Synopses of Master Papers

Bulletin of IISEE, 47, 7-12, 2013

SOURCE PROCESSES OF LARGE EARTHQUAKES
PRECEDING THE 2011 TOHOKU-OKI EARTHQUAKE,DETERMINED BY JOINT INVERSION OFTELESEISMIC AND NEAR-SOURCE DATALIU Rong ∗MEE11602

Supervisor:

Yuji YAGI ∗∗Bunichiro SHIBAZAKI ∗∗∗ABSTRACT
A sequence of M7-class interplate earthquakes (M6.7-7.3) occurred from 2003 to 2011 before theoccurrence of the M9 2011 Tohoku-oki earthquake. To investigate the relationship between theseprevious events and the giant earthquake, we determined the source processes of these events by joint
inversion method. We performed the inversions by using both near source and teleseismic body-wave
data. To perform a stable inversion, we applied smoothing constraints, and determined their optimal
relative weights on the observed data using an optimized Akaike's Bayesian Information Criterion(ABIC). The results show that the rupture areas of these M7-class events were nearly covered by therupture area of the 2011 Tohoku-oki earthquake determined by Yagi and Fukahata (2011). The
epicenter or the initial rupture of the giant earthquake was located in the un-rupture region surroundedby the rupture areas of the 2003 Fukushima-ken-oki earthquake, the 2005 Miyagi-ken-oki earthquake
and the 2011 Sanriku-oki earthquake. At the first stage of the source process, the rupture propagatedsoutheast and broke the asperity where the maximum slip occurred, and the asperity was also locatedin the un-rupture region which was close to the source regions of the 2003 Fukushima-ken-oki
earthquake and the 2011 Sanriku-oki earthquake.
Keywords: Teleseismic Wave, Strong Motion, Source Rupture Process, Joint Inversion

1. INTRODUCTIONOn March 11, 2011, a giant earthquake with magnitude 9.0 occurred off the Pacific coast of northern
Honshu. It resulted from thrust faulting on or near the plate boundary between the Pacific and North
America Plates. The strong ground shaking and large tsunami generated by this giant earthquake
caused significant casualties and damage. Before this M9 giant event, several M7-class earthquakes
occurred near the source region from 2003 to 2011: (1) October 31, 2003, Fukushima-ken-okiearthquake (M j 6.8, JMA magnitude); (2) August 16, 2005, Miyagi-ken-oki earthquake (M j 7.2); (3)May 7, 2008, Ibaraki-ken-oki earthquake (M j 7.0); (4) July 19, 2008, Fukushima-ken-oki earthquake(M j 6.9); (5) March 14, 2010, Fukushima-ken-oki earthquake (M j 6.7) and (6) March 9, 2011,Sanriku-oki earthquake (M j 7.3). The JMA results show that these six interplate earthquakes have
similar focal mechanism as the 2011 Tohoku-oki earthquake. They all occurred along the Japan
Trench because of the low-angle thrust faulting on the subduction zone. These events can beconsidered as the long-term pre-seismic signals of the Tohoku-oki earthquake. To gain understandingsof the preparation process of giant earthquake, it is necessary to investigate the source processes ofthese M7-class events.
∗Nanjing Municipal Earthquake Bureau, P. R. China.
Professor, Graduate School of Life and Environmental Sciences, University of Tsukuba, Japan.
∗∗∗Building Research Institute, Tsukuba, Japan.
∗∗

2. DATAIn this study, We obtained teleseismic body wave data recorded by IRIS-DMC. 177 components of 61teleseismic stations were selected. We chose the stations of epicentral distance ranges from 30° to 90°.
We also used 96 components of near-source data obtained from 23 strong motion seismograph stationsof the K-NET operated by NIED. The teleseismic body waveforms were converted into displacementwith sampling time 0.25 sec and the near-source data were integrated into velocity with sampling time

0.25 sec.
Figure 1. The distribution map of teleseismic stations: the stars represent the epicenters of theevents; red triangles indicate stations. a) teleseismic network, b) strong motion network.

