00:01Seattle today is a high-density metropolis, a multi-billion-dollar tech hub housing millions
00:07of people and a massive concentration of infrastructure.
00:11But the winter of 923 AD transformed this landscape into a site of absolute devastation.
00:18The ground buckled, shattering the coastline, and sending massive sections of forest sliding
00:24into the sound, while a tsunami swept across the region.
00:27This magnitude 7.5 earthquake occurred directly beneath the modern city center of Seattle.
00:35The culprit is the Seattle Fault Zone, a complex mesh of tectonic fractures threading through
00:41the crust, silently building pressure beneath the regional population center.
00:46For decades, geologists have estimated the risk based on a specific timeline.
00:50However, new research shows that our core assumptions about when this fault will snap are incorrect.
00:56To determine how often a 923-style cataclysm repeats, earthquake geologist Elizabeth Davis
01:03and her team looked for physical records of the deep past.
01:06These stepped, rocky formations along the coastline are known as marine terraces.
01:13They serve as a geological record, preserving the evidence of ancient earthquake activity.
01:18During the 923 event, the fault violently thrust these terraces upward, lifting sea-level rock
01:26roughly 26 feet into the air in a single moment.
01:29After mapping over 150 of these terraces, Davis' team discovered that the 923 quake was the
01:37only magnitude 7.5 event to occur in the last 11,000 years.
01:42The local geology is covered in thick sediment layers that would clearly show the scars of a rupture
01:48that large.
01:48There is no evidence of another massive quake hiding in the record.
01:52This provides a significant buffer.
01:55It suggests the interval between these catastrophic city-leveling events is measured in thousands
02:01of years.
02:02But another study, led by Stephen Angster, shifted the focus to Seattle's secondary faults, smaller,
02:09hidden fractures that never reach the surface.
02:11To understand tectonic pressure, think of the main fault as a massive steel spring taking
02:17millennia to compress and snap.
02:20Surrounding it are smaller, tighter springs.
02:22These secondary faults act independently, releasing energy on their own schedules.
02:28Using LIDAR and magnetic data, geologists identified three previously unknown earthquakes on these smaller
02:34faults that had never been recorded.
02:36These findings reveal a much more frequent danger.
02:39These smaller faults have been rupturing and causing earthquakes roughly every 350 years.
02:45Data also shows that seismic activity on these hidden fractures has actually increased
02:49over the last 2,000 years.
02:52Today, Seattle's most persistent, frequent danger comes from these smaller, rapid-fire
02:57ruptures occurring right beneath the modern pavement.
03:00If a secondary fault ruptures today, it would trigger a magnitude 6.7 earthquake directly
03:06under the Puget Sound area.
03:07An earthquake of this scale in an urban environment causes immediate heavy damage.
03:12Brick buildings can collapse, crushing vehicles and threatening lives while causing billions
03:18in infrastructure losses.
03:20While less destructive than a magnitude 7.5 event, this remains a statistical probability for
03:26the current generation.
03:27This chart shows the national seismic hazard model.
03:31It includes the massive 923 event, but these smaller, more frequent secondary faults are completely
03:37left out of the official projections.
03:39These faults are omitted because they fall below the model's minimum length requirements,
03:44and their specific rupture potential carries too much scientific uncertainty.
03:48This creates a paradox.
03:50The city is currently unprepared for its most frequent seismic threat, because the official
03:55models simply ignore it.
03:57We cannot stop tectonic plates from moving.
04:00However, mapping these blind faults is the necessary first step to surviving them.
04:05Do you live near a fault line or an area with a hidden geological history?
04:09If you found this helpful, please like the video and subscribe to the explainer.
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