Ryota Hino

Slip distribution of the 2003 Tokachi-oki earthquake estimated from tsunami waveform inversion

The slip distribution of the 2003 Tokachi-oki earthquake is estimated from the 11 tsunami waveforms recorded at 9 tide gauges in the southern Hokkaido and eastern Tohoku coasts and two ocean bottom tsunami-meters (pressure gauges) off Kamaishi, Tohoku. The largest slip of 4.3 m is estimated on the subfault located off Hiroo. A large slip of 2.1 m is also estimated on the subfault located near Kushiro. The total seismic moment of the 2003 Tokachi-oki earthquake is 1.0 × 1021 Nm. The slip distribution estimated from the tsunami waveform inversion is similar to the slip distribution deduced by Yamanaka and Kikuchi (2003) from the inversion of the teleseismic body waves. The rupture area of the 2003 Tokachi-oki earthquake is similar to the western part of the rupture area of the 1952 Tokachi-oki earthquake estimated by Hirata et al. (2003).

Micro‐tsunami from a local interplate earthquake detected by cabled offshore tsunami observation in northeastern Japan

A micro tsunami from an interplate earthquake (Mw 6.1) was observed in 1998 on ocean bottom tsunami meters (OBTMs) deployed east off the northeastern Japan. The offshore tsunami data without complex distortions due to the coastal topography enable us to estimate reliable tsunami source parameters. The observed amplitude was about 1.5 cm at epicentral distance less than 100 km. We numerically computed the tsunami waveform by solving the linear Boussinesq equations. The observed tsunami waveforms are well explained by synthetic waveforms assuming the fault width of 10–15 km. The depth of the fault is estimated as 5–10 km below the seafloor, which is in good agreement with the location of the plate boundary defined by previous seismic studies.

Deep crustal structure of the eastern Nankai Trough and Zenisu Ridge by dense airgun–OBS seismic profiling

Abstract An unprecedentedly extensive seismic refraction and wide-angle reflection survey using 65 ocean bottom seismographs revealed detailed crustal structure around the eastern Nankai Trough. A previously published crustal model shows an abrupt offset of the Moho at the south of the Zenisu Ridge, a prominent topographic high along the oceanward slope of the Nankai Trough. Our crustal model indicates that this offset of the Moho extends southwestward continuously to 138°E, decreasing its gap. The survey area experienced the last two great earthquakes in 1854 and 1944. However, the northeastern part of the survey area seems to have remained unruptured since the 1854 event. Factors controlling the size of the rupture area for great earthquakes are still a matter of debate. There are several candidates for these factors in the survey area: hypothetical tectonic boundaries that may or may not be oceanward prolongation of major on-land tectonic lines, estimated locations of slab disruption, and the extent of Moho offset along the strike of the Zenisu Ridge. The main purpose of this survey is to clarify the relation between the crustal structure and these geophysical and geological features bounding the rupture area. Our crustal model from the trough axis to the continental slope is characterized by a well-developed sedimentary wedge bounded by island arc crustal blocks, consisting of upper and lower crust, to the northwest. Furthermore, the subducting oceanic crust, which can be traced down to 25 km depth, shows that the down-dip angle steepens at 55 km landward from the trough axis. On the basis of compilation of our crustal model with previously published models around the eastern Nankai Trough, we derived an image of the entire subducting plate geometry for depths shallower than 20 km, which is still poorly constrained by the land observation of microearthquakes. Significant lateral variations of the crustal structure and the slab geometry are recognized along one prominent canyon, and the offset of the Moho at the south of the Zenisu Ridge disappears to the southwest of the canyon. Moreover, it seems that the slab disruption recognized at a depth greater than 20 km is connected to this canyon. Therefore, the lateral variation of the crustal structure along the canyon may be one of the causes to stop rupture propagation of great earthquakes. Furthermore, the crustal variation may also form a tectonic boundary that distinguishes the subduction pattern of the Philippine Sea plate, including the influence of the Izu–Ogasawara collision, in the eastern Nankai Trough from the simple subduction pattern of the western Nankai Trough.

