Tanio Ito

Delamination‐wedge structure beneath the Hidaka Collision Zone, central Hokkaido, Japan inferred from seismic reflection profiling

In the Hidaka Collision Zone of Hokkaido, northern Japan, the Kuril island arc collides with the northeast Japan arc. In order to better understand the collision process, new high resolution information about lithospheric structure was obtained by means of a series of seismic reflection surveys. These seismic profiles reveal distinct zones of seismic lamination in the lower crust of the Kuril arc. The upper portion of the lower crust is characterized by numerous east-dipping reflectors. In contrast, west-dipping reflectors dominate the lower part of the lower crust. From this reflector configuration, the lower crust of the Kuril arc is interpreted to be delaminated by the collision. The geometry of delamination is consistent with other geophysical data, as well as the peak metamorphic grades exposed in the Hidaka mountains. As earthquakes indicate that delamination here is ongoing, the Hidaka Collision Zone represents an active model for continental growth by arc accretion with delamination of lower arc crust.

Gently north-dipping Median Tectonic Line (MTL) revealed by recent seismic reflection studies, southwest Japan

Abstract The Median Tectonic Line (MTL), with a length of more 1000 km, is the most significant fault in Japan. It juxtaposes the high- P/T Sambagawa metamorphic rocks against the low- P/T metamorphic rocks of the Ryoke belt. The MTL was probably formed in the Cretaceous with many subsequent reactivations. The western segment of the MTL is still active with an almost pure right-lateral sense of motion. Although a great amount of geological information on the MTL has been accumulated, information about the subsurface, especially the deep-seated structure of the MTL, is still insufficient. It has been generally assumed that the MTL is vertical or steeply dipping at depth because of its straight surface trace and its recent lateral motion. Recently, new geophysical data have suggested that the MTL dips gently northward at depth. We have acquired a complementary set of geophysical profiles (seismic reflection and refraction, gravity and MT) across the MTL in east Shikoku. Our results confirm that the MTL dips northward at about 30 to 40 degrees from the surface to about 5 km depth, where it becomes listric. This fault geometry more reasonably explains the reactivation history of the MTL: the motion has occurred on a listric-type fault in so-called oblique or lateral ramp manner.

Seismological features of island arc crust as inferred from recent seismic expeditions in Japan

Abstract Crustal studies within the Japanese islands have provided important constraints on the physical properties and deformation styles of the island arc crust. The upper crust in the Japanese islands has a significant heterogeneity characterized by large velocity variation (5.5–6.1 km/s) and high seismic attenuation (Qp=100–400 for 5–15 Hz). The lateral velocity change sometimes occurs at major tectonic lines. In many cases of recent refraction/wide-angle reflection profiles, a “middle crust” with a velocity of 6.2–6.5 km/s is found in a depth range of 5–15 km. Most shallow microearthquakes are concentrated in the upper/middle crust. The velocity in the lower crust is estimated to be 6.6–7.0 km/s. The lower crust often involves a highly reflective zone with less seismicity, indicating its ductile rheology. The uppermost mantle is characterized by a low Pn velocity of 7.5–7.9 km/s. Several observations on PmP phase indicate that the Moho is not a sharp boundary with a distinct velocity contrast, but forms a transition zone from the upper mantle to the lower crust. Recent seismic reflection experiments revealed ongoing crustal deformations within the Japanese islands. A clear image of crustal delamination obtained for an arc–arc collision zone in central Hokkaido provides an important key for the evolution process from island arc to more felsic continental crust. In northern Honshu, a major fault system with listric geometry, which was formed by Miocene back arc spreading, was successfully mapped down to 12–15 km.

