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28 September 2004

Finally! The magnitude 6.0 Parkfield earthquake occurred!

18 September 2004

A sequence of small to moderate earthquakes occurred this morning in the California-Nevada border region about 23 miles SE of Bodie, CA, and 24 miles SSW of Qualeys Camp, NV. The sequence started with small event at 12:02 AM PDT, Md=2.3. This was followed by a mild event of ML=3.2 which occurred at 12:07 AM, and a moderate earthquake of ML=4.1 less than a minute later at 12:08 AM. Several small events have followed, with an earthquake of ML=3.0 occurring at 04:28 AM, and an earthquake of ML=3.6 which occurred at 05:39 AM. As of this writing (11:00AM) the small earthquakes are continuing. The largest event at 12:08 AM had a maximum intrumental intensity of IV, with intensity II recorded over an area stretching almost to Bishop. See a 6-minute, 3-component, velocity record of this event (and four others). These were recorded at CMB (Columbia College) at about 94 miles west of the sequence.

(11:00 PM Update, 2004/09/18) There have now been about 205 earthquakes in this sequence, including two ostensibly larger shocks. The first of these occurred at 4:02 PM and had a moment magnitude Mw=5.5. This had a maximum instrumental intensity of VI . The U.S.G.S. National Earthquake Information Center notes that this event was felt in California and western Nevada, including the cities of Oakland, San Jose and Reno.

The second slightly smaller earthquake, Mw=5.4, occurred at 4:43 PM, and also had a maximum instrumental intensity of VI .

Ground displacement seismograms of both of these strong earthquakes, one pair from KCC (58 miles away, azimuth 217 degrees) and the other pair from CMB (94 miles away, azimuth 272 degrees). It is interesting to note how different these appear, not only between the two stations, but also between the two earthquakes. The larger amplitudes on the CMB records indicate that its azimuth is much more important than its distance from the event in defining ground motion at this location.

Of the total number of earthquakes in the sequence to this point, there have been 18 events of magnitude greater than equal to 3.0, including two of magnitude 4.1 and the magnitude 5.5 and 5.4 earthquakes mentioned above.

(05:00 PM Update, 2004/09/20) The Northern California Network has now located a total of about 413 earthquakes in this sequence, now officially called the Adobe Hills Swarm. There have been a total of 24 events of magnitude greater than or equal to magnitude 3.0. The largest earthquake occurring since the last update was a magnitude 4.8 shock on Monday morning, September 20, at 09:51 AM PDT.

Further information on the swarm can be found at the CISN event page .