J. Phys. Earth, 36, 69-87, 1988

SEISMIC ACTIVITY IN THE SUBDUCTING PHILIPPINE

SEA PLATE ALONG THE SURUGA TROUGH

REVEALED BY OBS OBSERVATION

Motoo UKAWA,* Takao EGUCHI, and Yukio FUJINAWA

National Research Center for Disaster Prevention, Tsukuba, Ibaraki, Japan

(Received September 22, 1987; Revised April 20, 1988)

A seismicity observation with 10 pop-up-type ocean bottom seismometers were conducted along the Suruga trough, south off central Japan, for about one month in 1984. This observation detected more than 300 , among which about 60 earthquakes were located around the Suruga trough. In this observational

period, seismically the most active area is the middle part of the Suruga trough. Applying station corrections, we precisely located the earthquakes around the Suruga trough. The hypocenters show a thin seismic zone dipping westward from the trough axis. The focal depths at the trough axis, 0-10 km, are shallower by 5 to 10 km than the principal focal depths of hypocenters obtained by the land-based seismic network. The hypocentral distribution by the present survey is interpreted as the seismic activity in the uppermost part of the plate subducting from the Suruga trough axis. In contrast to the high seismicity at the middle part of the trough, only a small number of earthquakes were located in the northern and southern parts. The microearthquakes along the Suruga trough observed in the

present survey tended to occur simultaneously in the segment over about 50 km length, synchronizing with the activation of the small swarm around the northeast- ern edge of the .

1. Introduction

The Suruga trough is a northeastern extension of the Nankai trough, south off central Japan (Fig. 1), and is an interesting region for studies both in tectonics and in prediction. Tectonically, the Suruga trough is considered to be a northern end of the plate boundary of the Philippine Sea (PHS) plate (e.g.,

SUGIMURA, 1972), along which the PHS plate is subducting beneath the Eurasian

(EUR) plate. The subducting velocity of the PHS plate along the Suruga trough is estimated as 3 to 5 cm/yr in the N50•‹W direction (SENO, 1977; MINSTER and JORDAN, 1979). From a viewpoint of earthquake prediction study, the western side of the Suruga trough has attracted a great deal of attention as a hypothetical source

area of a future great earthquake and is considered to be in the stage of high seismic

* Now staying at University of Washington , Seattle, WA98195, U.S.A.

69 70 M. UKAWA, T. EGUCHI, and Y. FUJINAWA

Fig. 1. Location map of the Suruga trough. The arrow shows the direction of motion of the PHS plate. The unshaded rectangle shows the hypothetical rupture area of the future Tokai earthquake (ISHIBASHI,1981). potential at present (e.g., ISHIBASHI,1981; MOGI, 1987). The hypothetical source area by ISHIBASHI(1981) is indicated in Fig. 1. Although the Suruga trough is joined to the Nankai trough topographically, the subducting nature along the Suruga trough seems to be different from that along the Nankai trough. Along the Nankai trough, the of the PHS plate is supported by the following geophysical evidence: the subducting basement detected by the reflection and refraction studies (e.g., YOSHIIet al., 1973; KATOet al., 1983); the occurrence of the great thrust earthquakes (e.g., KANAMORI,1972); the repeated occurrence of historical great earthquakes with the intervals from 100 to 150 years (ANDO, 1975; AOKI, 1977); and the seismic zone beneath southwestern Japan dipping from the Nankai trough (SHIONO,1977; UKAWA,1982). On the other hand, the subduction along the Suruga trough seems to be more complicated for the following reasons. The subducting feature is not clear in Suruga Bay from the multi-channel reflection survey (SAKURAIand MOGI, 1980). The source areas of some historical great earthquakes along the Nankai trough, for example the 1498 Meio Tokai earthquake, extended to the south off the and the recurrence-time intervals of the great earthquakes along the Suruga trough are very variable (e.g., AOKI, 1977; UTSU, 1977). Seismicity of a long term along the Suruga trough has been studied on the basis Seismic Activity in the Subducting 71

Fig. 2. Epicentersaround the Suruga trough routinely obtained by the NRCDP network for the period from 1980 to 1986. In this map, events shallower than 30 km are plotted.

