We modeled the mainshock by a finite fault model (Figure 3) that was constrained using broadband seismological records at teleseismic and regional distances, and using interferometric synthetic aperture radar (InSAR) data. In total, the data from 33 broadband stations of the Federation of Digital Seismograph Network (FDSN) were retrieved from the IRIS data center (http://www.iris.edu/wilber). Two advanced land-observing satellite/phased array Type L-band synthetic aperture radar (ALOS/PALSAR) interferograms, one ascending (path 138, February 28, 2009–January 16, 2010) and one descending (path 447, March 9, 2009–January 25, 2010), each of which partially covered the rupture zone, were processed and released by the Japan Aerospace Exploration Agency and Ministry of Economy, Trade and Industry of Japan. The finite fault modeling of teleseismic body waves (P and SH) constrains the strike (264°), dip (64°), and initial rake (43°) of the single rectangular fault-plane solution. The strike agrees with the trend of the EPGFZ and with the elongation axis of the InSAR fringes. The dip of 64° to the North, agrees with the preliminary distribution of aftershocks; on the other hand, this is at odds with the near-vertical dip expected from the linear trend of the EPGFZ. The oblique rake angle also implies a large deviation from the left-lateral motion expected for the EPGFZ. The positioning of the fault model was determined by optimizing the modeling of the InSAR data, with the top of the model at the surface. The slip distribution in space and time is obtained from a joint inversion of the seismological and InSAR data, following the approach described by Delouis et al. . The USGS epicenter is clearly not compatible with the rupture plane dipping to the North. In the absence of local seismic stations, the epicenter determined with the global network can be erroneous by several kilometers. Accordingly, we explored new hypocenter positions in a series of trial-and-error joint inversions. An optimal fit of the joint datasets was obtained with the rupture initiation (the hypocenter) shifted 7 km to the NE with respect to the USGS location. Interestingly, this correction of the location agrees well with the aftershock sequence, as most of the major aftershocks have been located NE of their NEIC epicenter (Figure 1b). The resulting slip distribution is shown in Figure 3, together with a comparison between a subset of observed and simulated data. The rupture propagated unilaterally towards the West, with an average rupture velocity of 2.6 km.s−1. The main slip zone has slip values between 2 m and 5 m, and extends for about 40 km. The absence of significant slip in the uppermost part of the model, which is a result of the inversion because we did not constrain the slip to taper to zero near to the surface, agrees with the lack of clear coseismic offsets observed at the surface. Near to the rupture initiation and in the upper part of the slip distribution, the reverse component is predominant. In the deeper part of the model and in the west, left-lateral slip predominates (Figure 3). The final average rake angle in the model is 37°, and the total seismic moment is 4.9e+19 N.m, corresponding to Mw = 7.1. The deep slip patch at the north-western end of the rupture (Figure 3) accounts for less than 5% of the variance reduction; it is thus not likely to be a real feature. The hypocenter depth is constrained by the teleseismic data, as in the range of 12 ± 4 km. Therefore, the NEIC epicenter is incompatible with a north-dipping plane because a rupture plane containing the hypocenter would be too deep under the northern coastal area of the Southern Peninsula, which is in disagreement with the sharp gradient of the displacement field shown by the InSAR data. Only a south-dipping plane, as proposed by Hayes et al. , can reconcile the NEIC epicenter with the mainshock rupture. However, with the corrected epicenter, a complex rupture model like the one proposed by Hayes et al.  that has fault segments changing in azimuth and dip direction, does not appear to be necessary to satisfactorily explain the first-order characteristics of the seismological and InSAR data. Hashimoto et al.  reported an optimal dip of 42° to explain the InSAR data. However, we have verified that such a low dip produces reverse polarities or large amplitude mismatches in the initial parts of the seismograms at many of the teleseismic stations. We show here that the ALOS data are tolerably compatible with a steeper rupture plane, which is in agreement with the seismological data. A possible explanation for the differences in dip found by these two studies might be related to a certain amount of post-seismic deformation that is included in the InSAR data.
Figure 3. (a) Result of the joint inversion of the seismological and InSAR (© Japan Aerospace Exploration Agency and Ministry of Economy, Trade and Industry of Japan, see text) data for the Haiti main shock. The rectangular finite-fault model is projected onto the surface. Black arrows, direction of motion of the hanging wall with respect to the footwall. The focal mechanism (double-couple component) of the mainshock from the GCMT [Nettles and Hjörleifsdóttir, 2010] and from the present study (strike, dip, rake, 264°, 64°, 37°, respectively) are also shown. Yellow star, point of rupture initiation (hypocenter). (b) Different frames illustrating comparisons between the observed and simulated data. Upper panels: red lines, fringes of the two SAR interferograms; heavy dashed lines, two North-South profiles, P1 and P2. The agreement with the line-of-sight (LOS) displacements along these two profiles is also shown. Lower panels: illustrations of the modeling of the seismic records. (c) Teleseismic P-wave displacements (Z, vertical component) at stations ASCN, COLA, POHA, ARU, LPAZ, COR. (d) Three components (N, E, and Z, vertical) of the complete displacement waveforms at regional-distance stations MTDJ (Jamaica) and SDDR (Dominican Republic).
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