1999 Satsop Earthquake

On Friday, July 2nd, 1999 at 6:43 PM PDT, the Fourth of July weekend started off with a shake for many in the Pacific Northwest. Loud rumbling struck the region as a result not of fireworks, but of a magnitude 5.8 (ML 5.7) earthquake.

This earthquake struck about 8.2 km north of Satsop, WA, 103.5 km west-southwest of Seattle, WA. This event was the region’s largest since the 1965 Puget Sound earthquake (magnitude 6.7). Occurring at a depth of 40.7 km, the quake cracked walls, broke gas mains, and left some people without power. Four people were slightly injured.

The earthquake was felt throughout most of western Washington and northwest Oregon, and in parts of southwest British Columbia, Canada. The strongest shaking corresponded to a level of about VI on the Modified Mercalli Intensity scale.

The 1999 Satsop earthquake was what is known as a deep intraplate earthquake (Deep Earthquakes). In the Pacific Northwest, these types of earthquakes occur within the subducting Juan de Fuca plate along the Cascadia Subduction Zone.

As tectonic plates move across the earth’s surface, they spread apart, slide past each other, and collide. Off the Pacific Northwest coast, the oceanic Juan de Fuca Plate is colliding into and subducting under the North American Plate. As the Juan de Fuca plate descends beneath the North America plate it bends and encounters increasing temperature and pressure. Stress from this process results in the rupture of normal faults, creating deep intraplate earthquakes. Due to their depth, the energy released during these events is able to spread out. This results in less severe shaking directly at the epicenter, but moderate shaking is felt over a larger area.

The hypocentral depth of the Satsop earthquake, which was 40.7 km, indicates that it occurred within the Benioff Zone. The Benioff Zone is located anywhere from 30 to 90 kilometers below the earth in the Pacific Northwest. In this area, the Earth’s minerals begin to undergo a change in their configuration. During this morphing of minerals, the volume of the rocks begin to change and get smaller. Stress changes associated with this mineralogical change likely triggered this earthquake. This is also the general area where the 2001 Nisqually earthquake nucleated.

It is typical of deep earthquakes to have very few significant aftershocks. For several days following the event, the earthquake had no detectable aftershocks, until 6 days later. There were three small aftershocks with epicenters and depths almost identical to the main shock. This included earthquakes occurring at 10:13 pm PT on July 8th (magnitude 1.6), 12:45 am PT on July 9th (magnitude 2.5), and 01:07 am PT also on July 9th (magnitude 2.3). Later, another aftershock occurred on August 11 at 10:37 am PT (magnitude 1.7). Both the focal mechanism and lack of sizable aftershocks for this event are typical of deep intraplate (i.e., Benioff Zone) earthquakes.

At the time, PNSN was developing and upgrading its network and web pages and produced a number of different information products about this earthquake, including an early version of "Did-You-Feel-It", strong-motion data, a preliminary version of "ShakeMap" and other related information. See our Legacy Web Page.

This earthquake was reported to have caused $8.1 million dollars in damage to Western Washington. The city of Montesano in Grays Harbor County, WA, suffered structural damage which included cracked chimneys and broken windows. The Grays Harbor County Courthouse, which was built in 1910, experienced extensive damage and cracked interior walls. The Montesano fire station, which was built around 1979, also bore the same fate. 

In Aberdeen, there were reports of major cracks to several older brick buildings, shattered windows, and some structural cracks of walls and beams. In addition, power outages and water main breaks occurred. Among the badly affected businesses was Moore's Appliance Store, which sustained large-scale damage. The top part of the building caved inward and took out part of the roof, the attic, the suspended ceilings, lighting, as well as damaged merchandise. Fortunately, this damage happened just after the store owner left the building and was driving away.

The Satsop earthquake serves as a reminder that being properly prepared for a damaging earthquake can save lives and reduce damage. There is evidence that new buildings built to modern building codes as well as seismically retrofitted older buildings experienced less damage during this event. Retrofitting older buildings and performing non-structural hazard mitigation can greatly reduce hazards to human life and property during an earthquake. 

Individuals should also make sure they know what to do during and after an earthquake. If you feel shaking, Drop, Cover, and Hold On! Have an emergency kit prepared and accessible. Simple preparation steps can greatly reduce injuries, loss of life, and damage.

For more information about how to prepare for an earthquake, visit this page: Preparedness.