Triggering of the 2001-2002 Mexico Slow-slip Event, Mw 7.5, by the Shallow Normal Earthquake of 8 October 2001, Mw 5.8, and Its Aftershocks

Abstract

The Guerrero coast in the subduction zone of west-central Mexico has been a seismic gap for almost 100 years. Large aseismic slow-slip events (SSE) have been frequently observed since GPS stations were established in this area (Lowry et al. 2001; Kostoglodov et al. 2003). The tectonic mechanism of slow-earthquake episodes is not well understood: some are preceded by significant earthquakes, others are not. The coincidental occurrence on 8 October 2001 of a shallow coastal earthquake of magnitude 5.8 in the same area as the slow earthquake of magnitude Mw 7.5 is attributed to a triggering effect. Previous investigators have dismissed the triggering hypothesis for various reasons, mainly because the earthquake was judged to be too small to trigger such a large SSE. However, reexamination of the seismic, tilt, and GPS data suggests that the hypothesis of triggering cannot be excluded. The 2001–2002 SSE ( Mw 7.5) has been extensively described and investigated (Kostoglodov et al. 2003; Yoshioka et al. 2004; Mikumo 2004; Iglesias, Singh, et al. 2004; Iglesias, Pacheco, and Singh 2004; Franco et al. 2005). It generated a long-duration aseismic slip episode over an area of about 550 km by 250 km comprising the southeastern part of the Guerrero gap and extending into the active coast of northwestern Oaxaca (Figure 1). The slip episode was monitored by seven GPS stations over a period of more than eight months. Surface displacements were estimated at about 1.3 cm to 6 cm (Franco et al. 2005). The Coyuca earthquake of 8 October 2001 was an important seismic event. Shallow, normal-faulting earthquakes are rare on the Guerrero subduction coast. However, the Coyuca earthquake has received only cursory attention. We suggest that the triggering of large SSEs by normal earthquakes in the overriding plate may represent a previously …