Multi-scale study of ground motion coherence in Piñon Flats Observatory

Lei Qin, Christopher W. Johnson, Frank L. Vernon, & Yehuda Ben-Zion

Submitted August 11, 2018, SCEC Contribution #8394, 2018 SCEC Annual Meeting Poster #299

Characterizing ground motions between closely spaced broadband stations can reveal new insight on variations in the crustal response to earthquake, seasonal and ambient sources of seismic energy. We study the coherence using three years (2015-2017) of continuous waveforms from the Piñon Flats Observatory Array (PY). The array includes 13 broadband sensors arranged on three circles with station separated by about 65m, 330m, and 780m. The examined data include ambient noise and earthquake signals from local and teleseismic sources recorded by PY sensors. The coherence values are sensitive to signal source properties, local structures and sensor performance. The ambient noise coherence remains high in the 0.1-1.0 Hz band if the interstation distance is small (<65 m), and decreases quickly outside the dominant frequency band with increasing station distances. The horizontal coherences are reduced to ~0.2 at frequencies <0.1 Hz likely due to interstation ground tilt. The noise coherence in 1-10 Hz band is affected by incoherent high frequency signals including anthropogenic activities and local earthquakes. The local earthquake signals exhibit high coherence up to 50 Hz, while the teleseismic waves increase the coherence in the low frequency band (<0.1 Hz). In addition, high-coherence (>0.95) frequency bands in the two horizontal components exhibit seasonal variations likely related to strains associated with ocean loading and thermoelastic deformation. The coherence signals at PY contain anomalies in ~2-4 Hz band starting 30 days before the 2016 Mw5.2 Borrego Springs earthquake; however, careful analysis indicates that they result from anthropogenic activities and not precursory earthquake signals.

Key Words
Coherency; multitaper spectra

Citation
Qin, L., Johnson, C. W., Vernon, F. L., & Ben-Zion, Y. (2018, 08). Multi-scale study of ground motion coherence in Piñon Flats Observatory. Poster Presentation at 2018 SCEC Annual Meeting.


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