SSE induced seismic gap: A permanently reduced seismic hazard.

Abstract:

In Guerrero, Mexico large SSE’s (> M7) occur every 4 years and appear to at least partially invade the seismogenic zone in the Guerrero Seismic Gap. Previous studies have suggested that the SSE’s there reduce the seismic slip deficit by about four times. We examine the mechanism that allows for the large SSE’s to invade the Guerrero Gap. The tectonic geometry of Mexico is unique in the world, which allows for this study. It is the only place in the world where there is 1000 km of coastline within 100 km of the trench with a seismic gap in the middle. The large dynamic earthquakes (> M7) rupture at least partially beneath the coast so that characteristics of the seismogenic zone can be measured from land based studies. We take advantage of that to look for changes throughout the seismogenic zone that coincide with the gap. We combine evidence from magnetotelluric (MT) profiles and a geochemical study to analyze structural changes across the Guerrero Seismic Gap. Four MT profiles were performed. One was inside the Gap and the rest were outside. The profiles outside the Gap revealed fluids, as detected by high conductivity, are free to leave the slab and enter the crust within the seismogenic zone. Inside the gap the fluids were trapped below the crust and a very highly resistive (almost zero conductivity) body was found inside the crust. The geochemical study tracked magmatism over time along the coast within the Sierra Madre del Sur. It showed that magmatism lasted for less than 10 million years along the majority of the coast however in Gap there was continuous magmatism for 40 million years. We suggest that the highly resistive body found in the MT profile in the Gap is the result of the 40 million years of magmatism in the zone. This body has acted as a seal to trap fluids and over-pressurize the plate interface thereby allowing for SSE’s to invade the seismogenic zone. The geochemical dating shows that the body has been in place for the last 30 million years allowing for SSE’s to invade the seismogenic zone during that time. This would suggest that the slip and therefore the seismic seismic hazard are permanently reduced in the Guerrero Seismic Gap.