A Geodetic Study of the 22 January 2003 Tecoman, Colima, Mexico Earthquake

We use horizontal and vertical crustal displacements derived from GPS measurements at 26 sites in western Mexico to determine the characteristics of slip during the 22 January 2003 Mw=7.1 Tecomán, Colima earthquake in the Middle America Subduction Zone. The GPS network completely encompasses the onshore area of the earthquake's coseismic elastic strain release. Eight sites operated during the earthquake and displacements for the remaining 18 sites are inferred from GPS time series for up to 4~yr preceding the earthquake. We invert the GPS displacements using a realistic finite element model (FEM) to solve for the coseismic slip distribution. Total moment release was 5.9·1019~N·m, with maximum slip of 3.3~m occurring at 19~km depth. The complete extent of coseismic rupture was between 12~km and 40~km depth in a 65~km wide area not exceeding the edges of the Manzanillo Trough. In the best coseismic slip solution, misfits exceeding the average displacement uncertainties remain, so we perform tests of solution robustness with respect to subsets of data. Alternative FEM geometries are also explored. The Tecomán earthquake is then compared to the 9 October 1995 Mw=8.0 Jalisco earthquake, which occurred immediately to the northwest. As a pair, the 1995 and 2003 earthquakes bear a similarity to the Jalisco-Colima earthquake doublet of 3 June 1932 (Mw~7.9) and 18 June 1932 (Mw~7.8). This similarity suggests the Manzanillo Trough may be a mechanical limit to earthquake rupture size, and also may be relevant to local seismic hazard assessments.