Aseismic slip and low-frequency earthquakes in the Bungo channel, southwestern Japan

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L769, doi:1.19/3gl19381, Aseismic slip and low-frequency earthquakes in the Bungo channel, southwestern Japan Shinzaburo Ozawa, 1 Yuki Hatanaka, 1 Masaru Kaidzu, 1 Makoto Murakami, 1 Tetsuo Imakiire, 1 and Yuzo Ishigaki Received 9 December 3; revised 7 March ; accepted 11 March ; published 6 April. [1] Analysis of Global Positioning System (GPS) data has shown transient crustal deformation in the Bungo channel, in southwestern Japan, since late August 3, six years after a similar event in Considering the southeastward motion at many of the GPS sites and the similarity between the and 3 events, it is highly likely that the transients were caused by aseismic interplate slip between the Philippine Sea plate and the Amurian plate. The 3 aseismic slip is estimated beneath the Bungo channel and the relative slip increased with time on the west side, with a moment magnitude (Mw) amounting to 7.. In contrast to the gradual subsidence of low-frequency earthquakes which began and ended in coincidence with the 3 event, the 3 transient crustal deformation proceeded at a roughly constant rate until the end of November 3. The Bungo channel area may release energy accumulated from the subduction of the Philippine Sea plate by aseismic slip. INDEX TERMS: 16 Geodesy and Gravity: Crustal movements interplate (8155); 1 Geodesy and Gravity: Seismic deformations (75); 13 Geodesy and Gravity: Space geodetic surveys. Citation: Ozawa, S., Y. Hatanaka, M. Kaidzu, M. Murakami, T. Imakiire, and Y. Ishigaki (), Aseismic slip and low-frequency earthquakes in the Bungo channel, southwestern Japan, Geophys. Res. Lett., 31, L769, doi:1.19/3gl Introduction [] The Philippine Sea plate is subducting beneath southwestern Japan at the Nankai trough at an annual rate of about 7 cm yr 1 toward the NW direction [Miyazaki and Heki, 1]. Figure 1 shows the tectonic setting around the Bungo channel between Shikoku and Kyushu, two major islands of Japan. The Nankai trough runs parallel to Shikoku and Kyushu, changing its strike angle from the east to the west. [3] Because of the differences in the tectonic setting between Shikoku and Kyushu [e.g., Ozawa et al., 1], the transitional area around the Bungo channel, flanked by these two islands, has been of great interest to researchers. Although seismic activity in this area is low, the recent discovery of long-period earthquakes with frequencies ranging from 1 to 1 Hz along isodepth contours of the Philippine Sea plate indicates frequent occurrences of lowfrequency earthquakes in this Bungo channel area [Obara, ; Katsumata and Kamaya, 3]. 1 Geographical Survey Institute of Japan, Tsukuba, Japan. Japan Meteorological Agency, Tokyo, Japan. Copyright by the American Geophysical Union //3GL19381 [] In the period between late 1996 and 1997, anomalous crustal deformations occurred at several GPS sites facing the Bungo channel. Hirose et al. [1999] examined this event by curve fitting adopting a rectangular fault, and concluded that a silent earthquake occurred in this region. Ozawa et al. [1] employed Kalman filtering, following the time-dependent inversion technique, to estimate how the silent earthquake began and propagated over time. Their results indicated slip propagation from the southwestern part of Shikoku toward the southwest to the Bungo channel from late 1996 to late Transient crustal deformation similar to that of the event started in the Bungo channel in August 3. Associated with the observed 3 transient crustal motion, the number of low-frequency earthquakes with an epicenter depth of approximately km increased in the western part of Shikoku. In this study, we estimate the aseismic interplate slip for the 3 transient crustal deformation in the Bungo channel between Shikoku and Kyushu, compare it with the event, and investigate the relationship between the low-frequency earthquakes and the 3 silent earthquake.. Time Series Data and Silent Thrust Event [5] GPS data are analyzed with Bernese GPS software version. using the International GPS service for Geodynamics (IGS) precise ephemerides and earth orientation parameters from the International Earth Rotation Service (IERS) Bulletin B. Figure 1. (a) Tectonic setting in and around Japan. Solid lines indicate plate boundaries. (b) Enlargement of rectangular area in Figure 1a. The solid lines with numerals represent the contours of the upper depth of the Philippine Sea plate. Depth contours are from Figure 1 in Ozawa et al. [1]. L769 1of5

