A geodetic study of the 2003 January 22 Tecomán, Colima, Mexico earthquake

Abstract

We use coseismic displacements and aftershock information from Global Positioning System (GPS) measurements at 27 sites in western Mexico and a 12-station local seismic network to determine the characteristics of the 2003 January 22 M_w = 7.2 subduction thrust earthquake near Tecomán, Colima, Mexico. Estimates of the earthquake moment, slip direction and best-fitting slip distribution are derived by optimizing the fit to the GPS displacements for a 3-D finite element mesh that simulates the study area. The calculated moment release is 9.1 × 10^(19) N m (M_w = 7.2) , with maximum slip of 2 m at a depth of 24 km and a maximum rupture depth of 35–40 km. The inversion indicates that coseismic rupture extended downdip from depths of 9 to 40 km along a 80 km along-strike region that is bounded by the edges of the Manzanillo Trough. The optimal solution is robust with respect to plausible changes in the subduction interface geometry and differing subsets of the data. A comparison of the cumulative post-seismic slip that can be inferred separately from earthquake aftershocks and GPS measurements within a year of the earthquake indicates that 95 per cent or more of the post-seismic deformation was aseismic. Near-term post-seismic measurements indicate that slip propagated downdip to areas of the subduction interface beneath the coastline within days following the earthquake, as also occurred after the nearby M_w = 8.0 Colima-Jalisco subduction earthquake in 1995. The similar behaviours and locations of the 1995/2003 earthquake sequence to two earthquakes in June of 1932 suggests that thrust earthquakes along the subduction interface northwest of the Manzanillo Trough may trigger earthquakes in the vicinity of the Manzanillo Trough; however, our modelling of Coulomb stress changes caused by the 1995 earthquake indicate that it induced only modest unclamping of the subduction interface in the vicinity of the Tecomán rupture. In addition, GPS measurements indicate that elastic shortening characterized areas onshore from the Tecomán rupture from mid-1997 up until the time of the rupture, consistent with progressively stronger clamping of the subduction interface during this period. This precludes any obvious triggering relationship with the 1995 earthquake. The apparent coincidence of the edge of both the 1932 and 1995/2003 rupture sequences with the edge of the Manzanillo Trough may indicate that the trough is a mechanical barrier to along-strike rupture propagation. This implies a limit to the area of potential slip and hence rupture magnitude during future large earthquakes in this region.

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