We used the broadband waveforms automatically downloaded from the IRIS DMC to constrain the slip history of this giant Earthquake. During this analysis, we selected 23 teleseismic broadband P waveforms, 14 broadband SH waveforms, and 41 long period surface waves. These waveforms are converted to displacement by removing the instrument response. We use the hypocenter of the USGS (Lon.=142.369 deg.; Lat.=38.322 deg; Depth= 24 km).
Focal Mechanism Analysis
We used the long period surface waves (period from 166 s to 333 s) to constrain the focal mechanism of this giant earthquake. Our algorithm approached an earthquake source as multiple double-couples and can automatically determine the number of point sources by performing F test. We notice that a single pure thrust double couple is sufficient to match the long period radiation of this earthquake (Table 1). The low angle nodal plane inferred has a strike of 199 deg., a dip angle of 10 deg, and a slip angle of 92 deg. The total seismic moment is about 5.0x10^22 Nm, or a moment magnitude of 9.1, the largest event since the 2004 Mw 9.15 Sumatra-Andaman island earthquake.
Both the small centroid depth (14 km) and dip angle (11 degrees) suggested that the rupture occurred near the trench axis, consistent with the centroid location (lat=38.1 deg., lon=143.0 deg). It is noteworthy that the low angle fault plane is consistent with the plate interface constrained with wide angle reflection and refraction profile across the source region (Miura et al., 2005).
Figure 1. Ratios of peak amplitude of data (Ad) and synthetics (As). The red triangles and green circles denote the Rayleigh and Love waves, respectively.
We define the fault plane based on our point source solution, USGS hypocenter, and trench axis. The preferred low angle fault plane orients 198. deg. north and dip 10 deg. to the west. The seismic moment release based upon this plane is 5.6 x10^22 Nm using a 1D crustal model interpolated from CRUST2.0 (Bassin et al., 2000). Most of slip occurred at shallow depth from 8 to 20 km.
Figure 2. Cross-section of slip distribution. The strike direction of fault plane is indicated by the black arrow and the hypocenter location is denoted by a red star. The slip amplitudes at subfaults are showed in color and slip directions of the hanging wall relative to the footwall are indicated by white arrows. Contours show the rupture initiation time in sec.
Figure 3. Moment rate function.
Figure 3. Comparison of teleseismic body waves. The data is shown in black and the synthetic seismograms are plotted in red. Both data and synthetic seismograms are aligned on the P or SH arrivals. The number at the end of each trace is the peak amplitude of the observation in micro-meter. The number above the beginning of each trace is the source azimuth and below is the epicentral distance.
Figure 3.1. Comparison of long period surface waves. The data is shown in black and the synthetic seismograms are plotted in red. Both data and synthetic seismograms are aligned on the earthquake origin time. The number at the end of each trace is the peak amplitude of the observation in millimeter. The number above the beginning of each trace is the source azimuth and below is the epicentral distance in degrees.
Figure 3.2. Comparison of long period surface waves. The data is shown in black and the synthetic seismograms are plotted in red. Both data and synthetic seismograms are aligned on the earthquake origin time. The number at the end of each trace is the peak amplitude of the observation in millimeter. The number above the beginning of each trace is the source azimuth and below is the epicentral distance in degrees.
This earthquake has relatively long rise time (16s) and slow rupture velocity (< 2 km/s). Most of its seismic moment occurred at region shallower than the hypocenter. These features are quite similar to the 2006 Mw 8.3 Kuril Islands earthquake. The peak slip is not well constrained due to the lack of high frequency in radiated body waves. But it must larger than 30 m for the given seismic moment.
Figure 4. Surface projection of the slip distribution superimposed on ETOPO2. The black line indicates the major plate boundary [Bird, 2003]. The GCMT focal mechanisms of 51 magnitude 6 and larger earthquakes since 1990 are denoted as well.
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This work was supported by National Earthquake Information Center (NEIC) of United States Geological Survey. This web page is built and maintained by Dr. C. Ji at UCSB.