The Joshua Tree earthquake struck along a right-lateral fault in the Late Quaternary system — a fault that had not been previously mapped, which is itself a lesson in the incomplete picture that seismic hazard assessments can provide. The rupture occurred about 20 kilometers south of the Mount Pinto Fault and about 10 kilometers northeast of what geologists described as a mission-area location. Maximum fault slip was estimated from the surface deformation that followed. The epicenter was within the network of faults associated with the broader San Andreas system, specifically linked to the creek systems that feed into that network. Geologists noted the proximity to the Coachella Valley segment of the San Andreas — a section of fault that has not experienced a major rupture in historical times and is accordingly considered overdue.

The earthquake generated more than 6,000 aftershocks. The largest reached magnitude 4.8. A foreshock of 4.6 had occurred about two hours before the main event — though it would only be recognized as a foreshock in retrospect. The aftershock sequence spread northward from points near the San Andreas Fault northwest of Indio to the east-striking Mount Pinto Fault, tracing the subsurface geometry of the fault system in ways the seismographs could read even when the surface offered no clear indication. The California Department of Emergency Services, reading the same data seismologists were reading, issued a warning of possible future seismic activity in the affected counties. The warning was prudent. The future, it turned out, had more to offer.

What happened two months after the Joshua Tree earthquake put the April event in a different light. On June 28, 1992, the Landers earthquake struck — magnitude 7.3, one of the largest earthquakes to hit California in the 20th century, followed hours later by the Big Bear earthquake at magnitude 6.5. The aftershock sequence of the Joshua Tree quake had, in the weeks before the Landers rupture, shown a clear relative calm near the southern end of what would become the Landers aftershock zone. Seismologists later recognized this as meaningful — the quiet before the larger rupture, the Joshua Tree sequence essentially priming the fault system for the June event. Whether the Joshua Tree earthquake triggered Landers, or simply reflected the same regional stress increase that eventually produced it, remained a subject of research. What was not in question was that April 22, 1992, was not the end of anything.

The mountains where the 1992 Joshua Tree earthquake occurred frame the northern edge of the Coachella Valley — the same mountains visible from Palm Springs, the same geological structure through which the Palm Springs Aerial Tramway climbs. The Little San Bernardino Mountains are not a single range but a series of fault-bounded blocks, the landscape above reflecting the constant motion below. Driving south toward Palm Springs from Joshua Tree on Route 62, you descend from the high desert into the valley along a gradient created partly by the very fault systems that produced the 1992 earthquake and that will, seismologists agree with considerable certainty, produce larger ones.