of the data by the Japan Meteorological Agency (JMA) that suggests a quiescence in Suruga Bay for the events of magnitude larger than 5 (e.g., UTSU, 1977). Distribution of microearthquakes is obtained by the National Research Center for Disaster Prevention (NRCDP) and by Nagoya University. Figure 2 shows the hypocentral distribution obtained by the Kanto-Tokai observational network of the NRCDP (hereafter referred to as the NRCDP network) from 1980 to 1986, showing a low seismic activity in the northern half of Suruga Bay and relatively high activity in the southern half. The details of the NRCDP network are given in HAMADAet al. (1985). The hypocenters by Nagoya University also show the same features (e.g., YAMAZAKIand OOIDA, 1985). However, the hypocentral data around the Suruga trough are less accurate than those of the land area, especially in focal depth, due to the lack of seismic stations in the sea area. The detection ability of the land seismic networks is not high for the Suruga trough region for the same reason. In order to reveal the microseismicity along the Nankai-Suruga trough, the 72 M. UKAWA,T. EGUCHI, and Y. FUJINAWA

NRCDP has conducted one-month surveys by pop-up-type ocean bottom seis- mometers (OBSs) every year since 1981 (e.g., FUJINAWAet al., 1983; UKAWAet al., 1985 a). The first survey in the Suruga trough was done in 1983 and revealed the following seismicity features (UKAWAet al., 1985 a); (1) the middle part of the Suruga trough is active in microseismicity, whereas both the northern and southern parts are quiet; (2) most of the earthquakes near the trough axis occur in the depth range shallower than 15 km. Focusing on the Suruga Bay region, we conducted the second survey along the Suruga trough in 1984. In this paper, we report the result of the 1984 survey and try to elucidate whether the earthquakes along the Suruga trough occur within the subducting plate, within the overriding plate, or at the plate interface.

2. The 1984 OBS Survey and Data We deployed 10 OBSs and successfully recovered all of them after a one-month observation. The locations of the OBSs are shown in Fig. 3. In order to investigate precise seismicity and to detect microearthquakes with magnitude 1 or less, the instrument spacing was set at about 20 km. The water depths at the OBS stations ranged from 800 to 3,600 m. The observational period was from June 15 to July 13, 1984. Table 1 summarizes the observational data for each OBS. The OBS location was assumed to be the same as the ship position at the time of dropping of the OBS from the ship, which was positioned by using the DECCA system. The depth was determined from the water depth measured by PDR

Fig. 3. Locations of the OBS stations in the 1984 survey and the land station NSI belonging to the NRCDP network. Seismic Activity in the Subducting Philippine Sea Plate 73

Table 1. Locations and observational periods of OBS stations.

(Precision Depth Recorder). The accuracy of the OBS location is estimated to be 200 m or less on the basis of the previous experiments of the 1983 survey, when we compared the locations positioned by two methods: by ship position, and the more accurate method by acoustic distance measurements from a ship to an OBS at more than three points. The pop-up-type OBS used in the present survey includes two seismometers (vertical and horizontal components). The seismic data are recorded continuously by a direct-recording magnetic tape recorder with clock signal. The natural frequency of the seismometer is 2 Hz with critical damping. The total frequency range is from 2 to 15 Hz and the dynamic range is 30 dB. The OBS clock was corrected just before and just after the operation of the OBS on the ship by using the JJY standard time. The arrival time data are corrected by linear interpolation. The deviation rates range from —0.27 s/day to 0.12 s/day in the present survey. The detailed description of the OBS specification is given in EGUCHIet al. (1986). The seismic data recorded on the magnetic tape are played back by the following procedure. (1) The full data of the vertical component and clock signals are reproduced on light-developed recording papers and seismic events are picked up from them. (2) We select the events which were detected by at least three stations. (3) The data of the selected events are digitized from the magnetic , tape, and are compiled for each event by using a mini-computer. (4) For examining arrival times of seismic phases and duration times, we reproduce the compiled data on pen- recorder charts. In order to improve signal-to-noise ratio, filtered seismograms are also constructed on the same charts. The detailed procedure of the reproduction of the record is explained in UKAWAet al. (1985 b). In the present study, we compiled 342 events. As is seen in Fig. 4, clear onsets are detected for the events in Suruga Bay. We can read arrival times with the accuracy better than 0.1 s in the case of sharp onset. We rank the arrival time data into four classes in accordance with the reading accuracy as shown in Table 2. 74 M. UKAWA, T. EGUCHI, and Y. FUJINAWA

Fig. 4. Examples of vertical seismograms for an event in Suruga Bay (right panel). The epicenter and the locations of the OBS stations are indicated by a solid circle and crosses, respectively (left panel).

Table 2. Reading ranks, accuracies, and weights, WR.

For supplementing the OBS data, we use the seismic record obtained at NSI of the NRCDP network. The location of NSI is shown in Fig. 3.

