The 2023.4.24 Hambantota-offshore earthquake and microearthquakes in Sri Lanka and the landslide risk evaluation in a nearby slope by post-rainfall earthquakes

The 2016 Aranayake landslide in Sri Lanka occurred in heavy rain, killing 127 people and destroying 75 houses with rapid and long travelling motion. It had an impact on Sri Lanka and the ICL community. The ICL and the NBRO (National Building Research Organisation) jointly applied for a SATREPS (Science and Technology Research Partnerships for Sustainable Development) Project funded by the JICA (Japan International Cooperation Agency) and JST (Japan Science Technology Agency); the project “Development of Early Warning Technology of Rain-Induced Rapid and Long-Travelling Landslides in Sri Lanka” (Konagai et al. 2023 ). The five-year project (from 2019 to 2025) was approved and is ongoing. The 2016 Aranayake Landslide and 2017 Atwelthota Landslide sites were selected as pilot study sites. In 2021 and 2022, earthquake events and microtremors were increasingly reported in Sri Lanka. In recent years, NBRO has been concerned about the increasing trend of earthquakes and microtremors. The 2006 Leyte landslide in the Philippines that killed over 1,000 people was triggered by a small earthquake (Magnitude Ms=2.6) 22 km west of the source area of the landslide. The mechanism of this landslide was elucidated through the undrained dynamic-loading ring shear test using the real monitored earthquake record and also the new landslide simulation model LSRAPID analysis using the observed earthquake record (Sassa et al. 2010 ; Ajimera et al. 2023 ). To adapt to the current situation in Sri Lanka with increasing seismic activities, the ICL produced and installed nine sets of velocity-type seismometers to monitor microearthquakes in addition to the acceleration-type seismometers on 10 April 2023 in the Aranayake landslide area, the Atwelthota landslide area and others. Soon after the installation, an earthquake occurred offshore of the Hambantota coast on 24 April 2023. The magnitude was 4.4, according to the Geological Survey and Mines Bureau (GSMB), Sri Lanka. The 2023.4.24 Hambantota-offshore earthquake was monitored in both Aranayake and Atwelthota areas. Surprisingly, local microearthquakes were also observed in both landslide areas. In our LS-RAPID simulation, we applied the monitored earthquake waves to a nearby slope with landslide topography to examine whether a landslide can occur with a post-rainfall earthquake (namely, a certain amount of pore water pressure + a certain level of seismic shaking). This article reports the 2023.4.24 earthquake ground motions recorded