An earthquake with M6.6 occurred about 50 km southeast off Kikai-jima which is located near Amami-Oshima in Kagoshima prefecture at 19 : 37 October 18, 1995. About 16 hours after the event, an another earthquake with M6.5 occurred in the same region. Seismic intensity of 5 in the JMA scale was recorded at Takigawa in Kikai-jima for the two earthquakes. Focal mechanisms of the two earthquakes were normal fault-type, and they were estimated to have had occurred within the Philippine Sea plate. Tsunamis were observed at Pacific coasts in southwest Japan. Tsunami waves were higher than usual at Kikai-jima, relative to seismic magnitude, at the two earthquakes. Characteristics of the seismic activity before and after the near Amami-Oshima earthquake are summarized as follows : 1. Preceding seismic activity and foreshocks were observed. 2. Another earthquake with almost the same magnitude succeeded after a short interval. 3. Following characteristic patterns were recognized in seismicity : Seismicity was decreased noticeably in the northeastern part of the aftershock area just before larger shocks over M5. While conspicuous decrease in seismicity was observed in the southwestern part of the aftershock area after the earthquake of M6.5.
A shallow earthquake of MJMA 6.7 on October 18, 1995, at about 50km southeast off Kikai-jima in the Ryukyu Islands caused slope landslides, subsidence of concrete slabs at fishing ports and collapses of walls piled up with coral ragged stones. Several fishing boats moored to wharves were damaged by tsunami; four boats were turned over at ports in Amami-Oshima. From May 19 to 30, 1995, earthquakes of MJMA≤4.1 had occurred in the focal area after a long seismic quiescence since the 1911 Kikai-jima Earthquake of M=8.0. Some of them were felt in JMA intensity 1 or 2 at Kikai-jima. Numerous felt shocks also struck the island immediately after the main shock; 62 and 125 shocks on October 18 and 19, respectively. Numbers of aftershocks decreased rapidly, though they lasted through the end of 1996. The damage distribution of the 1911 event being compared with that of the 1995 event, their focal areas were considered to overlap with each other. It was, however, possibly an interplate earthquake of a thrust fault type in contrast with the 1995 event of a normal fault type in the subducting plate, because the first motions of P wave at Naze and of tsunami wave at Kikai-jima were reversed between the two events.
A field survey of the damage of the Amami-Oshima-Kinkai earthquake (Mw 7.1) and its tsunami of October 18, 1995 was conducted for four days from the next day of the main shock. A large aftershock (Mw 6.8) occurred in the next day and was also accompanied with a small tsunami. We conducted interviews of the inhabitants, and measured heights of the both tsunamis. The maximum height of the tsunami of the main shock was 3.0 m at Urahara Port on the south coast of Kikai-jima. Earthquake damage mainly occurred on Kikai-jima, where stone walls were fallen down at 91 places, the water supply facility of the island was disordered, and several concrete slabs slid down at Wan Port. Several fishing boats were wrecked due to the both tsunamis at ports on Kikai-jima and Amami-Oshima. Imamura-Hatori's magnitudes of the main shock and the large aftershock are m=1.0 and m=0.0, respectively. Abe's magnitude of the tsunamis the main shock is estimated to be Mt=7.6 and is large for the earthquake magnitude. A gigantic earthquake (M8.0) with a larger tsunami occurred on June 15, 1911 in the sea region close to the present event, and its folktale is handed down by the inhabitants of Kikai-jima.
