Spatial distribution and focal mechanisms of aftershocks of the 2011 off the Pacific coast of Tohoku Earthquake
© 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: 15 April 2011
Accepted: 10 June 2011
Published: 27 September 2011
We estimated centroid moment tensors of earthquakes that occurred from 2003 to 2011 in and around the focal area of the 2011 Mw 9.0 megathrust earthquake in eastern Japan. The result indicates that earthquakes occurring before the mainshock, which included foreshocks off Miyagi, were basically interplate earthquakes with thrust-type focal mechanisms. On the other hand, the aftershocks exhibited a variety of focal mechanisms. Interplate aftershocks with thrust focal mechanisms did not occur within the large coseismic slip area estimated from GPS data but instead occurred in the surrounding regions. This implies that slip could no longer occur in the coseismic slip area due to the large amount of stress release during the mainshock rupture, whereas the aftershocks in the surrounding regions were caused by a stress concentration in these regions due to the large co-seismic slip associated with the mainshock asperity. Normal-fault-type aftershocks were widely distributed in the overriding plate and the outer-rise of the Pacific Plate. These aftershocks may have been due to a tensional stress change caused by the coseismic slip. Thrust-fault-type aftershocks in the subducting Pacific Plate were also interpreted as being due to compressional stress change as a result of the coseismic slip.
Key wordsForeshock aftershock centroid moment tensor focal mechanism
The 2011 off the Pacific coast of Tohoku Earthquake with a moment magnitude (Mw) of 9.0 (e.g., Japan Meteorological Agency, 2011; United States Geological Survey, 2011) occurred at 5:46 (UTC) on March 11, 2011 along the boundary between the subducting Pacific Plate and the overriding plate. Preceding this Mw 9.0 earthquake, an MJMA 7.3 earthquake occurred on March 9. This earthquake and its aftershocks can be regarded as foreshocks to the Mw 9.0 event. Since large earthquakes are likely to produce stress concentrations in neighboring regions along the plate boundary, the mainshock might have been triggered by this sequence of foreshocks and this mainshock have caused aftershocks. It is thus very important to determine the detailed locations and focal mechanisms of the foreshocks, mainshock, and aftershocks, in order to understand the physical relationship among them.
Ito et al. (2004) investigated the focal mechanisms of aftershocks of the 2003 Tokachi-oki earthquake (MJMA 8.0) using moment tensor inversion. They revealed that aftershocks with similar focal mechanisms to that of the main-shock were distributed to the northeast of the mainshock and this aftershock area did not overlap with the coseismic slip area of the mainshock. In addition, aftershocks with different types of focal mechanisms were distributed above and below the plate boundary. In their study, the horizontal locations of the centroids were set to those of the hypocen-ters determined by arrival time data. Ito et al. (2006), on the other hand, successfully estimated centroid moment tensor solutions providing centroid locations in addition to mechanism parameters using newly developed grid search and inversion techniques. They applied their method to data for inland earthquakes observed at stations distributed in onesided regions to simulate the station coverage of off-shore earthquakes, and compared the obtained centroid locations with the hypocenters precisely determined by dense observations. The estimated horizontal and vertical locations of the centroid moment tensors (CMTs) were respectively located within 10 and 15 km from the hypocenters. Their result showed that CMTs can be stably estimated even for off-shore earthquakes, for which hypocenter estimation is difficult using only P- and S-wave arrival times due to poor coverage above the source area. Thus, such a CMT analysis can be a powerful tool for locating off-shore earthquakes and estimating their focal mechanisms. In the present paper, we apply this method to estimate the CMTs of earthquakes in and around the focal area of the Mw 9.0 earthquake and we discuss their spatial distribution.
2. Data and Analysis
Parameters for the CMT inversion.
Classification by amplitude magnitude
3.5 ≤ M < 5
6 ≤ M excepting 12 events
12 large events
Epicentral distance for station selection (km)
Waveform lengths for the CMT inversion (s)
Pass-band of the filter (Hz)
Initial centroid time from origin time (s)
3. General Features of the Obtained CMTs
We obtained CMTs for 1,970 earthquakes with variance reductions of ≥70% using at least 20 stations. Large earthquakes, their later phases, and frequent aftershock occurrences often contaminate seismic signal from target earthquakes to estimate CMTs. Unfeasible locations and times of initial centroids also give rise to difficulties in CMT estimation. In the present study, we set the initial centroid locations to be the hypocenter locations from the Hi-net catalogue, which includes automatically determined hypocen-ters, especially for aftershocks. Some of these are likely to be inaccurate, which would prevent stable estimation during the CMT inversion process.
