Geowissenschaftliches Kolloquium der Erdwissenschaften Graz

Understanding megathrust earthquakes through ocean drilling

Paola Vannucchi

Earth Sciences Department, Royal Holloway, University of London, Surrey, UK

In the past few years, we have learned much more about earthquakes and tsunami at subduction

boundaries. Major events such as the 2004 Sumatra, 2008 Maule and 2011 Tohoku Great Quakes

have led to enhanced interest – and major surprises – in our understanding of megathrust events.

Direct drilling of the megathrust has been a prime focus of the first decade of IODP. Three large

projects have received multiple legs of drilling. NantroSEIZE is targeting the shallow-dipping

plate boundary of the accretionary margin in the Nankai Trough, offshore southwestern Japan. It

is the largest project ever carried on in the history of scientific ocean drilling: 10 expeditions and

the deepest hole, >3000 mbsf, ever drilled so far, with the goal of reaching the seismogenic plate

interface ~5200 mbsf. CRISP has focused on the erosive subduction margin where the Cocos

Ridge subducts beneath Middle America. There the plan is to reach the seismogenic plate

interface in a tectonic setting similar to the Japan Trench responsible for the Tohoku Earthquake

in 2011. The first phase of shallow drilling in Costa Rica was completed in 2012. Finally, JFAST

has studied the region of the 2011 Great Tohoku Quake, and penetrated the décollement in

the area of the frontal rupture. NantroSEIZE and CRISP will continue during the next phase of

IODP, with their ultimate goal being to drill through the seismogenic megathrust fault zone at

each of these sites. After the surprising discovery of slow-slip by ‘episodic tremor’ along the

plate interface, an additional site, Hikarangi offshore New Zealand, has received intense interest

as a possible place where a tremor-region may be reachable by deep-sea drilling from the

Chikyu.

This talk focusses on plate boundary seismogenesis; what we are discovering and can discover

from drilling-based studies. It presents these ideas within the broader context of what we can

learn from complementary geological and geophysical studies of fossil and active megathrust

systems. Several major new findings from drilling-based research include: the discovery that the

frictional properties of fault zone material, not fluid overpressure, are the controlling factor

responsible for fault strength and rupture propagation to the toe; the discovery of striking

differences and similarities between erosive and accretionary forearcs such as the finding that

both can have high rates of sediment input, but with different distributions of sediment

accumulation within the forearc; and the finding of complex principal stress orientations within

these diverse forearc study areas.