A rupture model 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: 7 April 2011
Accepted: 18 May 2011
Published: 27 September 2011
The 11 March 2011 Tohoku Earthquake ruptured the interplate boundary off-shore of east Japan, with fault displacements of up to 40 m and a rupture duration of 150–160 s. W-phase inversion indicates a moment of 3.9 × 1022 N m (Mw 9.0) and a centroid time of 71 s. We invert teleseismic P waves and broadband Rayleigh wave observations with high-rate GPS recordings from Japan to characterize the rupture. The resulting rupture model begins with a steady increase of moment rate for the first 80 s, and a rupture speed of 1.5 km/s. Then the rupture expands southwestward at a speed of about 2.5 km/s. The model’s primary slip is concentrated up-dip from the hypocenter, with significant displacement extending to the trench. The seaward location of large slip is consistent with estimates of the tsunami source area from regional and remote tsunami observations. The region with large slip is approximately 150-km wide by 300-km long, which is relatively compact compared with the 200-km wide, 500-km long aftershock region. The model’s initial updip rupture expansion and the location of most slip updip of the hypocenter differs from P-wave array back-projections, which map high-frequency radiation along the downdip of the hypocenter, closer to the coast.
The great 11 March 2011 Tohoku Earthquake ruptured a large segment of the Pacific-North American (Okhotsk) plate boundary off the coast of northeastern Japan. This boundary has hosted large and deadly earthquakes in the past, but the 2011 event is the largest (Mw 9.0–9.1) in the recent history of Japan, and one of the five largest earthquakes of the modern era. Convergence across the boundary is close to trench-perpendicular at roughly 92 mm/yr (DeMets et al., 2010). The earthquake excited a large tsunami that devastated coastal communities in Japan. The earthquake is likely to be one of the most important seismic events recorded thus far because of the relatively long record of information on earthquakes in the region, and because of the extensive investment by Japan in geophysical observational facilities that recorded diverse signals from the event with unprecedented detail.
The Tohoku sequence began at 02:45:20.28 UTC on 9 March 2011, when an Mw 7.2 (USGS) to 7.5 (glob-alcmt.org) underthrusting foreshock located at 38.42N, 142.83E struck just to the north of the subsequent great earthquake epicenter. Depth estimates for the initiation of the major foreshock and the great earthquake are ~30 km. Aftershocks of the 9 March 2011 foreshock spread to the north, but several of the larger events appear to have migrated towards the eventual nucleation region of the 11 March 2011 mainshock, which struck at 05:46:24.51 UTC at 38.30N, 142.34E (USGS). Moment tensor inversion of 123 W-phase recordings (Kanamori and Rivera, 2008) from 71 global seismic stations indicates a mainshock seismic moment of 3.9×1022 N m (Mw 9.0), with small non-double couple component and a best nodal plane striking 202°, dipping 12.9°, a rake of 92.5°, and a 71 s centroid time shift. The great event initiated a vigorous aftershock sequence that included an Mw 7.9 underthrusting aftershock near the southern end of the rupture approximately 28 minutes later (06:15:40.88 UTC, 36.18N 141.17E; USGS); ten minutes after that, (06:25:50.79 UTC) a large, Mw 7.7, ex-tensional faulting earthquake occurred in the outer rise region (38.06N, 144.60E) of the Pacific plate. As of 25 March 2011, the continuing aftershock sequence has included 3 aftershocks with magnitudes of 7.0 or larger, nearly 40 with magnitudes between 6.0 and 7.0, and over 350 events in the magnitude range from 5.0 to 6.0.
2. The Observations
The R1 RSTFs were estimated from the mainshock observations using the Mw 7.2 foreshock signals as empirical Green functions (e.g., Ammon et al., 1993). We isolated the R1 signals with a wide group velocity window, and deconvolved the signals using an iterative time-domain procedure based on the method of Kikuchi and Kanamori (1982). We convolved each signal with a low-pass Gaussian filter to reduce short-period noise in the resulting time series. The resulting time series isolate the differential effects of the larger and smaller event ruptures, with accurate estimation of the main event properties for periods longer than about 30 s. Use of theoretical Green functions results in similar R1 source functions, but aspherical effects are more directly accounted for using the EGF approach. For many large earthquakes, R1 STFs show strong directivity patterns because the phase velocity of the R1 signals is relatively close to the rupture speed, enhancing directivity effects in the signals. For predominantly along-strike unilateral or bilateral ruptures the azimuthal directivity patterns are quite simple and can be analyzed in a straightforward way (e.g., Furlong et al., 2009; Lay et al., 2010b). In the case of the 2011 event, no simple azimuthal pattern in the R1 RSTFs was observed, suggesting that rupture propagation was significant in at least two, non-parallel directions.
3. The Rupture Model
We modeled the observations using a least-squares local-search algorithm that is designed to construct the broad features in the rupture first, then refine the broad features by adding detail as the search progresses (Ammon et al., 2005, 2006). For each step in the search, part of the model is randomly perturbed, the effect on the fits to the observations is assessed, and if the fits improve, we keep the new model, otherwise we try another perturbation. The rupture speed is specified during each search, and numerous searches are used to explore the sensitivity to the assumed rupture kinematics. Time functions in the search are restricted to a set of half-cosine shapes (Ji et al., 2002) that range in duration from 20 to 40 s, in 4 s increments.
