Direct Seafloor Imaging of the 2012 Wharton Basin Great Strike-slip Earthquakes rupture zones

Abstract:

The 2012 Wharton Basin earthquakes (Mw=8.6 and Mw=8.2) were the largest intra-plate strike-slip earthquakes ever recorded. Based on seismological and geodetic studies, different, and partly contradictory, models have been proposed for the fault geometry requiring a complex faulting mechanism with several faults, oblique to one-another. These earthquakes occurred in the Wharton Basin, which is considered to be a broad diffuse zone of intra-plate deformation with deformation taking place along re-activated N5ºE striking fracture zones, which was inconsistent with most of the seismology or geodesy based rupture models. In May-June 2015, we acquired 13 high-resolution seismic reflection profiles and more than 8500 km² of bathymetric data to the south and southwest of the main N-S segment of the Mw=8.6 earthquake rupture and across the Mw=8.2 earthquake rupture zone. We find that the epicenter of the Mw=8.2 earthquake lies on a re-activated fracture zone, expressed as a ~50-km wide region with four N5ºE striking left-lateral sub-faults. The easternmost sub-fault is most active and might be the master fault, where the maximum deformation might be taking place. The deformation along the other sub-faults becomes more diffuse moving westward. We also imaged a set of N110ºE trending 2-km wide right-lateral shear zones, which might act as transfer zones between the re-activated N5ºE striking fracture zones, and have orientations in agreement with aftershock focal mechanisms. We suggest that the 2012 great Wharton Basin earthquakes ruptured N5ºE re-activated fractures. Furthermore, the rupture of the Mw=8.6 event proceeded in en échelon fashion with this suite of N110ºE striking shear zones connecting the re-activated fracture zone imaged in this study with another N5ºE trending re-activated fracture zone on the Ninety East Ridge. Our model explains the discrepancy between direct observations on the seafloor and distant seismological and geodetic results.