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Source process of the 2011 off the Pacific coast of Tohoku Earthquake inferred from waveform inversion with long-period strong-motion records
© 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: 28 June 2011
Published: 27 September 2011
We have investigated the rupture process of the 2011 off the Pacific coast of Tohoku Earthquake using a multi-time-window linear waveform inversion method using long-period strong-ground motion data. From the record section of the long-period motion of the 3 phases, it is indicated that the rupture process mainly consists of the 3 stages. We have assumed a single planar fault model of 468 km long in strike and 228 km wide in dip. The seismic moment of this earthquake was estimated to be 4.3 × 1022 N m (Mw 9.0). The inverted slip distribution shows a large asperity with a maximum slip of about 47 m which is located on the shallower part of the fault plane. The rupture process is divided into three stages: a first stage with moderate slip; a second stage with large and long-duration slip in the shallow part of the fault; and a third stage with relatively small and short-duration slip in the southern part of the fault. The feature of the ground motion suggested from the record section is well represented by the peak moment rate distributions, rather than the slip distributions. The rupture velocity is around 3–4 km/s in the first and third stages, while rupture progression was suspended for a while before the rupture of the asperity.
On 11 March 2011, the Tohoku Earthquake of MW 9.0 (Japan Meteorological Agency) struck the Tohoku region, Japan, and generated a huge tsunami. Historical and seismic records indicate that interplate earthquakes with magnitudes of about 7.5 have occurred repeatedly around the hypocenter of this event with a recurrence interval of about 37 years (the Headquarters for Earthquake Research Promotion, MEXT, Japan, 2001). Recent studies have pointed out that the magnitude of the AD 869 Jogan earthquake, which occurred around the off coast of Miyagi area, was greater than Mw 8.4 (e.g., Satake et al., 2008). The 2011 Tohoku Earthquake (the 2011 off the Pacific coast of Tohoku Earthquake named by JMA) is the first M 9 earthquake to have been recorded by many near-field strong-motion stations. During previous historical M 9-class earthquakes (e.g., 2004 Sumatra, 1964 Alaska, 1960 Chile), few strong-motion seismometers near the source region were operated. It is important to discuss whether the asperity estimated from waveform inversion using long-period strong motion is the same as those using the tsunami, and to discuss the relationship between large slip area (asperities) expected to excite tsunami, and strong-motion generating areas (SMGAs).
In this study, we first investigate the characteristics of the rupture from the observed seismograms directly. Then, we estimate the source process using the long-period strong-motion waveform data in the near-field region of the event.
2. Observed Seismograms
A large number of strong motion records were obtained during the mainshock. In this section, we use the data obtained from the strong-motion seismometers of F-net and KiK-net (Aoi et al., 2004; Okada et al., 2004) operated by the National Research Institute for Earth Science and Disaster Prevention. From the KiK-net stations, we chose stations located on relatively hard rock to avoid local site effects.
3. Inversion of the Source Process
The data were windowed for 300 s, starting at P-wave arrival time, and band-pass filtered between periods of 200 s to 20 s (0.005–0.05 Hz) for waveform inversion. The accelerograms obtained from the KiK-net were integrated into ground velocities with a sampling time of 4 s.
We assumed a single fault plane for the waveform-inversion analysis. The fault size is assumed to be 468 km long in strike and 228 km wide in dip, referring to the aftershock distribution. We assume N193°E and 10° as the strike and dip angles, respectively, referring to the JMA CMT solution. The rupture starting point is located at the hypocen-ter determined by JMA: 38.103°N, 142.861°E, 23.7 km.
