Aseismic slip history during the interseismic period between the 2012 M 7.5 Ometepec and the 2018 M 7.2 Pinotepa earthquakes

Abstract:

The Ometepec-Pinotepa region is one of the most seismically active areas in Mexico with a well-documented history of large megathrust earthquakes (M>7) that frequently occurred within a relatively short time interval. For instance, in the last century, 8 earthquakes with magnitude larger than 6.9 occurred in 1928, 1937, 1950, 1982 (x2), 1995, 1996 and 1997. Such a short recurrence times are not well explained as a simple mainshock aftershock relation, but as a series of nearby asperities that interchange stresses causing large variations in the slip rate that boosts the rupture in the adjacent areas. In the past decade, once again, two large earthquakes with a similar magnitude and relatively close hypocenters (~47km) rupture within six years: The M 7.4 2012 Ometepec earthquake and the 2018 M 7.2 Pinotepa earthquake. In this study, we examine the aseismic slip variation inferred from characteristic repeating earthquakes during the interseismic period. Before the 2012 Ometepec earthquake, only two repeating earthquake clusters were detected around the main patch within more than ten years before the event. In contrast, after the 2012 Ometepec, more than two hundred clusters of repeating earthquakes were activated south of the rupture zone near the trench, which suggests that the mainshock promoted aseismic slip. This variation of the slip rate likely played an important role in the stress buildup of the 2018 Pinotepa earthquake six years later. These two events offer a unique opportunity to understand what are the mechanisms that promote the stress release that largely exceeds the seismic rate in nearby areas.