HR: 1400h
AN: S33A-07    [Abstracts]
TI: Seismicity pattern and first b-value mapping of the Caribbean - South American plate boundary in North-eastern Venezuela
AU: * Sobiesiak, M
EM: polar@gfz-potsdam.de
AF: GFZ Potsdam Department2, Section 2.1, Telegrafenberg, Potsdam, 14473, Germany
AU: Clark, S A
EM: stoney@rice.edu
AF: Dept. of Earth Sciences Rice University, 6100 Main Street, Houston, Tex 77005, United States
AU: Levander, A
EM: alan@pop.mail.rice.edu
AF: Dept. of Earth Sciences Rice University, 6100 Main Street, Houston, Tex 77005, United States
AU: Palma, M
EM: mpalma@funvisis.gob.ve
AF: FUNVISIS Dept. of Seismology, Calle Mara, El Llanito, Caracas, Venezuela
AU: Romero, G
EM: gromero@funvisis.gob.ve
AF: FUNVISIS Dept. of Seismology, Calle Mara, El Llanito, Caracas, Venezuela
AB: The implementation of the National Seismological BB Network in Venezuela gave way to much more precise determinations of earthquake hypocentres. This applies especially to the crustal events in North-eastern Venezuela related to the major El Pilar fault and its sub-parallel fault system, expanding over a width150 to 200 km. However, the improved depth calculations effected also the deeper seismicity locations. A pronounced seismicity gap separating crustal from deeper seismicity could be detected for the first time. The deeper seismicity exhibits a column-like shape reaching a depth of approximately 120 km thus having a width of ~ 50 - 70 km. These features coincide well with observations from the reflection/refraction/teleseismic transect at 64 deg W from the BOLIVAR (Broadband Ocean-Land Investigation of Venezuela) project. The seismicity column is located beneath the area where the profile denotes a step in Moho depth and the seismic gap agrees with the area around the Moho. We used the same seismicity data to do a spatial seismic b-value study. Spatial variations in the seismic b-value obtained from the Gutenberg-Richter relation are known to map out changes in the state of stress and/or material properties. This capability of b has been used to determine structural heterogeneity in faults and related stress inhomogeneities. In general, low b-values are related to increased stress or applied pressure whereas high b-values more likely indicate low stress environments or volumes of high crack densities. The b-value map we derived shows statistically significant variations. The deeper seismicity column is characterized by pronounced low b-values (0.5 and lower) indicating a high stress environment or high strain rates. This supports the hypothesis that the step change in Moho depth observed in the 64deg W profile marks the lower crustal plate boundary between the Caribbean and South America and coincides with a shear tear through the South American lithosphere. We propose that the actively propagating shear tear produces the column-like seismicity pattern and creates low seismic b-values along the entire width of the tear.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8106 Continental margins: transform
SC: Seismology [S]
MN: 2007 Joint Assembly