3. Hypocentral Distribution

The hypocenters of the events picked up were determined by using the weighted least squares method. When the focal depth becomes negative in the course of iteration, we determine only its epicenter and origin time by fixing the focal depth at 3 km. The arrival-time data were weighted according to the reading rank and the epicentral distance. We defined the weight W= WR• WEfor each station, where WR is the weight depending on the reading rank and WE on the epicentral distance. WR is given in Table 2 and WE is shown in Fig. 5. Theoretical travel times were calculated by assuming a horizontal layered structure. The seismic velocity model used here is the same one as that in UKAWAet al. (1985 a), which is constructed on the basis of the velocity study across Suruga Seismic Activity in the Subducting Philippine Sea Plate 75

Fig. 5. Weight function, WE, versus epicentral distance.

Table 3. Velocity structure model SRG.

Bay (IKAMI,1978). This model named SRG is a five-layered structure given in Table 3. The first and the second layers correspond to the sedimentary layers. The third and the fourth layers are the upper and the lower crustal layers, respectively. The fifth layer is the uppermost mantle. The Moho lies at a depth of 25 km, which is shallower than a typical continental Moho and deeper than a typical oceanic Moho. The magnitude, MF-p, is estimated from F-P times (duration times), TF-p, by using the following equation: (1) where coefficients, A and B, were determined for each OBS station by fitting the magnitude data of the NRCDP network for the events detected by both the OBS and the NRCDP network. The resulting epicenter map is shown in Fig. 6, in which epicenters are indicated by four different symbols according to their focal depths. Among the 342 events compiled in the present study, about 60 events are located in and around the Suruga trough. The distinctive feature is the high seismicity in the southern part of Suruga Bay, that is, around the middle part of the Suruga trough. The most acitve area is the segment of about 50 km along the trough axis. As is seen in Fig. 6, most of the earthquakes detected in and around the OBS array are shallower than 30 km. In contrast to the high seismicity around the middle part of the Suruga trough, the northern and the southern parts are extremely quiet in seismicity. We could not locate any earthquakes in the northern part of Suruga Bay. Whereas we found a small number of earthquakes in the southern part of the Suruga trough. 76 M. UKAWA, T. EGUCHI, and Y. FUJINAWA

Fig. 6. Epicenters obtained by the present survey. The epicenters are shown by four types of symbols, according to their focal depths. Size of the symbols depends on MF_p. Crosses indicate the locations of the OBS stations.

Figure 7 shows the cumulative number of earthquakes versus the magnitude for the region S in Fig. 6. The largest event in the region S is magnitude 2.5 and most of the events are smaller than 1.5 in magnitude. Figure 7 indicates that the detectable magnitude limit of the present observation is about 0.7. Judging from the detectability above, there were no microearthquake with magnitude greater than 0.7 in the northern part of the Suruga trough and few in the southern part, during the observation period. We estimate the accuracy of the hypocenters to be better than 3 km for most events in and near the OBS array. In Fig. 6, we plotted the hypocenters with errors less than 10 km in latitude, longitude, and depth, and 2 s in origin time. In general, because of possible systematic deviation of hypocentral parameters due to the velocity heterogeneity, the errors should be considered not as an absolute deviation from the true hypocenter but as a relative indicator.

4. Relocation with Station Corrections

In order to clarify the hypocentral distribution of the most active area, the Seismic Activity in the Subducting Philippine Sea Plate 77

Fig. 7. Cumulative number of the earthquakes versus MF_p for the region S in Fig. 6.

middle part of the Suruga trough, we attempt to relocate the earthquakes by applying station corrections. The OBS observation in and around the subduction zone is considered to be influenced most seriously by the heterogeneity in shallow depths, especially by the variety of the thickness of trough-fill sediment with low seismic velocity. In the present study, we estimate station corrections by the same method as UKAWAet al. (1985 a), in which station corrections and hypocentral parameters are simultaneously inverted from arrival-time data of selected earth- quakes by using the damped least squares method. The station correction set estimated by this method is useful for reducing the effect of the velocity hetero- geneity throughout the seismic ray paths, especially the effect of structure just beneath the stations, in the case of a relatively small area of hypocenters. We select nine earthquakes along the middle part of the Suruga trough, whose original and relocated hypocenters are shown in Fig. 8. To obtain reliable station corrections, seven OBS stations and one land station belonging to the NRCDP network, NSI, are selected on the condition that more than three arrival-time data are available for both P and S phases. Figure 8 shows the selected stations with the inverted station corrections. The station correction cannot be estimated uniquely because of the trade-off with the origin times by this method (CROSSON,1976). Therefore, we fix the station correction of P wave at the station C at 0 s. Table 4 summarizes the result, indicating that the range of the station corrections of the OBS stations is from —0.03 to 0.62 s for P wave and from —1.43 to 0.85 s for S wave. The land station, NSI, has the largest station corrections for both P and S waves. The large difference between the station corrections of the land station and those of the OBS stations was also obtained in the previous study (UKAWAet al., 1985 a). This difference is mainly due to the lack of a low-velocity sedimentary layer at the land area, which is included in the standard velocity model SRG. 78 M. UKAWA, T. EGUCHI, and Y. FUJINAWA

