Spatial and temporal influence of sea level on inland stress based on seismic velocity monitoring

Abstract

Earth’s crust responds to perturbations from various environmental factors. To evaluate this response, seismic velocity changes offer an indirect diagnostic, especially where velocity can be monitored on an ongoing basis from ambient seismic noise. Investigating the connection between the seismic velocity changes and external perturbations could be useful for characterizing dynamic activities in the crust. The seismic velocity is known to be sensitive to variations in meteorological signals such as temperature, snow, and precipitation as well as changes in sea level. Among these perturbations, the impact of variations in sea level on velocity changes inferred from seismic interferometry of ambient noise is not well known. This study investigates the influence of the ocean in a 3-year record of ambient noise seismic velocity monitoring in the Chugoku and Shikoku regions of southwest Japan. First, we applied a bandpass filter to determine the optimal period band for discriminating among different influences on seismic velocity. Then, we applied a regression analysis between the proximity of seismic station pairs to the coast and the ocean influence, as indicated by the correlation of sea level to seismic velocity changes between pairs of stations. Our study suggests that for periods between 0.0036 to 0.0155 cycle/day (64–274 days), the ocean’s influence on seismic velocity decreases with increasing distance of station pairs from the coast. The increasing sea level deforms the ocean floor, affecting the stress in the adjacent coast. The stress change induced by the ocean loading may extend at least dozens of kilometers from the coast. The correlation between sea level and inland seismic velocity changes is negative or positive. Although it is difficult to clearly interpret the correlation based on a simple model, they could depend on the in situ local stress, orientation of dominant crack, and hydraulic conductivity. Our study shows that seismic monitoring may be useful for evaluating the perturbation in the crust associated with an external load. Graphical abstract

Space‐Time Monitoring of Seafloor Velocity Changes Using Seismic Ambient Noise

“…Other studies (Andajani et al, 2022;Kramer et al, 2023) suggest that in saturated conditions (e.g., at the seafloor), when cracks close from increasing stress, the fluid within is pushed to the pores, which leads to an overall increase of the pore pressure and a decrease of seismic velocity. In addition, a phase delay of seismic velocity changes relative to the applied strain also has been observed by previous studies (Sens-Schönfelder & Eulenfeld, 2019;Takano & Nishida, 2023).…”

“…As a result of insitu stress changes induced by loading, cracks may close when oriented horizontally and open when oriented vertically. Therefore local heterogeneities (Andajani et al, 2022) in the seafloor could be the reason for the observed polarity oscillation in the space (Figure 8). We also acknowledge that some of the anomalies could be seismic inversion artifacts.…”

“…We also note the polarity oscillations in the dv/v images (Figure 8), for example, negative anomalies in the seafloor images corresponding to positive velocity changes. Andajani et al (2022) found that the correlation between sea level height and nearby inland seismic velocity changes can be negative or positive, depending on the in-situ local stress, orientation of dominant crack, and hydraulic conductivity. When the ocean loading is applied vertically downward due to the increase of the sea level, the structure will expand horizontally.…”

Space-time monitoring of seafloor velocity changes using seismic ambient noise

“…at the seafloor), when cracks close from increasing stress, the fluid within is pushed to the pores, which leads to an overall increase of the pore pressure and a decrease of seismic velocity. Furthermore, Andajani et al (2022) found that the correlation between sea level height and nearby inland seismic velocity changes can be negative or positive, depending on the in-situ local stress, orientation of dominant crack, and hydraulic conductivity. The local heterogeneities (Andajani et al, 2022) in the seafloor could also be the reason for the observed negative anomalies in the time-lapse seafloor images (Figure 7) associated with positive velocity changes and the positive anomalies in the images corresponding to negative velocity changes, although we acknowledge that some of the anomalies could also beyond the resolution of seismic inversion.…”

Spatiotemporal Variations and Postseismic Relaxation Process Around Mt. Fuji, Japan, During and After the 2011 Tohoku‐Oki Earthquake

“…We can use the information from the cross-correlation functions (CCFs) of coda waves (i.e., the scattered waves in the subsurface due to crustal heterogeneities) to obtain stable estimates of the velocity changes (e.g., Meier et al, 2010;Sens-Schönfelder & Wegler, 2006). The ambient noise-derived monitoring data can provide continuous information that is relevant to volcanic activity because the observed velocity changes are related to the crustal stress state, pore pressure changes, crack generation, and/or the presence of volcanic and hydrothermal fluids (Andajani et al, 2020(Andajani et al, , 2022Brenguier et al, 2014;Wang et al, 2017). However, most studies of ambient noise monitoring have used a longer period of data (i.e., longer data window) for the velocity monitoring, such that the monitoring results generally possess a lower temporal resolution (e.g., several to 10 days; Hutapea et al, 2020).…”