Crustal Earthquakes In Japan Research Articles

Evidence for the location and cause of large crustal earthquakes in Japan

Large crustal earthquakes (magnitude 5.7–8.0, depth 0–20 km) during a period of 115 years from 1885 to 1999 in Japan occurred in or around zones of low seismic velocity revealed by seismic tomography. The low‐velocity zones may represent weak sections of the seismogenic crust. The crustal weakening is considered to be closely related to the subduction of the oceanic Pacific and Philippine Sea plates in this region. Along the volcanic front and in back‐arc areas the crustal weakening may be caused by active volcanoes and magma chambers. In the forearc areas, fluids are detected in the earthquake source zones, which may have contributed to the crustal weakening and the rupture nucleation. The fluids may be related to the dehydration of the subducting Philippine Sea slab. These results indicate that large crustal earthquakes do not strike anywhere, but only anomalous areas that may be detected with geophysical methods.

地震発生域の地震波速度

We present a method to abtain the in situ seismic velocity in the sourece region of crustal earthquakes. The procedures of the method are as follows.1. Determination of hypocenters of earthquakes in a small area using data at only nearer stations.2. Calculation of a horizental apparent velocity and an emergent direction of seismic waves using travel times of earthquakes at each distant station. When we use the direct waves at a distant station, a horizontal apparent velocity is nearly equal to the true velocity.3. Calculation of the true velocity from horizontal apparent velocity and vertical one which is determined by the focal-depth dependence of travel times at each station.We applied this method to the eastern Yamanashi earthquake in central Japan. The focal depths of these earthquakes are 16-20km, and are somewhat deeper than those of usual crustal earthquakes in Japan. Direct waves were obseved as first-arriving waves at 12 stations with wpicentral distances of 55-156km which we used in this analysis. The horizontal apparent velocity and the true valocity were determined 6.17±0.14 and 6. 09±0.12km/sec, respectively. We can conclude, therefore, that the eastern Yamanashi earthquakes have occirred in the upper crust with the P-wave velocity of 6. 1km/sec.We can also obtain the azimuthal anisotropy in the media, the P-wave velocity is 6.09+0.08 When we assume the azimuthal anisotropy in the velocity is reduce from 0.115 to 0.095km/sec. THe maximum velocity is obtained in the NE-SW direction which is also the orientation of the maximum tension axis of earthquakes in this region.