Fig. 8. Inverted P and S wave station corrections, and the original and relocated epicenters of the events used for the estimation of the station corrections. The P and S wave station corrections are indicated by the length of bars with solid and open triangles, respectively. The inset explains the station correction. The original and inverted epicenters are indicated by open and solid circles, respectively.

Table 4. Estimated station corrections.

St. Cr.(P) and St. Cr.(S) are station corrections for Pand Swaves, respectively. TCOR = TOBS+St. Cr.,where TCOR and Tonsare corrected and observational arrival times, respectively.

We relocate the earthquakes, using the above 8 stations and their station corrections estimated here. Figure 9 shows the relocated hypocentersfor the region S in Fig. 6. In this figurethe westwarddipping seismiczone is clearly seen on the E- W cross section, which is almost perpendicularto the strike of the Suruga trough. The thickness of the seismic zone is about 10km. This thickness seems to be comparable to that obtained for the deeper part of the subducting PHS plate Seismic Activity in the Subducting Philippine Sea Plate 79

Fig. 9. Relocated hypocenters by applying the station corrections; (a) epicenter map, (b) cross section projected onto a vertical plane along E-W direction, and (c) cross section projected onto a vertical plane along N-S direction. The symbols for the epicenters in (a) are the same as in Fig. 6. T.A. indicates the trough axis. beneath the Tokai district (e.g., UKAWA,1982; YAMAZAKIand OOIDA, 1985). It should be noted that the upper boundary of the seismic zone meets the trough axis at the surface. Recently, HORI et al. (1985) found evidence that the seismically most active part in the subducting PHS plate is the subducting oceanic crust. If the same condition takes place near the trough axis, the seismicity feature observed in this survey is in conformity with the hypothesis that the PHS plate is subducting from the Suruga trough. It is reasonable to consider that the earth- quakes along the Suruga trough occur in the uppermost part of the subducting PHS plate or partly at the interface between the subducting and overriding plates and that the overriding plate near the trough axis is seismically quiet. In Fig. 9, several events near the trough axis are determined to be shallower than 4 km, in the depth range of the sedimentary layer of SRG. Judging from the standard deviations of focal depths, less than 2 km for most of them, it is certain that these events occurred in very shallow depths. According to the result of multi- channel seismic study (KATOet al., 1983), the thickness of the sedimentary layer of the PHS plate east of the axis of the Suruga trough is about 1 km or less. Hence, most of the shallow events are considered to have occurred in the basement layer of the PHS plate, not in the sedimentary layer. For the events outside of the OBS array, however, the accuracy of focal depth is not good enough to compare with the velocity structure. 80 M. UKAWA, T. EGUCHI, and Y. FUJINAWA

Fig. 10. Space-timeplots of the eventsdetected in the present survey (right panel). Left panel shows the epicenters and the three regions: I, a region along the Suruga trough; II, a region of the west coast of Suruga Bay; III, swarm area south off the Tokai district.

