Comparing Pre-, Co- and Post-Seismic Offsets Measured by Continuous GPS for the September 5, 2012 Costa Rica Earthquake (Invited)

The giant subduction megathrust earthquakes and tsunamis of 2004 (Sumatra) and 2011 (Japan) are reminders that our ability to make useful estimates of about future earthquake size remains weak. Reid's [1910] elastic strain hypothesis states that strain accumulated during the interseismic period is released during the earthquake. While we cannot predict the precise timing of future earthquakes, this hypothesis implies that measurement of interseismic strain might allow us to estimate the size of the next earthquake, assuming that all the accumulated strain was suddenly released and that the strain accumulation 'clock' was reset to zero after the last major earthquake. One problem is that we do not understand what controls the area of fault rupture, and linkage of adjacent segments. In subduction zones, smaller segments of faults may rupture in M ~7-8 events several times over periods of a few hundred years, but a subsequent event may involve rupture of multiple adjacent segments, leading to a M~9 event. Another problem, at least for subduction zones (the ones we would most like to understand because of their ability to generate destructive tsunamis) is that the critical zone of strain accumulation lies far offshore, where on-land sensors have limited resolution. A continuously recording GPS network in the Nicoya Peninsula of northwest Costa Rica allows detailed measurement of strain accumulation and release associated with the seismic cycle. Here we summarize GPS data on slow slip events for the decade prior to the September 5 2012, M 7.6 earthquake; seismic displacements associated with the 2012 event; and post-seismic motions for the first year after the earthquake. We show that the 2012 rupture area was outlined by adjacent slow slip events occurring in the prior decade. If this pattern continued through the entire interseismic period (since 1950) slow slip may have relieved sufficient strain to limit the rupture area of the 2012 event.