The Source Mechanism of the October 1950 Ambon Tsunami Revealed by Geological Data and Tsunami Modeling

The 1950 Ambon tsunami warrants further studies due to several uncertainties surrounding its source mechanism and coastal impacts. Historical records and eyewitnesses suggest that the entire coast of Ambon Bay was not uniformly affected. This led to the hypothesis that the tsunami may have originated from submarine landslides. Geological evidence is needed to confirm the hypothesis. We use foraminifera as key geological data, along with numerical tsunami modeling, to understand the 1950 Ambon tsunami mechanism. Our geological data indicate that the initial tsunami wave was directly caused by the earthquake, as evidenced by the presence of foraminifera typically found outside Ambon Bay. The subsequent waves most likely resulted from a combination of the earthquake and landslide, supported by the abundance of Pliocene-aged foraminifera within the tsunami deposit. These Pliocene-aged species are attributed to the submarine landslide, not originating from sedimentary rock exposed on land. The third wave was solely generated by the landslide, characterized by the presence of Pliocene-aged foraminifera only. These geological data are supported by tsunami modeling, which reveals tsunami waves and inundation patterns consistent with both geological evidence and archival records. This study is the first to confirm that foraminifera can be used to determine tsunami source mechanisms.