iPlex Lunch Winter-2018

Slip distributions during large underthrusting earthquakes on subduction zone plate boundaries are now commonly determined using seismic, geodetic, and/or tsunami observations. A persistent challenge has been to constrain the up-dip extent of rupture, or how close to the trench coseismic sliding occurs. This issue is important for several reasons. Assessing the potential for shallow tsunami earthquakes that rupture the megathrust near the trench requires knowledge of whether prior deeper ruptures have extended to the trench. This is illustrated by the 2010 Mentawai tsunami earthquake, which ruptured up-dip of the 2007 Sumatra event. Earthquake slip at shallow depth also characterizes the frictional properties of the shallow megathrust and its seismogenic potential. On-land geodetic observations have little resolution of rupture if it occurs relatively far off-shore, as was demonstrated by analyses of the great 2011 Tohoku earthquake. Seismological observations alone also may have a quite limited resolution of very shallow faulting on the up-dip portion of the megathrust. New approaches using water reverberations in the P coda hold the promise of improving seismological resolution. Off-shore seafloor geodetic observations (GPS-Acoustic or ocean-bottom pressure sensors) significantly improve the determination of trenchward rupture extent, as shown for the 2011 Tohoku event, but only a few regions currently have sufficient instrumentation. For a number of recent large earthquakes, joint analysis of seismic and tsunami observations, in some cases including on-shore and off-shore geodetic data, appears to provide reliable estimates of up-dip slip extent (corroborated by direct imaging of seafloor offsets for 2010 Maule, Chile and 2011 Tohoku earthquakes). For the tsunami data to be best interpreted, corrections for path effects that have commonly been ignored must be included.