2 Figure. Detrended ground displacements at GPS stations relative to the station in the Chugoku district, Japan. Error ellipses at arrow heads show one standard deviation. Solid arrows represent observations while white arrows show values computed using the estimated model. (a) From November 1996 to February Detrended crustal movements at GPS sites near the Bungo channel showed southeastward motion of up to 3 cm during this period. Southeastward displacements in central Kyushu are attributed to postseismic deformation (see text). (b) From July 1, 3 to February 1,. Southwestern part of Shikoku and northeastern Kyushu show southeastward transient crustal deformation of up to 3 cm, which is similar to the pattern in (a). average annual velocities removed. We can clearly see a deviation in ground motion at GPS sites near the Bungo channel for both the period between late 1996 and 1997 and the period from late August 3. For the transient crustal movements in 3, the maximum speed of deformation at these stations seems faster than that of the event, and the 3 event seems to have subsided in December 3 (Figure 3c). We believe that the small and gradual changes in early 3 recorded at site 37 are unreliable, since we corrected offsets caused by antenna changes at the GPS sites during this period. [1] Considering the simultaneous occurrence of abnormal crustal motion at several GPS stations facing the Bungo channel, the displacement vectors that are opposite to the motion of the Philippine Sea plate, and the similarity between the and 3 events, it is most likely that the 3 abnormal crustal deformation is due to a silent thrust rebound at the plate boundary in this region, as was inferred for the event. 3. Estimation of Rupture Area and Distribution of Aseismic Slip [11] We investigate spatiotime slip distribution of the 3 event as follows. In this study, the fault surface and slip distribution are represented by superposition of the B spline functions, as was described by Ozawa et al. [1]. [1] We place the fault patch at the upper boundary of the Philippine Sea plate, as shown in Figure 1b. The adopted fault patch is composed of 1 1 B-spline functions with node points shown in Figure a. [13] By adopting the fault patch shown in Figure a, we applied Kalman filtering to the detrended time series data, following the time-dependent inversion technique and esti- [6] Since the raw data includes annual and linear trend components, we estimate them for the period between 1999 and and remove them from the raw data, as described by Ozawa et al. []. [7] Figure a shows detrended crustal deformation for the period between November 1996 and February 1998 corresponding to the aseismic thrust event. Southeastward displacements at GPS sites in the Bungo channel indicate a rebound process between the subducting Philippine Sea plate and the overriding Amurian plate [e.g., Ozawa et al., 1]. Southeastward displacements at GPS sites in central Kyushu are thought to be caused by the postseismic interplate slip after the 1996 Hyuga-nada earthquakes [e.g., Ozawa et al., 1]. [8] Six years later, a similar transient crustal deformation began in the Bungo channel area. Figure b shows the detrended crustal deformation around the Bungo channel for the period between July 1, 3 and February 1,, relative to the sites. As shown in Figure b, southeastward crustal deformations in the southwestern part of Shikoku and northeastern part of Kyushu are clearly observed, with the area around Cape Ashizuri showing about 3 cm displacement at maximum. The spatial pattern of abnormal crustal deformation in 3 is very similar to that of the event. [9] Figure 3 shows detrended time series data at the selected stations shown in Figure b, with seasonal and Figure 3. (a) (b) Detrended time series data of east-west (EW) and north-south (NS) displacements at selected GPS stations (a) 7 and (b) 37 denoted by the white squares in Figure b. The selected stations clearly show transient motion for the period between late 1996 and 1997 and from late August 3. (c) EW components of sites 37 (upper part) and 7 (lower part). (d) Cumulative number of lowfrequency earthquakes whose locations are plotted in Figures a and b (data are from JMA). of5

