Explosion Seismic Studies of the Underground Structure in the Matsushiro Earthquake Swarm Area

Abstract

The underground structure in the Matsushiro Earthquake Swarm Area was derived from the data of explosion seismic observations in profiles A and B. First, the number of layers and the velocity in each layer are determined by using the T' curve. Then on the assumption that the dip of each interface is small, the structure of the first approximation was derived. The final models were derived by the method of trials and errors so as to reduce (O-C), the difference between observed and calculated travel times. The underground structure in profile A consists of the following three layers on the whole: the first layer : 1.6-2.3 km/s the second layer : 3.3-4.75 km/s the third layer : 5.9-6.0 km/s In profile B the underground structure also consists of three layers, but there is a surface layer in some places: the surface layer : 0.36-1.2 km/s the first layer : 1.7-3.2 km/s the second layer : 4.0-4.4 km/s the third layer : 6.0 km/s The underground structures derived by using above velocities are given in Fig. 2 for profile A and in Fig. 3 for profile B. The underground structure in profile A, which is in the Central Belt of Uplift named geologically and is parallel to its strike, is relatively simple. Generally speaking, the top of the third layer (with the velocity of 6.0km/s) is unusually shallow, about 1.5km from the earth's surface even at the deepest, and has a tendency getting shallower a little toward the southwest. Or it may be said that the depth to the top of the third layer is smaller by 0.3-0.5km in the portion from the middle of E2 to D8 than the other portions. The velocity in the third layer is 6.0km/s in the area northeast of Matsushiro and 5.9km/s in the area southwest of Matsushiro. There is a possibility of existence of anomalous structure or a fault between E4 and E5 although there are no observation points in this area because of the topography. This anomalous struc-ture may correspond to the low Bouguer anomaly around Mt. Minakami. The underground structure in profile B, which passes the central part of the epicentral area and crosses the Central Belt of Uplift, is fairly complicated. The velocity in the first layer is 1.7-3.2km/s and this layer becomes thick abruptly near the Chikuma River toward the northwest. The velocity in the second layer is 4.0-4.4km/s and it is noteworthy that the thickness of this layer increases by about 3km almost discon-tinuously around the middle of E1 near Nagano City. From another point of view, the top of the third layer becomes very shallow in the central part of the profile and reaches only about 1km from the earth's surface. This interface becomes deep toward the southeastern end of the profile, so that the deep structure of the Central Belt of Uplift may be defined. At least the layer with the velocity of 6.0km/s might be related to the formation of the Central Belt of Uplift, or it might be said that this layer played an important role for its formation. This profile passes the foot of Mt. Mina-kami at D7 and the underground structure has anomaly near Mt. Minakami which may correspond to low Bouguer anomaly. There is a large possibility that the anomalous structure in this profile is of the same origin with that in profie A since the locations of anomalous structures are close with each other and their manners are quite similar. It is quite interesting that there exist anomalous underground structures in the area where Matsushiro swarm earthquakes have occurred most frequently. Also it is interest-ing to note that there is no layer with the velocity of about 5.5km/s in the area concerned. On the other hand by the comparison of the underground structure in this paper with results from other geophysical and geological investigations, several interesting relations are pointed out.