The mainshock, measuring Mw 6.5, struck at 17:54 PST, along the Superstition Hills segment of the San Jacinto Fault Zone.

The earthquake generated a 27 km (17 mi) surface rupture with additional postseismic slip continuing for a year.

Buildings in El Centro, Imperial, and Westmorland sustained collapsed chimneys, ruptured pipes, and shattered windows.

This region represents a transition zone between continental transform faulting and oceanic rifting in the Gulf of California.

The fault ruptured bilaterally for 20–25 km (12–16 mi) in a northeast towards the Brawley Seismic Zone and southwest, where it joined the SHF.

[1] A Mw  6.5 occurred at 7.5 km (4.7 mi) depth,[9] 12 hours later, with an epicenter at the northwestern termination of the SHF, where it intersects the ERFZ.

Its aftershocks illuminated a zone roughly parallel to the SHF, although situated several kilometers southwest of the main rupture trace.

[13] At the time of observation in 1988, the Wienert Fault's southern portion produced vertical offsets of 25 cm (9.8 in) and was actively increasing.

Installed creepmeters indicated several millimeters of slip occurred on the San Andreas Fault, but this was insufficient to create a surface rupture.

[16] Damage from the Mw  6.0 shock in Calipatria, El Centro, Heber, and Westmorland corresponded to VI (Strong) on the Modified Mercalli intensity scale (MMI).

Intensity VI was felt at Brawley, Calexico, Calipatria, Heber, Holtville, Imperial, and Seeley.

The shaking was also felt more than 300 km (190 mi) away in Las Vegas, Nevada, and Tempe, Arizona, and in Mexico's Tijuana and Ensenada.

An Immigration and Naturalization Service station between the city and Calexico experienced heavy interior damage.

A runway at the Naval Air Facility El Centro was closed for inspections, later revealing small cracks.

[19][20] Collapsed chimneys, severed pipes, shattered windows, and highways were offset in El Centro, Imperial, and Westmorland.

Near Westmorland, operations at the Desert Test Range Control Center ceased because of a damaged water tank and equipment.

[12] Several buildings in Calexico's business district sustained damage, and a brick wall toppled onto some cars.

[20] The two-story Imperial County services building, a steel frame structure, only received nonstructural damage.

In El Centro, detainees of the Immigration and Naturalization Service were relocated to the courtyard due to damage at the station.