Results of Post-Field Survey on the Mw 8.3 Illapel Earthquake Tsunami in 2015

On 16 September 2015, a Mw 8.3 earthquake occurred off the coast of Illapel, Central Chile, generating a tsunami that caused moderate damage along the coast of Central Chile. To investigate tsunami damage and evacuation responses, we conducted a post-tsunami field survey between La Serena and Concon. The average tsunami inundation height in the survey area was 3-4 m; however, in the innermost corner of Coquimbo Bay, which was shielded from the tsunami source by the Coquimbo Peninsula, the tsunami inundation height was 6.0-6.2 m, decreasing gradually along the coast away from the corner. This phenomenon was also confirmed around Tongoy Bay, which is geographically similar to Coquimbo Bay. Based on our results, we hypothesize that the tsunami was diffracted and refracted by the peninsula, and then converged on the innermost corner of the bay, located behind the peninsula. Interviews with local residents confirmed that most people evacuated immediately after feeling strong seismic motion, but before the tsunami warning and evacuation orders were issued. However, several people did not evacuate, either because they believed that they were located in a safe place, or because they wanted to protect their property.