Seismogenic zone structure of the southern Middle America Trench, Costa Rica.
Journal of Geophysical Research-Solid Earth. 108 10.1029/2002jb002294 Abstract
 The shallow seismogenic portion of subduction zones generates damaging large and great earthquakes. This study provides structural constraints on the seismogenic zone of the Middle America Trench offshore central Costa Rica and insights into the physical and mechanical characteristics controlling seismogenesis. We have located similar to 300 events that occurred following the M-W 6.9, 20 August 1999, Quepos, Costa Rica, underthrusting earthquake using a three-dimensional velocity model and arrival time data recorded by a temporary local network of land and ocean bottom seismometers. We use aftershock locations to define the geometry and characteristics of the seismogenic zone in this region. These events define a plane dipping at 19degrees that marks the interface between the Cocos Plate and the Panama Block. The majority of aftershocks occur below 10 km and above 30 km depth below sea level, corresponding to 30 - 35 km and 95 km from the trench axis, respectively. Relative event relocation produces a seismicity pattern similar to that obtained using absolute locations, increasing confidence in the geometry of the seismogenic zone. The aftershock locations spatially correlate with the downdip extension of the oceanic Quepos Plateau and reflect the structure of the main shock rupture asperity. This strengthens an earlier argument that the 1999 Quepos earthquake ruptured specific bathymetric highs on the downgoing plate. We believe that subduction of this highly disrupted seafloor has established a set of conditions which presently limit the seismogenic zone to be between 10 and 35 km below sea level.
Seismogenic zone structure beneath the Nicoya Peninsula, Costa Rica, from three-dimensional local earthquake P- and S-wave tomography.
Geophysical Journal International. 164:109-124. 10.1111/j.1365-246X.2005.02809.X Abstract
The subduction plate interface along the Nicoya Peninsula, Costa Rica, generates damaging large (M-w > 7.5) earthquakes. We present hypocenters and 3-D seismic velocity models (V-P and V-P/V-S) calculated using simultaneous inversion of P- and S-wave arrival time data recorded from small magnitude, local earthquakes to elucidate seismogenic zone structure. In this region, interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes. Plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern Nicoya Peninsula, respectively. Microseismicity offset across the plate suture of East Pacific Rise-derived and Cocos-Nazca Spreading Center-derived oceanic lithosphere is similar to 5 km, revising earlier estimates suggesting similar to 10 km of offset. Interplate seismicity begins downdip of increased locking along the plate interface imaged using GPS and a region of low V-P along the plate interface. The downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental Moho intersection, possibly due to the onset of ductile behaviour. Slow forearc mantle wedge P-wave velocities suggest 20-30 per cent serpentinization across the Nicoya Peninsula region while calculated V-P/V-S values suggest 0-10 per cent serpentinization. Interpretation of V-P/V-S resolution at depth is complicated however due to ray path distribution. We posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes.
Web browser applet allows visualization of three-dimensional models.
EOS, Transactions of the AGU. 83(18) 10.1029/2002EO000136 Abstract
A new, machine-independent Java Applet, a program that runs in a browser, downloads both byte code and three-dimensional models from a remote Web site and displays them on a local computer. The code is a few hundred kilobytes in size and allows the viewer to control a two-dimensional view of a three-dimensional array, which can be represented by files as small as one byte per node. This code allows both the angle of view and the color map to be controlled by the user.
The gravitational edge effect.
Journal of Geophysical Research. 80:2949-2950., Washington, DC, United States (USA): American Geophysical Union, Washington, DC 10.1029/JB080i020p02949 Abstract
The knowledge that a gravity anomaly is due to an edge effect is sufficient to resolve the inherent ambiguity of the inverse potential problem. Thus given the gravity field across the contact between two laterally uniform structures, the density difference between the adjacent sections can be calculated by means of integral transforms operating on the data. The Backus-Gilbert inversion technique allows a rational trade-off between accuracy and resolution. The kernels associated with the physics of the problem indicate a resolution comparable to that of surface waves.
Deep-water sea-floor array observations of seismo-acoustic noise in the eastern Pacific and comparison with wind and swell.
Natural Physical Sources of Underwater Sound. ( , Ed.).:165-174., Holland: Kluwer Academic Publishers 10.1007/978-94-011-1626-8_14 Abstract
We report results from the analysis of data from an array of Ocean-Bottom Seismographs (OBSs) employed in an array of 150 meter aperture at a depth of 3800 meters off the California coast. The array recorded noise samples four times per day for a month using pressure and three-component inertial sensors.
Comparison of the month-long noise spectrograms with swell spectrograms and wind hind-casts shows marked similarities. In the 0.05–1.0 Hz range the frequency-doubling of swell energy into sea-floor noise predicted by the wave interaction theory is evident. In the 1–10 Hz range the wind-related effects dominate. Lulls in the wind produce deep notches in the noise level. During times of high wind, saturation of the wind wave spectrum causes limiting and reduces the size of the noise maxima.
The wind estimates are from the meteorological model of the U.S. Navy Fleet Numerical Oceanography Center and the swell estimates are from their Global Spectral Ocean Wave Model.