Crustal response to heavy rains in Southwest Japan 2017-2020
Kosuke Heki, Syachrul Arief
Abstract
Weather fronts stationary above Southwest Japan often bring disastrous heavy rains in early summer. Here we study four such episodes in each of four summers between 2017 and 2020, and investigate transient lithospheric subsidence caused by rainwater loads using the daily coordinates of a dense network of continuous GNSS stations. After applying a network filter to remove common mode errors, we isolated subsidence signals of 1-2 centimeters in flooded regions. Such subsidence recovered mostly in a day as rainwater drained rapidly to nearby ocean promoted by large topographic slopes. Spatiotemporal correlation between subsidence and precipitation was weak due possibly to rapid post-precipitation migration of rainwater. However, a strong correlation was found between subsidence and rain spatially integrated over the entire Southwest Japan, i.e., bulk subsidence of ∼0.1 km3 (equivalent to the uniform subsidence ∼0.6 mm) occurred for every 1 Gt rainwater per day. This linearity breaks down for rains exceeding ∼10 Gt/day as rainwater possibly exceeds the water-holding capacity of forest catchments.