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Rocking the Boat: Poro-elastic Stress Change at Seismogenic Depth Associated with Oil Production in the Los Angeles Basin in the Early 20th Century

Susan E. Hough, & Roger Bilham

Published July 25, 2017, SCEC Contribution #7334, 2017 SCEC Annual Meeting Poster #040

The Newport-Inglewood Fault and other faults in the southwestern Los Angeles Basin are characterized by low slip and seismicity rates; the rate of moderate (Mw4.0-6.4) earthquakes on these faults was, however a factor of ≈5 higher between 1920-1960 than since 1960. In addition to the 1933 Mw6.4 Long Beach earthquake, notable events included the 14 Nov. 1941 Mw5.1 Torrance and the 22 Oct. 1941 Mw4.7 Dominguez Hills earthquakes. Analyzing available data for six ML≥4.0 earthquakes between 1938 and 1944, we conclude that during the early instrumental era in southern California (1932-1960), detailed macroseismic data provide better constraint on epicentral location than do instrumental data. We then consider the revised locations of the events relative to oil production in the region. A close spatial and temporal association between most of the moderate earthquakes and notable industry activity – typically significant increases in production, and/or expansion of fields, and/or deepening of existing wells – suggests that many of the earthquakes might have been induced by primary oil production. Before 1960, primary production commonly induced subsurface deflation and significant surface subsidence. We calculate strain changes associated with production in 8 oil fields in the southwestern Los Angeles Basin; the predicted subsidence surrounding production horizons resulted in strains ≥ 10-4, the failure strain of typical shallow rocks. Significant stress changes, upwards of 0.1 MPa, extended more than 2 km away from production horizons, which by the late 1930's extended as deep as ≈4 km in the southwestern basin. Models thus suggest that pre-1950 oil production would have increased poro-elastic stress significantly on nearby faults at the top of the seismogenic crust. Our elastic models suggest that the magnitudes of shallow oil field induced earthquakes may have been limited by the spatially rapid reversal of strain polarity in contiguous lobes of increased stress, making it difficult for ruptures to propagate farther than a few km. The advent of fluid injection recovery methods, or “water-flooding,” around 1960 reduced or reversed aquifer subsidence in existing fields, and prevented it in others. The widespread adoption of water-flooding around 1960 appears to have mitigated induced earthquake risk as well.

Key Words
Induced earthquakes

Hough, S. E., & Bilham, R. (2017, 07). Rocking the Boat: Poro-elastic Stress Change at Seismogenic Depth Associated with Oil Production in the Los Angeles Basin in the Early 20th Century. Poster Presentation at 2017 SCEC Annual Meeting.

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