducted three times during 2014–2016 each time with changing OBS numbers and array distribution. As a result, one sVLFE episode, containing successive Rayleigh wave pulses with slow velocities due to marine sediments, could be detected at a frequency band of 0.1–0.15 Hz per observation, resulting in a total of three episodes. Rayleigh wave amplitudes of ordinary earthquakes in the continuous records were suppressed using earthquake catalogues. We estimated the dispersion curve for the Rayleigh wave group velocity for each array, which represented the averaged group velocity within the array, using coda interferometry, and applied an envelope correlation method (ECM) using the group velocities to continuous records. These processing provided sVLFE epicentres with horizontal distance errors of <5 km. Our results showed that sVLFEs depths, which were inferred from the contour line of the top of the Phillipine Sea Plate, had increased from <10 km to 10–15 km in the region of the subducted Kyusyu-Palau Ridge (KPR). It was also apparent that migration of sVLFE epicentres occurred in 2015 from a depth of 15 km to shallower depths along the northern margin of the subducted KPR. These results identified the subducted KPR as a structural factor controlling the excitation conditions of sVLFE activities. [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]

We documented regional and local variations in basement relief, sediment thickness, and sediment type in the Shikoku Basin, northern Philippine Sea Plate, which is subducting at the Nankai Trough. Seismic reflection data, tied with ocean drilling program drill cores, reveal that variations in the incoming sediment sequences are correlated with basement topography. We mapped the three-dimensional seismic facies distribution and measured representative seismic sequences and units. Trench-parallel seismic profiles show three regional provinces in the Shikoku Basin that are distinguished by the magnitude of basement relief and sediment thickness: Western (<200–400 m basement relief, >600 m sediment thickness), Central (>1500 m relief, ∼2000 m sediments), and Eastern (<600 m relief, ∼1200 m sediments) provinces. The total thickness of sediment in basement lows is as much as six times greater than that over basement highs. Turbidite sedimentation in the Shikoku Basin reflects basement control on deposition, leading to the local presence or absence of turbidite units deposited during the middle Oligocene to the middle Miocene. During the first phase of sedimentation, most basement lows were filled with turbidites, resulting in smooth seafloor morphology that does not reflect basement relief. A second phase of turbidite deposition in the Eastern Province was accompanied by significant amounts of hemipelagic sediments interbedded with turbidite layers compared to the other provinces because of its close proximity to the Izu–Bonin Island Arc. Both regional and local variations in basement topography and sediment thickness/type have caused lateral heterogeneities on the underthrusting plate that will, in turn, influence lateral fluid flow along the Nankai accretionary prism. [ABSTRACT FROM AUTHOR]

When seamounts and other topographic highs on an oceanic plate are subducted, they cause significant deformation of the overriding plate and may act as asperities deeper in the seismogenic zone. Kashinosaki Knoll (KK) is an isolated basement high of volcanic origin on the subducting Philippine Sea Plate that will soon be subducted at the eastern Nankai Trough. Seismic reflection imaging reveals a thick accumulation of sediments (∼1200 m) over and around the knoll. The lower portion of the sedimentary section has a package of high-amplitude, continuous reflections, interpreted as turbidites, that lap onto steep basement slopes but are parallel to the gentler basement slopes. Total sediment thickness on the western and northern slopes is approximately 40–50% more than on the summit and southeastern slopes of KK. These characteristics imply that the basal sedimentary section northwest of KK was deposited by infrequent high-energy turbidity currents, whereas the area southeast of KK was dominated by hemipelagic sedimentation over asymmetric basement relief. From the sediment structure and magnetic anomalies, we estimate that the knoll likely formed near the spreading center of the Shikoku Basin in the early Miocene. Its origin differs from that of nearby Zenisu Ridge, which is a piece of the Shikoku Basin crust uplifted along a thrust fault related to the collision of the Izu–Bonin arc and Honshu. KK has been carried into the margin of the Nankai Trough, and its high topography is deflecting Quaternary trench turbidites to the south. When KK collides with the accretionary prism in about 1 My, the associated variations in sediment type and thickness around the knoll will likely result in complex local variations in prism deformation. [ABSTRACT FROM AUTHOR]

Megathrust earthquakes have occurred repeatedly at intervals of 100 to 150 years along the Nankai Trough, situated in the southwest of Japan. Given that it has been 70 years since the last event, the occurrence of the next devastating earthquake is anticipated in the near future. On April 1, 2016, a moderate earthquake (Mw 5.9, MJMA 6.5) occurred off the southeastern coast of Mie Prefecture in the source region of the 1944 Tonankai earthquake (Mw 8.2). In this study, we investigated the influence of the 2016 earthquake on future megathrust earthquakes. We first determined the hypocenter distributions using a precise velocity structure obtained from seismic surveys in the source region. Using data obtained from the DONET ocean-bottom observation network, we found that this earthquake occurred along the plate boundary fault, which is also believed to have slipped during past megathrust earthquakes. We then performed a preliminary numerical simulation to reproduce the occurrence of a moderate earthquake in the middle of a megathrust earthquake cycle. We used a hierarchical asperity model, in which smaller asperities causing moderate earthquakes are embedded in a hyper-asperity that serves as the source region of megathrust earthquakes. The simulation shows that moderate earthquakes, caused by ruptures of a smaller asperity, occur as a result of shrinkage of strongly coupled areas in the hyper-asperity. This result is consistent with the observation that the hypocenter of the 2016 earthquake was located at the edge of a strongly coupled region along the plate boundary. The simulation also reproduced postseismic slip (afterslip/slow slip) along the plate boundary updip of the hyper-asperity, which is consistent with the observations of slow slip events and very-low-frequency earthquakes after the 2016 earthquake. Thus, the occurrence of the moderate earthquake offshore southeastern Mie Prefecture in the middle of the megathrust earthquake cycle implies that the shrinkage of the strongly coupled area along the plate boundary is occurring as a preparatory process for the next megathrust earthquake in the region. [ABSTRACT FROM AUTHOR]