Deep seismic reflection experiment using a dense receiver and sparse shot technique for imaging the deep structure of the Median Tectonic Line (MTL) in east Shikoku, Japan

A seismic experiment was carried out in east Shikoku, Japan, to detect deep reflections across the Median Tectonic Line (MTL), which juxtaposes low-P/T metamorphic rocks with high-P/T metamorphic rocks. Our experiment employed an unconventional technique: sparse shot spacing, a strong energy source (dynamite) and a dense array of seismometers. The above specifications produce only single fold coverage without common midpoint (CMP) stacking. Nevertheless, the reflection profile provides essential information on the deep structure of the MTL, of other major faults, and of the Moho in east Shikoku. On the MTL, this profile is the first to delineate the MTL from the surface to about 12 km depth. The following three factors were essential to the success of our experiment. First, the receiver interval was sufficiently small to provide horizontal resolution that was able to detect deep reflectors. Second, the simple crustal structure does not require CMP stacking to enhance data quality. Third, a thin weathering layer at the surface reduced the attenuation of seismic waves and minimized the generation of the surface waves that often obscure deep reflectors. In these conditions, the technique can be an effective means of probing the deep crust while substantially reducing survey costs.

Seismic reflection profiling across the seismogenic fault of the 1995 Kobe earthquake, southwestern Japan

Abstract The 1995 Kobe (Hyogo-ken Nanbu) earthquake (Mw 6.9) surface rupture appeared along the Nojima fault on the northwest coast of Awaji Island. The aftershock epicentres in northern Awaji Island are not aligned with the surface faulting. suggesting a complicated active fault structure. To reveal the structure of the seismogenic fault and its associated active faults, a perpendicular 41.6-km-long seismic reflection survey was undertaken across northern Awaji Island. The Trans-Awaji seismic reflection profile reveals the fault geometry beneath the area. Awaji Island was uplifted by movement on the Kariya fault along its east coast, which produced an asymmetry in Neogene basin that is shallower in the western part than in the eastern part. The faults beneath Awaji Island show a ‘pop-up’ or a ‘positive flower structure’ and the basement is bounded by two active faults, the Nojima along the west coast and the Kariya along the east coast. Both are high-angle, reverse faults with a right-lateral, strike-slip movement. The cluster of aftershocks in the northern part of Awaji Island has a Y-shaped hypocentral distribution. The Nojima and Kariya faults thus appear to connect in the middle of the upper crust at about 7 km depth, forming a seismogenic master fault. Three major seismic events have been reported on the active faults in northern Awaji Island: a prehistoric event dated 2000 yr BP, the 1596 Keicho-Fushimi earthquake, and the 1995 Kobe earthquake. Based on the structural relationship of the active faults that we observed in this study, we suggest that these events occurred on the proposed master fault at mid-crustal depth.

Geologic fault model based on the high-resolution seismic reflection profile and aftershock distribution associated with the 2004 Mid-Niigata Prefecture earthquake (M6.8), central Japan

The Mid-Niigata Prefecture earthquake in 2004 (MJMA 6.8) generated surface ruptures along the eastern rim of the Uonuma Hills. To elucidate the structural linkage between the surface ruptures and the source fault at depth, the high-resolution seismic reflection profile across the surface ruptures and nearby active faults, and the data of aftershock distribution are examined. The 5.2-km-long, high-resolution, depth-converted seismic section reveals an emergent thrust beneath the surface ruptures. A two-dimensional model of the fault geometry has been constructed based on the aftershock distribution and the shallow reflection profile. The development of the main geologic structure are well explained by forward modeling using a balanced cross-section method. In detail, the fault system generated the main shock dips at a steep angle (60°) below 5 km depth and more shallowly (30°) near the surface.

Crustal structure of the southwestern margin of the Kuril arc sited in the eastern part of Hokkaido, Japan, inferred from seismic refraction/reflection experiments

Hokkaido Island in northern Japan is located at the intersection of the Kuril and Northeast Japan arcs. Eastern Hokkaido represents the southwestern margin of the Kuril arc that evolved as an oceanic island arc, but its crustal structure has remained unclarified. In the summer of 2000, a highly dense onshore-offshore integrated seismic experiment was conducted in order to reveal the entire crustal section covering from the Kuril Trench to the back-arc basin of the Okhotsk Sea crossing eastern Hokkaido. High-quality refraction/wide-angle reflection data collected for the onshore survey line delineated a detailed crustal structure of easternmost part of Hokkaido Island. Our seismic velocity model shows a good correlation with the surface geology along the profile. The notable feature of the velocity model is the existence of a 6.0-km/s layer beneath the onshore survey line. A reflective middle to lower crust is also found beneath eastern Hokkaido. These results indicate that the southwestern margin of the Kuril arc is in a matured state of oceanic island-arc crust.