Threat Hierarchy Assessment
The United States faces multiple significant earthquake threats with varying probability and potential impact. Determining the "biggest" threat requires evaluating both probability and consequence—a low-probability mega-earthquake may pose greater total risk than a high-probability moderate earthquake.
Cascadia Subduction Zone - Highest Consequence
The Cascadia Subduction Zone represents the highest-consequence threat, capable of magnitude 8.7-9.2 earthquakes producing massive tsunamis and widespread damage across the Pacific Northwest. A major Cascadia rupture could cause $1+ trillion in direct damage, trigger cascading failures in transportation and utility infrastructure, displace 2+ million people, and generate transpacific tsunamis affecting Japan and Asia. However, the probability in any given year remains low.
San Andreas Fault - Highest Probability
The San Andreas Fault poses the highest probability threat, with 72% probability of a magnitude 6.7+ earthquake in California within 20 years. The Southern Segment hasn't ruptured in 300+ years, indicating stress accumulation. Such an earthquake would cause $400+ billion in direct damage to the Los Angeles and San Francisco metropolitan areas.
Hayward Fault - High Proximity Risk
The Hayward Fault (magnitude 7.0-7.5 potential) poses particular risk due to proximity to 7+ million people in the San Francisco Bay Area. The USGS estimates 33% probability of major earthquake within 30 years. Historical activity suggests 150-year recurrence intervals; the last major rupture was 1868.
Overall Assessment
In terms of expected annual risk (probability × consequence), the Cascadia Subduction Zone and San Andreas Fault pose comparable aggregate threats. California's dense population in seismically active areas amplifies earthquake consequences, making West Coast earthquake risk the most critical in the nation.