On the Origin of Slow and Large Earthquakes in South-Central Mexico

Víctor M. Cruz-Atienza , Carlos Villafuerte, Vladimir Kostoglodov, Josué Tago, Raymundo Plata-Martinez, Sara Franco, Ekaterina Kazachkina, Jorge Real

Slow slip events (SSEs) in Guerrero and Oaxaca, Mexico, have likely triggered five of the last six M7+ earthquakes in the region since 2012. This interaction, however, is non-systematic, as evidenced by the preceding 17 years of large earthquake quiescence, when multiple SSEs occurred without consequence. The Mexican catalog since 1800 reveals that large earthquakes cluster in time every ~15 years, suggesting that the subduction zone periodically reaches a critical state where SSEs become seismic precursors. The regional inter-SSE coupling pattern is highly consistent with the SSEs bimodal spatial distribution, with the maximum associated stresses being released by successive SSEs at depths of 25 to 40 km. In the eastern and western bordering regions where SSEs are negligible, the coupling becomes shallower and encompasses the historical rupture areas. The occurrence of tectonic tremor, low-frequency and repeating earthquakes, together with the friction condition of the plate interface, indicate (1) a major velocity weakening zone where long-term SSEs develop; and (2) two limiting zones of transition to free-slip where velocity strengthening friction becomes critically stressed, seismic radiation is maximum, and slip is intermittent producing short-term SSEs. The offshore segment of the Guerrero seismic gap where shallow SSEs were recently discovered is the most frictionally stable region, a condition that partly explains the absence of large earthquakes since 1911. The long-term slip deficit rate at the plate interface is found to be consistent (within 14% error) with sequences of large (M7+) repeating earthquakes throughout the region.