[J07-2] Tracking the sea floor in motion II

[J07-2-05] Studying fault slip during and after the 2012 M 7.6 Costa Rica earthquake using land-based GNSS and near-trench fluid pressure observations

^○Tianhaozhe Sun¹, Kelin Wang^{2, 1}, Earl Davis², Yan Jiang², Martin Heesemann³ (1.School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, Canada, 2.Pacific Geoscience Centre, Geological Survey of Canada, Sidney, BC, Canada, 3.Ocean Networks Canada, University of Victoria, Victoria, BC, Canada)

invited

Knowing whether the subduction fault slips to trench during or after an earthquake is important to the study of fault mechanics and tsunami generation, and it requires near-trench geodetic observations. In Costa Rica, two CORK borehole observatories were installed <1 km apart on the seaward and landward sides of the trench. During the 2012 M 7.6 earthquake, seafloor pressures on the two sides showed no relative change, indicating that rupture was limited to the epicentral area near the coast. Starting from 1.5 days after the rupture, several episodes of uplift of the subduction prism toe were observed as inter-site seafloor pressure changes. This uplift was accompanied by a subseafloor pressure rise (contraction) within the prism toe and a pressure drop (dilatation) within the incoming plate, suggesting pulses of afterslip propagating to the trench along the plate interface. Over >1 years, this uplift showed the same temporal characteristics as GNSS time series recorded on land, implying that the offshore and onshore deformation was controlled by the same afterslip process. We use both data to determine the co- and post-seismic slip distributions of this event. For the coseismic deformation, peak slip of ~5 m is determined at a depth of 25-30 km. Calculation of shear stress change on the subduction fault suggests up to 8 MPa stress drop in the rupture zone but stress increase (<1 MPa) further updip. Afterslip is found to be shallower than the coseismic rupture with a cumulative peak (>1 m over 1.3 years) located in the area of coseismic stress increase just offshore and fairly large slip (~0.7 m) at the trench. While postseismic stress decrease occurs at shallow depths, stress variation near the trench is smaller because of the low rigidity of sediments. The trench-breaching afterslip in Costa Rica, together with the trench-breaching rupture in the 2011 Tohoku-oki earthquake, provides important information on the slip behaviour of the shallowest part of megathrusts.