3. THEORY AND METHODOLOGY
To construct the source model and obtain the source processes of the six interplate earthquakes, weused a numerical method developed by Yagi et al. (2004), whose basic theory follows the standard
waveform inversion scheme (Hartzell and Heaton, 1983). The rupture process is represented as a
spatiotemporal slip distribution on a fault plane. We divided the fault plane into multiple subfaults.
Then we described the slip-rate function on each subfault by a series of triangle functions with risetime. We calculated the Green's function for teleseismic body wave data by the method of Kikuchi andKanamori (1991). For near-source strong ground motion we used the discrete wave number method
developed by Kohketsu (1985). We applied the velocity structure based on a simple-layered oceanic
model referred to the Jeffreys-Bullen model (Jeffreys, 1970). To simplify the calculation and obtain
more stable results, we applied smoothing constraints to the slip distribution with respect to time andspace to obtain more stable results. With the constraints, we can determine the model parameters thatminimize the residual sum of squares. To get the values of variances we applied the minimum
Akaike’s Bayesian information criterion (ABIC) (Akaike, 1980). The optimal ABIC formulation oftwo types for partially-dependent prior information was developed by Fukahata et al. (2003). We
applied a grid-search method to obtain optimal values of variances which minimize ABIC, andemployed the non-negative least squares (NNLS) algorithm of Lawson and Hanson (1974) to solve theleast squares problem with a positivity constraint on the model parameters.

4. RESULTS AND DISCUSSIONAssuming the faulting occurs on the single fault plane and that the slip angle remains unchanged
during the rupture, we adopted the focal mechanism and the epicenter determined by JMA. We
constructed our fault model with a length along the strike and a width along dip to obtain a roughestimate of the rupture area. Then we set the number and size of the subfaults on the fault plane. Since
the focal depth is not adequately constrained by the local seismological network, we varied the value
in the inversion procedure with the focal mechanism and epicenter fixed, and found its minimumvariance.

Figure 2. Joint inversion results of M7-class earthquakes.
a) Focal mechanism, b) source time function, c) slip distribution
Figure 2 gives the inversion results of these M7-class events before Tohoku-oki Earthquake.
The source durations, the focal mechanisms of these earthquakes and the final dislocation on the fault
plane are shown here. The source time functions indicate the total seismic moments of these events.
Our results seem almost in agreement with the CMT moments determined by JMA. The comparison ofthe observed data and the synthetic waveform is shown in Figure 3: The overall fit is quite good.
The relationship between these M7-class events and the giant earthquake was discussed by
combining the results obtained from the preceding analysis. Figure 4 superimposes these slip
distributions of the M7-class events with that of the 2011 Tohoku-oki earthquake (Yagi and Fukahata,2011). The colored contour interval of each coseismic slip is 10 cm and the Tohoku-oki earthquake’s
black contour interval is 5 m. The yellow stars represent the epicenters of the M7-class events; the redstar indicates the epicenter of the giant one. The green broken lines indicate isodepth contours of theplate interface with 10-km intervals (Nakajima and Hasegawa, 2006) and the black one represents theJapan Trench.

Figure 3. Comparison of the observed waveforms (black curve) with the calculated
waveforms (red curve)

Figure 4. Coseismic slip distribution of six
events and the Tohoku-oki earthquake

In this figure the rupture areas of theseM7-class events were nearly covered by that of theTohoku-oki earthquake. The epicenter of the giantearthquake was located at the gap of the rupture areas ofthese events. The rupture propagated southeast during thefirst 20 s, broke the asperity to the northeast of the source
region of the 2003 Fukushima-ken-oki earthquake and thesoutheast of that of the 2011 Sanriku-oki earthquake. Therupture was greatly accelerated around this area where themaximum slip finally got over 50 m. At the same time,
the rupture started at 20 s in the source region of the 2005
Miyagi-ken-oki earthquake. After 40 s, the rupture
extended widely towards both north and south, as shownin Figure 5. In this figure, which compares the slip
distributions for the six M7-class events and initiation
process of the 2011 Tohoku-oki earthquake, it can beseen that the Tohoku-oki earthquake initiated the region
surrounded by the rupture areas of the 2003