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: We performed 3D seismic tomography of the Hyuga-nada region, western Nankai subduction zone, to investigate the relationship of the subducted part of Kyushu-Palau Ridge (KPR) to coseismic rupture propagation, seismicity, and shallow very low frequency earthquakes. Combining active-source and passive-source data recorded both onshore and offshore, we imaged the deep slab from near the trough axis to the coastal area. Our results show the subducted KPR as a low-velocity belt oriented NW–SE extending down the plate boundary to around 30km depth. At this depth, we suggest that the subducted KPR detaches from the slab and becomes underplated on the overriding continental plate. As the coseismic slip areas of past large earthquakes do not extend into the subducted KPR, we suggest that it may inhibit rupture propagation. The interior of the subducted KPR shows active intraslab seismicity with a wide depth distribution. Shallow very low frequency earthquakes are continuously active above the location of the subducted KPR, whereas they are intermittent to the northeast of the subducted KPR. Thus, the subducted KPR appears to be an important factor in coseismic rupture propagation and seismic phenomena in this region. [Copyright &y& Elsevier]

Abstract: We have performed a two-dimensional numerical simulation to elucidate the physical processes governing earthquake behavior when significant stress perturbations are produced by interaction between fault segments. Our model involves two seismogenic segments separated down-dip on a subduction zone plate boundary and incorporates a rate- and state-dependent friction law. Based on repeated simulations under different scenarios, we find that the rate- and state-dependent frictional parameters (b–a) and d c in the deeper seismogenic segment are required to be smaller than those in the shallower segment in order that more deep earthquakes occur than shallow ones. Our simulations show that slip amounts in either seismogenic segment increase in each of the co-, pre- and post-seismic stages when an earthquake occurs shortly after another earthquake in the other seismogenic segment. Conversely, when earthquakes occur in a single seismogenic segment several times in succession while the other segment remains locked, all three pre-, co-, and post-seismic slip amounts become smaller. These results imply that precursory changes do not necessarily occur at the same level on every occasion. In cases of multiple rupturing, the co-seismic slip of the later earthquake in a pair is approximately characteristic when frictional stability in the aseismic segment between the two seismogenic fault segments is strong enough to produce different rates of seismicity on each segment. Our simulation also shows that the degree to which rupture initiation points vary between different earthquakes is higher in low seismic coupling regions. This result may prove useful in estimating seismic coupling coefficients from observations of interplate hypocenters. [Copyright &y& Elsevier]

Abstract: Large earthquakes have repeatedly occurred in the area off southeastern Hokkaido Island, Japan, as the Pacific Plate subducts beneath the island, which is on the North American Plate. The most recent large earthquake in this area, the 2003 Tokachi-oki earthquake (Mw=8.0), occurred on September 26, 2003. In order to investigate aftershock activity in the rupture area, 47 Ocean Bottom Seismometers (OBSs) were quickly deployed after the main shock. In the present study, we simultaneously estimate the hypocenters and 3-D seismic velocity models from the P- and S-wave arrivals of the aftershocks recorded by OBSs. The subducting plate is clearly imaged as a northwest dipping zone in which Vp is greater than 7 km/s, and the relocated hypocenters also show the subducting Pacific Plate. The aftershock distribution reveals that the dip angle of the plate boundary increases abruptly around 90 km from the Kuril Trench. The bending of the subducting plate corresponds to the southeastern edge of the rupture area. The island arc crust on the overriding plate has P-wave velocities of 6–7 km/s and a Vp/Vs of 1.73. A region of Vp/Vs greater than 1.88 was found north of the epicenter of the main shock. The depth of the high Vp/Vs region extends about 10 km upward from the plate interface. The plate boundary just below the high Vp/Vs region has the largest slip at the main rupture. A high Vp anomaly (~7.5 km/s) is found in the island arc crust in northeast part of the study area, which we interpret as a structural boundary related to the arc–arc collisional tectonics of the Hokkaido region, as the rupture of the main shock terminated at this high Vp region. We suggest that the plate interface geometry and the trench-parallel velocity heterogeneity in the landward plate are principal factors in controlling the rupture area of the main shock. [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]

We investigated the state of plate coupling at the subduction zone beneath the Nicoya Peninsula, northwestern Costa Rica, from 1.5 years of trans-peninsula GPS campaign experiments. This area is recognized as a seismic gap located between the rupture areas of two M≥7 earthquakes that occurred in the early 1990s, and it has not ruptured since 1950. We carried out campaign GPS observations beginning in autumn 2001 to obtain the profile of site velocities across the peninsula for the interseismic period. The obtained velocity field indicates a strong coupling at the plate interface. However, velocity directions deflect counterclockwise, which suggests a trench-parallel forearc sliver motion of Nicoya Peninsula at ∼8.5 mm/year. An inversion analysis to infer inter-plate coupling at Nicoya seismic gap shows that the strongly coupled zone agrees well with seismogenic zone inferred from seismicity. Cumulative slip deficits since 1950 at the Nicoya seismic gap would produce a thrust earthquake as large as MW≥7.5. [Copyright &y& Elsevier]

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]

Seismic reflection profiles reveal steeply landward—dipping splay faults in the rupture area of the magnitude (M) 8.1 Tonankai earthquake in the Nankai subduction zone. These splay faults branch upward from the plate-boundary interface (that is, the subduction zone) at a depth of ∼10 kilometers, ∼50 to 55 kilometers landward of the trough axis, breaking through the upper crustal plate. Slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion. [ABSTRACT FROM AUTHOR]

• 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]