Interplate fault slip along the Japan Trench before the occurrence of the 2011 off the Pacific coast of Tohoku Earthquake as inferred from GPS data
A sequence of M 7-class interplate earthquakes and transient postseismic slips following each of these earthquakes occurred along the Japan Trench before the occurrence of the giant earthquake on March 11, 2011. Continuous GPS data detected a significant postseismic deformation with displacements that were much larger than that of the coseismic offset. Moreover, the geodetic inversion results using these observations revealed that the total moment released by these transient slips was much larger than the coseismic ones. This differs from our understanding of the postseismic process that the postseismic deformation and slip are smaller than the coseismic ones. These transient processes may help us understand not only the postseismic process but also the pre-seismic signals indicating the occurrence of the giant earthquake.
Key wordsInterplate fault slip postseismic deformation the 2011 off the Pacific coast of Tohoku Earthquake pre-seismic signal the Japan Trench
A nation-wide continuous GPS network, GEONET (GPS Earth Observation Network System), operated by the Geospatial Information Authority (GSI) of Japan, includes more than 1300 stations spread over the entire region of Japan. GEONET data are routinely analyzed using the BERNESE Ver. 5.0 software and the IGS (International GNSS Service) final ephemeris. The daily coordinates of the GPS stations are published as “F3 solutions” (Nakagawa et al., 2009). Here, we used the daily coordinates with respect to the site 950154 (Iwasaki, see Fig. 1(a)). Annual, semi-annual, and linear-trend components in the GPS time series were estimated by least-square fitting for the period from January 1, 1997, to January 1, 2002, when there were no abnormal events. Subsequently, we removed them from the original time series. Note that coseismic offsets by large earthquakes are not eliminated in this time series, including those of the September 26, 2003, Tokachi oki earthquake (Mj 8.0).
3. Results and Discussions
3.1 Time series
The time series of the GPS data in the east-west component clearly reveals coseismic offset and subsequent transient deformation at each earthquake (Fig. 1(b)). There was no significant subsidence or uplift signals in the vertical component except for the coseismic subsidence by the 2005 Miyagi earthquake, which is not displayed here. The westward step in 2008 at the stations 950172, 950175, and 960550 is a coseismic offset associated with the June 14, 2008, Iwate-Miyagi Nairiku earthquake (Mj 7.2), which occurred in the inland shallow crust (Fig. 1(a)). We do not discuss the deformations associated with this earthquake.
In the case of the 2005 Miyagi earthquake, postseismic transient deformation decayed rapidly with time and ended in the middle of 2007, as is seen at the sites 950172, 950175, 960550, and 950179. Its duration seems to be about 2 years, as Miura et al. (2006) reported. We refer to this event as a reference of postseismic deformation produced by an afterslip in this study. A number of studies have reported that the postseismic signals produced by an afterslip rapidly decay with time and the amount of displacement is less than or equal to the coseismic offset, although the decay time and duration of the signal are different for each earthquake and may also depend on its mechanisms (e.g., Heki et al., 1997; Gahalaut et al., 2008; Perfettini et al., 2010). On the other hand, large transient displacements were seen after the 2008 Ibaraki earthquake at the stations 970800, 950214, and 950216, and after the 2008 and 2010 Fukushima earthquakes at the stations 950179, 940038, and 950203, located in Fukushima and Ibaraki prefectures (Fig. 1). As described in greater detail in the next section, it is noted here that the total transient deformations are extremely large compared to those of the coseismic offset at these stations. Moreover the decay-time constant, assuming exponential decay at these stations, is estimated to be more than 300 days for the 2008 Ibaraki, and the 2008 and 2010 Fukushima earthquakes. This constant is much larger than in the case of the 2005 Miyagi earthquake, which is estimated to be about 200 days or less at stations 950172, 950175, 960550, and 950179 (Fig. 1(b)).
3.2 Coseismic and postseismic deformations
3.3 Estimated coseismic and postseismic slip
Using these displacement data, we performed the inversion method devised by Yabuki and Matsu’ura (1992) to estimate the fault slip of this sequence of earthquakes and postseismic events on the plate interface. The results are shown in Fig. 2 with the observed horizontal displacements. The estimated postseismic slip is distributed over a broad area compared to the coseismic slip distribution in all events. The peak locations of the coseismic and postseismic slips are nearly identical in most cases. Previous studies reported that the location of the maximum slip of the coseis-mic and postseismic areas is not in the same place. Most of the postseismic slip occurred on the deeper extent or surrounding area of the main rupture at the plate interface for M 8 class earthquakes in the Japan Trench (e.g., Yagi et al., 2003; Ozawa et al., 2004) and for M 8 ~ 9 class events in other subduction zones (e.g., Perfettini et al., 2010; Vigny et al., 2011). Baba et al. (2006) reported that postseismic slip was estimated on the surrounding area including that at the same depth as the main rupture of the 2003 Tokachi-oki earthquake using the offshore geodetic data.
