The Loma Prieta earthquake was a magnitude 6.9 event on the moment magnitude scale (with a surface‐wave magnitude of about 7.1), making it a strong to major earthquake.

Quick Scoop

  • Magnitude: 6.9 (moment magnitude, often compared to 6.9–7.1 “Richter‑type” values in popular descriptions).
  • Date & time: October 17, 1989, at 5:04 p.m. local time in California.
  • Duration of strong shaking: Roughly 10–20 seconds.
  • Epicenter: Near Loma Prieta peak in the Santa Cruz Mountains, about 14 km (9 miles) northeast of Santa Cruz and around 96 km (60 miles) south‑southeast of San Francisco.
  • Depth: About 16–18 km (around 10–11 miles) beneath the surface.
  • Fault: Slip on the San Andreas Fault, rupturing roughly 30–35 km (about 20–22 miles) of the fault zone in that area.

In everyday terms, a 6.9 earthquake is powerful enough to cause serious damage over a wide region, especially where soft soils amplify shaking, which is exactly what happened around the Bay Area in 1989.

So if you’re wondering “how big was the Loma Prieta earthquake?”: it was a magnitude 6.9 quake, strong enough to severely shake both the San Francisco and Monterey Bay regions and leave a lasting mark on California’s earthquake planning.

Information gathered from public forums or data available on the internet and portrayed here.