Exploring the Gravitational Impact of Spring Tides on Earthquake Occurrence in Central Asia

Abstract In this study, we analyzed variations in seismic activity in relation to the syzygy of the Sun, Moon, and Earth, corresponding to the semilunar or spring tide (ST) cycle. We focused on two regions within the Central Asian Orogenic Belt: the North Tien Shan region (NTSR) and the Xinjiang Uygur tectonic province (XUTP), using data from two regional earthquake catalogs. For the NTSR, we used data from the Seismological Experimental and Methodical Expedition in Almaty, Kazakhstan, covering 1 January 1970 to 31 July 2022. For the XUTP, we utilized data from the Data Management Centre of China National Seismic Network at the Institute of Geophysics China Earthquake Administration, spanning 1 January 1987 to 30 June 2019. We applied the nonparametric Kuiper’s test to evaluate cyclic variations in seismic activity, focusing on different magnitude thresholds in the regional catalog. For both regions, the original catalog as well as the catalogs of mainshocks and aftershocks were analyzed. The nearest-neighbor declustering method was employed to decompose the seismic catalogs. Kuiper’s test statistics revealed varying trends in the occurrence times for mainshocks and aftershocks during ST cycles. The test showed “the good-to-fit” to a uniform distribution for the timing of mainshocks within the ST cycle. Kuiper’s test revealed notably consistent trends in aftershock occurrence times during ST cycles with a significance level α of 0.001 or less in both regions analyzed. These results suggest a potential correlation between aftershock activity and the gravitational effects of the ST cycle. The most substantial decrease in aftershocks occurs during the micromoon ST cycle, when the distance between the Moon and the Earth exceeds 400,000 km.