3 Figure. (a) Estimated slip distribution for the period between July 1, 3 and September 1, 3. The solid line indicates the adopted fault patch. Arrows represent the motion of the overriding Amurian plate against the subducting Philippine Sea plate at the node points of B-spline functions (see text). Red dots represent low-frequency earthquakes observed by the Japan Meteorological Agency. Broken lines show isodepth contours of the plate boundary. (b) Estimated slip distribution for the period between September 1, 3 and February 1,. Symbols are the same as in (a). Relative slip magnitude in the western Bungo channel area increased in this period. (c) Estimated slip distribution on the fault patch denoted by the solid line for the period between November 1996 and February Southeastward slip near the Bungo channel reflects the slow thrust event for the same period. The slip area off the central part of Kyushu is that of postseismic slip after the 1996 Hyuga-nada earthquakes. mated time evolution for the period between July 1, 3 and February 1, [Appendix A]. We used east-west, north-south, and up-down components at the GPS sites denoted by white arrows in Figure b. We set slip to zero at the edge of the fault patch. We also constrained the interplate aseismic slip to be southward and eastward. [1] We also estimated the slip distribution of the event by employing the analytical method described by Ozawa et al. [1] for the total crustal deformation data for the period between November 1996 and February In this case, we used the larger fault patch shown in Figure c, since the event was accompanied by the postseismic deformation associated with the 1996 Hyuga-nada earthquakes.. Results and Discussion [15] Figures a and b show the slip distribution for the period between July 1 and September 1, 3 and for the period between September 1, 3 and February 1,, respectively, derived from Kalman filtering analysis of the data between July 1, 3 and February 1,. The slip arrows indicate the motion of the overriding Amurian plate against the subducting Philippine Sea plate. The slip area beneath the southwestern part of Shikoku and the Bungo channel shows southeastward motion and the relative slip magnitude increased over time on the westerner side of the Bungo channel area. This relative slip increase in the west also occurred at the time of the event [e.g., Ozawa et al., 1]. The M w is estimated to be around 7.. [16] Figure c shows the estimated slip distribution of the event for comparison. In Figure c, the slip area in the Bungo channel corresponds to a thrust event that occurred between 1996 and 1997, whereas slip distribution in an area offshore of central Kyushu is attributed to the postseismic interplate slip after the 1996 Hyuga-nada earthquakes. The moment magnitude of the aseismic slip beneath the Bungo channel is estimated at around 7.. [17] Compared with Figure c, the slip distributions in Figures a and b are very similar and overlap that of the event. The and the 3 models reproduce the observations well within 3 standard deviations, as shown in Figure, in the Bungo channel area. Discrepancy in the central Kyushu area in Figure a is due to the small size of the adopted fault patch and has no influence on the following discussion. [18] Simultaneously with the 3 silent earthquake, low-frequency earthquakes started from late August 3. Low-frequency earthquakes which are associated with transient crustal motion are distributed near the northern part of the estimated slip area of the 3 aseismic interplate slip, as shown in Figures a and b (epicenter data from Japan Meteorological Agency (JMA) [3]). The relative locations between the low-frequency earthquakes and the estimated slip area include some uncertainties, since the accuracy of locations of low-frequency earthquakes is about km in horizontal coordinates with very poor resolution of depth [JMA, 3], while one standard deviation of slip magnitude in Figures a and b is about cm. Figure 3d shows the cumulative number of low-frequency earthquakes whose locations are plotted in Figures a and b. As shown in Figure 3d, there is a rapid increase of the number of lowfrequency earthquakes from