We used teleseismic P waves only and a non-negative least squares algorithm to initially explore the likely rupture speeds and found that a slow initial rupture expansion followed by a faster expansion resulted in relatively simple slip distributions and good fits to the data. We adopted a slow initial rupture speed (1.5 km/s) with which the rupture expands to a distance of 100 km from the hypocenter, after which the rupture propagates outward with increased speed of 2.5 km/s. Our results are generally consistent for small (0.5 km/s) variations in these parameters—as always, a faster rupture speed spreads the moment further over the fault, a slower rupture speed results in a more compact pattern and larger estimates of slip (Lay et al., 2010c). We included 39 P waveforms, 31 R1 RSTFs, and 15 three-component hrGPS signals. We initiated the rupture at the USGS hypocenter, aligned the observed and predicted P waveforms by picking the P onsets, shifted several of the R1 RSTFs to improve alignments, and used absolute time to fit the hrGPS signals. We chose a strike of 202°, consistent with the W-phase solution, to align the top of the model with the local trend of the trench, and assumed a dip of 12°, which is within the range (10–14°) estimated by various early long-period point-source inversion efforts. We fixed the rake at 85°, which is again consistent with the available long-period point-source estimates of the average rupture process. P wave only solutions with non-negative least squares suggest minor variations in rake over the fault plane, but these are not well resolved. Future inversions can include variations in rake but that approach becomes more complicated when working with the R1 RSTFs, which have been estimated using Green functions for a single rake. Perhaps the most complicated part of modeling multiple, complementary data is interpreting the results from data modeled with different approaches and different earth model pa-rameterizations. The P waves were modeled using a 2 km thick water-layer over a halfspace with a P-wave speed of 6.2 km/s. Clearly a half-space is a first order approximation to the lateral variation in the structure over the great event source region. The R1 STFs are modeled using the true earth model for the foreshock location, since we used an empirical Green Function approach. The geodetic observations were modeled using spheroidal mode summation for the anisotropic PREM structure with all modes up to 50 mHz (Luis Rivera, personal communication, 2011).
The moment rate function has a peak with a value 6.4 × 1020 N m/s; a temporal centroid of 75 s, and a seismic moment of 3.6×1022 N m (Mw 9.0), quite compatible with the W-phase solution. The majority of slip in the rupture models is contained in a region approximately 150-km wide by 300-km long, which is relatively compact compared with the 200-km wide, 500-km long aftershock region. The region of substantial slip extends to the trench, and is centered on a segment of the trench that protrudes noticeably more eastward than areas to the north and south. The Mw 7.7 normal fault aftershock is located seaward of this bulge in the upper plate and most intraplate trench-slope seismicity is concentrated seaward of the region of large slip. The slip extends about 100 km to the north and about 200 km to the south. We recover little slip from deeper portions of the fault model except immediately down dip of the hypocenter. We also recover little slip to the south-southwest in the region of the Mw 7.9 aftershock.
A peak slip of 40 meters and a broad region with slip values from 30–40 meters suggests that at least part of this rupture had not failed for three-to-four hundred years or more, possibly back to the historic 869 tsunamigenic earthquake. The long time between such ruptures may explain why an event of this size was not expected in the region, but makes clear the problems geoscientists face when trying to extrapolate imperfect historical and geologic earthquake records to future earthquake forecasts. Recent geodetic work used GPS measured surface velocities to infer strong interseismic coupling along the coast of eastern Japan (Suwa et al., 2006; Loveless and Meade, 2010), landward of where we recover large megathrust slip (off Miyagi prefecture). Our models suggest that the locked region of the megath-rust extended farther updip than indicated by the land-based geodetic signals. Resolution of the frictional coupling at the shallowest regions of the trench is an important goal to assess the tsunami potential along subduction boundaries.
Although our model has its maximum slip downdip from the trench, the slip near the trench is substantial, which helped produce the large observed tsunami. Still, our smooth model and simple mapping of moment to slip using a uniform shear modulus probably underestimates the slip at the shallowest regions of the plate boundary. We compare the model predictions with far-field tsunami observations in Lay et al. (2011b), resolving more details of the slip in this region may require local tsunami observations (e.g. Fujii et al., 2011). One of the important applications of early models of large earthquake slip patterns is to estimate the effects the large earthquake may have on adjacent faults. Recent large earthquake sequences have illuminated the importance of such interactions during great earthquake sequences (Ammon et al., 2008; Lay et al., 2009, 2010a). We refer the reader to Lay et al. (2011c) for a discussion of the implications such large slip may have on processes in adjacent Pacific plate. Finally, an additional intriguing aspect of the model is its difference from short-period tele-seismic P-wave back projection results that suggest strong short-period P-wave radiation occurred along the megathrust, downdip of the hypocenter. Koper et al. (2011) discuss the details of what appears to be a compound rupture with large slow slip and predominantly long period seismic radiation updip and conventional slip with broadband seismic wave radiation downdip.
Seismic data were distributed by the IRIS Data Center. Preliminary GPS time series provided by the ARIA team at JPL and Caltech. All original GEONET RINEX data provided to Caltech by the Geospatial Information Authority (GSI) of Japan. We also thank Luis Rivera for sharing his normal mode summation software and Gavin Hayes for discussions. This work was supported by NSF grant EAR0635570, AFRL Contract FA8718-09-C-007, and Award Number 05HQGR0174. Comments from J. Loveless, K. Yomogida, and K. Koketsu helped improve this manuscript.
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