Imbricated Beachrock Deposits Adjacent to the Java Trench, Indonesia: Influence of Tsunami and Storm Waves

Abstract

We discovered several Imbricate Beachrock Deposits (IBD), one of which was observed to have formed during the tsunami caused by the 1994 7.8 Mw earthquake in East Java. Similar IBD were also found along the southern coastlines of central Java, Bali, Lombok, Sumba, Kisar, Leti and Nailaka Islands. The largest imbricated beachrock slabs are around 3m3. Most IBD are composed of thin, rectangular (2.5 x 1.7 x 0.4 m) slabs of calcareous beachrock dislodged from the intertidal platform during one or several powerful wave impactions. Ages of coral boulders incorporated into the IBD match with historical records of known tsunamigenic earthquakes offshore and candidate paleotsunami sand deposit ages onshore. To test for the influence of storms on the IBD we measured the positions of boulders over a 3-year period at one site by overlaying digital surface models created from UAV surveys. During the 3 years there were multiple uncommonly high wave events including two tropical cyclones. Around 113 individual beachrock slabs of the approximately 1220 slabs in the IBD were moved slightly or flipped within the deposit, but no conclusive evidence exists for addition or removal of beachrock slabs. The combination of data from various sources favors the hypothesis that the IBD are emplaced by tsunamis. If this is the case, then the IBD provide a durable record of previous tsunamis that should be incorporated into tsunami risk assessments for the highly populated coastlines of the eastern Sunda and Banda Arcs. We estimate from tsunami models that at least 6 million people inhabit likely inundation zones of worst-case scenario tsunamis generated by a Java Trench mega-thrust earthquake.