5. Synchronous Seismic Activation along the Suruga Trough Figure 10 shows the space-time plots of the events in and around Suruga Bay detected by the present observation. In this figure, the events are classified into three groups as indicated in the left panel; I, events along the Suruga trough; II, events beneath the west coast of Suruga Bay; III, events of the small swarm detected from June 18 to July 1, 1984 by the present survey. For the swarm activity, we use the data by the NRCDP network, because the epicenters are beyond the present OBS array. Figure 10 shows that the seismicity of region I was activated from June 19 to 23, 1984. Twenty-nine events out of a total of 50 events occurred in the above five days, that is, 58 percent of the total events occurred in the interval of 18 percent of the total observational period. Figure 10 also shows that the area was activated in seismicity extended over about 50 km length along the Suruga trough. The swarm activity of region III occurred in this active period. JMA (1985) reported that the swarm started on June 12, 1984 and continued to mid-August, and that it was most active on June 20, 1984. On the other hand, the events of region II seem to have occurred independently. In order to investigate the seismicity along the Suruga trough for a longer period, we examine the data obtained by the NRCDP network. Figure 11 shows the space-time plots and the epicenters from 1980 to 1986 by the NRCDP network data. In this case, the lower limit of magnitude is about 1.5 (MATSUMURA,1985). From March to June, 1983, the seismicity was activated simultaneously over 40 km length along the Suruga trough. As for this activity, AOKI (1985) pointed out that the earthquakes in April migrated northward along the trough and finally a middle- class-magnitude event (M=4.9, April 29, 1983) occurred beneath the southern foothill of Mt. Fuji beyond the seismically quiescent region at the northern part of Suruga Bay. Figure 11, however, does not show any other distinct, simultaneous Seismic Activity in the Subducting Philippine Sea Plate 81

Fig. 11. Space-timeplots for the events detected from 1980to 1986,based on the NRCDP network. The rectangle in the left panel shows the plotted area. occurrence of earthquakes along the Suruga trough. The synchronous activation of the microseismicity along the Suruga trough and the northern end of the Nankai trough in 1984 may indicate that the tectonic stress changes simultaneously in the wide region along the Nankai-Suruga trough. As no other geophysical evidence suggesting the stress change at the same period has been reported, the level of the stress change is considered to be too small to be detected by other observations but possibly large enough to induce microearth- quakes. It should be noticed that the synchronous activation of microseismicity in 1984 could not be detected by the NRCDP network due to the relatively poor detectability for the off-shore area. Many observations of synchronous activation of earthquakes have been reported for various scales in space, time, and magnitude, and they have been explained by relating corresponding tectonic background; for example, stress increase by subducting plate (e.g., KANAMORI,1981; MOGI, 1981), movement of active faults (e.g., OIKE, 1979), and stress change along the volcanic front (MATSUMURA,1987). In comparison with these observations in other regions, the present case is smaller in magnitude and shorter in period. But the width of the region is as comparable as other cases observed for larger earthquakes. We suggest that a small variation of the movement of the PHS plate caused the present synchronous seismicity activation. Although it is not certain that such a small but wide stress change is a common feature at subduction zones because of our poor knowledge about the microseismicity in off-shore areas, detecting a small stress change over a long term may be quite useful for earthquake prediction study. A 82 M. UKAWA, T. EGUCHI, and Y. FUJINAWA

Fig. 12. Verticalcross section of the relocatedhypocenters (solid circles)and the hypocentersroutinely obtained by the NRCDP network (open circles). The plotted area is shown by the rectanglein Figs. 2 and 9. continuous observation with higher detectability in the off-shore area is required in order to elucidate the detailed nature of the synchronous seismic activation.

6. Discussion The present OBS observation revealed a thin seismic zone dipping westward from the Suruga trough axis. We compare the present hypocenters with the hypocenters obtained by the Land seismic network. Figure 12 is a vertical cross section of hypocentral distribution across the Suruga trough, in which the present result and the hypocenters routinely obtained by the NRCDP network are plotted. At the trough axis, the focal depths of the present result range from 0 to 15 km. The focal depths by the 1983 OBS survey are also in this depth range (UKAWA et al., 1985 a). On the other hand, the focal depths by the NRCDP network range mostly from 10 to 25 km as is shown in Fig. 12, being deeper by 5 to 10 km than those by the OBS array. In the subducting PHS plate, earthquakes occur in the uppermost part of the subducting plate, mainly in the subducting oceanic crust (HORIet al., 1985). Around the Izu Peninsula and the western part of Sagami Bay, the focal depths are shallower than 15 km (TSUMURAet al., 1978; UKAWAet al., 1987). These obser- vations indicate that the most seismically active portion of the PHS plate is its uppermost part, in the depth range down to 15 km from the upper surface of the plate in both the landward subducting side and the oceanic side of the plate. The deeper focal depth at the trough axis obtained by the NRCDP network contradicts the above tendency, if the PHS plate is subducting from the trough axis. The results Seismic Activity in the Subducting Philippine Sea Plate 83

