The 1968 Borrego Mountain earthquake occurred on April 8, at 18:28 PST in the geologically active Salton Trough of Southern California.

The moment magnitude (Mw ) 6.6 strike-slip earthquake struck with a focal depth of 11.1 km (6.9 mi).

The zone of surface rupture was assigned a maximum Modified Mercalli intensity (MMI) of VII (Very strong).

[3] The tectonic boundary in Southern California is complex—plate motion is accommodated by the SAF and a network of subparallel faults.

The SAF terminates at the Salton Trough, a transtensional zone (pull-apart basin) that separates it from the Imperial Fault in the south.

[1] West of the Salton Trough is a largely offshore fault system that accommodates approximately 20 percent of the plate motion.

[6] East of the California Borderland, the plate boundary is a complex zone of faults that run parallel to the SAF.

[2] Further east, near the border with Nevada and Arizona, the Eastern California Shear Zone (ECSZ) takes up to 25 percent of the plate motion.

[9] The ECSZ consists of north–west trending right-lateral faults in the Mojave Desert, and Walker Lane, which lies at the western margin of the Basin and Range Province.

The mainshock which measured 6.6 Mw ,[11] was the result of shallow strike-slip faulting which initiated from the hypocenter at 11.1 km (6.9 mi) depth.

A focal mechanism analysis indicated the rupture plane had a northwest strike and dipped steeply (80°) to the south.

[12] A 33 km (21 mi)-long surface rupture through Quaternary alluvium and lake bed sediments was revealed.

[14] A maximum horizontal offset of 38 cm (15 in) was measured along the northern rupture located 5 km (3.1 mi) northwest of Ocotillo Wells, at the foothills of Borrego Mountain.

[15] Along the southern rupture, the maximum offset was 20 cm (7.9 in), measured 10 km (6.2 mi) southeast of Ocotillo Wells.

[13] There were also left-lateral displacements 1–2 km (0.62–1.24 mi) from Ocotillo Badlands north of Highway 78 and at the northern base of Borrego Mountain.

Whether these left-lateral offsets were part of the rupture mechanism or environmental changes unrelated to tectonic processes could not be determined.

[16][18] The maximum peak ground acceleration (pga) recorded by a seismometer at El Centro was 0.14 g. A maximum MMI of VII (Very strong) was assigned in the Borrego Mountain–Ocotillo Wells area (northeastern San Diego County) where surface rupturing occurred.

[19] In the meizoseismal area, small offsets occurred along the Coyote Creek Fault and Highway 78 near Ocotillo Wells cracked.

Large transformers 5.1 km (3.2 mi) from Ocotillo Wells were displaced, snapping anchor bolts and X-bracings.

Plaster fell from the walls and ceilings on the second level of the Balboa Hotel in El Centro.

Shifting furnitures; falling objects; rocking vehicles, trees and bushes; and minor cracks were reported.

[22] After the earthquake, the fault displayed a phenomenon known as aseismic creep, observed only along the central and southernmost section of the rupture.

[23] Evidence of movement along the Imperial Fault (70 km (43 mi) from epicenter) was discovered on April 13 on Interstate 8 when cracks appeared.

Creep occurred for 22 km (14 mi), although its actual length is unknown as dunes and urban developments obstructed any possible rupture trace.

[1] Movement along the Southern California segment of the SAF (50 km (31 mi) northeast of the epicenter) was documented on April 24.