Intensive seismic activity around the Nankai trough revealed by DONET ocean-floor seismic observations
Megathrust earthquakes anticipated in the Nankai trough will likely cause severe damage in central and western Japan. The Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET), a network of permanent ocean-bottom seismic stations for the early detection of earthquakes and tsunamis developed by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), is in place above the expected source region of such earthquakes. Data from DONET sensors are transmitted in real time to our laboratory at JAMSTEC. Intensive ongoing seismic activity is being detected off the Kii Peninsula by DONET, mainly distributed in three clusters that overlap with the aftershock distribution of the 2004 off the Kii Peninsula earthquakes (MJMA = 7.1 and 7.4), and most of them are also aftershocks of the 2004 earthquakes. Some are linearly distributed on a different trend from the strike of the 2004 foreshock and mainshock fault planes. This result implies that the 2004 events triggered seismic activity on different faults on the subducting Philippine Sea plate. We also observed changes in seismic activity caused by the 2011 Tohoku-oki earthquake. These results could not have been obtained with on-land observations alone, which indicates the importance of DONET for monitoring seismic activity along the Nankai trough.
Key wordsOcean-bottom seismograph seismicity Kumano fore-arc basin
Along the Nankai trough, southwest of Japan, where the Philippine Sea plate is subducting beneath the overriding Eurasian plate, megathrust earthquakes have occurred repeatedly at intervals of 100–150 years (e.g., Ando, 1975), causing severe damage in western and central Japan. The 1944 Tonankai (Mw = 8.1) and 1946 Nankai (Mw = 8.4) megathrust earthquakes ruptured the eastern and western segments of the trough, respectively. This repeated megathrust earthquake activity implies that another great earthquake may occur in the near future that would cause serious widespread damage in central to western Japan.
To monitor seismic activity in this region, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) developed the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) above the Tonankai earthquake source region off the Kii Peninsula (Kaneda et al., 2009; Kawaguchi et al., 2010). DONET ocean-bottom seismic and water-pressure observation stations are connected with an optical fiber cable, and data from the sensors are transferred in real time to our laboratory at JAMSTEC. The seismic and water-pressure observations made by the DONET stations immediately above the source region of megathrust earthquakes improve our ability to detect earthquakes and tsunamis.
Seismic airgun surveys have been repeatedly carried out in this region to reveal the seismic velocity structures in the subduction zone (e.g., Mochizuki et al., 1998; Nakanishi et al., 2002, 2008; Park et al., 2002; Kodaira et al., 2006), and natural earthquake activity has been investigated by using observations made by pop-up-type ocean-bottom seismographs (OBSs) (Obana et al., 2004, 2005, 2009). On 5 September, 2004, a sequence of large earthquakes (off the Kii Peninsula earthquakes, MJMA = 7.1 and 7.4) occurred in this region. The foreshock and mainshock are considered to have ruptured different fault planes, although the fault model is still controversial (e.g. Saito et al., 2010). The aftershock distribution was obtained from OBS observations (Sakai et al., 2005; Obana et al., 2009), and their centroid moment tensor (CMT) solutions were determined (Ito et al., 2005). Very low-frequency (VLF) earthquakes (Obara and Ito, 2005; Ito and Obara, 2006a, b; Ito et al., 2009) and episodes of low-frequency tremor (Obana and Kodaira, 2009) are also known to have occurred in the shallow accretionary prism in this region. These anomalous events are considered to be caused by the dynamic deformation of the accretionary prism (Ito and Obara, 2006a).
Numerical studies have also been conducted to clarify the mechanisms of megathrust earthquakes along the Nankai trough. Hori et al. (2004) showed that it is important to incorporate the geometry of the subducting oceanic plate into the model space, because lateral variations in the frictional properties of the subducting plate can control the nucleation position of giant earthquakes. Kodaira et al. (2006) showed that weak fracture zones in the subducting oceanic plate are the key to the segmentation of megathrust earthquakes, because deformation in these zones releases accumulated stress. Mapping such structures from seismic observations is therefore important for constructing detailed structural models for the simulation of megathrust earthquakes.
In this study, we used data from DONET to determine hypocenters and to investigate the spatiotemporal distribution of seismic activity off the Kii Peninsula. We found much more active seismicity around the Nankai trough than had been detected by on-land seismic observations. We then examined the seismo-tectonic implications of the present seismic activity.
2. DONET Ocean-Floor Observation Network
The construction of DONET began in 2010. Observations began when the first station was installed in March 2010. By January 2011, eight DONET stations were in operation. Three stations were added in March and six more in May 2011. By August 2011, with the installation of another three stations, all 20 planned stations were in operation.
