M4.5 Earthquake at Orcas Island, Washington, March 3rd, 2025

By Nate Stevens, Steve Malone, Alex Hutko, Renate Hartog, Harold Tobin, Doug Gibbons, and Amy Wright

 

Summary

 

The Ml4.5 earthquake beneath Orcas Island, WA on March 3rd 2025 was widely felt across the Puget Sound region, from North Seattle to North Vancouver, BC (Figure 1). This light earthquake is an interesting event both from scientific and public safety perspectives because:

 

• It provided a successful, live test of the ShakeAlert Earthquake Early Warning System.

 

• It demonstrated the engagement of PNW citizen scientists in earthquake reporting.

 

• It has, thus far, produced far smaller and fewer aftershocks than anticipated.

 

• It occurred in an area with little history of seismicity and is not associated with a previously known active fault.

 

Figure 1: A map of the Puget Sound in northern Washington and southernmost British Columbia showing the location of the  M4.5 earthquake (star) and seismometers that observed the event (triangles). Select seismograms from the M4.5 event are shown for P-waves on vertical channels (blue) and S-waves on horizontal channels (green) at select stations.

 

 

Successes with ShakeAlert EEW and DYFI Citizen Science

 

This event was small enough that it did not cause any significant damage, but large enough to provide a natural test of the USGS ShakeAlert Earthquake Early Warning (EEW) system for Washington State. ShakeAlert performed as expected, producing an initial estimate of the earthquake’s location and magnitude 5.4 seconds after it started. Alerts were not released to the public immediately because the initial magnitude (M3.8) was below the public alerting threshold (M4.5). In the next 5 seconds, with additional data, ShakeAlert’s algorithms updated the magnitude to M4.5 and notifications were made available to cell phone applications (e.g., MyShake). Replies to our automatic social media posts on X, Bluesky, and Facebook, alongside anecdotal accounts from friends and family, indicate that the alert was received!

 

Figure 2: A map of community-sourced shaking intensity estimates for the M4.5 earthquake near Orcas Island, WA on March 3rd 2025 produced using 11033 Did You Feel It responses. Accessed March 3rd, 2025 at 9:45 pm.

 

 

The USGS Did You Feel It (DYFI) system received over 5000 responses within 30 minutes of this event occurring, and over 11000 responses within 24 hours (Figure 2). These data from US and Canadian citizen scientists across the Puget Sound provide valuable information about shaking intensity, damage, and community resilience that are incorporated into earthquake hazard assessments. 

 

 

Aftershock Monitoring

 

In the 24 hours following this event, the Pacific Northwest Seismic Network (PNSN) located 47 tiny earthquakes within 10 kilometers of the M4.5 event (Figure 3). None of these aftershocks were detected by PNSN’s automated system because they are so small. Thanks to stations near Olga (OLGA; Figure 1) and Deer Harbor (TURTL), installed as part of the ShakeAlert program, and rapid manual review by PNSN staff, we have a detailed view of this developing aftershock sequence. This analysis gives us confidence that larger aftershocks have not gone undetected, and presents a point of scientific interest. The largest aftershock thus far was 8 hours after the main shock (M0.92) and the second largest occurred 6 minutes after the main shock (M0.77). 

Figure 3: (Above) A map of earthquake locations with the M4.5 earthquake source mechanism shown in white and blue and aftershocks shown as red circles. All event markers are scaled by magnitude. (Below) A time series of main shock (blue) and aftershock (red) magnitudes relative to the origin time of the main shock. The green line indicates the smallest magnitude earthquakes that the PNSN reliably detects automatically for this part of the Puget Sound.

 

 

Based on the size and general tectonic setting of the M4.5 earthquake, the initial aftershock forecast by the USGS for the week following this event had a 39% likelihood of a M3+ aftershock, a 9% likelihood of a M4+ aftershock, and a 1% likelihood of a larger (M5+) event. The Cascadia region has relatively low aftershock productivity compared to other subduction zones (Gomberg and Bodin, 2021), so the forecast may be a slight overestimation. However, this forecast will be regularly updated as the aftershock sequence progresses, incorporating new information from close monitoring by the PNSN.

 

 

Tectonic Setting and Initial Interpretation

 

Crustal earthquakes in the Pacific Northwest occur on faults in the North American plate, releasing stresses built up by interactions with the Pacific and Juan de Fuca plates, as well as interactions between rigid blocks within the North American plate. These interactions produce a generally north-south oriented compression across Washington, which is reflected by the distribution and source mechanisms of earthquakes in this region. Many of the mapped (known) faults with evidence of geologically recent activity are aligned roughly perpendicular (i.e., east-west) to this compression. However, the source mechanism of the March 3rd earthquake suggests east-west compression on a north-south trending fault (Figure 3). The geologic history of the San Juan Islands is long and complex, and some of the very old faults exposed on Orcas Island are aligned with this earthquake’s source mechanism. Although seismicity on “inactive” faults like those in the San Juan Islands is less common, it is not unheard of. To determine whether this earthquake was on the deeper part of one of these old faults requires more careful analysis.