Apparent Weekly and Daily Earthquake Periodicities in the Western United States

Abstract

Analysis of apparent seismicity rate (ASR) using magnitude ≥ 1 earthquakes located in the western United States confirmed the existence of prominent spectral peaks with periods of 1 and 7 days. The number of recorded earthquakes on Sundays for the duration of 1963-2008 is about 5% higher than that on weekdays, and, more significantly, there is a 9% increase of ASR in the early morning compared with that in the middle of the days. Significant similarities in the spatial distributions of the weekly and daily variations suggest that the two types of variations have the same sources and both originate from periodic variations in cultural noise that lead to periodic variations in the detectability of the seismic networks. Comparisons with freeway traffic flow data suggest that traffic flow on the freeways is not the only significant factor in the observed periodicities. Instead, ambient noise from all the ground traffic, operating machineries, and building shaking is probably the major cause of the observed apparent periodicities. The observed temporal variations in ambient noise as reflected by the ASR can be used as objective guidelines for choosing the best time/day for noise-sensitive scientific experiments.

Anatomy of the high‐frequency ambient seismic wave field at the TCDP borehole

“…Source processes that excite noise at these frequencies include anthropogenic activities [ Ringdal and Bungum , 1977; Gurrola et al , 1990; Young et al , 1996; Atef et al , 2009; Lewis and Gerstoft , 2012], wind acting on topographic irregularities [ Withers et al , 1996; Hillers and Ben ‐ Zion , 2011], precipitation and runoff [ Burtin et al , 2008], and thermoelastic straining [ Berger , 1975; Ben ‐ Zion and Leary , 1986; Hillers and Ben ‐ Zion , 2011]. Scattering and attenuation properties of the crustal material control the randomization of energy propagation directions and relative mode excitation [ Margerin et al , 1998, 2001; Larose et al , 2008].…”

Seasonal variations of observed noise amplitudes at 2-18 Hz in southern California

“…This is also supported by the existence of clear signals in both horizontal and vertical components (Figs 3 and S1). Changes in earthquake activity may also produce seasonal variations of noise, but apparent variations of seismicity have been associated with changes in network sensitivity (Atef et al 2009), which may result in part from seasonal noise as demonstrated by our analysis.…”

“…Energies at frequencies above 1 Hz are usually attributed to cultural noise, which propagates mainly as rapidly‐attenuating high‐frequency surface waves, manifested by significantly reduced noise amplitudes in boreholes (Gurrola et al 1990), tunnels, and mines. Diurnal and weekly variations of cultural noise (Ringdal & Bungum 1977) affect earthquake detection capabilities (Atef et al 2009). Wind acting on topographic irregularities or trees and posts can also transmit energy into the ground at high frequencies and the associated ground tilting may produce longer period noise.…”

Daily Variation in Earthquake Detection Capability: A Quantitative Evaluation

“…These plots also show a short recovery of the detection capability around noon. [28] has found a similar pattern of the detection capability in California through the hourly distribution of the number of reported earthquakes in the Advanced National Seismic Systems (ANSS) catalogue. Note that, however, both magnitude-time plots and hourly distributions provides us with only a qualitative pattern of the detection capability.…”

“…In this chapter we considered only the daily variation of detection capability, but human activities also have a weekly periodicity [28]. Additionally, we often observe seasonal (annual) variations in seismic noise level [39,40], which are mainly caused by meteorological factors.…”