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Precise measurement of seismic traveltimes ‐ Investigation of variation from tidal stress in shallow crust
Author(s) -
Liu HsiPing,
Westerlund Robert E.,
Fletcher Jon B.
Publication year - 1983
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/gl010i005p00377
Subject(s) - geology , seismology , geophone , amplitude , geodesy , induced seismicity , physics , quantum mechanics
We have conducted 8 precise seismic surveys near Hollister, CA, over a period of 1 y in an attempt to detect the traveltime variation caused by the solid‐earth tidal stress. The surveys were conducted along a 600 m baseline located in quartz monzonite hills 2 km west of the San Andreas fault. A 656 cm³ air gun fired in a mud‐filled pit 2 m deep provided a repeatable seismic source. The signals from two 2.3 Hz vertical‐component geophones 600 m apart were digitized at a nominal rate of 600 samples/s by two cassette recorders modified for precise synchronization of data sampling against a master clock. Each survey consists of ∼ 100 traveltime measurements over the 12 h period between 6 p.m. and 6 a.m. local time; the time frame of each experiment was limited by daytime cultural noises. Analysis of traveltime variation is done either by timing of amplitude extrema or by cross‐correlation of a waveform constructed from the digital data by a cubic‐spline interpolation. Fractional error of the repeatability of traveltime measurement is typically ±3.3 × 10 −4 for the first high‐frequency, large amplitude arrival following the direct body waves. The first survey, conducted at a spring tide in August, 1981, showed a variation of Δt/t ∼ 2 × 10 −3 and correlated in time with the extensional tidal strain component along the baseline direction. The next two surveys, conducted at two neap tides, showed variation of Δt/t ∼ 6 × 10 −4 and also correlated with the same tidal strain component. However, the other 5 surveys conducted after the onset of 1981 rainy season and into the 1982 dry season, 4 at spring tides and 1 between a spring and a neap tide, showed traveltime constant to within 1 standard deviation. These results corroborate only partially the previously reported tidal stress variation of traveltimes in the shallow crust.