We have developed a method for three-dimensional simultaneous velocity and density inversion using traveltimes of local earthquakes and gravity data. The purpose of this method is to constrain the velocity inversion and increase the spatial resolution of shallow velocity structures by introducing additional gravity data. The gravity data contributes to the P- and S-wave velocity models by imposing constraints between seismic velocities and density. The constraint curve is constructed so as to fit the data for porous rock samples, and deviations from the curve are taken into account in the inversion. The constraint is imposed at only the first layer, because density structure is well resolved at shallower parts and it is difficult to determine uniquely at greater depths. Synthetic inversion tests indicate that gravity data can improve the resolution of the velocity models for this layer. The method is applied to investigate the subsurface structure of Izu-Oshima volcano, Japan and velocity structures with high spatial resolution are obtained. The additional gravity data contribute primarily to improvement of the S-wave velocity model. At 0.25 km depth, a high velocity anomaly due to caldera-filling lava flows is observed. At 1.25 and 2.5 km depths, high velocity intrusive bodies are detected. A NW-SE trending high velocity belt at 1.25 km depth is interpreted as being caused by repeated intrusion of dikes.

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On October 6, 2000, the 2000 Western Tottori Earthquake (Mjma 7.3) occurred in the western Tottori prefecture area, southwestern Japan. It initiated at a depth of 12 km at the bottom of the seismogenic zone, which was derived from aftershock distribution. The aftershocks extend over a 35 km length in a north-northwest direction. Spatial and temporal distribution of the aftershocks exhibits local characteristics in the fault region. The northern part consists of earthquake clusters while the southern part consists of a rather simple lineament of aftershocks, and the spreading and decaying rate of the aftershocks is slower in the northern part. This contrast is possibly due to the heterogeneity of the fault system and probably affected the rupture process of the mainshock. Two swarm sequences occurred in the surrounding region after the mainshock. One initiated 48 hours after the mainshock 25 km southwest of the main aftershock distribution. The other started 20 hours after the mainshock northeast of the mainshock on the southeast flank of Daisen volcano. These activities are probably induced seismicity due to stress changes in the focal region. Pre-seismic swarm activities occurred in the focal region from 1989 and deep low-frequency earthquakes were observed since 1999. It is important to understand the relationship between these possible precursory phenomena and the occurrence of the mainshock.

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Swarm-like seismic activity including six moderate events (Mj = 5.1-5.4) occurred in 1989, 1990 and 1997 in the same area as the 2000 Western Tottori Earthquake (Mj = 7.3). For each time period, we carried out temporary seismic observations in and around the source area and processed the data together with data from permanent stations, to determine the hypocenters precisely. In this study we also redetermined the earthquake locations in each seismic activity using a two-step master event technique with common master events, so that the accuracy in the relative locations of the events was improved. The purpose of this study is to clarify the relationship between the preceding seismic activity and the mainshock in 2000 by comparing the hypocenter distributions. The relocated hypocenter distributions show that the three preceding swarms occurred in different parts of the same fault plane as the 2000 Western Tottori Earthquake. The b-values of the preceding swarms were low (0.51-0.67), suggesting a high stress level in the area. The mainshock initiated in the area of the preceding swarms. The rupture propagated with relatively small slip (-1 m) in the area for the first three seconds. Then, it developed to main rupture with large slip (2-4 m) outside the area toward the southeast.

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The b-value of the frequency-magnitude distribution and the parameters in the modified Omori law, describing the decay rate of aftershock activity, are investigated for more than 4000 aftershocks identified in the first four months after the Western Tottori earthquake (October 6, 2000). We used the JMA data catalog, containing aftershocks with magnitude larger than or equal to 2.0. The studied area is first divided into three areas: one region (A) corresponding to the main aftershock area and other two (B and C) corresponding to seismic activity probably triggered by the stress change caused by the main shock. For region A, the magnitude of completeness (Mc) decreases with time, from the largest value of 3.2 in the first two hours of the sequence, to 2.0, about four days after the main shock. Taking the threshold magnitude as 3.2, we estimated the b-value for the whole region A to be about 1.3 and p-value around 1. However, highly significant variations in both b and p values are found when analyzing their spatial distribution in region A. The seismic activity in the regions B and C started about 2.5 days after the main shock. The b-value for region B (Mc = 2) is 1.05. The decay rate of earthquake activity in Region B is well modeled by the modified Omori law and the p-value is found to be relatively low (0.83). The number of events in region C is too small for a meaningful study. The physical interpretation of the spatial variation of the parameters is not straightforward. However, the variation of b-value can be related to the stress distribution after the main shock, as well as the history of previous ruptures. Thus, the relatively low stress in the regions that have already experienced rupture is probably responsible for the larger value of b found in these areas. Regions with relatively low b-value, on the other hand, are probably regions under higher applied shear stress after the main shock. Alternatively, one can hypothesize that the areas that experienced slip are more fractured, favoring higher b-values. The larger p-values correlate well with the regions that experienced larger slip during the main shock, while small p-values are found generally in regions that have not ruptured recently. The variation of p-value can be related with the frictional heating produced during rupture. The crustal structure may explain some local features of b and p value spatial distribution. In order to verify our hypothesis we also analyzed the seismic activity that occurred before the Tottori earthquake, starting in 1978, using the data of DPRI, Kyoto University. It seems that the previous seismic activity associated with some moderate events in 1989, 1990 and 1997 had an influence on the following seismicity in the area-in particular on the spatial distribution of b and p values observed for the aftershocks of the Tottori earthquake. The aftershocks of the 1997 M5.5 earthquake have a larger p-value than previous aftershock sequences, while the b-value has a clear increase following the M5.5 event.

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The degree of damage to wooden houses in the vicinity of the source area of the 2000 Western Tottori Earthquake was very low compared to the 1995 Hyogo-ken Nanbu Earthquake. Shimoenoki in Hino Town, however, suffered remarkable damage compared to other villages, particularly to residential wooden houses. Furthermore, although Shimoenoki is a small area of only 300 × 400 m², the damage varied markedly from the foot of the mountains through central Shimoenoki to the Hino River. From the damage distribution based on a survey of all wooden houses, the local site-amplification characteristics estimated from aftershock records, and the transfer functions of wooden houses evaluated using microtremors, the spatial variation of damage appears to be attributed to the variation in site-amplification factors at frequencies between 2 and 5 Hz, which is close to the first natural frequency of wooden houses.

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