1989 Loma Prieta Earthquake

On Tuesday 17 October 1989 at 5:04:15 p.m. local time, a magnitude 6.9 (moment magnitude; surface-wave magnitude 7.1) earthquake struck near Loma Prieta peak in the Santa Cruz Mountains — about 14 km (9 mi) northeast of Santa Cruz and 96 km (60 mi) south-southeast of San Francisco, according to the US Geological Survey. The rupture began about 18 km (11 mi) deep, slipped as much as 2 metres (7 ft) along the San Andreas fault system, and ran 35 km (22 mi) without breaking the Earth's surface. Because the fault ruptured in both directions at once, shaking lasted only about 10 seconds — yet it was the strongest earthquake to hit the San Francisco Bay region since 1906.

63 people died and 3,757 were injured, with an estimated $6 billion in property damage (USGS). One USGS Professional Paper gives 62 deaths, and official damage estimates range from $6 billion to $10 billion depending on scope. The worst failures were far from the epicentre, on poor ground: the double-decker Cypress Street Viaduct on Interstate 880 in Oakland collapsed, killing 41; a span of the San Francisco–Oakland Bay Bridge fell, killing one; and in San Francisco's Marina District, built on uncompacted sandy fill over a shallow water table, the ground liquefied — sand erupted in "sand volcanoes" and more than 20% of the wastewater collection lines had to be repaired or replaced.

The quake struck minutes before the third game of the 1989 World Series in San Francisco. It became a wake-up call: post-1989 research prompted California's 1990 Seismic Hazards Mapping Act. Engineered buildings performed well even near the epicentre; the most severe damage was to older structures on poor ground — a lasting lesson in where, not just how strong, an earthquake strikes.

Sources & references

• USGS - M 6.9 October 17, 1989 Loma Prieta Earthquake
• USGS DDS-29 - Selected Photographs
• USGS Fact Sheet 151-99 - Progress Toward a Safer Future
• National Academies - Practical Lessons from the Loma Prieta Earthquake