Quake clamps down on slow slip at the Hikurangi subduction margin, New Zealand

Friday, 19 December 2014: 8:00 AM
Laura M Wallace, University of Texas at Austin, Institute for Geophysics, Austin, TX, United States, Noel M Bartlow, Scripps Institution of Oceanography, La Jolla, CA, United States, Ian J Hamling, GNS Science, Lower Hutt, New Zealand and Bill Fry, GNS Science-Institute of Geological and Nuclear Sciences Ltd, Lower Hutt, New Zealand
Some studies suggest that slow slip events (SSEs) may play a role in triggering dangerous megathrust earthquakes by increasing stress on the adjacent locked, seismogenic portion of the plate boundary. However, the role that earthquakes play in the evolution and occurrence of SSEs themselves is not clear, although there is some evidence that SSEs can be triggered by local or regional earthquakes. We show for the first time that stress changes induced by a local earthquake can arrest an ongoing SSE, using a recent example from the Hikurangi subduction margin, New Zealand. The Kapiti SSE is a large (Mw > 7.0), long-term slow slip event that began at the southern Hikurangi margin in early 2013, and continued into 2014. We show that an abrupt halting of the Kapiti SSE observed at cGPS sites in late January 2014 occurred due to an increase in the normal stress (~0.1 MPa) and a decrease in Coulomb failure stress (~.02-.03 MPa) in the SSE source area, induced by a nearby Mw 6.3 intraslab earthquake that occurred on January 20, 2014. We also show that stress changes induced by the Kapiti SSE may have promoted the occurrence of the Mw 6.3 earthquake, revealing surprising feedbacks between SSE evolution and local earthquake occurrence.

Halting of the Kapiti SSE via small changes in stress suggests that stresses needed to drive SSEs are relatively small--not much larger than 0.01 MPa. Moreover, a decrease in SSE slip rate due to an increase in normal stress can also be explained in terms of the direct effect in rate and state friction laws. Our observation of an abrupt halting/slowing of the SSE in the stress shadow of a local earthquake also adds to the debate over dynamic vs. static stress triggering as drivers for earthquake aftershocks, and supports the premise that static stress changes are an important ingredient in triggering (or delaying) fault slip.