Special Earthquake Report No: 99-015

Regional Location:

Preliminary Magnitude:

Moment Magnitude:

Greenwich Mean Date:
Greenwich Mean Time:
Focal Depth (km):
Analysis Quality (A-D):


5.4 Ms, Mb 5.5 (NEIC), 5.1 ML, Mb 5.5 (PNSN)

Mw 5.9 (NEIC), 5.9 Mw (Harvard), 5.7 Mw (Oregon State)

40.6 km

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An earthquake registering a preliminary magnitude of Ms 5.4, Mb 5.5 (NEIC), ML 5.1 (PNSN), Mw 5.9 (Harvard), Mw 5.7 (Oregon State) was detected at 01:43:54 UTC, July 3 (6:43 p.m. PDT, July 02), in western Washington. The focal depth was placed at 41 km beneath the surface and the fault plane solutions from NEIC, Harvard and Oregon State suggested a complex oblique normal-slip motion along a generally north-south trending fault plane.
The quake was centered about 68 miles southwest Seattle, 45 miles west-southwest of Olympia, 20 miles east-northeast of Aberdeen, or 5 miles north of the small town of Satsop on Highway 6, near the Grays Harbor - Mason County Line at Shafer State Park. (See Maps).
The quake was widely felt in the Puget Sound and in the Olympic Mountains, and as far away as Ellensburg and Wenatchee on the eastern side of the Cascade Range, the central Oregon coast and Victoria, B.C., Canada. While reports have varied from area to area, some said the shaking lasted for what seemed like 15-20 seconds and some said as long as 30 seconds. Some reports said there was a loud bang prior to the shaking and many said the shaking was accompanied by a rumbling or a loud roar like a freight train passing close to their house. Many were frightened by the quake, especially from the long shaking duration.
Damage reports came in from all over the epicentral area where dishes were thrown from cabinets, tables and shelves, many windows were broken, and a few walls were cracked. There have been numerous reports of items toppling from table and shelves in the southwestern Puget Sound.
Ken Urie from the Emergency Supply Center in Olympia, WA, said most of the damage reports came in from the Aberdeen (on the coast) and from the epicentral (Satsop-Elma) area. Visit his web site for information on earthquake safety supplies. Other areas with damage included Hoquiam, Brady, and Montesano.
A water main ruptured in downtown Aberdeen and at least one building shifted 6 inches off its' foundation. Gas leaks, toppled chimneys and power outages throughout Grays Harbor County (Aberdeen area). One structure burned down. In Aberdeen, a roof at a furniture store collapse and the local hospital reported four people were treated with quake-related injuries, including a man who fell off a roof and a baby dropped by a woman - none of which were serious injuries. The Gray's Harbor County Courthouse sustained some minor damage to court rooms.
Most people were preparing for the 4th of July Holidays when the quake struck. Some were not sure what it was but as it continued to shake, it became quite obvious it was an earthquake. People described furniture dancing about rooms, stereos being thrown to the floors and in some cases, windows exploding. Most common was cars on the streets were bouncing around followed by the pervasive sound of car alarms going off.

Interestingly, the quake follows two quakes during the last 24 hours. A
M3.1 quake Thursday night, July 1, shook the south Puget Sound area. It was centered near Maury Island north of Tacoma. Earlier on July 2, a Ms 5.7, Mw 6.0 earthquake struck 76 miles off the southwestern coast of Vancouver Island in the Explorer tectonic plate. It was centered too far from shore to have been felt.
The variability in the magnitude reporting by the several monitoring institutions is because of the different methods in how they measured the earthquake. Because the focal point of this temblor was at 41 km beneath the surface and the seismic waves had to pass through several different rock types before getting to the surface, thus becoming distorted and attenuated, the best estimate of the magnitude would come from a Body wave or Moment magnitude analysis. The Pacific Northwest Seismic Network (PNSN) used a local (ML) or Richter magnitude which is usually reserved for earthquakes that are near the surface or less than 15 km deep with the seismic waves passing through homogeneous rock mass. This method would be the least reliable method in measuring a deep earthquake like this one.
Although the focal depth of the Western Washington earthquake suggests it may have occurred near the Cascadia megathrust, the subduction zone boundary between the Juan de Fuca plate and the overriding North American plate (See Generalized Pacific Northwest tectonic map; See Generalized Cascadia subduction zone sketch), the NEIC, Harvard and Oregon State fault plane solutions implied the quake occurred on some nearby fracture perhaps within the subducted Juan de Fuca plate. The megathrust would show thrust-style (up and over) motion but the fault plane solution indicated predominately normal-slip (down and away).
The Cascadia megathrust is believe to have ruptured in the year 1700 with a powerful M9+ earthquake which created a powerful tsunami that crashed on shore in Japan, killing thousands of people. This is the first M5+ quake to have occurred this deep in the Pacific Northwest since the 1976. Other deep notable events include the 1949 Olympia M7.1 earthquake and the Seattle M6.5 earthquake in 1965.

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Subtract 8 hours from Greenwich Mean Time to obtain PST or 5 hours for EDT
Location Quality: A (good), B (fair), C (poor), D (bad)
Magnitude: Ml (local or Richter magnitude), Lg (mblg), Md (duration), Mb (body wave), Ms (surface wave), Mw (moment)

Standard Sources Include:
U.S. Geological Survey, National Earthquake Information Center, Golden, CO (NEIC)
Harvard Geophysical Observatory, Harvard University, Cambridge, MA (HRV)
U.S. Geological Survey, Northern California Seismic Network (NCSN)
University of California, Berkeley, Seismological Laboratory (UCBSL)
Southern California Seismic Network (USGS & Caltech), Pasadena, California (SCSN)
California Institute of Technology, Pasadena, California (Caltech)
various wire reports and/or personal communications

All data are preliminary and subject to change.
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