Abstract: Precise Leveling Evidence for Aseismic Growth of Ventura Avenue Anticline (1978 to 1997) Implies Anelastic Strain Release in Ventura Basin, Southern California

ARTHUR G SYLVESTER, J. SCOTT COOMBS, ERIK D. JOHNSON, ERIK C. RONALD, NICHOLAS P. SIMON, and FRANK W. SPADA, Department of Geological Sciences and Institute for Crustal Studies, University of California, Santa Barbara, CA

Ventura Avenue anticline is a 22 km-long segment of a 50 km-long anticlinorium that grew 2-5 mm/yr to its present form and height in perhaps only the last 200,000 years (Rockwell, et al., 1988; Grigsby, 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 an additional 40 mm relative to its flanks in the past 19 years, without the benefit of earthquakes, and 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 geologic estimates and GPS measurements (1988 to 1994) indicate horizontal shortening of 6-10 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). This geodetic observation of aseismic fold growth supports the concept that regional strain is partitioned between elastic and anelastic deformation, and it provides permissive evidence to negate prevailing dogma that folds only grow coseismically in this region. It counters as well the conclusion (Dolan et al., 1995) that accumulated shortening strain will be released only elastically in future, frequent, Northridge-type earthquakes, or infrequent, large shocks. Thus if aseismic fold growth and possibly fault creep are also occurring elsewhere in the mountains north of Santa Barbara and Ventura, thereby releasing some fraction of accumulated strain energy anelastically, then impending earthquakes will be smaller and/or less frequent than maintained recently by other investigators.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California