Infrasonic observations of the Northridge, California, earthquake Page: 4 of 21
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INFRASONIC OBSERVATIONS OF
THE NORTHRIDGE, CALIFORNIA, EARTHQUAKE
J. Paul Mutschlecner and Rodney W. Whitaker
Los Alamos National Laboratory
Los Alamos, NM 87545
Infrasonic waves from the Northridge, California, earthquake of 17 January 1994
were observed at the St. George, Utah, infrasound array of the Los Alamos National
Laboratory. The distance to the epicenter was 543 kilometers. The signal shows a com-
plex character with many peaks and a long duration. An interpretation is given in terms of
several modes of signal propagation and generation including a seismic-acoustic secondary
source mechanism. A number of signals from aftershocks are also observed.
On 17 January 1994 a large earthquake occurred in Southern California near the
town of Northridge in the San Fernando Valley. We report here on the detection of infra-
sonic waves from the earthquake, which were observed by an array operated by the Los
Alamos National Laboratory, near St. George, Utah. The Northridge earthquake was a
large, very destructive event with seismic magnitude ML (local system) = 6.4. The esti-
mated depth of the disturbance was 16.4 km. The principal event (12:30:55 UTC) was
followed by many aftershocks; over 400 were observed during the following 8-day period,
The Los Alamos National Laboratory has for some years operated several infra-
sound arrays. Some details of these arrays and our operating and analysis procedure have
beer. discussed by Whitaker et al. (1988). Because these arrays nominally operate continu-
ously, unanticipated signals, such as those from earthquakes, may be observed. We have,
in fact, detected the signals from a number of earthquakes covering a large range in seismic
magnitude. Unfortunately, only the St. George array was operating during the Northridge
Figure 1 presents results from our analysis for the time period 10:00 to 13:00 (all
times herein UTC). The plots, from top to bottom, show the azimuth, trace or horizontal
velocity across the array, correlation coefficient, and power resulting from a beam-steering
algorithm analysis. The correlation coefficient is essentially an average of pairwise
correlations among the channels. The power level is given in relative units; typically
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Mutschlecner, J. P. & Whitaker, R. W. Infrasonic observations of the Northridge, California, earthquake, article, September 1, 1994; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1389876/m1/4/: accessed April 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.