Stephen Hicks*, Andreas Rietbrock, Isabelle Ryder School of Environmental Sciences, University of Liverpool, UK

Chao-Shing Lee National Taiwan Ocean University, Taiwan

Matt Miller Universidad de Concepción, Chile

*Email: s.hicks@liverpool.ac.uk

A high-resolution 3D seismic velocity model of the 2010 Mw8.8 Maule, Chile earthquake rupture zone Using land and OBS networks

The overriding / underlying question…

Can we physically identify asperities and barriers along the megathrust?

Megathrust dynamics Material properties

6th largest recorded earthquake

magnitude 8.8 rupture length 500 km max slip 16m a heterogeneous megathrust?

Mainshock hypocentre location: Hayes et al. (2013), GJI

Co-seismic slip model: Moreno et al. (2012), EPSL

Tomographic inversion algorithm: SIMUL2000 (Thurber,1983, JGR)

An unprecedented dataset

160 land + 37 OBS stations

670 aftershocks

38,000 P-wave picks

14,000 S-wave picks

Forearc 2D velocity structure

Focal mechanisms from: Agurto et al. (2012), EPSL Hayes et al. (2013), GJI Coastline

Resolution limits

Forearc 2D velocity structure

Focal mechanisms from: Agurto et al. (2012), EPSL Hayes et al. (2013), GJI Coastline

Resolution limits

Forearc 2D velocity structure

Coastline

Resolution limits

Forearc 2D velocity structure

Coastline

Resolution limits

Down-dip segmentation of the megathrust

Automatic picks from Rietbrock et al. (2012)

3D velocity structure

Coastline

Resolution limits

3D velocity structure

Coastline

Resolution limits

Automatic picks from Rietbrock et al. (2012)

Shedding light on megathrust dynamics

Pre-seismic locking (90%, 95%): Moreno et al., 2010

Post-seismic Afterslip: Lin et al., 2013

Interface aftershocks: Rietbrock et al., 2012

Co-seismic rupture Nucleation: Hayes et al., 2013

Slip: Moreno et al., 2010

High freq: Kiser & Ishii (2011)

Shedding light on megathrust dynamics

Shedding light on megathrust dynamics

Post-seismic Afterslip (>1m): Lin et al., 2013

Interface aftershocks: Rietbrock et al., 2012

Pre-seismic locking (90%, 95%): Moreno et al., 2010

Co-seismic rupture Nucleation: Hayes et al., 2013

Slip: Moreno et al., 2010

High freq: Kiser & Ishii (2011)

Shedding light on megathrust dynamics

Forearc body: composition & origin

Gravity model from Hicks et al. (2012), GRL

vp ~ 7.7 km/s; vp/vs ratio ~ 1.8 Positive gravity anomaly

Observations

Ultramafic - weakly serpentinised Christensen & Mooney (1995), JGR; Hacker & Abers (2004), G3

Composition

Origin Subducted topographic anomaly? Hicks et al. (2012), GRL

Forearc body: composition & origin

vp ~ 7.7 km/s; vp/vs ratio ~ 1.8 Positive gravity anomaly

Observations

Ultramafic - weakly serpentinised Christensen & Mooney (1995), JGR; Hacker & Abers (2004), G3

Composition

Origin Subducted topographic anomaly? Hicks et al. (2012), GRL

Root of Paleozoic granite batholith?

Forearc body: composition & origin

vp ~ 7.7 km/s; vp/vs ratio ~ 1.8 Positive gravity anomaly

Observations

Ultramafic - weakly serpentinised Christensen & Mooney (1995), JGR; Hacker & Abers (2004), G3

Composition

Origin Subducted topographic anomaly? Hicks et al. (2012), GRL

Root of Paleozoic granite batholith?

Mantle upwelling during Triassic extension Vásquez et al. (2011), J. Geol.

1

2

3

Ultramafic bodies beneath coast inhibit rupture propagation

Low vp gradient and lower vp/vs values associated with high slip

High vp/vs controls up-dip limit of seismogenesis

4 Afterslip may be compositionally-driven

Implications for Maule seismogenesis

Can we physically identify asperities and barriers along the megathrust?

Up-dip barrier: Fluid-saturated sediments

Down-dip barrier: Long-lived

ultramafic bodies in crust

3D velocity structure

Coastline

Resolution limits

A segmented seismogenic zone!