Fig. 13. Schematicseismicity map around the Suruga trough region. Dotted area is seismicallyactive. Areas indicated by Q are seismicallyquiet regions. Line EF indicates the tectonic line proposed by MOGI(1977). Inset is a cross- sectional view of the seismicityalong the dotted line AB. of the present study based on the OBS survey clearly indicate that the earthquakes occurred in the uppermost part of the downgoing PHS plate from the trough axis. The deeper focal depths by the NRCDP network are considered to be a result of poor depth control due to a lack of observation in Suruga Bay. Although the clear subducting basement was not detected in Suruga Bay by the multi-channel reflection survey (SAKURAIand MOGI, 1980), the subduction of the PHS plate in Suruga Bay is supported by the bathymetric feature in Suruga Bay (Mom and SAKURAI,1980), the tectonic landform in the western side of the Suruga trough (YONEKURA,1984), and the P wave-velocity structure in Suruga Bay (MURAUCHIet al., 1981). The present hypocentral distribution supports the subduction of the PHS plate along the Suruga trough, at least in the southern part of Suruga Bay. As discussed above, it is certain that the earthquakes around the Suruga trough occur mainly in the uppermost part of the PHS plate. We cannot, however, reject the possibility that the earthquakes occur at the interface between the subducting PHS and overriding EUR plates. For examining this possibility, focal mechanism data are useful. If an earthquake occurs at the plate interface, the expected focal mechanism is a reverse fault type with a nodal plane coincident to the plate interface. The observed focal mechanisms in this area (NRCDP, 1986) show that the predominant fault types are strike-slip and reverse fault with P axes in N-S to NE-SW direction, not indicating the thrust-type faulting along the plate interface. Hence, it is most probable that the earthquakes in the middle part of the Suruga trough occur mainly inside the subducting PHS plate. Figure 13 summarizes schematically the seismicity along the Suruga trough. We note that the seismicity in the overriding plate is relatively quiet over the whole 84 M. UKAWA,T. EGUCHI,and. Y. FUJINAWA region along the Suruga trough. As for the subducting plate, the middle part of the Suruga trough is active,.and the northern and southern parts are quiet. MOGI(1977) proposed a tectonic line, extending northwestward from the NW-SE, right-lateral strike-slip fault trace of the 1974 Izu-Oki earthquake to the west coast of Suruga Bay (the line EF in Fig. 13). This tectonic line separates the PHS plate in the northern part of Suruga Bay from that in the southern part. The seismically active area of the middle part of the Suruga trough includes this tectonic line. The movement along the tectonic line is expected to be right lateral (MOGI, 1977). But, the observed focal mechanisms of recent earthquakes (NRCDP, 1986) do not support the right lateral movement along the tectonic line. The focal mechanisms also suggest that the bending stress of the PHS plate (NAKAMURAet al., 1984) is not a dominant cause of the seismicity along the Suruga trough, because of the existence of reverse fault earthquakes with sub-horizontal P axes in the N-S to NE-SW direction, which may indicate the horizontal, compressional stress state rather than horizontal, tensional stress state. Our observation reveals that the spatial variation of seismicity in Suruga Bay comes mainly from the seismic activity in the subducting PHS plate. However, the reason why the seismicity varied in the subducting plate along the Suruga trough is not yet known.

7. Conclusion

Seismic observation with the 10 pop-up-type OBSs was conducted for about one month in 1984 and microearthquakes with magnitude greater than 0.7 were located in the Suruga trough region. This OBS survey revealed a high microseis- micity along the middle part of the Suruga trough. The relocated hypocenters with the station corrections show that the earthquakes along the middle part of the Suruga trough form a seismic zone of about 10 km thickness dipping westward from the trough axis. The principal focal depths at the trough axis by the OBS survey ranged from 0 to 15 km, being shallower by 5 to 10 km than those routinely obtained by the NRCDP network. The seismic zone shows that the earthquakes along the Suruga trough occurred in the uppermost part of the subducting PHS plate. In contrast to the high activity along the middle part, the northern and southern parts are quiet even in respect to the microearthquakes. The microearthquakes along the Suruga trough tended to occur simultaneously in the segment over about 50 km in length, suggesting that they were caused by the same stress change. Because the microearthquakes seem to be sensitive to the stress change, the monitoring of microseismicity at the middle part of the Suruga trough would provide useful information for the precursory stress change prior to the future great earthquake.

We thank Prof. M. Ohtake for critical reading of the manuscript and for useful comments.Dr. H. Takahashi continuouslyencouraged us in our OBS survey.We also thank SeismicActivity in the Subducting PhilippineSea Plate 85 the crew of the Tokai-Daigaku-maru II and the members of Tokai University who participated in our OBS survey, for their help in deploying and recovering the OBSs.

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