late August until early September 3 followed by gradual subsidence until the end of November, though the GPS sites show almost linear behavior of transient motion from late August until the end of November (Figure 3c). [19] In the Bungo channel and northeastern part of Shikoku, swarms of low-frequency earthquakes which last more than several days occur with a time interval of about six months, whereas the seismicity of low-frequency earthquakes at other times is minor [Obara and Hirose, 3]. Hirose and Obara [3] and Obara and Hirose [3] reported steplike changes in a tiltmeter, in association with the observed low-frequency earthquake swarm. The reported tiltmeter changes of about.1 microradian occur within several days, which is below the detection level of the GPS.Based on the tiltmeter results, they inferred that aseismic interplate slip occurred lasting several days with a recurrence time of about six months, coinciding with the activation of low-frequency earthquakes in the area near the Bungo channel. With regard to the 3 event, tiltmeter data also showed steplike changes followed by gradual signal changes in the same direction. Aseismic interplate 3of5

4 slip with duration of several days is estimated to have occurred slightly north of the resolved slip area of this study, from late August to early September 3, on the basis of these tiltmeter data [National Research Institute for Earth Science and Disaster Prevention (NIED), 3]. [] Rogers and Dragert [3] reported that deep aseismic slip is associated with low-frequency earthquakes for Cascadian subduction zone events [Dragert et al., 1; Rogers and Dragert, 3]. In the case of the Bungo aseismic slip, the low-frequency earthquakes showed a good temporal correlation with the start or acceleration time of transient crustal deformation in 3, although the exact start time of transient crustal motion is still unclear. An analysis developed by Rogers and Dragert [3] could be used to clarify this temporal correlation more precisely. However, the time evolutions of low-frequency earthquakes and transient crustal motion seem to differ in that transient motion proceeded at a roughly constant rate, until around November 3, whereas the number of low-frequency earthquakes increased sharply from late August to early September and then gradually subsided to the end of November 3 (Figures 3c and 3d). [1] With regard to spatial correlation, the estimated aseismic slip area comes to a relatively shallow region at a depth of between and 15 km compared with the lowfrequency-earthquake area, although this relative location remains to be resolved precisely in a future study, as discussed above. In particular, it is essential to improve the accuracy of vertical crustal motion and densification of spatial coverage of GPS sites. [] From the activity of low-frequency earthquakes in Figure 3d, we divide the 3 event into two periods. From late August to early September, aseismic interplate slip occurred, according to NIED, slightly north of the resolved slip area of this study, causing a sharp increase of the number of low-frequency earthquakes and tiltmeter changes. Simultaneously or following this event, an aseismic event occurred in a shallower area until the end of November, accompanied by less active low-frequency earthquakes and transient crustal motion detected by GPS. From these estimates, we hypothesize that aseismic slip first occurred in a deeper part of the plate boundary between the Philippine Sea plate and the overriding Amurian plate near the low-frequency earthquake area and accelerated or prompted the occurrence of slip in the shallower part, which lasted until the end of November 3, though this hypothesis remains to be verified through future studies. Considerable changes in low-frequency earthquake activity between the first and second stages of the 3 event suggest that these two steps contributed differently to lowfrequency earthquakes. In this respect, uncertainties remain in the temporal correlations between the low-frequency earthquakes and transient aseismic interplate slip in the case of the 3 Bungo silent earthquake. With regard to the cause of low-frequency earthquakes, we believe slow stress changes caused by aseismic slip to be a main factor, as is proposed by Rogers and Dragert [3], considering the occurrence of low-frequency