HR: 0800h
AN: T31A-1259    [Abstracts]
TI: The World Stress Map Project - Stress Orientations Near Plate Boundaries From Focal Mechanisms
AU: * Heidbach, O
EM: oliver.heidbach@gpi.uni-karlsruhe.de
AF: Geophysical Institute, Karlsruhe University, Hertzstr.16, Karlsruhe, 76187 Germany
AU: Reinecker, J
EM: john.reinecker@gpi.uni-karlsruhe.de
AF: Geophysical Institute, Karlsruhe University, Hertzstr.16, Karlsruhe, 76187 Germany
AU: Reinecker, J
EM: john.reinecker@gpi.uni-karlsruhe.de
AF: Heidelberg Academy of Sciences, Karlstr.4, Heidelberg, 69117 Germany
AB: The World Stress Map (WSM) is a global compilation of contemporary tectonic stress in the Earth's crust using a wide range of geophysical and geological stress indicators. In general there is no systematic deviation of SH azimuth (maximum horizontal stress) between the different stress indicators. However, near the San Andreas fault the SH azimuth from focal mechanisms give conflicting information to those derived from borehole breakouts. This had led to the hypothesis that the San Andreas fault is a `weak' fault and casts doubt on the accuracy of focal mechanisms as stress indicators. Assuming that all plate boundaries are potentially weak affects the data quality of stress data from focal mechanisms nearby. We proceed a statistical analysis of the data from the WSM release 2004 to develop criteria which detect focal mechanisms which are probably inaccurate for deducing the SH azimuth. We conclude that the SH azimuth from focal mechanisms is at a greater risk of being inaccurate if the earthquake has a) prevailing strike-slip component, b) occurred within a 100 km corridor around a presumably weak transform plate boundary, and c) if the azimuth from one of the two possible horizontal slip vectors of the focal mechanism is within 20ø of the relative plate motion azimuth. This applies for 605 out of approximately 9000 focal mechanisms used in the WSM database. We mark these data sets as `possible plate boundary events', or PBE herein, indicating that these events are probably controlled by the fault geometry and not by the stress field. Case studies at transform plate boundaries reveal that the mean SH azimuth changes in the 100 km corridor by deselecting the PBE. Along the El Pilar fault in Northern Venezuela the change of mean SH azimuth is 33ø. The impact on the San Andreas fault is very small (2.4ø) due to the exceptionally high amount of stress data available from other sources than focal mechanisms. Examples from the southern Pacific ocean and the central Atlantic ocean show a change in mean SH azimuth of 22.0ø and 33.3ø respectively.
DE: 7230 Seismicity and seismotectonics
DE: 8150 Plate boundary--general (3040)
DE: 8164 Stresses--crust and lithosphere
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting