A high-resolution 3D seismic velocity model of the 2010 Mw 8.8 Maule, Chile earthquake rupture zone using land & OBS networks.
Knowledge of seismic properties along a subduction megathrust can shed light on the composition and structure of rocks along the fault. By comparing seismic velocity structure with models of interseismic locking, co-seismic slip and afterslip, we can begin to understand how physical properties may affect fault dynamics throughout the subduction seismic cycle. The Maule earthquake, which hit the coast of central Chile in 2010, is the 6th largest earthquake ever recorded, rupturing a 500 x 80 km area of the Chilean megathrust. Published models demonstrate a complex bilateral rupture, with most co-seismic slip occurring to the north of the mainshock epicentre, although significant slip likely stopped short of the trench and the continental Moho. Here, we show a new high-resolution 3D velocity model (vp and vp/vs ratio) of the central Chilean margin Our velocity model is based on manually picked P- and S-wave arrival times from 670 aftershocks recorded by the International Maule Aftershock Deployment (IMAD) network. Seismic properties of the marine forearc are poorly understood in subduction zones, but by incorporating picks from two ocean-bottom seismometer (OBS) networks, we can resolve the velocity structure of the megathrust as far as the trench. In total, the catalogue used for the tomographic inversion yields a total of ~50,000 high quality P- and S-wave picks. We analyse the quality of our model by analysis of the resolution matrix and by testing characteristic models. The 3D velocity model shows the main structures associated within a subduction forearc: the marine forearc basin (vp < 6.0 km/s), continental mantle (vp > 7.5 km/s), and subducting oceanic crust (vp ~ 7.7 km/s). The plate interface is well defined by relocated aftershock seismicity. P-wave velocities along the megathrust range from 6.5 km/s beneath the marine forearc to 7.7 km/s at the intersection of the megathrust with the continental Moho. We infer several high vp anomalies within the South American forearc that are also expressed in the forearc Bouguer gravity field. One of these (vp ~ 7.5 km/s) is located in the north of the rupture zone and is associated with a region of intense crustal seismicity in the Pichilemu region. Another anomaly (vp~ 8.0 km/s) lies along the plate interface, close to the nucleation point of the 2010 mainshock. Values of vp/vs appear to be related to aftershock seismicity at both the up- and down-dip limits of the seismogenic zone. We speculate that fluid saturated sediments in the marine forearc (vp/vs > 1.95) act as the up-dip limit to aftershock seismicity, and dehydration of oceanic crust (vp/vs~ 1.88), starting at 45 km depth, terminates the rupture at the lower end. By defining a new plate interface model for the central Chile megathrust, we show how the structure of the forearc may have influenced nucleation and rupture of the Maule earthquake, and explore the implications for aftershock seismicity and afterslip. Using our new 3D velocity tomography model and the spectral element method, we will generate full waveform simulations for large aftershocks, forming the basis of an adjoint inversion in the near future.