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Weekly Earthquake Reports
Earthquake Alert Bulletin No: 97-194

Preliminary data indicates a significant earthquake has occurred:

Regional Location:

Preliminary Magnitude:

Moment Magnitude (Mw):

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


4.0 M1


25.1 km
An earthquake registering a preliminary magnitude of Ml 4.0 occurred at 03:18 UTC, July 23 (8:18 p.m. PDT, July 22), in the Klamath Mountains of Northern California. The quake was centered about 14 miles east of Willow Creek in the New River drainage, near Denny. The depth was determined to be 25 km, suggesting the temblor occurred along or near the Cascadia subduction zone under the Klamath Mountains. This is the largest earthquake to have occurred here since a ML 4.4 on March 9, 1993. Since detailed monitoring began in Northern California during the 1970's, the strongest of eleven M4+ quakes in this area occurred January 18, 1990 and measured ML 4.9. Prior to the 1970's, records for this region are rather scant, but the University of California, Berkeley, detected a ML 5.0 event on May 5, 1945. Although the Cascadia subduction zone is a major structural feature that shows some microseismicity, moderate to large activity during historic times has been rare.
Subtract 7 hours to obtain PDT or 4 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)

Sources Include:
U.S. Geological Survey, National Earthquake Information Center, Golden, CO (NEIC)
Harvard Geophysical Observatory, Harvard University, Cambridge, MA (HRV)
International Data Center, Center for Monitoring Research, Arlington, VA (IDC)
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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