Diurnal and Semidiurnal P‐ and S‐Wave Velocity Changes Measured Using an Airgun Source

Abstract

Monitoring the subtle seismic velocity changes provides a promising tool to study the Earth's dynamic processes. However, the precise monitoring of seismic velocity with high temporal resolution is still challenging especially at large distances. Here, we continuously monitor P-wave and S-wave velocities with a precision of up to 10 −5 using an airgun source. During a 1-week experiment conducted in the Yunnan Province, Southwestern China, unambiguous diurnal and semidiurnal velocity changes with amplitudes of~10 −4 -10 −3 were observed for both P-and S-waves at distances up to 21 km. The amplitudes of P-wave velocity (Vp) change are 1.2 to 3.8 times the amplitudes of S-wave velocity (Vs) changes. The velocity variation decreases with the increase in distance and is larger in the basin region than that in the mountainous region. Seismic velocity change shows less correlation with volumetric tide strain but strong correlation with changes of air temperature and air pressure. We propose that the thermal strains from air temperature changes are the primary cause of the observed diurnal and semidiurnal seismic velocity changes. The stronger Vp than Vs change is explained by the higher Vp sensitivity to the saturation rate. The velocity changes in mountainous area are ascribed mainly to the crack density changes, while both crack density change and saturation change are required to account for the velocity changes in the basin. Key Points:• Diurnal and semidiurnal P-and Swave velocity changes of 10 −3 -10 −4 are observed with a precision of 10 −5 using an airgun source • The change in Vp is larger than that in Vs, and velocity changes in basin are stronger than in mountainous area • The thermal strain-induced crack density or/and saturation ratio changes are likely responsible for the observed velocity changes Supporting Information:• Supporting Information S1 . (2020). Diurnal and semidiurnal P-and S-wave velocity changes measured using an airgun source.

Fine Structure of the Chenghai Fault Zone, Yunnan, China, Constrained From Teleseismic Travel Time and Ambient Noise Tomography

“…Journal of Geophysical Research: Solid Earth semidiurnal P and S wave velocity changes with amplitudes of 10 −4 to 10 −3 were derived from a 1-week data set of the air gun source in Binchuan (Wang et al, 2020).…”

Seismic Velocity Changes Caused by Water Table Fluctuation in the New Madrid Seismic Zone and Mississippi Embayment

“…Estimating seismic velocity change has been done by measuring the travel time or phase difference from active sources including explosions (Li et al., 1998, 2003, 2006; Nishimura et al., 2000), airguns (Wang et al., 2020; Wegler et al., 2006), and repeating earthquakes (Peng & Ben‐Zion, 2006; Poupinet et al., 1984; Rubinstein & Beroza, 2004a, 2004b; Rubinstein et al., 2007; Schaff & Beroza, 2004), and by computing the dephasing of the ambient noise cross correlations (CCs) (Brenguier et al., 2008; Sens‐Schönfelder & Wegler, 2006) or autocorrelations (ACs) (Minato et al., 2012; Ohmi et al., 2008). We prefer ambient noise analysis because it not only circumvents the uncertainty of repeating earthquakes and high expense of the active sources but also allows for long‐term velocity monitoring over time periods of months to years.…”

Impacts of Reservoir Water Level Fluctuation on Measuring Seasonal Seismic Travel Time Changes in the Binchuan Basin, Yunnan, China

“…Seismic velocity changes are usually used as a proxy to quantitatively understand crustal deformation. Temporal changes in seismic velocity can reflect fault zone co-seismic damage and post-seismic healing [6][7][8], volcanic eruption [6,9], groundwater level changes [10][11][12], temperature and atmospheric pressure variations [13][14][15][16][17][18], and solid earth and oceanic tidal deformation [17,[19][20][21]. Thus, high temporal resolution monitoring and high precision measurements of seismic velocity changes are necessary.…”

“…Seismic velocity changes have been estimated through measuring travel time or phase difference from active sources including explosion [21][22][23][24][25], electronic hammer [26], airguns [18,27,28], repeating earthquakes [29][30][31][32][33][34], and dephasing of ambient noise crosscorrelations (CCs) [6,35]. Among these methods, temporal changes in ambient noise amplitude and heterogeneous distribution of noise sources [12,17] may introduce a bias to the measurements of seismic velocity changes.…”