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United States Geological Survey

DATA AND METHOD Earthquakes observed by a network of the National Research Institute for Earth Science and Disaster Prevention, NIED (Hamada et al. 1985) were used in our analyses. Using initial P-wave readings within an accuracy of 0.1 s, we analysed earthquakes located in the ranges 601xs110km, 501ys100km and 30~z~80km by the NIED in the period from January 1983 to May 1989 and observed at six or more stations among 31 stations shown in Fig. 1. These stations are at distances less than 120 km from the centre of the epicentral region. The origin of (x, y) coordinates is (139.0°E, 35.5″N). Fig. 2 shows the hypocentre distribution of earthquakes in the southwestern Ibaraki prefecture routinely determined by the network. The total number of earthquakes is 753, and serial numbers are used to label mechanism diagrams in the following figures. The P-wave velocity model used in this study is shown in Table 1 (after Hurukawa & Imoto 1987). We used a method of modified joint hypocentre determination (MJHD) developed by Hurukawa & Imoto (1990) in relocation of local earthquakes. Since the medium is very heterogeneous in this area and focal depths are large, solutions by a traditional JHD method (Douglas 1967; Freedman 1967) are unstable and unreliable because of the trade-off between station corrections and focal depths of earthquakes. Hurukawa & Imoto (1990) modified the JHD Downloaded from by guest on 25 July 2019 Subducting oceanic crusts and weak-zone-normal compression 641 I 0 I I 0 0 0 I I 1 I m 60. 00 80. 00 100. 00 60. 00 80. 00 100. 00 X. KM LO 01 I I I I 0 0 tn 30. 00 50. 00 70. 00 Z, KM Figure 2. Hypocentre distribution of earthquakes in the southwestern Ibaraki prefecture routinely determined by the NIED. Top left: epicentral distribution. Top right: depth distribution along NS cross-section. Bottom: depth distribution along EW cross-section. Earthquakes shallower and deeper than about 60 km in depth are supposed to occur at the plate boundary between the Eurasian and Philippine Sea plates and at that between the Philippine Sea and Pacific plates, respectively. The radius of a circle is proportional to the magnitude. The origin of coordinates (x, y) is located at 139.UoE, 3S.S”N. K: Kinugawa cluster.