3. Development of the DONET Hypocenter Catalogue
3.1 Method of hypocenter determination
We determined the hypocenters of earthquakes observed by DONET after 8 January 2011, when eight DONET stations had been installed, by using three-component seismograms obtained from the broadband seismometers. We also used data obtained from permanent OBSs deployed by the Japan Meteorological Agency (JMA) off the Kii Peninsula (Fig. 1).
We used the method of Hirata and Matsu’ura (1987) for the hypocenter determinations. The local magnitude (ML) was determined from the maximum amplitude of the vertical velocity seismogram (Watanabe, 1971). We did not use any data from on-land stations because the velocity structure of the shallower crust may differ significantly from that used in this study. We determined the hypocenters for earthquakes for which we had at least four P and/or S readings. We incorporated station corrections for P-and S-wave arrival times, estimating the correction times for each station from the averaged differences between the observed and calculated travel times (O − C times). The root mean square (RMS) of the O − C times was reduced from 0.275 to 0.204 s for the P-wave arrival time, and from 0.665 to 0.539 s for the S-wave arrival time, by incorporating the station corrections. The averaged estimation error of the hypocenter location was 1.4 km in the NS direction, 1.6 km in the EW direction, and 2.8 km in depth.
One-dimensional velocity structure used for the hypocenter determinations.
P velocity (km/s)
3.2 Comparison with other hypocenter catalogues
4. Hypocenter Relocation by the Double-Difference Method
To examine the hypocenter distribution in more detail, we applied the double-difference (DD) method (Waldhauser and Ellsworth, 2000) to the DONET data. The DD method minimizes errors due to unmodeled velocity structures and improves the precision of relative source locations in an activity swarm. For the hypocenter relocation, we used the manually picked P and S arrival times, with P and S arrival time differences for 27,440 and 38,883 pairs, respectively. We then selected neighboring events, within 10 km of each other, and relocated the hypocenters. After the relocation, the RMS of the double-difference time residual was reduced from 276 to 63 ms. We note that with the DD method the absolute location of earthquake clusters depends on the initial hypocenter distribution. We removed from the catalogue some scattered earthquakes that were not well grouped with other events in the obtained hypocenter distribution.
The seismic activity in this region occurs mainly in three clusters (Fig. 8). The earthquakes in cluster A, which is between station KMB06 and the trough axis, are distributed in a region extending EW about 50 km and NS about 30 km. The source depth is between 10 and 30 km. Cluster B is south of cluster A, just below the trough axis. This cluster extends NE-SW about 60 km and is about 30 km wide. The earthquake hypocenters are relatively deep, between 20 and 50 km. The earthquake magnitudes are also relatively large compared with those of other clusters, possibly because small earthquakes in this cluster were not well detected owing to the large distance between the cluster and the stations. Southwest of cluster B is a linear cluster of seismic activity with an NS alignment (cluster C). The source depth is about 10 km in the northern part of the cluster and more than 20 km in the southern part. The two largest earthquakes that occurred in this region during the analysis period (ML = 4.4, 18 January, 2011, and ML = 4.8, 12 March, 2011) are in this cluster. Next, we describe the activity in each cluster in detail.
North of cluster A, we observed a distinct linear band, trending NNW-SSE, of a relatively low number of earthquakes (cluster A′ in Fig. 8). In this region, aftershocks were also relatively few (Sakai et al., 2005; Obana et al., 2009), but their CMT solutions, which were well determined by Ito et al. (2005), indicate vertical dip-slip or normal faults with the T -axis trending WNW-ESE. Both of these focal mechanisms have a nodal plane that strikes NNW-SSE, which corresponds to the direction of the earthquake alignment and implies that a fault with that orientation was the source of the earthquakes. These focal mechanisms are clearly different from the F-net CMT solutions of the 2004 foreshock and mainshock, which indicate a reverse fault with the P-axis trending NS. Therefore, these aftershocks occurred on a different fault from the 2004 fore-shock and mainshock.
The cluster-B activity is rather deep, and the earthquake hypocenters are mostly in the uppermost mantle (Fig. 9(a)). Before the 2004 earthquakes, seismic activity was observed in this region, but it was not as frequent (Obana et al., 2004). Since the epicenters of the 2004 foreshock and mainshock were in this region, the present activity is also considered to be aftershocks of the 2004 events. The hypocenter distribution of the present activity well overlaps the aftershock distribution (Sakai et al., 2005; Obana et al., 2009). The aftershock activity in this region extended westward to the region of cluster C immediately after the mainshock, but, at present, there is a gap between clusters B and C. The CMT solutions of the aftershocks are similar to those of the fore-shock and mainshock (Ito et al., 2005). These earthquakes seem to be distributed on two planes, one in the southeastern region dipping to the north and one in the northwestern region dipping to the south, thus forming a “V” in cross-section (Fig. 9(a)). These planes are consistent with the fault planes of the CMT solutions of the 2004 foreshock and mainshock.
