A sequence of earthquakes 8 miles NNW of Burney, CA culminated on December 20, 2000 with a light magnitude 4.6 earthquake. The time of the largest earthquake (M 4.6) of the sequence was 03:39 PM PST Wednesday, Dec 20, 2000 (or 23:39 UTC Dec 20, 2000).

This sequence began on December 15 21:07 PST with a M3.1 earthquake. A few small aftershocks followed over the next 4 days, but on December 20 at 14:55 PDT activity resumed with a M3.8 earthquake. As of December 21, there were a total of 30 earthquakes recorded, the largest being the M4.6 at 03:39 PM PST Wednesday and the second largest M4.3 at 03:22 PM PST, 17 minutes earlier. There were 8 earthquakes above M3.0. This sequence occurred in the same location as a M3.7 earthquake on July 22, 1987 and two quakes of M3.5 and 3.6 on March 27, 1998.

Figure 1: The location of the December 2000 earthquakes NNW of Burney has been seismically active in the past. The Burney earthquakes from December 20 are shown with yellow dots, and the blue dots are catalog earthquakes from 1980 until December 19, 2000. The cluster of (blue) earthquakes in the upper left corner is the Stout's Meadow swarm of 1984. This swarm is about halfway between the Burney swarm and Mt. Shasta. There are numerous mapped faults in the Burney region, but it is unknown at this time whether the December 20 earthquakes can be associated with one of these faults.

Figure 2: The seismic station KHMB (at Horse Mountain, near the California coast south of Cresent City) prominently recorded the four largest earthquakes of the sequence. Each horizontal segment of the seismogram (which covers the entire day) is 15 minutes long. The segments are in four different colors according to which part of the hour they are in. The four largest earthquakes are in four different colors only because each happened in a different 15 minute segment. The seismic station (location map of the station) also recorded a heavy background of microseism noise, which is the 7 second period signal recorded throughout the day. This microseism noise is typical of stations located near the coast and is caused by ocean swells and surf from waves of 7 second period at sea.

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Figure 3: The Burney earthquakes are within the diffuse band of seismicity extending from south of Lake Tahoe up to the Mt. Shasta area. This band appears to be an extension of the eastern Sierra seismic zone, but is not caused by a single fault. This seismic zone includes some major volcanic centers, such as Long Valley, Lassen and Shasta. Most earthquakes in this zone are not related to these volcanoes, but probably just a slightly weaker zone in the earth's crust. The cluster of earthquakes halfway between Burney and Mt. Shasta is the Stout's Meadow swarm of 1984, and is also visible in figure 1.

The earthquakes were felt throughout the Hat Creek - Burney region. The felt intensities are summarized in the "Did you feel it?" maps for the two largest earthquakes (M4.6 and M4.3) in the sequence.

Instrumental intensity maps based on the expected shaking from an earthquake of the calculated size and location and from shaking recorded by seismometers were also produced for the two largest earthquakes (M4.6 and M4.3) in the sequence.

The Burney earthquake sequence is best described as an earthquake swarm. An earthquake swarm is a sequence with several earthquakes of comparable magnitude which builds in intensity, rather than a mainshock-aftershock sequence with a single large earthquake followed by smaller aftershocks. Mainshock-aftershock sequences are typical of the large faults in California such as the San Andreas or Hayward. Earthquake swarms as well as mainshocks can occur in many parts of the state.

Volcanic eruptions or buried magma movement are generally preceded by earthquake swarms, but a swarm DOES NOT mean magma movement is occurring. Swarms associated with eruptions (such as Hawaii or Mt St. Helens) or buried magma movement (such as Long Valley) are very intense with hundreds of earthquakes lasting for days, months or years. The Burney earthquakes DID NOT occur in sufficient numbers to suggest an eruption or even magma movement is imminent. The earthquakes are not directly under a known volcano, but there numerous volcanic centers to the southeast in the Hat Creek and Lassen volcanic fields. Generally, we look for sustained seismicity and other observations as a harbinger of volcanic activity. As to the earthquakes cause, it seems premature to point to a volcanic processes. The swarm nature of the sequence and the geology of the region suggests the earthquakes may be in an area of weak crust.