Basic Information

     8.2 km   N of Satsop, WA
    28.9 km ENE of Aberdeen, WA
   103.5 km WSW of Seattle, WA

A Note on The Magnitude

The magnitude estimation routinely used by the Pacific Northwest Seismograph Network is Mc or "Coda magnitude". Although the coda magnitude scale works well for small and moderate size earthquakes it has not been well calibrated for deep and large earthquakes. In order to do a better job of comparing this earthquake with other large earthquakes around the world it is useful to determine the magnitude using other methods commonly used for large earthquakes. Coda magnitude for the Satsop earthquake is Mc 5.1

Other magnitude scales include the body wave magnitude (mb), surface wave magnitude (Ms) and moment magnitude (Mw). Other agencies including the National Earthquake Information Center, and Harvard University have computed the following estimates of magnitude for this earthquake: mb=5.5, Ms=5.5, and Mw=5.8.

Earthquake Felt Reports for the Main Shock

Aftershocks

Normally, deep earthquakes have very few significant aftershocks. This earthquake had no detectable aftershocks for several days. On July 9, 1999 we recorded 3 small aftershocks with epicenters and depths almost identical to the main shock. They included earthquakes at 05:13UT (Mag 1.6), 07:45UT (Mag 2.5), and 08:07UT (Mag 2.3).

Detailed Information and Graphics for the Main Shock

Our data base of earthquake hypocenters contains information from the 19th century to the present. Included are:

• A regional map showing the location of the earthquake (in red) plus previous earthquakes M > 3.5 since 1990.
• A Intermediate scale map showing the location of the earthquake and prior historical earthquakes with magnitude 3.5 or greater.
• A list of 10 big earthquakes preceding the mainshock that are closest to it and have a magnitude greater than 3.5. These are the most significant historical earthquakes in the vicinity of the mainshock (see intermediate scale map above).
• A map showing the location of the earthquake relative to cities.
• A map showing the location of the earthquake relative to the nearest seismograph stations that recorded it.
• A detailed list of all earthquakes (M > 1.0) that occurred in the vicinity of the special earthquake in the previous two years. For the locations of these events see the next map.
• A map of all earthquakes (M > 1.0) that occurred in the two years prior to the special earthquake. Detailed area shown is just few kilometers around the mainshock.
• A List of the nearest 20 strong motion sites

Earthquake Mechanism

If sufficient P-wave first motion data are available, a fault plane determination for the earthquake will be available in the form of

• A diagram of the fault plane solution(s). In some cases more than one solution may fit the data.
• A table describing the fault plane solution(s) summarizing the characteristics of possible fault planes ("A" and "B") that are consistent with the recorded first-motion data.

Moment Tensor Estimates of the Earthquake size may also be available. To see, check at Oregon State U.

• Moment tensor summary from OSU.
• Moment-Tensor Web site at Oregon State University

Preliminary Strong Motion Information

• Strong-motion information from digital accelerographs.

• Preliminary map of Ground Acceleration.

Related Information: Most Up-to-Date Local, National, and Global Earthquake Summaries

• Catalog and Map of Pacific Northwest earthquakes, Mag>2 from the PNSN
• Catalog and Map of United States earthquakes, from the ANSS
• Catalog and Map of world earthquakes, from the IRIS