Abstract
The Gravity Recovery and Climate Experiment (GRACE) has monitored global gravity changes since 2002. Gravity changes are considered to represent hydrological water mass movements around the surface of the globe, although fault slip of a large earthquake also causes perturbation of gravity. Since surface water movements are expected to affect earthquake occurrences via elastic surface load or pore-fluid pressure increase, correlation between gravity changes and occurrences of small (not large) earthquakes may reflect the effects of surface water movements. In the present study, we focus on earthquakes smaller than magnitude 7.5 and examine the relation between annual gravity changes and earthquake occurrences at worldwide subduction zones. First, we extract amplitudes of annual gravity changes from GRACE data for land. Next, we estimate background seismicity rates in the epidemic-type aftershock sequence model from shallow seismicity data having magnitudes of over 4.5. Then, we perform correlation analysis of the amplitudes of the annual gravity changes and the shallow background seismicity rates, excluding source areas of large earthquakes, and find moderate positive correlation. It implies that annual water movements can activate shallow earthquakes, although the surface load elastostatic stress changes are on the order of or below 1 kPa, as small as a regional case in a previous study. We speculate that periodic stress perturbation is amplified through nonlinear responses of frictional faults.
Highlights
Fault slips of earthquakes can perturb the Earth’s gravity field (Imanishi et al 2004; Han et al 2006), as formulated in elastomechanics (Okubo 1991; Sun and Okubo 1998)
Since gravity changes mainly reflect annually varying shallow water storage as discussed in Hydrology (e.g., Wahr et al 2004), we expect another relation between earthquakes and gravity changes: some shallow earthquakes may be triggered by water movement
Examination of effects of pore pressure and surface load What causes the positive correlation between the annual gravity amplitudes and the background seismicity rates? As described in the introduction section, the relation between gravity change and occurrence of small earthquakes may reflect the earthquake triggering due to water movement around the surface
Summary
Fault slips of earthquakes can perturb the Earth’s gravity field (Imanishi et al 2004; Han et al 2006), as formulated in elastomechanics (Okubo 1991; Sun and Okubo 1998). Larger earthquakes cause greater perturbation of gravity. Large earthquakes remain the gravity signals, because of limitation in accuracy of gravity measurement. The Gravity Recovery and Climate Experiment (GRACE) satellites have allowed us to monitor large-scale mass movements. The GRACE satellites investigate 100- to 1000-km-scale gravity changes with an interval of approximately 1 month and can detect great perturbations of gravity due to large (> Mw 8.2) earthquakes (Han et al 2006; Heki and Matsuo 2010; Han et al 2013; Tanaka et al 2015b). The relation between gravity changes and small earthquakes that
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