We use a multi-time-window linear waveform-inversion procedure (e.g., Hartzell and Heaton, 1983) in which the moment-release distribution is discretized in both space and time. For discretization in space, we divide the fault plane into 39 in the strike direction and into 19 in the down-dip direction (making a total of 741 subfaults over an area of 12 km × 12 km). We use 8 smoothed ramp functions with a duration of 16 s separated by 8-s intervals to represent the slip history of each subfault. The triggering front of the first-time-window propagates at a constant velocity, and the multi-time-window analysis allows a variable rupture velocity and slip duration in an inverted rupture model by weighted time windows. To suppress instability or excessive complexity, a smoothing constraint is introduced to reduce differences in moment release values close in space. The smoothing constrained parameter used in the inversion was determined to choose the solution with a total slip which agrees with that expected from the geodetic data (e.g. 24 m at off coast of Miyagi, by Sato et al., 2011). Non-negative constraints (Lawson and Hanson, 1974) to limit the rake-angle variation are also adopted. The rake angles are allowed to vary within 45° centered at 90°. We use the first time window triggering velocity (FTWTV) as 2.5 km/s, and the rupture velocity will be discussed in Section 4.
4. Result and Discussion
Slip models by tsunami inversion are proposed by Satake et al. (2011). The overall slip patterns and the average slip in the shallow part of the fault along the trench in this study are nearly the same as their results (the maximum slip of about 30 m).
Phase 2 (around 100 s in Fig. 7) is expected to be generated during the second stage of the rupture. The synthetics calculated from Area 2 (the asperity area) explain the largest waves observed at the northern station (e.g. TMR). At KSN, Phase 2 consists of the superposition of the waves calculated from Areas 1 and 2 with similar amplitudes. The pulse width coming from Area 1 is short (~20 s), whilst that from Area 2 is long (>30 s). The high moment rate on the deeper part (I in Fig. 4(B)) just after the rupture of the asperity (frame 75–90 s in Fig. 6) also contributes to Phase 2, which is overlapping on the seismic waves coming from Area 2.
Phase 3, which is the largest wave at the southern station (e.g. IBRH19), is not explained by the synthetics calculated from Area 1 or 2. The synthetics calculated from Area 3, which is active in the third stage, explain Phase 3. Although there is no clear slip in the total slip distribution in Area 3, a moderate peak moment-rate area (II in Fig. 4(B)) is found in Area 3.
The distribution of the peak moment rate indicates some strong peak areas of the moment-rate functions. Two high peak moment-rate areas are located on the deeper side of the rupture starting point (I) and one moderate peak area is recognized on the southern part of the fault plane (II). The strong motion-generating area derived by EGF modeling (Kurahashi and Irikura, 2011) agrees well with the peak moment-rate distribution, rather than the slip distribution. The moment-rate function gives far-field displacement waveforms of P and S waves (e.g. Aki and Richards, 2002). Thus, ground motions reflect moment rate rather than slip distribution; in particular, short-period motions depend on the peak moment-rate distribution.
We have investigated the rupture process of the 2011 Tohoku Earthquake by examining the long-period components of the near-source region strong-motion waveforms and by multi-time-window linear-waveform inversion using the long-period (20–200 s) strong-motion records. The record section of the long-period strong ground motion implies that the rupture process consists of 3 stages. The inverted-source process model shows that a large asperity (about 300 km × 100 km) located on the shallower part of the fault was estimated, and maximum slip is about 47 m. This source model with a shallow large asperity is consistent with the tsunami source model (Satake et al., 2011). The rupture process is divided into three stages: the first stage with moderate slip in the deeper area of the northern part of the fault; the second stage with large and long-duration slip in the shallow area of the northern part of the fault (the asperity area); and the third stage with the rupture of the southern part of the fault. The feature of the ground motion suggested from the record section is well-represented by the peak moment-rate distributions, rather than the slip distributions. The rupture velocity is around 3–4 km/s in the first and third stages, while rupture progression is suspended for a while before the rupture of the asperity. As a result of the heterogeneous rupture propagation, the average rupture velocity in the entire fault is about 2.2 km/s.
We thank the National Research Institute for Earth Science Disaster Prevention and Hokkaido University to provide the strong-motion data. The clarity and completeness of the article was improved by reviews from Dr. Yoshiaki Shiba and an anonymous reviewer. Many of the figures in this paper were produced using GMT (Wessel and Smith, 1998).
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