Cluster C is located at the western end of the aftershock activity of the 2004 earthquakes (Sakai et al., 2005). On the seismic section along survey line NT0405 (Nakanishi et al., 2008), the earthquakes can be seen to occur in the oceanic crust and uppermost mantle (Fig. 9(b)). The aftershock distribution of the 2004 earthquakes is also distinctly shallower than 10 km in this region (Sakai et al., 2005). Accordingly, we also consider the present activity in this cluster to be aftershocks of the 2004 events. The CMT solutions of the aftershocks in this region, obtained by Ito et al. (2005), display strike-slip faults with P- and T-axes trending NNE-SSW and WNW-ESE, respectively. The F-net focal mechanisms of the two ML > 4 earthquakes in this cluster similarly indicate strike-slip faults (Fig. 8). One of the nodal planes strikes NNW-SSE, corresponding to the earthquake alignment in the cluster. These focal mechanisms differ from those of the foreshock and mainshock of the 2004 event. Therefore, the seismic activity may be occurring on a fault different from the faults that caused the 2004 events, as in the case of cluster A′. Activity was also observed in this region before the 2004 events, although it was limited to a very narrow region (cluster D in figure 6 of Obana et al., 2005). The focal mechanisms of this early activity are very similar to those of the aftershocks, and the activity is attributed to deformation along pre-existing faults of the incoming Philippine Sea plate (Obana et al., 2005). West of cluster C, Obana et al. (2005) observed some earthquake clusters, but we could not find any distinct activity corresponding to these clusters in the present data.
5.1 Faults in the subducting Philippine Sea plate
As explained in Section 4, we consider most of the present seismic activity off the Kii Peninsula to be aftershocks of the 2004 earthquakes. The F-net CMT solutions of the foreshock and mainshock in 2004 indicated reverse faults with the P-axis trending NS (see Fig. 8). This focal mechanism is obviously different from those of the aftershocks in clusters A′ and C, suggesting the existence of strike-slip faults in the subducting Philippine Sea plate which differ from the faults responsible for the mainshock and aftershock of the 2004 event. Seismic activity on these faults would have been triggered by stress perturbations caused by the 2004 earthquakes. Saito et al. (2010) have suggested that a different fault model, a reverse fault with the P-axis trending NE-SW, for the mainshock better explains the dispersion of tsunami waves than the F-net CMT solution, but this fault model does not match the inferred faults of the cluster A′ and C aftershocks, either.
Pre-existing weak zones in a subducting oceanic plate play a key role in the segmentation of megathrust earthquakes. Kodaira et al. (2006) successfully simulated the segmentation and/or synchronization of giant earthquakes along the Nankai trough by incorporating a weak zone inferred from their findings of strike-slip faults in the oceanic plate. Without such a weak zone, Hori et al. (2004) could not reproduce the segmentation of the megathrust earthquakes, but found that they were always synchronized over the trough. Therefore, it is important to map such weak zones from seismic observations to understand the mechanism of giant earthquakes in a subduction zone.
5.2 Temporal changes in seismic activity
The DONET ocean-bottom seismic observation network, deployed immediately above the source region of earthquakes along the Nankai trough, has much enhanced our ability to detect earthquakes off the Kii Peninsula compared with detection based on land observations alone. The hypocenter distribution obtained from the DONET observation data shows that most of the present seismic activity is from aftershocks of the 2004 off the Kii Peninsula earthquakes (MJMA = 7.1 and 7.4). The seismic activity can be grouped into three main clusters. Detailed investigations of the source distributions indicate that there are several faults within the subducting Philippine Sea plate which are different from those that caused the foreshock and mainshock of the 2004 event. We also observed seismicity in this region triggered by the 2011 Tohoku-oki earthquake. These results indicate the value and importance of DONET for monitoring seismic activity along the Nankai trough.
We used the Japan Meteorological Agency Earthquake Catalogue in our analysis. We Thank F. Waldhauser for sharing the hypoDD program code. We thank K. Takizawa for the preparation of the seismic cross-section data KR9806 and NT0405. We greatly appreciate the comments of an anonymous reviewer. All the figures were drawn with Generic Mapping Tools (Wessel and Smith, 1998).
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