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Characteristics of ground motion generated by interaction of wind gusts with trees, structures and other obstacles above the surface

Christopher W. Johnson, Haoran Meng, Frank L. Vernon, Nori Nakata, & Yehuda Ben-Zion

Published August 13, 2018, SCEC Contribution #8443, 2018 SCEC Annual Meeting Poster #305

Detection of small seismic events requires the ability to separate them from other sources of weak ground motion. Toward this goal we deployed 42 Fairfield ZLand geophones from 9 February to 17 March 2018 at the SGB site on the San Jacinto fault near Anza, CA. The instruments were arranged in a 350 x 300 m area and configured to target interactions of wind with obstacles above the surface producing ground motion. Located on the property is the PBO borehole seismometer B946 installed at a depth of 147 m and an anemometer was installed to record the wind velocity. Temperature data was obtained from a weather station located 3 km to the north. The analysis is performed using 1 minute intervals of seismic data by estimating the spectra to produce spectrograms and group the waveforms by the reported wind velocity using a 1 m/s interval from 0 to 4 m/s. We select two days as a low and high wind day and compare the spectra during different wind conditions using a transect of geophones extending from a structure on the property. The results show a decrease in the spectral amplitude correlating with the wind velocity, where the maximum amplitude is observed during the strongest winds. The peak amplitudes decay three orders of magnitude over the 125 m spacing to the farthest geophone. The spectra indicate a peak in amplitude between 35-45 Hz, regardless of wind velocity, that shifts to higher frequencies as the distance increases. Daily spectrograms also indicate an increase in amplitude in the 35-45 Hz band that is present even in low wind conditions. During periods of elevated temperature, the peak amplitude shifts to higher frequencies with overtones observed clearly on days with sustained wind velocity >2m/s. The spectrogram for a geophone covered from direct sunlight does not display the same spectral shift. The geophone results indicate both the wind and temperature modify the instrument noise, which is not constant in time, and the noise amplitude is dependent on the distance to a local source as well as site specific conditions. Additionally, the borehole station B946 shows increased energy in the 1-5 Hz band that corresponds to increased wind speed and is not anthropogenic since one observation occurs at 2 am local time. The results indicate that wind produced ground shaking is observable to depths of 147 m and show an increase in energy in the bandwidth of interest for earthquake monitoring and detection.

Johnson, C. W., Meng, H., Vernon, F. L., Nakata, N., & Ben-Zion, Y. (2018, 08). Characteristics of ground motion generated by interaction of wind gusts with trees, structures and other obstacles above the surface. Poster Presentation at 2018 SCEC Annual Meeting.

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