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.


Geosciences and Geological and Petroleum Engineering

Keywords and Phrases

Ambient noise; Cultural noise; Detectability; Earthquake periodicity; Freeway traffic; Operating machineries; Periodic variation; Seismic networks; Significant factors; Spatial distribution; Spectral peak; Temporal variation; Traffic flow; Western United States; Machinery; Remelting; Traffic surveys; data processing; earthquake magnitude

International Standard Serial Number (ISSN)


Document Type

Article - Journal

Document Version

Final Version

File Type





© 2009 Seismological Society of America (SSA), All rights reserved.

Publication Date

01 Aug 2009

Included in

Geology Commons