 |
|
|
|
 |
 |
 |
|
 |
|
 |
|
 |
|
 |
|
 |
|
 |
|
 |
|
 |
|
 |
|
 |
|
>Return to Research
Dynamics of subduction zones from earthquake to geological time scales
Subduction zones are both the primary source of buoyancy driving plate tectonics and the focus of intense seismic and volcanic activity. Generally, our goal is to understand the force balance in subduction zones across different time and length scales. As a corollary, we need to describe the rheological behavior of the entire system, including the megathrust, forearc, arc, and backarc (e.g., 18). On the shortest time scales, we use geodesy and seismology to constrain the distribution, rates, and style of co-seismic, post-seismic, aseismic, and inter-seismic deformation. For example, we undertook a detailed series of kinematic models of the seismic cycle in the Central Andes (12, 35, S2). These models use GPS, InSAR, and broadband seismic data to look at a suite of recent large magnitude earthquakes (Mw 6.8 to 8.4) in Peru and Chile, as well as postseismic and aseismic deformation associated with some of these events. Through the Caltech Tectonic Observatory and colleagues in Chile and Peru, we are now beginning to construct a network of continuous GPS sites in this region to further constrain the secular strain field from the megathrust all the way across the arc, as well as to observe any sign of transient deformation (aseismic as well as coseismic). This deployment is combined with continued satellite InSAR analysis. This observational effort is part of a larager project focused on Andean subduction dynamics.
On a smaller scale we have been using the GPS-derived strain field to develop models of the elastic strain accumulation across the Central Range and western foothills of Taiwan (20). While we are able to explain the horizontal strain field in a model that is consistent the release of elastic strain in the western foothills, our model fails to predict the nearly 1 cm/yr uplift of the central range. We are now undertaking more complicated dynamic models designed to understand both the observed co-seismic and post-seismic deformation from the 1999 Chi-Chi earthquake and the long-term uplift of the Central Range.
On a global scale we recently found a correlation between intra-subduction zone variations in seismogenic behavior and large amplitude variations of gravity, topography, coastal morphology, and basins (21). A similar correlation was found by Ray Wells (USGS) and his colleagues. The typical amplitudes of gravity and topography anomalies involved are of order 40 mGals and 750 m, respectively - both of which require Myr timescales to develop. Based on these correlations we conclude that: 1) Areas on the megathrust with large seismogenic moment release will occur in areas associated with relatively large amplitude negative gravity and topography anomalies, 2) Areas of relatively large positive anomalies will not be seismogenic, 3) The frictional character of the megathrust must vary rapidly along-strike within a given subduction zone, and 4) The seismogenic behavior of the megathrust must be fairly stationary in time (21).
We are currently attempting to further constrain the link between dynamic processes active on the earthquake time scale and those on the geologic time scale. To this end, we are continuing to develop a suite of finite slip models for large earthquakes and trying to correlate their temporal evolution during rupture as well as their total slip distribution with variations in gravity and topography. We are also reassessing kinematic models of intereseismic strain accumulation using a variety of model geometries and inversion parameterization. In particular, we are reprocessing the extraordinary data set available from GEONET, the 1000 site Japanese GPS network. Complimenting these observational approaches, we are beginning to explore finite element based dynamic models that attempt to explain the link across time scales. These models are exploring the role of different fault zone rheology and anelastic deformation in the forearc.
See listed publications below for additional information.
S2Teleseismic, geodetic, and strong motion constraints on slip from recent southern Peru subduction zone earthquakes, M. E. Pritchard, C. Ji, R. Boroscheck, D. Comte, M. Simons and P. A. Rosen, J. Geophys. Res., in preparation, 2005.
35 Distribution of slip from Mw 11 > 6 earthquakes in the northern Chile subduction zone, M. E. Pritchard and C. Ji and M. Simons, J. Geophys. Res., submitted, 2005.
34 An aseismic slip pulse in northern Chile and along-strike variations in seismogenic behavior, M.E. Pritchard and M. Simons, J. Geophys. Res., 111, doi:10.1029/2006JB004258, 2006. [PDF]
21 Large trench-parallel gravity variations predict seismogenic behavior in subduction zones, T.A. Song and M. Simons, Science, 301, 630-633, 2003. (Includes supplementary online material). [PDF]
20 A two-dimensional dislocation model for interseismic deformation of the Taiwan mountain belt, Y. Hsu, M. Simons, S. Yu, L. Kuo, H. Chen, Earth Plan. Sci. Lett., 211, 287-294, 2003. [PDF]
18 Multiscale Dynamics of the Tonga-Kermadec Subduction Zone, M.I. Billen, M. Gurnis, and M. Simons, Geophys. J. Int., 153, 359-388, 2003. [PDF]
12 Co-seismic slip from the July 30, 1995, Mw 8.1 Antofagasta, Chile, earthquake as constrained by InSAR and GPS observations, M. Pritchard, M. Simons, P. Rosen, S. Hensley, and F. Webb, Geophys. J. Int., 150, 362-376, 2002. [PDF]
Mark Simons' Paper Collection: Entire paper including figures are all made available online (within the bounds of copyright restrictions).