After the occurrence of the 2011 magnitude 9 Tohoku earthquake, the seismicity in the overriding plate changed. The seismicity appears to form distinct belts. From the spatiotemporal distribution of hypocenters, we can quantify the evolution of seismicity after the 2011 Tohoku earthquake. In some earthquake swarms near Sendai (Nagamachi-Rifu fault), Moriyoshi-zan volcano, Senya fault, and the Yamagata-Fukushima border (Aizu-Kitakata area, west of Azuma volcano), we can observe temporal expansion of the focal area. This temporal expansion is attributed to fluid diffusion. Observed diffusivity would correspond to the permeability of about 10 À15 (m 2 ). We can detect the area from which fluid migrates as a seismic low-velocity area. In the lower crust, we found seismic low-velocity areas, which appear to be elongated along N-S or NE-SW, the strike of the island arc. These seismic low-velocity areas are located not only beneath the volcanic front but also beneath the fore-arc region. Seismic activity in the upper crust tends to be high above these low-velocity areas in the lower crust. Most of the shallow earthquakes after the 2011 Tohoku earthquake are located above the seismic low-velocity areas. We thus suggest fluid pressure changes are responsible for the belts of seismicity.

Earthquake focal mechanisms before and after the 1995 Hyogo-ken Nanbu earthquake have been investigated using seismic records from regional seismic networks. Before the mainshock, seismicity was very active at the Tamba Plateau, a neighboring area of the Hyogo-ken Nanbu earthquake rupture zone. In contrast, the seismicity along the Hyogo-ken Nanbu earthquake rupture zone was not so active. Most earthquakes in these regions had source mechanisms of E-W compression and were of the strike-slip or reverse-fault type. Most aftershocks along the Hyogo-ken Nanbu earthquake rupture zone have strike-slip solutions with P-axis in the E-W or ESE-WNW direction, which is compatible with the trend of aftershock distribution and the strike of active faults the same as the mainshock mechanism. Simultaneously, many other aftershocks were of the reverse-fault type with E-W compression. This area is still controlled by the regional stress field of E-W compression observed before the mainshock. Although, we could find various types of mechanisms in the aftershock sequence, some normal fault-type events were also observed in the mainshock rupture zone. We could find events of SE-NW compression, and this direction is nearly perpendicular to the trend of the mainshock rupture zone. Some aftershocks that occurred near the epicenter of the mainshock had solutions of N-S compression. The geometry of the active fault systems and/or local stress change induced by the mainshock may cause these complex features of focal mechanisms. After the mainshock, the focal mechanisms of earthquakes in the Tamba Plateau were approximately E-W compressional; the same as that before the mainshock.

We carried out high density aftershock observations a week after the 2000 Western Tottori Earthquake for 40 days. We deployed 72 seismic stations in and around the aftershock area. The average spacing of the stations in the aftershock area was 4-5 km. We determined accurate hypocenters and focal mechanisms for ∼1,000 aftershocks and obtained a high resolution 3-D velocity structure in the source region. High P and S wave velocity anomalies (> 4%) near the southeasternmost aftershock area at 2 km depth correlated with Jurassic to Late Cretaceous plutonic and high pressure metamorphic rocks. The depth distribution of the P and S wave velocities along the mainshock fault showed that high velocity anomalies were located at the shallow southeastern edge and the deeper central part of the aftershock area. The ratio between P and S wave velocities in the high velocity anomalies was a little higher (∼1.75) than the average value (∼1.70) in the upper crust. These results indicate that the high velocity anomalies could correspond to the plutonic or metamorphic rocks. The distributions of the high velocity anomalies and large slips of the mainshock were complementary. These suggest that the high velocity anomalies could be stronger than the surrounding materials and might behave as barriers to the mainshock rupture.

An earthquake with surface magnitude (Ms ) 7.0 occurred 100 km off the Nicaraguan coast on September 2, 1992 (GMT). Despite its moderate size, this earthquake generated a sizable tsunami, which caused extensive damage along the coast of Nicaragua. In late September, about 170 people, mostly children, were listed dead or missing; 500 were listed injured; and over 13,000 were listed homeless, with more than 1500 homes destroyed. Damage was the most significant since the 1983 Japan Sea earthquake tsunami, which killed 100 people in Japan. The Flores (Indonesia) earthquake and tsunami of December 12, 1992, were more destructive than the Nicaragua or Japan Sea events.

Abstract. The seismicity in the Tamba region, northeast of the Hyogo-ken Nanbu earthquake in Japan (January 17, 1995; MjM A 7.2), increased significantly following this earthquake. This increase suggests that the static stress change due to a large earthquake causes a change in the crustal condition or dynamics. In order to reveal the changes quantitatively, we investigate the temporal variation in coda Q A temporal change in coda Q-• was first found by Chouet The 1995 Hyogo-ken Nanbu earthquake in Japan (MjM A 7.2) provides high-quality data with which to study the variations in coda Q-1 and b value produced by a stress change in the crust. In this study we carefully examine the response of coda Q-• and b value due to the static stress change in the Tamba region, which is located to the northeast of the mainshock rupture zone. Tectonic and Seismic Features in the Tamba RegionThe Tamba region is located in central Honshu, Japan (Figure 1). This region has the densest distribution of Quaternary active faults in Japan and has very high seismicity (Figure 1 Since the Hyogo-ken Nanbu earthquake, the seismicity in the Tamba region has increased (Figure 2). Hashimoto [1996, 1997] calculated the change in the Coulomb failure function ACFF due to the Hyogo-ken Nanbu earthquake using a fault model with six fault segments derived mainly from Global Positioning System (GPS) observations. He reported that the rapid increase of microearthquake activity in the Tamba region was induced by an increase in ACFF (-0.04 MPa for strike-slip faults trending N45øE and their conjugates). Katao et al. [1997] studied the focal mechanisms of microearthquakes in and around the rupture zone of the Hyogo-ken Nanbu earthquake in detail. They found no change in the direction of the regional stress field of the Tamba region after the Hyogo-ken Nanbu earthquake. Summary of the Mean Values and the Standard Deviations of log (Q•) and the Results of the Statistical t TestBefore After