Showing papers by "Mark Simons published in 2003"

Large Trench-Parallel Gravity Variations Predict Seismogenic Behavior in Subduction Zones

Abstract: We demonstrate that great earthquakes occur predominantly in regions with a strongly negative trench-parallel gravity anomaly (TPGA), whereas regions with strongly positive TPGA are relatively aseismic. These observations suggest that, over time scales up to at least 1 million years, spatial variations of seismogenic behavior within a given subduction zone are stationary and linked to the geological structure of the fore-arc. The correlations we observe are consistent with a model in which spatial variations in frictional properties on the plate interface control trench-parellel variations in fore-arc topography, gravity, and seismogenic behavior.

Neutral atmospheric delay in interferometric synthetic aperture radar applications: Statistical description and mitigation

Abstract: Variations in the refractive index of the atmosphere cause variations in satellite-based interferometric synthetic aperture radar (InSAR) observations. We can mitigate tropospheric effects by averaging N-independent interferograms. Because the neutral atmosphere is uncorrelated at timescales longer than 1 day, using this technique statistically reduces the variance, σ^2, of the noise by a factor of N. Using zenith neutral atmospheric delays from Global Positioning System (GPS) data from the Southern California Integrated GPS Network, we find that the average variance depends on the distance between observations, L, and height difference, H, as σ = c L^α + kH with estimated values for c, α, and k of about 2.5, 0.5, and 4.8, respectively, where σ is in mm and L and H are in km. We expect that the value of α is largely site-independent but the value of c will depend on the water vapor variability of the area of interest. This model is valid over a range of L between approximately 10 and 800 km. Height differences between 0 and 3 km have been used in this analysis. For distances of 100 and 10 km with negligible height differences, σ is estimated to be approximately 25 and 8 mm, respectively. For a given orbit revisit time and image archive duration, we calculate the number and duration (assumed constant) of interferograms required to achieve a desired sensitivity to deformation rate at a given length scale. Assuming neutral atmosphere is the dominant source of noise, a 30° look angle, and an image revisit time of 7 days, detection of a deformation rate of 1 mm yr^(−1) over distances of 10 km requires about 2.2 years of continuous observations. Given our results, we suggest a data covariance structure to use when using InSAR data to constrain geophysical models.

Multiscale dynamics of the Tonga—Kermadec subduction zone

Abstract: Our understanding of mantle convection and the motion of plates depends intimately on our understanding of the viscosity structure of the mantle. While geoid and gravity observations have provided fundamental constraints on the radial viscosity structure of the mantle, the influence of short-wavelength variations in viscosity is still poorly understood. We present 2-D and 3-D finite-element models of mantle flow, including strong lateral viscosity variations and local sources of buoyancy, owing to both thermal and compositional effects. We first use generic 2-D models of a subduction zone to investigate how different observations depend on various aspects of the viscosity structure, in particular, the slab and lower-mantle viscosity and the presence of a low-viscosity region in the mantle wedge above the slab. We find that: (1) the strain rate provides a strong constraint on the absolute viscosity of the slab (10^(23) Pa s); (2) stress orientation within the slab is sensitive to the relative viscosity of the slab, lower mantle and the wedge; and (3) the stress state and topography of the overriding plate depend on the wedge viscosity and local sources of buoyancy. In particular, the state of stress in the overriding plate changes from compression to extension with the addition of a low-viscosity wedge. We then use observations of strain rate, stress orientation, dynamic topography and the geoid for the Tonga–Kermadec subduction zone as simultaneous constraints on the viscosity and buoyancy in a 3-D regional dynamic model. Together these observations are used to develop a self-consistent model of the viscosity and buoyancy by taking advantage of the sensitivity of each observation to different aspects of the dynamics, over a broad range of length-scales. The presence of a low-viscosity wedge makes it possible to match observations of shallow dynamic topography and horizontal extension within the backarc, and down-dip compression in the shallow portion of the slab. These results suggest that a low-viscosity wedge plays an important role in controlling the presence of backarc spreading. However, for a model with a low-viscosity and low-density region that provides a good fit to the observed topography, we find that a reduction of the slab density by a factor of 1.3 relative to the reference density model, is required to match the observed geoid. These results suggest that compensation of the slab by dynamic topography may be a much smaller effect at short to intermediate wavelengths than predicted by long-wavelength modelling of the geoid.

Plan for living on a restless planet sets NASA's solid Earth agenda

Abstract: What are the most important challenges facing solid Earth science today and over the next two decades? And what is the best approach for NASA, in partnership with other agencies, to address those challenges? A new report, Living on a Restless Planet, provides a blueprint for answering these questions. The top priority for a new spacecraft mission in the area of solid Earth science over the next 5 years, according to this report, is a satellite dedicated to Interferometric Synthetic Aperture Radar (InSAR). At the request of NASA, the Solid Earth Science Working Group (SESWG) developed a strategy for the highest priority objectives in solid Earth science for the space agency over the next 25 years. The strategy addresses six challenges that are of fundamental scientific importance, have strong implications for society, and are amenable to substantial progress through a concerted series of scientific observations from space.