Geology Bloggers Speculate on the Cause of Hawaii's Sunday Quake

After we polled a few geology bloggers, Chris at Highly Allocthonous bravely took on the challenge of proposing a possible tectonic explanation for Sunday's large earthquake in Hawaii. Chris noted (a) that the epicenter was northwest of the current volcanic source point that lies on the southeastern side of the Big Island, where most events associate with volcanism have been occurring on a regular (daily) basis; and (b) that the Molokai Fracture Zone (labeled OFZ in a slightly modified version of Chris' excellent map at right), happens to pass in the regional vicinity of the epicenter. Chris also pointed out that the newly revised depth for the earthquake by the USGS is 28 km, not 38.9 km as reported earlier. Twenty-eight kilometers is still deep considering that most oceanic crustal earthquakes are less then 10 km deep.

The USGS website has a good explanation of earthquake focal mechanisms represented by the red and white beachball. While OFZ in particular lies a bit northwest of the epicenter and trends west-southwest, the fracture zones do in fact roughly parallel the east-west trending nodel plane of the earthquake focal mechanism calculated by the USGS. Further, if you look closely, there is a more east-west trending fracture zone just south of OFZ that more closely parallels the east-west nodal plane. If the earthquake occurred on one of the fracture zones, it would've been a left-lateral strike-slip event. However, as most tectonics students learn, fracture zones aren't typically active away from the spreading center in a strike-slip sense. It could be that thermal weakening of the crust and mantle near the hotspot have permitted preexisting fracture zones to slip in response to intraplate stresses. (Thanks, Chris!)