HR: 1340h
AN: S43C-1027    [Abstracts]
TI: Geophysical Imaging of Active Tectonics: A Case Study From the Inter Andean Valley, Ecuador
AU: * Call, C
EM: cjcb@lehigh.edu
AF: Lehigh Universtiy, 31 Williams Dr, Bethlehem, PA 18015
AU: Meltzer, A
AF: Lehigh Universtiy, 31 Williams Dr, Bethlehem, PA 18015
AU: Alvardo, A
AF: Instituto Geofisico Escuela Politcnica Nacional, Casilla, Quito, 17-01-2759 Ecuador
AB: The Inter Andean Valley is a Pliocene-Quaternary basin filled with volcanic, lacustrine, fluvial and marine sedimentary deposits. A series of faults sometimes collectively referred to as the Delores-Guayaquil Mega Shear (DGM) traverses the length of the Inter Andean Valley posing a seismic hazard to a number of cities including the capitol, Quito. The DGM is a large right-lateral fault system similar in scale and seismicity to the San Andres Fault system which is understudied, especially in the subsurface. A site characterization study utilizing ground penetrating radar (GPR) and near-surface seismic reflection profiling was conducted in two areas of the Inter Andean Valley where geomorphic evidence suggests active faulting. One area, Nono Valley, exhibits extensional characteristics through basin bounding fault and the second area, Saquisili, has a structure consistent with the geometry of a fault propagation fold. Both areas are covered with thinly layered volcanic ash which is clearly seen in outcrop and the GPR profiles. Saquisili, in addition to the ash layers, has a nonuniform layer of pumice near the surface which was revealed in the drilled holes for the seismic source, which helps to account for the quick attenuation of the higher frequencies. The GPR profiles also image abrupt terminations and offset of horizontal layers, often associated with active faulting. We used a 48 channel multichannel seismograph with 30Hz geophones and a 20ft spacing to collect 24 fold common-midpoint profiles using a Betsy Seis-Gun firing 12 gauge blanks. Preliminary seismic data indicates that the frequency content ranged between 20 and 100 Hz with higher frequencies being systematically filtered out with depth. Seismic velocities range between 740 and 2600 m/s, producing a vertical resolution between 2 and 32.5m to a depth of approximately 900m. The GPR data was collected using a GSSI SIR-2 data accusation system with a 100 MHz antenna. The GPR signal penetrated between 120ns and 350ns (9 and 26m assuming a dielectric permittivity of 16). Combination of GPR and near-surface seismic techniques compliment each other by providing varying vertical resolutions and depths of penetrations. This case study provides valuable information that is relevant to future studies utilizing near-surface geophysics to identify and image active structures in the Inter Andean Valley or other geologically similar areas.
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2004 AGU Fall Meeting