On the other hand, for the 1,028 aftershocks that occurred in a wide area off Iwate, Miyagi, Fukushima, Ibaraki, and Chiba (Fig. 2(b)), the CMTs indicated that they were of mixed types. For example, the Mw 7.4 and 7.6 earthquakes at 6:08 and 6:15, respectively, on March 11 were similar thrust-type earthquakes along the plate boundary to the Mw 9.0 mainshock. However, an Mw 7.6 normal-fault-type earthquake occurred in the outer-rise of the Pacific plate at 6:26 on March 9, an Mw 7.1 thrust-type earthquake with a larger dip angle occurred in the subducting slab at 14:32 on April 7, and an Mw 6.7 normal-fault-type earthquake occurred in eastern Fukushima at 8:16 on April 11 (Figs. 2(b) and (d)).
4. Detailed Distribution of Interplate and Other Aftershocks
The Geospatial Information Authority of Japan (2011) has estimated the spatial distribution of coseismic and post-seismic slip from GPS data. They showed that the large co-seismic slip (>8 m) area extended from off southern Iwate to off Fukushima and that its largest peak (>24 m) was located off Miyagi. The epicenter distribution of the 315 interplate aftershocks does not overlap with this large co-seismic slip area (Fig. 3 (a)). This suggests that the large coseismic slip area, which is thought to correspond to the mainshock asperity, can no longer slip in the form of aftershocks due to the large amount of the stress release during the mainshock rupture. On the other hand, interplate aftershocks occurred in the northern, southern, and deeper extensions of the large coseismic slip area. These surrounding areas, which probably did not have large amount of co-seismic slips, were primarily loaded by the coseismic slip of the mainshock asperity. In addition, a large amount of postseismic slip was estimated in the deeper extension of the large coseismic slip area off southern Iwate and Miyagi (Fig. 3(a)), which would also promote the occurrence of aftershocks along the plate boundary. Consequently, many interplate aftershocks may have occurred in this region.
Of the 1,028 aftershocks, 713 were identified as being non-interplate types. Instead, a variety of different focal mechanisms were found. In order to discuss the relationship between the coseismic stress change and the spatial distribution of these focal mechanisms, we plot the distribution of these non-interplate aftershocks in the hanging and foot walls in Fig. 3(b) and 3(c), respectively. In the hanging wall, normal faulting is predominant, although the T -axis directions of these aftershocks are scattered. In the foot wall, normal faulting with a T axis along the east-west direction is predominant and these aftershocks are distributed along the Japan Trench near the large coseismic slip area (Fig. 3(c)). These normal-fault aftershocks near the trench and in the outer-rise occurred mainly in the up-dip portion (eastern part) of the foot wall of the large coseismic slip area; seismicity in this area may have been activated by a tensional stress change caused by the thrust faulting of the mainshock. The down-dip portion (western part) of the hanging wall was also subjected to a tensional stress change as a result of the mainshock. Therefore, some of the normal-fault-type aftershocks in these regions, such as the shallow (~10 km) aftershocks near the Pacific coast of Fukushima, Ibaraki, and Chiba in Fig. 3(b), might have been activated by such a stress change. The western part of the foot wall is expected to have been subjected to a compressional stress change. Thrust aftershocks in this area that occurred within the subducting Pacific Plate (e.g. Mw 7.1 at 14:32 on April 7, off Miyagi in Fig. 3(c)) may also have been activated by this compressional stress change.
We estimated the CMTs of earthquakes that occurred before and after the 2011 Mw 9.0 earthquake in eastern Japan. Most of the earthquakes before the mainshock had thrust-type focal mechanisms and occurred along the plate boundary. Foreshocks in the two days preceding the mainshock occurred in a localized area off Miyagi, and the Mw 9.0 mainshock occurred at the southwestern edge of this fore-shock area. On the other hand, the aftershocks exhibited various types of focal mechanisms. No interplate aftershocks occurred in the large coseismic slip area, but rather in the surrounding regions along the plate boundary, probably as a result of a stress concentration due to this large coseismic slip. Other aftershocks occurred in the Pacific Plate and the overriding plate, some of which are thought to have been caused by tensional and compressional stress changes, respectively, as a result of the mainshock.
In the present study, we used topographic data for inland and ocean areas respectively provided by the Geospatial Information Authority of Japan and the Japan Oceano-graphic Data Center. We thank L. Rivera and an anonymous reviewer for providing thoughtful reviews, which helped us to improve this manuscript. All figures were drawn using the software GMT (Wessel and Smith, 1995).
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