A system was launched last spring to give advance warning of the next major earthquake in the Pacific Northwest. Emergency alerts go out and phones buzz. The system's effectiveness will depend on the magnitude of the earthquake and its location. In 1700, the Pacific Northwest experienced its last magnitude-9 event in the offshore subduction zone. We have only a few clues about how it happened. However, the earthquake early warning system is being improved and expanded. Seismologists are curious to see what ShakeAlert would do in the event of a major earthquake.
Geological Survey have collaborated to simulate different magnitude-9 slips along the Cascadia fault. This simulation was used to assess how ShakeAlert would perform under 30 scenarios. The alerts work generally well, although the results show that there are ways to improve the system for high-risk events. The research will appear online at the American Geophysical Union's annual autumn meeting on December 13. It is a hybrid event that takes place in New Orleans. Lead author Mika Thomson is a UW student in Earth and Space sciences. An early warning system allows people to take a break and think before they react. This is especially important in the case of a major earthquake. This work was based on detailed computer simulations that simulate magnitude-9 earthquakes. These were created for an study which looked at how an offshore big event might play out depending on how deep and where the Cascadia fault slips. Thompson ran the simulations using an offline ShakeAlert program and calculated the alerts that would be sent across the region. “The alerts are generally performing well, but they are not perfect,” stated co-author Renate Hartog who is the manager of UW's Pacific Northwest Seismic Network. “This project aims to identify the system's weaknesses and make recommendations for future alerting strategies. Even though large offshore earthquakes can be difficult to locate and detect, the alerts worked well. However, there were instances when warnings arrived too late for some areas. When the simulated rupture occurred at the fault's southern end, the initial estimates for distant places, such as Seattle, were often below the shaking intensity threshold 5 threshold that would trigger an immediate alert. The shaking intensified as the slip moved northward, but the alerts were too late to be effective in Seattle for warning of higher shaking levels or level-5. Thompson stated that Magnitude-9 earthquakes can be challenging as alerts are generated as the seismic events unfold. Thompson said that the Nisqually earthquake had a magnitude-6.8 magnitude and lasted for only six seconds. A magnitude-9 earthquake, however, could take longer than five minutes to complete the rupture. Hartog suggests that users lower their thresholds for alerts to shaking intensities 3 and 4. This would help to reduce uncertainty. Although users might be alerted for minor events, they will also be alerted if there is a magnitude-9 earthquake. Thompson stated that for the scenario in Northern California, the threshold should be set to shake intensity-3. This will alert everyone in the West Coast ShakeAlert area, with some people getting warning times up to one minute. But if you set the threshold to shaking intensity-5, then you will see smaller alerting areas that will have missed alerts at the outer edges. If a rupture occurs in northern California or southern Oregon, alerts for the greater threshold would be missed by the entire Seattle-Tacoma area. Apps are expected to be available in Washington soon and will allow users the ability to set their alert thresholds.