Lucy Jones, USGS; Egill Hauksson, Caltech Seismological Laboratory
A local magnitude Mw5.2 earthquake occurred 9 km (6 miles) east-southeast of Anza, California at 8:41 am on Sunday, 12th of June 2005. The mainshock focal depth was 13 km (8 miles). The earthquake is located within the San Jacinto fault zone and within the Anza ‘gap’ (where future M>6 events are expected). The earthquake was widely felt across all of southern California and into Nevada and Arizona. The strongest shaking occurred to the northwest of Anza, about 25 km 15 miles) to the south of Palm Springs.
The earthquake probably occurred on a small “cross-fault” within the San Jacinto system. This means a small fault at a high angle to the main fault that accomplishes the same overall strain as the main fault. The San Jacinto fault strikes northwest with right-lateral strike-slip motion. The alignment of aftershocks to the Mw5.2 suggests a north-east strike to its fault. The depth of the earthquake is 13 km. This depth puts the earthquake below the section of the fault thought to be accumulating slip for a large future earthquake.
The San Jacinto fault is a 200 km long branch of the San Andreas system that accommodates horizontal motion between the North America and Pacific Plates. The past history of the region includes numerous M5 events, including a M5.1 on 31st of October 2001and a M6.0 earthquake 1937, located about 10 miles to the southeast of today’s epicenter.
The San Jacinto fault is the most active earthquake fault in California, producing an earthquake of at least magnitude 6 eight times in the last century, including:
- 2005 Mw5.2 Anza
- 1987 M6.6 Superstition Hills
- 1968 M6.5 Borrego Mountain
- 1954 M6.4 San Jacinto
- 1942 M6.5 Fish Creek Mountains
- 1937 M6.0 Terwilliger Valley
- 1923 M6.3 North San Jacinto fault
- 1918 M6.8 San Jacinto
- 1899 M6.5 San Jacinto