The Whittier fault is a principal strand of the San Andreas fault system near the southwest edge of the Puente Hills in the northeastern Los Angeles basin. Physiographic evidence indicates recent movement along the Whittier fault and its southeast extension, the Elsinore fault. Exposures in test trenches

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also indicate recent movement along a fault parallel with the principal trace of the Whittier fault in the Puente Hills. However, no major earthquakes or rupturing of the ground surface have occurred along the Whittier-Elsinore fault in historic time. A destructive earthquake occurred in 1929 along the Norwalk fault 6 mi south of the Whittier fault.

Analysis of precise level lines run by the Los Angeles County Engineers since 1951 indicates that the Puente and Montebello Hills area and the Santa Fe Springs-Coyote Hills trend are rising at a rate of 0.01 to 0.04 ft/year relative to the synclinal area between the Puente and Montebello Hills on the north and the Santa Fe Springs-Coyote Hills trend on the south. This relative motion may be caused by tectonic motion, or by withdrawal of ground water and compaction of sediments within the synclinal area. Because of oil production and waterflood activities, motion of the ground over oil fields also was detected.

Several oil fields are located along the Whittier fault and on anticlinal structures along the Santa Fe Springs--Coyote Hills trend southwest of the Whittier fault. The Whittier and Coyote (west) oil fields are undergoing extensive waterfloods. The Whittier fault provides the updip closure for oil sands in the Whittier oil field. Thus, the Whittier fault area was considered an ideal test site to search for a relation between subsurface pressure and the distribution and frequency of microearthquakes.

A network of portable seismometers was operated in the Whittier fault area between July, 1971 and April, 1972. Because of the background noise, the smallest event that could be reliably located had a magnitude of 1.0. Epicenters of 31 microearthquakes with a maximum magnitude of 3.0 were determined, but no direct evidence could be established for a relation between oil production and waterflood activities and the distribution of microearthquakes. Sufficient data were available to determine hypocentral depths for 17 events. Assuming a range of 60-70° north dip on the Whittier fault, 8 of the 17 hypocenters are on the subsurface projection of the Whittier fault; one hypocenter is on the Norwalk fault. Eight of the hypocenters cannot be related to any known structure. On the basis o the microearthquakes detected during this study, the Whittier fault must be considered active.

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