An earthquake of MJMA 6.7 occurred at 19 : 37 on October 18, 1995 at about 50 km southeast of Kikai-jima in the central part of Ryukyu-islands, Japan. About 16 hours after this event, another large earthquake of MJMA 6.6 occurred in the same area. We relocate hypocenters of two large events and their aftershocks using JMA data by considering the station corrections, which are obtained by joint using the temporary observation data of ocean bottom seismometers (OBS) and JMA data. The relocated hypocentral distribution obtained by joint using of OBS and JMA data show the possibility that two large events occurred on the adjacent faults. The events during four days after the MJMA 6.7 event occurred in the region of about 70 km in length in NNE-SSW direction and the epicentral area is divided into two sub-regions, NNE and SSW sub-regions. The aftershock activity of the MJMA 6.7 event is predominant in the SSW sub-region until 7 : 00 a.m. on the next day. After 7 : 00 a.m., the activity increases in the NNE sub-region and the MJMA 6.6 event at 11 : 41 a.m. on October 19 and its aftershocks occurred in this region. On the other hand, the activity in SSW sub-region becomes low in this period. Such the alternative rise and fall activities in two sub-regions are thought to be caused by the interraction of stresses on the different faults. Therefore, the present activity is concluded to be constituted by two foreshockmainshock-aftershock sequences occurred on the adjacent faults in a short time interval.
On October 18, 1995, an earthquake with magnitude Mj 6.7 occurred in the central part of Ryukyu Arc at 28°1.7'N, 130°23.0' E and 38 km in depth. The next day, another earthquake with magnitude Mj 6.6 occurred in the same area. The hypocenter parameters were determined by Japan Meteorological Agency (J.M.A.). For the purpose of investigating the aftershock activities finely, we put an array of twenty-two ocean bottom seismometers (OBSs) covering the area of aftershocks. The OBS observation started on October 28, ten days after the occurrence of the Mj 6.7 event, and continued about four weeks. In this study, we relocated hypocenters of the aftershocks using the data of eighteen OBSs. The results give us detailed view of the hypocenter distribution as follows : Most of aftershocks occurred (1) on the fault of Mj 6.7 event, and (2) in the focal area of Mj 6.6 (October 19, 1995). Considering the distributions of (1) and (2), we identified one of the nodal planes of the CMT solutions as the fault plane of Mj 6.7 event and Mj 6.6 event. For both events, the strike is almost parallel to the trench axis. Dip angle of the Mj 6.7 event is almost vertical. Therefore, it is obvious that this event is not a low-angle thrust-type interplate earthquake but an intraplate event.
Twin earthquakes (the Amami-Oshima-Kinkai Earthquakes) occurred at the southeast from Kikai Island on October 18 (MJMA 6.6) and 19 (MJMA 6.5), 1995. We observed the aftershocks at Kikai Island from October 21, 1995 to February 18, 1996 using the broadband strong-motion seismometer (VSE11C/12C). After all, we observed 40 events that include the second largest aftershock, the event of November 1 (MJMA 5.6). In this paper we model the strong-motion records of six events among the observed aftershocks, which have large amplitudes and rather simple waveforms to estimate their focal parameters (focal depth, strike, dip, rake, seismic moment and rise time), by try and error. The results suggest the following : (1) The strikes of the mainshocks of October 18 (the first mainshock) and 19 (the second mainshock) might be about N230°E and N185°E, respectively; (2) The hypocenter of the first mainshock might locate near the north edge of the fault which was ruptured from north to south along the fault strike, while the hypocenter of the second one might locate near the south edge of the fault which was ruptured from south to north.
Several earthquakes, which occurred near Kikai-jima from October 18 to November 1, 1995, are examined by means of teleseismic body waves. All the focal mechanisms determined for these earthquakes reveal normal-faults having a down-dip extension within a subducting slab. The main source parameters of the largest event are : (strike, dip, rake) = (208°, 75°, -92°); the seismic moment=5.9×1019 [Nm] (Mw=7.1); fault area =6.0×108 [m2];dislocation=1.5 [m]; rupture duration=23 [s]; stress drop =10 [MPa]. In this event the rupture propagated unilaterally to SW direction. The second largest event ruptured the NE region, where the fault plane did not smoothly connected but formed a step to the main fault plane. It may be noteworthy that these earthquakes did not trigger any significant inter-plate earthquakes, suggesting that the adjacent plate interface may be intrinsically aseismic.
