Possible correlation between annual gravity change and shallow background seismicity rate at subduction zone by surface load

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.

Effects of Variations in Earth’s Gravitational Force Fields on Climate Change

“…Figure 3 shows the monthly changes in gravity as equivalent water thickness and the number of earthquakes per month (in grey lines) at a grid of longitude 275° and latitude 5°. Gravity change was equated to water thickness because changes in gravity are congruous to the hydrological water mass movements around Earth"s surface (Mitsui & Yamada, 2017); as changes in hydrology are also affected by climate warming (Yao et al, 2022). Besides, the mass transport, energy exchange and gas emissions generated by earthquakes are part of the agents of climate change.…”

Analysis of GRACE Satellite Data in Terms of Geomonitoring of Strong Underwater Earthquakes

“…In [3] it is hypothesized that variations in the gravitational field reflect the hydrological movements of water masses around the solid Earth which are caused by an increase in the elastic surface load on faults. The EWH (Equivalent Water Height) parameter or the equivalent water height above the geoid contour (in cm) [4] is used as an amplitude of the gravity change in the GRACE and GRACE-FO data.…”

Possible correlation between the vertical component of lithospheric magnetic field and continental seismicity

“…Although timing is hard to predict, location does follow a few rules: Most middle and large earthquakes occur along boundaries of plates (Beroza and Kanamori 2007), or along boundaries of active tectonic blocks (Zhang et al 2003(Zhang et al , 2005. Seismicity also correlates with other geophysical parameters, such as lithospheric thickness (Chen et al 2013), gravity field (Zhang et al 2010;Mitsui and Yamada 2017), and aeromagnetic anomalies (Zhang et al 2010). A common feature is that large earthquakes tend to occur along the remarkable gradient belts of these parameters.…”