Source tomography by simulated annealing using broad-band surface waves and geodetic data: application to theM_w= 8.1 Chile 1995 event

Abstract

S U M M A R YWe invert surface-wave and geodetic data for the spatio-temporal complexity of slip during the M,=8.1 Chile 1995 event by simulated annealing. This quasi-global inversion method allows for a wide exploration of model space, and retains the nonlinearity of the source tomography problem. Complex source spectra are obtained from 5 to 45 mHz from first-and second-orbit fundamental-mode Rayleigh waves using an empirical Green's function cross-correlation technique. Coseismic displacement vectors were measured at 10 GPS sites near Antofagasta. They are part of a French-Chilean experiment which monitors the Northern Chile seismic gap. The spectra, together with the geodetic data, are inverted for the moment distribution on a 2-D dipping fault, under the physical constraints of slip positivity and causality. Marginal a posteriori distributions of the model parameters are obtained from several independently inverted solutions. In general, features of the slip model are well resolved. Data are well fitted by a purely unilateral southward rupture with a nearly uniform velocity around 2.5-3.0 km s-l, and a total duration of 65 s. Several regions of moment release were imaged, one near the hypocentre, a major one 80 km south of it and a minor one 160 km south of it. The major patch of moment release seemed to have propagated to relatively shallow depths near the trench, 100 km SSW of the epicentre. The region of major slip is located updip of the 1987, M,=7.5 earthquake, suggesting a causal relationship. Most of the slip occurred updip of the hypocentre (36 km), but the entire coupled plate interface (20-40 km) ruptured during the Chile 1995 event.

Controls on earthquake rupture and triggering mechanisms in subduction zones

“…Our centroid location and rupture length are consistent with other slip models [Ihmlé and Ruegg, 1997;Pritchard et al, 2006], although the orientation of the moment release is slightly different.…”

“…This event has been the subject of a number of studies, which generally found that rupture initiated just south of the Mejillones peninsula and propagated unilaterally to the southwest [e.g., Delouis et al, 1997;Ihmlé and Ruegg, 1997;Carlo et al, 1999;Pritchard et al, 2006]. Our measurements and model fit are shown in Figure 7a.…”

Asperity generating upper crustal sources revealed by b value and isostatic residual anomaly grids in the area of Antofagasta, Chile

“…However, this local study showed a more detailed picture of the coseismic slip area separated in two patches of large slip values. Ihmlé and Ruegg [1997] suggested a similar structure from results of a simultaneous inversion of teleseismic and GPS data also showing two patches of high slip.…”

Rupture Process of the 1995 Antofagasta Subduction Earthquake ( M w = 8.1)

“…The location of the 1995 event raises the issue of whether the event increased the probability of failure of the 1877 gap (RUEGG et al, 1996). It has been proposed that the plate interface beneath the Mejillones peninsula acts as a structural barrier for rupture propagation; the nucleation site of the 1995 event perhaps supports this inference (IHMLÉ and RUEGG, 1997). RUEGG et al (1996) calculated Coulomb stress perturbations associated with the 1995 event, inferring that it did increase the driving stress in the southern end of the 1877 gap.…”