Bangladesh lies at the meeting point of the Indian and Eurasian plates and experiences heavy seasonal rainfall. To study how rainfall affects earthquakes and the Earth’s crust, we used data from a network of seismic sensors that records both ground vibrations from earthquakes and natural background noise.

In earlier work, we identified seasonal changes in background noise and local site effects. Building on that, we used a method called ambient noise interferometry to track how seismic wave-speeds (seismic velocity) change over time. We found that seismic velocity increases during the monsoon season, meaning the upper crust becomes stiffer. This is different from other places where rainwater seeps deep underground, softening the crust. In Bangladesh, thick clay-rich sediments block infiltration, so the weight of rainwater on the surface compresses the crust instead.

Our earthquake data also show that during the monsoon, fewer earthquakes happen at middle depths in the crust, but they tend to be slightly stronger. This suggests that the added surface weight during the monsoon suppresses fault movement at those depths. These results match GPS studies showing that the ground shifts under rainfall weight, supporting the idea that surface loading, not pore pressure, drives seasonal changes in earthquakes here.