Active deformation in the San Salvador extensional stepover, El Salvador from an analysis of the April–May 2017 earthquake sequence and GPS data

Abstract

Modern and historic seismic observations indicate that earthquake swarms, often with similar characteristics, have occurred within a few tens of kilometers of the San Salvador volcano, which poses a major active hazard to the San Salvador metropolitan area. Here, we evaluate whether the April–May 2017 earthquake sequence near the San Salvador volcanic complex was tectonic or volcanic and more broadly consider the implications of up-to-date GPS observations and seismic data for earthquake and volcano hazards in the San Salvador metropolitan area. Based on magnitudes calculated for 532 earthquakes in the April–May sequence, we report a Gutenberg-Richter law b-value of 0.95 ± 0.12, consistent with b values for tectonic earthquakes, and based on temporal distribution of aftershocks we report an Omori law p-value of 1.5 ± 0.1. Focal mechanisms estimated for the mainshock and 14 largest foreshocks and aftershocks are all strike-slip with NNE-striking, left-lateral-slip and ESE-striking right-lateral-slip nodal planes. The best located foreshocks and aftershocks unequivocally indicate that the earthquake sequence accommodated left-lateral slip along a NNE-striking fault, compatible with the local tectonic setting. An absence of seismic and GPS evidence for any unrest of the San Salvador volcano during the years after the 2017 earthquake sequence is consistent with its tectonic origin. Continuous GPS measurements at a site 5 km south of the volcano reveal a previously unknown inflationary episode from mid-2010 to mid-2012, but no evidence for volcanic unrest in 2017 or later. Updated GPS velocities indicate that ~4–5 mm yr−1 of E-W-dominated stretching occurs between the San Salvador volcano and Ilopango caldera. That the 2017 earthquakes and Mw = 5.7 1986 San Salvador earthquake both accommodated left-lateral strike-slip along NNE-striking faults suggests that the GPS-measured E-W stretching is accommodated by bookshelf faulting between blocks that rotate clockwise in response to 10 ± 1.4 mm yr−1 (95%) of dextral motion across the volcanic arc at the location of San Salvador.