[S22-1] Lithosphere structure and dynamics

[S22-1-05] Potential for coincident megathrust and crustal earthquakes - an additional component of seismic hazard

^○Kevin Furlong, Matthew Herman (Penn State University, University Park, PA USA)

The Mw 7.8 Kaikoura,N.Z. earthquake presents two distinct characteristics to geophysical observers. Seismologically the earthquake is dominantly a megathrust event, apparently part of the southernmost segment of the Hikurangi subduction zone. Field observations show numerous surface rupturing fault segments, many of which show substantial surface offset. This presents the earthquake science community with a dilemma - is the earthquake a megathrust event and the surface faulting a byproduct or is the event primarily restricted to upper plate strike-slip (+ some thrust) faulting? Although seismological analyses indicate some strike-slip-like behavior in the initial and latter stages of the rupture (perhaps as much as 1/3 of the moment), most teleseismic analyses require significant thrust faulting consistent with rupture of the subduction megathrust. Thus it appears that there were synchronous megathrust and crustal faulting events. Similar situations potentially exist along other megathrust systems. Our model that reconciles these two concurrent deformational events includes an inter-seismic period of strain/slip-deficit accumulation along the megathrust plate interface. Co-seismically, crustal deformation conditions are changed by: (a) reduction of fault-normal stresses acting on the crustal faults, and (b) the decoupling of the upper-plate faults from their base (the underlying subducting slab). These effects combine to decouple the faults from their underlying foundation and significantly change the Coulomb stress conditions acting on the faults, allowing appropriately oriented crustal faults to slip concurrent with the megathrust rupture.