earthquake in a peripheral area of the resolved aseismic slip area shown in Figures a and b, where stress accumulates. However we cannot rule out the importance of the fluid interaction hypothesized by Obara []. [3] The 3 Bungo event was not accompanied by any large earthquakes, which suggests that aseismic slip in the Bungo channel is more similar to those in Cascadia aseismic events. On the basis of this fact and repeated occurrences of aseismic slip, the southwestern part of Shikoku Island and the Bungo channel area is hypothesized to release most of the strain energy accumulated due to the subduction of the Philippine Sea plate via aseismic interplate slip motion. Appendix A [] We adopt the state of x njn =(u,v,p 1,p,,,,p L ), where u, v, and p represent fault slip, slip velocity, and random walk at each station. The transition equation of the information square root filter is expressed as W 1 3 R 1 1 F 1 R 1 1 F 1 6 R 1 R 1 MJ 7 5 a t I w 1 w x nþ1jn 3 5 ¼ 6 R 1 1 x njn R 1 MJx njn where a, M, J, x n+1jn, x njn, and t represent a smoothing parameter in space, the smoothing matrix, the matrix used to select the fault slip component from a state, one step ahead predicted state, the state at time n, and the lapse time between step n and step n+1, respectively. w 1, w, R 1, F and I represent system noise of a transition equation in time with covariance of WW T, system noise of spatial smoothing, the upper triangular matrix with R 1 R T 1 = V njn which is the covariance of x njn, the transition matrix in an ordinary transition equation, and the identity matrix, respectively. R R T = MJV njn J T M T with only diagonal components. The observation equation uses the conventional formulation of the square root information filter. The covariance of initial velocity is set as a (M T M) 1. Backward smoothing has been described by Ozawa et al. []. We use the conventional formula of the log likelihood of Kalman filtering ; [5] Acknowledgment. We used epicenter data of low-frequency earthquakes determined by the Japan Meteorological Agency. References Dragert, H., K. Wang, and T. S. James (1), A silent slip event on the deeper Cascadia subduction interface, Science, Hirose, H., and K. Obara (3), Repeating slow slip events which correlated deep low-frequency tremor activity in southwest Japan, paper presented at Fall Meeting, Seismol. Soc. of Japan, Kyoto, Japan. Hirose, H., K. Hirahara, F. Kimata, N. Fujii, and S. Miyazaki (1999), A slow thrust slip event following the 1996 Hyuga-nada earthquakes beneath the Bungo channel, southwest Japan, Geophys.Res.Lett., 6, Japan Meteorological Agency (3), Low-frequency earthquakes in the Bungo channel area (in Japanese), Rep. Coord. Comm. Earthquake Predict., in press. Katsumata, A., and N. Kamaya (3), Low-frequency continuous tremor around the Moho discontinuity away from volcanoes in the southwest Japan, Geophys. Res. Lett., 3(1), 1, doi:1.19/gl Miyazaki, S., and K. Heki (1), Crustal velocity field of southwest Japan: Subduction and arc-arc collision, J. Geophys. Res., 16, National Research Institute for Earth Science and Disaster Prevention (NIED) (3), Slow slip event and low-frequency earthquakes (in Japanese), Rep. Coord. Comm. Earthquake. Predict., in press. Obara, K. (), Nonvolcanic deep tremor associated with subduction in southwest Japan, Science, 96, of5

5 Obara, K., and H. Hirose (3), Periodic activity of deep low-frequency earthquakes and correlated tilt changes observed in southwest Japan subduction zone, paper presented at Fall Meeting, Seismol. Soc. of Japan, Kyoto, Japan. Ozawa, S., M. Murakami, and T. Tada (1), Time-dependent inversion study of the slow thrust event in the Nankai trough subduction zone, southwestern Japan, J. Geophys. Res., 16, Ozawa, S., S. Miyazaki, T. Nishimura, M. Murakami, M. Kaidzu, T. Imakiire, and X. Ji (), Creep, dike intrusion, and magma chamber deflation model for the Miyake eruption and the Izu islands earthquakes, J. Geophys. Res., 19, B1, doi:1.19/3jb61. Rogers, G., and H. Dragert (3), Episodic tremor and slip on the Cascadia subduction zone: The chatter of silent slip, Science, 3, Y. Hatanaka, T. Imakiire, M. Kaidzu, M. Murakami, and S. Ozawa, Geographical Survey Institute, Crustal Dynamics Department, Kitasato-1, Tsukuba, Ibaraki , Japan. Y. Ishigaki, Japan Meteorological Agency, Tokyo, Japan. 5of5