Coupling, fluids and foreshocks – preparing megathrust ruptures at the Chilean plate boundary

Abstract:

Recent studies have suggested that geodetic locking at convergent plate boundaries is closely related to slip distribution of subsequent megathrust earthquakes. The nature of locking and its evolution towards rupture, however, remains a matter of debate. The international initiative IPOC (Integrated Plate Boundary Observatory Chile; ipoc-network.org) addresses these goals at the Chilean margin. We explore geophysical and geodetic data collected in the decade before an event to identify the petrophysical state as well as change along the plate interface leading up to a megathrust event.

Seismological data exhibit well defined changes of reflectivity and Vp/Vs ratio along the plate interface that released the M=8.8Maule earthquake of 2010. High Vp/Vs domains, interpreted as zones of elevated pore fluid pressure, spatially correlate with lower locking degree, and exhibit higher background seismicity as expected for partly creeping domains. In turn, unstable slip associated to a higher degree of locking is promoted in lower pore fluid pressure domains. We speculate that hydraulic loading during the terminal stage of a seismic cycle to close to lithostatic pore pressure with an equivalent reduction of effective strength may be as relevant for earthquake triggering as stress loading from long-term plate convergence.

In contrast to the Maule earthquake, the Pisagua Mw=8.1 earthquake of 2014, while also rupturing a geodetically well-defined major asperity, was preceded by a protracted series of foreshocks. Since July 2013 three seismic clusters hit this part of the plate boundary with increasing magnitudes in a domain that was transitional between a fully locked and a creeping portion. Leading up to this earthquake the b-value of the foreshocks gradually dropped during the years prior to the earthquake, reversing its trend a few days before the Pisagua earthquake. We conclude that gradual weakening of the central part of the Northern Chile seismic gap accentuated by the foreshock activity in a zone of intermediate seismic coupling was instrumental in causing final failure. In spite of similar geodynamic conditions, processes leading up to the rupture were distinct for the Pisagua and the Maule earthquake suggesting a diversity of evolutionary paths towards megathrust rupture.