Rapid short-term rates of intraplate seismicity related to episodic release of high pore pressure
- Geological Survey, Menlo Park, CA (United States)
- Stanford Univ., CA (United States). Dept. of Geophysics
One puzzling characteristic of intraplate seismicity is that centers of intense seismic activity appear to be geologically short-lived. Comparison of earthquake focal mechanisms with independent stress orientation data indicate that faulting in these intraplate areas is occurring in response to a relatively uniform regional stress field derived from far-field sources that are unlikely to change rapidly with time. The authors suggest that localization of intense seismic activity, as well as the marked discrepancy between the long- and short-term rates of activity at a given site, may be related to extremely slow buildup, but relatively rapid dissipation, of high fluid pressure at depth. Once the pore pressure in a region is high enough to cause faulting, repeated fault movements cause deformation and fracturing of the surrounding crust, allowing the high pore pressure to dissipate over time scales possibly on the order of thousands of years. After the period of intense deformation, natural processes of fluid flow in the crust could geochemically seal the newly opened fractures, allowing pore pressure to rebuild and begin the process again. Several observations suggest that locally small stress perturbations can markedly affect the style of faulting in intraplate seismic zones, implying the existence of small stress differences at seismogenic depths: diverse aftershock focal mechanisms in close proximity, and a contrast between thrust faulting in SE Canada and strike-slip faulting in the central and eastern US that generally coincides with the maximum glacial-rebound gradient, suggesting that very small superimposed rebound stresses (1--10 MPa) are responsible for this contrast. The easiest explanation of these small stress differences is locally elevated pore pressure. However, regional consistency of horizontal stress orientations imply that overall the crust is strong and that high pore pressure/small stress differences are temporally and spatially restricted.
- OSTI ID:
- 5769345
- Report Number(s):
- CONF-921058-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 24:7; Conference: 1992 annual meeting of the Geological Society of America (GSA), Cincinnati, OH (United States), 26-29 Oct 1992; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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