Relocation of the 2011 off the Pacific coast of Tohoku earthquake sequence and fault planes of M ≥ 7 earthquakes
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 7 April 2011
Accepted: 20 May 2011
Published: 27 September 2011
We relocated foreshocks, mainshock, and aftershocks of the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.1) using the modified joint hypocenter determination (MJHD) method in order to obtain their precise hypocenters and to identify fault planes of larger earthquakes. We used P-wave arrival times at stations worldwide reported by the U.S. Geological Survey (USGS). We confirmed by relocated hypocenters that the mainshock and aftershocks had occurred along the plate boundary between the North American and Pacific plates. We also confirmed that the Mw 7.5 foreshock, which occurred two days before the mainshock, and the largest aftershock (Mw 7.9), which occurred half an hour after the mainshock, were thrust earthquakes along the plate boundary. The second largest aftershock (Mw 7.6), which was a normal-faulting earthquake and was a bending-stress intra-plate event caused by the strain reduction on the subduction thrust, occurred at the outer rise of the Japan Trench and was well relocated with its aftershocks. We found that its fault plane dipped westward and it bounded the aftershock distribution on the seaward side. This implies that the western side of the fault plane had subsided, corresponding with the westward plate subduction.
An Mw 9.1 earthquake occurred off the Pacific coast of Tohoku on March 11, 2011 and caused severe damage in the Tohoku and Kanto districts of the northeastern part of Japan. This was a thrust earthquake occurring at the boundary between the North American and Pacific plates. An Mw 7.5 foreshock preceded the mainshock on March 9 and many large aftershocks including, two Mw 7-class aftershocks, followed the mainshock. Although the mainshock was a thrust earthquake as indicated in global centroid moment tensor (CMT) solutions as produced by Dziewonski et al. (1981) and later updates, preliminary hypocenters estimated by the U.S. Geological Survey (USGS) did not correspond well with the plate boundary thrust. This is to be expected if the depths determined by the USGS were not well resolved. Furthermore, it was quite difficult to identify the fault plane for the second Mw 7 aftershock, the mechanism of which was normal-faulting occurring at the outer rise of the Japan Trench.
An accurate and detailed hypocenter distribution map is important basic seismological information for understanding the nature of earthquakes, establishing, for example, the area of the mainshock rupture by estimating the extent of aftershocks in the first day. Therefore, using global phase data reported by the USGS and openly available from their website, we relocated the off the Pacific coast of Tohoku mainshock of March 11, 2011, and its foreshocks and aftershocks. We then identified the preferred fault planes of the M ≥ 7 earthquakes in the sequence, from the candidate fault planes provided by the global CMT solutions.
To precisely relocate hypocenters, we employed the modified joint hypocenter determination (MJHD) method developed by Hurukawa and Imoto (1990, 1992). Because of the lateral heterogeneity of the Earth, the assumption of the horizontally homogeneous velocity model, as normally used in standard hypocenter determination, is inadequate to obtain the precise location of earthquakes. The joint hypocenter determination (JHD) method (Douglas, 1967; Freedman, 1967) enables us to simultaneously relocate many earthquakes, thereby removing the effects of lateral heterogeneity within the Earth. Therefore, we define a station correction that accounts for lateral heterogeneity in the Earth. JHD solutions may, however, become unstable and unreliable as a result of the trade-off between station corrections and estimated focal depths, given the heterogeneous nature of the Earth’s structure and in cases of poor station coverage. For this reason, Hurukawa and Imoto (1990, 1992) developed the MJHD method for locating local earthquakes, in which station correction is independent of both the distance and azimuth from the center of the studied region to the given station, thereby improving the stability of the method. Subsequently, Hurukawa (1995) extended the MJHD method for locating teleseismic earthquakes; we use this extended version in this study, and we used the iasp91 model for the estimation of travel times.
We downloaded the mchedrqed.dat file of hypocenters and phase arrival data for the period from 1 to 31 March 2011 from the USGS website. We selected events at all depths within the region with bounds 35.0°N and 41.0°N in latitude and 140.0°E and 146.0°E in longitude. We have used here only first P-wave arrival times, because these are more reliably observed than later S-wave arrival times, and very few S-wave arrivals had been reported at the time. Since the aftershock region is broad, ray paths to each station from the events may be significantly different, in breach of the assumptions made for the method. This may mean that the relative location errors, especially those for focal depth, are poorly estimated. However, our main objective here is to establish the main features of hypocenter distribution, including focal depths, in the source region for this M 9-class earthquake. Thus, in analyzing this vast source region as a single set of events, we have sacrificed some of the precision normally expected in the study of a more compact region.
To calculate MJHD hypocenters, two parameters must be defined: the minimum number of stations (MSTN) that observed each earthquake and the minimum number of events (MEVN) observed at each station. Here, MSTN and MEVN were set to 60 and 60, respectively. During the relocation process, we isolated readings with arrival-time residuals exceeding 2 s and these observations were excluded from later stages. In this process, the number of events observed at some stations became less than 60, so we reset MEVN to 50 for the final hypocenter calculations.
Some earthquakes east of the Japan Trench were determined deeper than those near the Japan Trench. These earthquakes, including the Mw 7.6 aftershock, will be discussed in Section 4.4.
4.2 Largest foreshock on March 9, 2011
Two days before the mainshock, an Mw 7.5 foreshock occurred near the mainshock. The relocations for the 24 foreshocks are indicated in Fig. 3(b). The fault plane preferred for the Mw 7.5 foreshock is the nodal plane of the global CMT solution with a strike of 190° and a dip of 7° westward as shown projected in Fig. 3(b). The foreshocks were located ENE of the mainshock. The size of the fore-shock area is ~50 km × ~50 km. The mainshock rupture apparently started near the WSW extension of the rupture for the Mw 7.5 foreshock. Close comparison of Figs. 3(a) and 3(b) suggest that no immediate aftershocks occurred in the foreshock area.
4.3 Largest aftershock off Ibaraki
The largest aftershock of Mw 7.9 occurred off Ibaraki 29 minutes after the mainshock (Fig. 3(c)). Although we cannot distinguish between aftershocks of the mainshock and those of the Mw 7.9 aftershock, we can examine whether the largest aftershock could have occurred on an extension of the fault plane of the mainshock. Relocated aftershocks aligned with the nodal plane dipping WNW, shown as a solid line projected on cross-section A–B of Fig. 3(c), rather than the other nodal plane, which projects almost perpendicular to the solid line. This preferred nodal plane is almost parallel to the fault plane of the mainshock, with the aftershocks mainly located on the SWS extension of the mainshock fault plane. Therefore, the preferred fault plane for the largest aftershock in the global CMT solution is the plane with a strike of 199° and a dip of 17° WNW.
Crustal earthquakes near the coasts of Ibaraki and Fukushima are located well above the subduction thrust (distance — 50 to — 100 km in Fig. 3(c)). These are mostly normal faulting events with EW tensional axes, caused by EW relaxation after the mainshock slip on the subduction thrust.
Other shallower earthquakes occurred above the rupture of the Mw 7.9 aftershock on the eastern trench side, from 141.5°E to 142.0°E. Seismicity near the trench is very low in this region. Further studies are necessary to understand faulting here in the accretionary prism and to evaluate the potential for a tsunami-generating earthquake near the trench.
4.4 Second largest aftershock near the outer rise
Figure 3(d) shows event solutions in the vicinity of the solution for the Mw 7.6 outer rise aftershock, which occurred 39 minutes after the mainshock. The very different normal-faulting mechanism suggests it was a bending-stress intra-plate event caused by strain reduction on the subduction thrust and an increased tendency for Pacific plate sub-duction to the west after the mainshock rupture. Earthquakes near and west of the Japan Trench were aftershocks occurring on the mainshock thrust as shown in Figs. 2(b) and 3(a), while outer-rise earthquakes east of the Japan Trench distributed along the nodal plane along the A–B cross-section. Therefore, we conclude that the fault plane of this earthquake is the nodal plane of the global CMT solution with a strike of 182° and a dip of 42° westward. Since this large aftershock was the deepest located in this region, it seems the local normal-faulting rupture started here. No earthquakes occurred east of this fault plane. Hence, the western side of the fault plane of the outer rise aftershock subsided beneath the overriding North American plate and the bending stress is probably less significant further eastwards.
The 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.1) was the largest earthquake ever recorded in Japan. Two magnitude 8 or larger earthquakes occurred during the last two centuries. They were the 1896 (M 8.2) and 1933 (Mw 8.4) Sanriku earthquakes, which had epicenters as shown in Fig. 2(a). The 1896 (M 8.2) earthquake was a thrust and typical tsunami earthquake (Kanamori, 1972) occurring very close to the Japan Trench as shown in Fig. 2(a). Since no aftershocks of the 2011 off the Pacific coast of Tohoku earthquake occurred near the 1896 mainshock location west of the Japan Trench, it seems the rupture plane of the 1896 earthquake may not have been re-ruptured this time. The 1933 Sanriku earthquake was a normal-faulting earthquake in the Pacific plate beneath the Japan Trench (Kanamori, 1971), located north of the Mw 7.6 aftershock of 2011.
Since there is a dense seismic network in Japan, locations and mechanism solutions for large earthquakes in and around Japan should be well determined by the regional network. Earthquakes nearer the Japan Trench and its outer rise may have poorer regional solutions because of restricted azimuthal coverage and fewer closer stations. This study demonstrates that the global network, whose arrival times are available soon after earthquake occurrences, can be used to locate these earthquakes very precisely. The modified joint hypocenter determination (MJHD) method minimizes the effects of unmodeled lateral inhomogeneities, so that the MJHD hypocenters may serve to identify the fault planes of large earthquakes even in the outer rise regions. For further improvement of locations, it is important to combine both the local data and the global teleseismic data in the MJHD location.
We thank anonymous reviewers for their helpful comments. Figures and maps were prepared using Generic Mapping Tools software (Wessel and Smith, 1998).
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