Puente Hills Fault

The Puente Hills Fault (also known as the Puente Hills thrust system) is an active geological fault that is located in the Los Angeles Basin in California. The thrust fault was discovered in 1999 and runs about 40 km (25 mi) in three discrete sections from the Puente Hills region in the southeast to just south of Griffith Park in the northwest. The fault is known as a blind thrust fault due to a lack of surface features normally associated with thrust faults. Large earthquakes on the fault are relatively infrequent but computer modeling has indicated that a major event could have substantial impact in the Los Angeles area. The fault is now thought to be responsible for one moderate earthquake in 1987 (the 1987 Whittier Narrows earthquake) and another light event that took place in 2010, with the former causing considerable damage and deaths.

The Los Angeles Basin is situated along the coast of Southern California at the confluence of the Transverse Ranges and the Peninsular Ranges. The basin is under the influence of several strike-slip and blind thrust faults with geodetic studies providing evidence of the northern basin being shortened in the north-south or northeast-southwest directions at a rate of 4.5–5 millimetres (0.18–0.20 in) per year. Some of the shortening can be attributed to several known fault systems and two models have been proposed to account for the excess.

The Puente Hills Fault thrust system runs 40 km (25 mi) across the northern Los Angeles Basin from Downtown Los Angeles to near Brea and the Chino Hills in northern Orange County. It may account for some of the shortening in the northern Los Angeles Basin.

The Puente Hills thrust system was rendered in a collection of seismic reflection profiles that were produced by the petroleum industry. The images were made available to seismologists John Shaw and others for a comprehensive study of the region that was published in the bulletin of the Seismological Society of America in 2002. The thrust system is visually distinct as north dipping reflections in many of these representations. Stacking velocity measurements and sonic logging in the area helped to determine that the fault plane slopes at 25—30°. The system comprises three sections that strike generally east-west and are labeled the Los Angeles, Sante Fe, and Coyote Hills segments.

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