Aftershock observation of the 2003 Tokachi-oki earthquake by using dense ocean bottom seismometer network

The Tokachi-Oki earthquake occurred on September 26, 2003. Precise aftershock distribution is important to understand the mechanism of this earthquake generation. To study the aftershock activity, we deployed forty-seven ocean bottom seismometers (OBSs) and two ocean bottom pressure meters (OBPs) at thirty-eight sites in the source region. We started the OBS observation four days after the mainshock for an observation period of approximately two months. In the middle of the observation period, nine OBSs near the epicenter of the mainshock were recovered to clarify the depth distribution of aftershocks near the mainshock. From the data overall OBS, seventy-four aftershocks were located with high spatial resolution. Most of the aftershocks were located in a depth range of 15–20 km and occurred within the subducting oceanic crust, the 5.5-km/s layer of the landward plate and the plate boundary. No aftershocks were found in the mantle of the subducting plate. The low seismic activity beneath the trench area where the water depth is greater than about 2000 m suggests a weak coupling between the two plates. The depth of the mainshock is inferred to be 15–20 km from the aftershock distribution.

Hypocenter distribution of plate boundary zone off Fukushima, Japan, derived from ocean bottom seismometer data

Microearthquake observation using Ocean Bottom Seismometers (OBSs) was carried out to obtain a detailed distribution of microearthquakes beneath the area off Fukushima, in the middle section of the Japan Trench in the summer of 1997. The observation period spanned approximately one month. Almost all of the well-determined hypocenters occurred in the vicinity of the plate boundary in this region (approximately 100 km landward of the trench axis), while seismicity is markedly lower between this area and the trench itself. The seaward limit of the high seismicity region is close to the western end of the zone of direct contact between the oceanic crust and the overriding landward crust. Twenty-nine earthquakes were recorded within the overriding landward plate. Twelve earthquakes were recorded about 30 km below the plate boundary, and form a landward dipping plane that appears to be an up-dip continuation of the lower plane of the double seismic zone. The microseismicity characteristics of the plate boundary region are interpreted to be controlled by the geometry and physical properties of the plate interface. The seismic activity in the lower seismic plane near the trench is considered to relate to bending of the subducting plate and to dehydration of serpentinized Pacific Plate mantle.

Crustal structure around the eastern end of coseismic rupture zone of the 1944 Tonankai earthquake

Abstract The high potential for a major earthquake to occur in the Suruga Trough, “Tokai Earthquake”, has been recognized since 1969 and is closely related to how far the coseismic rupture during the 1944 Tonankai earthquake propagated eastward. To understand what kind of structure might have interrupted the rupture of the 1944 event, it is necessary to obtain the crustal structure around the eastern end of its rupture zone. The crustal model deduced from a wide-angle seismic survey shows characteristics of the subducting oceanic crust and the Neogene–Quaternary accretionary sediments bounded by the crustal block of the island arc around the eastern end of the rupture zone of the 1944 event. This model shows that the subduction oceanic crust has a particularly irregular geometry 40 km landward from the deformation front. Comparing our crustal models with a recently published coseismic rupture distribution of the 1944 event deduced from tsunami waveforms, there is no slip in the east of the irregularity of the slab. Moreover, it seems that coseismic rupture propagated northeastward avoiding this irregularity. The spatial relationship between the irregularity of the slab found in this study and the rupture distribution is analogous to that between the subducted seamount found off the Shikoku Island and the rupture distribution of the 1946 Nankai earthquake. Although the existence of a subducted ridge parallel to the Nankai Trough off the Tokai district has been suggested based on published geomagnetic data and seafloor topography, our crustal models may support the existence of a slab irregularity more suggestive of a subducted seamount, at least, at the eastern end of the coseismic rupture zone of the 1944 event. Such irregular geometry of the subducting oceanic crust, in particular subducted seamounts, may be one of the causes that control the rupture process of great earthquakes along the Nankai Trough.