We estimated the fault parameters of the 1995 Amami-Oshima-Kinkai Earthquake, which occurred along the Ryukyu trench with a normal fault mechanism. The earthquake generated larger tsunamis than expected from its magnitude (MJMA 6.7), about 3 m on nearby Kikai-jima Island. Among six fault models we considered, the low angle fault dipping to the east (model LA) best explains the observed data such as the aftershock distribution and the horizontal coseismic movement of Amami-Oshima Island inferred from GPS measurements. The fault is 60 km long, 30 km wide, 10-20km deep and the average slip on the fault is 1 m. Tsunami numerical computations indicate that the tsunami amplitudes, particularly at far-field, are insensitive to the fault parameters, suggesting that the large tsunami was not due to unusual source process. Numerical computations also show that the bathymetry between the source and Kikai-jima Island is responsible to the large tsunamis observed on the island.
The 1995 Amami-Oshima-Kinkai Earthquake occurred near the Nansei-Shoto Trench where the upheaval area of the Philippines Sea plate subducts beneath the Nansei-Shoto islands. The main shock was MJMA 6.6 and its largest aftershock was MJMA 6.5. The aftershock distribution for these two events by Yamada et al. (1996) corresponds to two distinct and nearly vertical fault zones. The focal mechanisms obtained by Kikuchi (1996) are consistent to the aftershock distribution. The authors propose that the seamount found beneath the trench-continental-slope indirectly triggered this earthquake activity. If a subducting oceanic plate is normal oceanic denser than an overriding island arc, the oceanic plate should be faulted near vertically priori to the plate subduction by horizontally tensional force due to plat bending. On the other hand, an oceanic plate with seamounts or an oceanic plateau lighter than a normal oceanic plate, might resist to plate subduction due to its small density and delaying normal faultings might occur in the subducting oceanic plate. The delaying normal faultings between a subducting seamount and a preceding normal portion of the oceanic plate can compensate the subduction process. The compressional convergence margin such as the Nankai Trough, however, may not generate such normal faultings due to the nature of stress field. The low seismicity area existing across the trench axis is also seen both in this aftershock activity and ISC hypocenters. This is the same result as those in other regions. This might imply low earthquake potential for this portion of plate interface due to the existence of low density sediments and water contained in the sediments.
The Amami Plateau is a remnant arc situated in the northwestern part of the Philippine Sea Plate. The convergent rate of the Philippine Sea Plate relative to the Ryukyu Arc is 5.0 to 7.0cm/yr in a N50° W direction (Seno et al., 1993). The morphology of the continental slope of the collision area between the Amami Plateau and the Ryukyu Arc is characterized by a wide variety of tectonic landform. Since 1983 HDJ (Hydorographic Department of Japan) has conducted geophysical surveys by S/ V TAKUYO and S/ V MEIYO under the Continental Shelf Surveys Project and the Earthquake Prediction Project in the adjacent water of Japan. The area surveyed by these projects includes the northern part of Ryukyu forearc and the northwestern part of the Philippine Sea Plate. HDJ obtained SeaBeam swath bathymetric data, single channel seismic reflection profiles, and magnetic and gravity data in the collision area. These surveys reveal the process of the formation of the tectonic landforms caused by the collision between the plateau and the arc. The continental slope of the collision area is characterized by two tectonic landforms, ridges and minor ridges. Ridges are distributed on the topographic high including the Kikai Island, which are NE-SW trending straight ridges and valleys. The north and south ends of the topographic high are consistent with these of the Amami Plateau. Minor ridges are distributed on the gentle continental slope in the east of the topographic high with the ridges. These minor ridges are divided into two groups by the strike of the ridge axis, which are NE-SW and NW-SE trending ridges. The NE-SW trending ridges are distributed on three blocks in the gentle slope bordered by NW-SE trending ridges and valleys. A subducted seamount, which is estimated from the magnetic anomaly, lies below the central block, and the Kikai Seamount borders on the northeastern block on the trench. The collision between the Amami Plateau and the arc results in large tectonic effects to the Ryukyu Arc. In the collision area the continental slope is uplifted in two difference processes correspondent with two tectonic landforms, one is uplift caused by the buoyant subduction of the plateau, the other is uplift caused by the subduction of seamounts on the plateau.