Coseismic model

The results presented on Figure 6.6 and in Table 6.3 can be divided into 3 categories:

• Sites that show no significant offset after the earthquake: CAPT, SROD, MONC, LVEG, CASX. These sites are located more than 50 km from the epicenter.
• Sites that show a significant offset after the earthquake and are located within 50 km of the epicenter: REUN, HIDA, CAMP, SAGO. These offsets are likely to be of coseismic origin.
• Sites that show a significant offset after the earthquake and are located more than 50 km from the epicenter: TERR, ARRO, LAVE. As shown below, these sites are too far from the epicenter to experience coseismic displacements.

In order to interpret these results, we calculate the expected surface coseismic displacements using a dislocation model in a uniform half-space. The model parameters are the geometry of the rupture (length, width, dip angle), its geographic location, its depth, and the slip distribution on the rupture plane. The dip angle (15 degrees) and fault strike (121$^o$) are taken from the Harvard CMT solution. We assume a rupture length of 25 km, a width of 11.5 km, and a uniform slip of 50 cm, purely dip-slip. These parameters are taken from the empirical scaling laws of Smith and Coppersmith (1994) that we slightly adjusted to match the observed seismic moment (4.8$\times$10$^{18}$ N.M). We then adjust the location and depth of the rupture plane by trial-and-error in order to best match coseismic offsets at sites REUN, HIDA, and CAMP. We find that the best fit is obtained with the center of the top portion of the rupture located at -70.65/19.90 at a 2 km depth (Figure 7.1).

Figure 7.1: Observed (black arrows) and modeled (white arrows) horizontal coseismic displacements. Rupture parameters are given in the text. Contours are modeled horizontal coseismic displacements, labels in mm. Background colors show modeled vertical coseismic displacements. Dashed white rectangle: surface projection of the rupture plane. Blue circles: aftershocks located by INDRHI (courtesy L.O. Gomez). Harvard and USGS CMT focal mechanisms are shown.

The model on Figure 7.1 shows a good agreement with the direction of the observed coseismic displacements at sites REUN, HIDA, CAMP, and SAGO. The agreement is fair in magnitude also, although the model tends to underestimate the oberved displacements.