Akiko Hasemi

Extensional structure in Northern Honshu Arc as inferred from seismic refraction/wide‐angle reflection profiling

A recent extensive seismic wide-angle experiment revealed a new image of crustal and uppermantle structure across Northern Honshu Arc, Japan. The western part of the arc recorded the crustal deformation by the Miocene back arc spreading of the Sea of Japan. The crust is composed of highly deformed Tertiary sedimentary layers, a relatively low velocity (5.75–5.9 km/s) crystalline basement and a 15-km thick lower crust with a velocity of 6.6–7.0 km/s. Clear westward crustal thinning from 32 to 27 km represents the extensional deformation by the backarc spreading. The crust attains the maximum thickness (32–35km) east of the backbone range for which the magmatic intrusion/underplating since 10–15 Ma is a predominant factor. The eastern part of the arc has a less deformed upper crust and a reflective middle/lower crust, probably remaining a stable block since the time of the backarc spreading.

Fine structure beneath the Tohoku district, northeastern Japan arc, as derived by an inversion of P-wave arrival times from local earthquakes

Abstract Three dimensional P-wave velocity structure beneath the Tohoku district, northeastern Japan arc, is investigated by an inversion of arrival times from local earthquakes using the method originally due to Aki and Lee (1976). In the crust (0–32 km depth) a low-velocity region is found along the volcanic front and its vicinity. Velocities at depths of 32–65 km are low beneath the regions where many Quaternary volcanoes and geothermal areas are distributed. In the region deeper than 65 km, the subduction of the Pacific plate is clearly revealed, and the mantle structure above the descending plate is rather uniform. These features suggest that volcanic activities have relation to the upper mantle structure. The results obtained in this study will be helpful in investigating the mechanism of magma generation in a subduction zone.

Three-dimensional P wave velocity structure of shallow crust beneath the northern Awaji Island derived from refraction seismic explosion

After the 1995 Hyogo-ken Nanbu (Kobe) earthquake (M7.2), the Research Group for Explosion Seismology (RGES) conducted a refraction seismic experiment in the northern part of the Awaji Island in order to investigate a detailed structure around the source region. In the surveyed area, 145 temporal observation stations were deployed two-dimensionally at 200–250 meters intervals, and 7 shots of 150 kg charges were detonated. Using the first arrival times, we derived a three-dimensional P wave velocity structure of the shallow crust in this region. A dominant feature was the low velocity in the northern part of the region as compared with the southern part. The averages of velocities in the northern area were 4.1 km/s (at a depth of 0.09 km) and 4.5 km/s (at 0.49 km). In the southern region, these were 4.9 and 5.2 km/s, respectively. The study region was mostly within the Cretaceous granitic rocks, and we cannot find a difference in geology that corresponds to the velocity variation. On the other hand, the Bouguer anomaly and the resistivity distributions are consistent with the lateral velocity difference.

Seismic reflector alignment along the volcanic front in northeastern Honshu, Japan

[1] We found seismic reflectors aligned along the east side of the volcanic front in northeastern Honshu, Japan, by analyzing seismograms of intermediate-depth earthquakes recorded at High Sensitivity Seismograph Network Japan stations. We identified an unusual high-frequency later phase on the seismograms, which arrives after the direct S waves and continues for >5 s. The characteristics of the waveforms suggest that this later phase is an S wave incidence. We plotted envelope waveforms and identified later phase arrivals on envelope waveforms. To estimate the location of the source of the later phases, we defined an event group that consisted of six to eight events distributed in a region of ∼50 km 3 . The source of the later phases was located by a combination of a station and an event group using later phase travel times. If we assume that later phases arriving at a station from an event group are generated by a common source, then we can use the observed travel times in a grid search to obtain the source location. The sources of the later phase were found to be aligned along the east side of a volcanic front at a depth of 20-30 km. The averaged ratio of the amplitudes of the later phases to the amplitudes of the incident waves on the later phase source was estimated to be ∼0.6. This result suggests the existence of unusual reflectors spread in the lower crust beneath the fore-arc region. It may be possible that the later phase sources are steeply inclined bodies filled with fluid.