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Text of 1997 AGU Fall Meeting Abstract - see relevant graph below the text. Reference citation:
Sylvester, A. G., 1997. Aseismic growth of Ventura Avenue anticline (1978 to 1997): Evidence of anelastic strain release in Ventura basin, southern California, from precise leveling. Transactions of the American Geophysical Union 78, F156-157.
Aseismic Growth of Ventura Avenue Anticline (1978 to 1997): Evidence of Anelastic Strain Release in Ventura Basin, Southern California, From Precise Leveling
Arthur G Sylvester (Dept. Geological Sciences and Inst. for Crustal Studies, Univ. California, Santa Barbara, CA 93106)
Ventura Avenue anticline is a geologically young, major fold that grew 2-5 mm/yr during Holocene time (Rockwell, et al., 1988). Comparison of three first-order leveling surveys across the anticline, 1978 (NGS) and 1991 (County of Ventura), 1997 (UCSB), indicate that the anticlinal crest rose nearly 40 mm relative to its flanks, despite continued pumping of oil and water from the anticline. By state law, produced fluids must be replaced to mitigate potential subsidence, and comparison of two levelings 10 years after the law went into effect in 1956 indicate that a re-injection program was successful in countering local subsidence (Buchanan-Banks, et al., 1975). Between 1978 and 1997 no earthquakes > M2.0 were recorded anywhere close to the anticline, even though GPS measurements from 1988 to 1994 indicate horizontal shortening of 7 mm/yr across the entire Ventura basin. The cumulative seismic moment of all earthquakes recorded near the anticline in that time period scarcely exceeds the equivalent of a M4 earthquake, which, if it occurred at the 12-16 km depths where the hypocenters are located, would be sufficient to cause only 10 mm of surface uplift (O'Connell et al., 1997). The aseismic growth of Ventura Avenue anticline provides permissive evidence to modify prevailing dogma that folds only grow coseismically in this region. It counters as well the assumption that strain accumulation inferred from GPS measurements will be released only elastically in future, frequent, Northridge-type earthquakes, or infrequent, even larger shocks (Dolan et al., 1995). This observation of aseismic fold growth supports the concept that regional strain is partitioned between elastic and anelastic deformation. If aseismic fold growth and possibly fault creep are occurring elsewhere in the greater Los Angeles area, thereby releasing some fraction of accumulated strain energy, then seismic hazard estimates for the Los Angeles region may be significantly different than inferred by Dolan et al (1995) based on a wholly elastic model. Impending earthquakes may be smaller and/or less frequent, therefore, than maintained by those investigators.