Introduction
Researchers have discovered that two faults near the city of Seattle, Washington ruptured simultaneously around 1100 years ago. If this were to happen again, it could result in an earthquake more than 30 times larger than previously predicted for the region.
The Study
A comprehensive study was conducted by Bryan Black and his colleagues at the University of Arizona to determine whether the faults ever ruptured close together, causing larger earthquakes. Ancient tree rings found around the Puget Sound were analyzed to date past earthquakes. The rings were linked to specific calendar years by measuring radiocarbon concentrations in the wood.
The Findings
All the trees were killed within a 6-month period between AD 923 and 924, suggesting that the past ruptures occurred either simultaneously or in quick succession. Such events can have devastating consequences, as witnessed in previous earthquakes around the world.
Potential Impact
A major earthquake on the Seattle faults, closer to the city than the well-known “Big One” on the Cascadia megathrust fault, would result in more intense shaking and less warning time for a tsunami. The researchers estimate that back-to-back ruptures could cause earthquakes with magnitudes of 7.5 and 7.3, while a multi-fault rupture could produce a magnitude-7.8 quake. This is significantly larger than the earthquakes currently included in the region’s seismic hazard models.
City Preparedness
Experts agree that no city in the world is fully prepared for a 7.8 magnitude earthquake. Although Seattle is taking steps to protect against seismic hazards, such as reinforcing old masonry buildings, the impact of such an earthquake would be catastrophic.
Conclusion
The potential for a repeat of the 1100-year-old earthquake that shook Seattle is a serious concern. The study’s findings highlight the need for increased preparedness and mitigation strategies to minimize the impact of future earthquakes.