Our synthetic test of the case that postseismic slip distributes around the coseismic slip shows that the coseismic slip is reasonably well resolved by the geodetic data. In the case of the postseismic slip, deep slip near or beneath the land is well resolved when the data include enough signals in the vertical component. However, it is undeniable that the land geodetic data do not resolve shallow off-shore slip on the plate interface. Because the observed horizontal displacements are small, the signal-to-noise ratio of the used data is low. And there are no clear vertical signals in our data. Consequently, the resolving power of our inversion is poor and this may affect out results regarding the distribution of the coseismic and postseismic slip. Therefore, we emphasize the ratio of moment release between coseismic and postseismic slip, rather than their spatial distribution, in this study.
The released moment by each postseismic slip is also larger than the coseismic slip (see Table 1). For example, it is more than three and a half times larger than the 2008 Fukushima earthquake, though the postseismic slip following this earthquake is included in the postseismic slip induced by the 2008 Ibaraki earthquake, which occurred just two months earlier and was located 200-km south from the epicenter of the 2008 Fukushima earthquake. However, it should be noted that the moment released by this postseis-mic slip is larger than the sum of the coseismic moments of these two earthquakes. Furthermore, the postseismic signals apparently did not end in this time period, because the next earthquake occurred before the end of the postseismic signal (see Fig. 1(b)).
3.4 Interpretation of sequence events
Five M 7-class interplate earthquakes occurred before the Mw 9.0 giant earthquake, as we have described in the foregoing. The total moments released by the interplate earthquakes and postseismic slips are 26.9×1019 N m and 34.2×1019 N m, respectively. This is only 1.5% of that of the Mw 9.0 giant earthquake (see Table 1).
Our understanding of the postseismic slip following the coseismic rupture is that of a strain-release process of the remnant strain that is not released by the coseismic rupture. Therefore, the postseismic slip proceeds on the deeper part or adjacent to the main rupture area, and the moment release is equal or less than the coseismic. In addition, the observed postseismic signal reveals a strong reduction with time.
Based on our understanding of the postseismic process, we look again into the sequence of the interplate fault slip. The postseismic process of the three earthquakes in 2008 and 2010 seems to be slightly different from the 2005 Miyagi earthquake case. Certainly, the postseismic signals following these three earthquakes decay with time (Fig. 1(b)). However, the decay-time constant is much longer than that of the 2005 Miyagi earthquake case, and the total postseismic displacement is much larger than the co-seismic displacement even though the postseismic process does not appear to end in each period (Figs. 1(b) and 2). Moreover, the moment released by the postseismic slip is also larger than that of the coseismic rupture in each earthquake (Table 1).
Strong interplate coupling has been estimated along the
Japan Trench using GPS data and this region corresponded to the M 7 ~ 8 class earthquake rupture area (Nishimura et al., 2004; Suwa et al., 2006; Hashimoto et al., 2009). However, the average strain rate released in historical earthquakes is considerably lower than the strain accumulation rate estimated from contemporary deformation. An aseis-mic slip, including an afterslip, has been suggested as a possible mechanism for the accumulated strain release in the Japan Trench (e.g., Heki et al., 1997; Kawasaki et al., 2001). In the Sanriku-oki region, Kawasaki et al. (2001) reported that a large moment was released by an aseismic slip following the earthquakes, including the 1989 and 1992 Sanriku-oki earthquakes. However, the duration of these aseismic events is less than a few weeks.
Considering these temporal and spatial differences from the postseismic process and strain budget in the Japan Trench, we may interpret these sequence events not only as part of the postseismic strain-release process but also as the preparation or pre-seismic signal of the giant earthquake. Recently, Hori and Miyazaki (2011) numerically simulated earthquake cycles in which several M 7 events occur between recurrences of M 9 events. They reported that post-seismic slip following the M 7 events became larger and wider in the later part of the M 9 earthquake cycle. However, even if this interpretation is correct, it is difficult to distinguish between the postseismic and pre-seismic process based on the data before the 2011 Tohoku earthquake.
Based on the GPS data and geodetic inversion, a sequence of M 7-class interplate earthquakes and subsequent transient events prior to the giant earthquake were investigated to discuss the long-term pre-seismic signals of the giant earthquake. The main results of this paper is that the observed postseismic deformations and the estimated moment released by the transient slip following the M 7-class earthquakes, which occurred in 2008 and 2010, are much larger than the coseismic ones. These results are different from our understanding of the postseismic process as the postseismic deformation and slip are smaller than that of the main rupture. One of the possible interpretations is that these sequence events provide insights not only on the post-seismic process but also on the pre-seismic signal of the giant earthquake.
We appreciate Dr. Miyazaki and an anonymous referee for useful comments. We also thank the Japan Meteorological Agency for providing their earthquake catalogue. All the figures were created with the GMT (Generic Mapping Tools) software by Paul Wessel and Walter H. F. Smith.
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