Stress evolution within the seismogenic zone due to Slow Slip Events, Nicoya Peninsula, Costa Rica

Abstract:

Slow Slip Events (SSEs) are an important part of the seismic cycle in many subduction zones. However, their role in earthquake triggering is still unclear. Down-dip events may load higher parts of the seismogenic zone, perhaps promoting seismic rupture. Alternately, slow slip could release a portion of accumulated strain, making seismic rupture less likely. In September 2012, a M=7.6 earthquake took place beneath the Nicoya Peninsula in Costa Rica. Approximately 1 month prior to the 2012 main shock a SSE took place, representing the first time that a SSE was directly imaged in close spatial and temporal proximity to a large earthquake. We investigate the stress changes within the seismogenic zone associated with this preceding SSE along with previously recorded SSEs, and compare them with the spatial extent of the 2012 earthquake. Using inversions for fault slip from a dense C-GPS network located on the Peninsula, we compute the Coulomb Failure Stress Change (Delta CFS) associated with this inferred slip using Coulomb 3.3 (Toda et al., 2011). We find that Delta CFS due to the preceding SSE was as high as 0.5 Bars, however these values were well down dip of the co-seismic rupture location. Using earthquake values from a published coseismic inversion (Yue et al., 2012), the preceding SSE induced Delta CFS of +0.128 Bars. Varying the receiver fault geometry does not generate Delta CFS values greater than 0.2 bars at the updip rupture location, below commonly accepted values for static triggering of earthquakes or aftershocks. This leads us to believe that the preceding SSE did not trigger the M 7.6 earthquake. These calculations only consider an elastic half space. We are developing a Finite Element Model of the subduction zone to consider the possible impact of more complex rheologies on the stress transfer process.