ShakeAlert Warning Times for earthquakes in the Pacific Northwest

The ShakeAlert earthquake early warning system is now active throughout the Pacific Northwest and California. So if an earthquake were to occur tomorrow somewhere in Washington or Oregon, how much warning time would you expect to get from ShakeAlert
A new animation from the USGS, PNSN, and IRIS answers that question by diving into the plate tectonics that ultimately cause most earthquakes in the Cascadia region. 
The short answer is that ShakeAlert gives you anywhere from a few seconds to tens of seconds of advanced warning before strong shaking arrives. But as you might expect, the long answer to how much warning time ShakeAlert can offer is, “It depends.”
ShakeAlert detects earthquakes by using a network of seismic sensors across the West Coast to rapidly detect earthquakes. Depending on where a given earthquake starts relative to the positions of the sensors, the earthquake may be detected by ShakeAlert algorithms within a few seconds or less. It also matters where you are located relative to the earthquake. An earthquake that begins below your feet will be detected by seismic sensors at the same time as it is detected by you; in other words, no advanced warning time. But if you are 100 miles away, you’ll have much more time to get a ShakeAlert-powered warning.
Ultimately, it’s always a question of where you are relative to the earthquake. So what kinds of earthquakes are likely to occur near you? That’s where it’s important to consider the unique tectonic environment of the Pacific Northwest. There are three main types of earthquakes that cause major shaking in our region.
  1. Shallow crustal earthquakes that grow up to magnitude 7.5.
  2. Deep intraslab earthquakes that also can get up to magnitude 7.5
  3. Massive subduction zone earthquakes that can rupture all the way from Northern California to British Columbia. These can be magnitude 9 or above.
Shallow earthquakes can occur almost anywhere in Washington and Oregon, near major metro areas or in rural regions. Examples include the 1993 Scotts Mills quake in the Portland area, or the 1872 North Cascades earthquake near Wenatchee. This type of earthquake can be challenging for an earthquake early warning system - since it occurs near the surface, there will always be some people who won’t get any advance warning. Those who do receive a warning will probably get 10 seconds or less.
Deeper earthquakes are quite common in the Pacific Northwest, the most recent being the 2001 Nisqually quake. They tend to occur in the highly populated western parts of Washington and Oregon, directly below our feet. But these earthquakes can be deep enough that faster moving (but less damaging) P-waves can reach ShakeAlert’s sensors with enough time to provide some warning to most people before the damaging S-waves reach the surface. Computer simulations show that 10 seconds of warning is typical for these types of earthquake as well.
Sooner or later, a massive Cascadia earthquake will happen offshore. We don’t know where it will start, but it is likely to cause strong shaking across the entire Pacific Northwest region. The good news is that if you happen to be located far away from the start of the earthquake, you may receive over a minute of advanced warning from ShakeAlert. That sounds like plenty of time, but again, it’s impossible to predict where exactly this (or any) type of earthquake will start. So it’s better not to wait for a ShakeAlert notification to start packing your emergency kit.
More detailed information about likely warning times in the Pacific Northwest, including estimates of the relative likelihood of the three main types of earthquakes in cities like Seattle and Portland, can be found in this USGS open file report