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The Land-Atmosphere Interactions from Barometers and Seismometers

Anne Valovcin, Jiong Wang, & Toshiro Tanimoto

Published August 14, 2016, SCEC Contribution #6796, 2016 SCEC Annual Meeting Poster #224

Our understanding of seismic noise has improved dramatically in the last 50 years. Also the cross-correlation approach now allows us to make the Green’s functions and apply it to practical applications. However, depending on the frequency band, the cross-correlation approach does not necessarily work. Clearly, this situation requires a better understanding of the nature of seismic noise.

In this study, we focus on the direct interaction between the atmosphere and the solid earth, or the land-atmosphere interaction. The well-known microseisms (0.05-0.4 Hz) are generated by ocean waves, but the land-atmosphere interactions also generate an important part of low-frequency seismic noise, such as tilt. There are now many stations that have both barometers and seismometers from which we can study the nature of the land-atmosphere interaction. We summarize our findings from the local Southern California stations and the Earthscope TA stations. For the latter stations, we examine two aspects, one on the data when tropical cyclones hit the Earthscope TA and the other is on the special subset of TA stations that have had barometers, wind measurements and other environmental sensors (temperature, humidity etc.).

We specifically focus on three aspects in our findings: (i) Threshold pressure, (ii) tilt effects on horizontal components and (iii) seasonal bifurcations that are seen at some stations.

The threshold pressure is easily recognized in vertical-component data when they are plotted against pressure (from co-located instruments). In Tropical Cyclone data, it is about SP=10(Pa2/Hz) where SP is the power spectral density (PSD) of surface pressure. Below this pressure value, vertical amplitudes are basically constant and above this value seismic amplitudes increase with pressure. This critical, threshold pressure can be recognized at many stations.

In horizontal-component data, tilt dominates the signals, showing increasing horizontal amplitudes with pressure for the entire range of pressures. The effects of threshold pressure show up in the changes of gradient in the plot of horizontal amplitudes vs. pressure.

There are some stations for which horizontal amplitudes vs. pressure show different behaviors from season to season; in one-year long data set, we often see bifurcated patterns in the amplitude vs. pressure plot; typically, in winter, horizontal amplitudes show high constant values and do not change with pressure. But in summer, horizontal amplitudes change with pressure. This fact means that in summer atmospheric pressure controls noise level, but in winter there is some noise generated by another mechanism and because they are at higher amplitudes, they dominate the horizontal-component data.

Key Words
land atmosphere interactions,seismometer barometer,pressure

Valovcin, A., Wang, J., & Tanimoto, T. (2016, 08). The Land-Atmosphere Interactions from Barometers and Seismometers. Poster Presentation at 2016 SCEC Annual Meeting.

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