Fukushima-ken-oki earthquake, the 2005 Miyagi-ken-oki earthquake and the 2011 Sanriku-okiearthquake. Therefore, the initial rupture area of the Tohoku-oki earthquake is characterized by theoccurrence in the un-rupture region surrounded by the large earthquakes preceding it. At this firststage of the source process, the rupture broke the asperity or sub-faults where the maximum slip
occurred.

Figure 5. Snapshots of the rupture propagation of the 2011 Tohoku-oki
earthquake at 10-s intervals from 0 – 40 s (Yagi and Fukahata, 2011)
5. CONCLUSIONS
Applying the joint inversion method developed by Yagi et al. (2004) that uses the near source strong
motion data and teleseismic body wave data, we determined detailed source fault models andspatiotemporal slip distributions of a sequence of M7-class interplate earthquakes from 2003 to 2011
along the Japan Trench preceding the 2011 Tohoku-oki earthquake.
The results indicate that the slip distributions of the 2008 Ibaraki-ken-oki earthquake, the2008 Fukushima-ken-oki earthquake and the 2010 Fukushima-ken-oki earthquake are relatively

similar: The source process consisted of a main rupture in the vicinity of the hypocenter, followed bypropagation of the rupture toward the east, and extended area is limited. The inversion results of the2003 Fukushima-ken-oki earthquake, the 2005 Miyagi-ken-oki earthquake and the 2011 Sanriku-okiearthquake show different characteristics of source process. The source processes of these events arecharacterized by the large rupture.
Comparison with the slip distribution for the M7-class events revealed that the rupturearea of the 2011 Tohoku-oki earthquake determined by Yagi and Fukahata (2011) covered most of
those of the M7-class earthquakes. The epicenter or the initial rupture of the giant earthquake was
located in the un-rupture region surrounded by the rupture areas of the 2003 Fukushima-ken-oki
earthquake, the 2005 Miyagi-ken-oki earthquake and the 2011 Sanriku-oki earthquake. At the firststage of the source process, the rupture propagated southeast and broke the asperity where themaximum slip occurred, and the asperity also was located in the un-rupture region which was close to
the source regions of the 2003 Fukushima-ken-oki earthquake and the 2011 Sanriku-oki earthquake.
ACKNOWLEDGEMENT
Waveform data obtained by GSN-BROADBAND (Global Seismographic Network broadband stations)were supplied by the IRIS-DMC. We used hypocenters determined by JMA and strong ground motion
data observed by K-net, NIED.
REFERENCES
Akaike, H., 1980, pp. 143-166, University Press, Valencia, Spain.
Fukahata, Y., et al., 2003, Geophys. Res. Lett., 30, 10.1029/2002GL016293.
Hartzell, S. H. and T. H. Heaton., 1983, Bull. Seism. Soc. Am., 73, 1553-1583.
Jeffreys, H., 1970, The Earth Cambridge University Press, Cambridge, 525 pp.
Kikuchi, M., and Kanamori H., 1991, Bull. Seism. Soc. Am., 81, 2335-2350.
Kohketsu, K., 1985, J. Phys. Earth, 33, 121-131.
Lawson, C. L. and R. J. Hanson., 1974, Prentice-Hall, Inc., Englewood Cliffs., New Jersey.
Nakajima, J. and A. Hasegawa, 2006, Geophys. Res. Lett., 33, L16309, doi:10.1029/2006GL026773.
Yagi, Y. and Y. Fukahata, 2011, Geophys. Res. Lett., 38, L19307, doi:10.1029/2011GL048701.
Yagi, Y., et al., 2004, Bull. Seism. Soc. Am. 94:1795-1807.