SCEC Award Number 11069 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Widespread Anthropogenic Seismic Tremor: Sources and Effects
Investigator(s)
Name Organization
Duncan Agnew University of California, San Diego
Other Participants Barbour, Andy (GSR)
SCEC Priorities A4, B3, B4 SCEC Groups Seismology
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
 As part of a study of the sensitivity of borehole strainmeters, we have analyzed data from the accompanying borehole seismometers in the Anza area of southern California. Spectra of the data from these borehole seismometers show a clear peak, which we originally thought might reflect tremor signals on the San Jacinto fault. Even though this is a rural area, this peak is actually of
anthropogenic origin. It shows a clear daily and weekly fluctuation, with much lower levels on weekends. Overall traffic levels in Southern California do not have this pattern, but the level of long-haul truck traffic does. We
tentatively attribute the noise peak to truck traffic in the adjacent Coachella Valley. The frequency peak is typical of the normal modes of translation and rotation of the vehicle frame.
Intellectual Merit Because this noise source has frequencies similar to tremor, it is important to understand its source so that it can be identified and separated from actual fault-related tremor signals. Its temporal variations also limit the sensitivity of the seismic network and thus need to be understood for proper evaluation of seismicity statistics.
Broader Impacts These methods can be extended beyond the Anza area to examine
tremor and other "noise-like" fluctuations seen in seismic data.
Exemplary Figure Figure 3

Fluctuations in spectral level of seismic energy recorded on a PBO borehole seismometer at Pinon Flat Observatory. The time series of individual values (top panel) is ifficult to interpret because of the frequent large values
created by transients (earthquakes). The bottom panel shows the same data smoothed with a 2-hour non-overlapping running median, which is insensitive to the timeseries' highly skewed distribution of values. This panel shows a very clear day-night cycle, which might be thought to be meteorological if it did not shrink every weekend, with the values at midday on Sunday being comparable to those at midnight during the week.