Discovery of imbricated beachrock deposits adjacent to the Java trench, Indonesia: influence of tsunami and storm waves, and implications for mega-thrust earthquakes

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, Indonesia. Similar IBD were also found along the southern coastlines of central Java, Bali, Lombok, Sumba, Kisar, Leti and Nailaka Islands. Most IBD are composed of thin, rectangular (2.5 × 1.7 × 0.4 m) slabs of in situ calcareous beachrock dislodged from the intertidal platform during powerful wave impacts. The largest imbricated beachrock slabs are around 3 m3. Ages of coral boulders incorporated into the IBD generally match with historical records of known tsunamigenic earthquakes and candidate paleotsunami sand deposit ages. 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 small uncrewed aerial system surveys. During the 3 years there were multiple uncommonly high wave events including two tropical cyclones, which are rare in Indonesia. Of the approximately 1220 slabs in the IBD around 113 moved slightly or flipped within the deposit, but no beachrock slabs were added or removed. The combination of data from various sources (eyewitnesses, consistent boulder characteristics, lack of storms or their effects on boulders, and age analyses) favors the hypothesis that the IBD are emplaced by recurring large wave events. The most probable causes of these events are tsunamis generated by the Java Trench and other submarine faults and landslides. If this is the case, then the IBD may provide durable records of previous mega-thrust earthquakes and tsunamis that should be incorporated into tsunami risk assessments for the highly populated coastlines of the eastern Sunda and Banda Arcs. We include two tsunami models that estimate > 6 million people inhabit likely inundation zones of a worst-case scenario tsunami generated by a Java Trench mega-thrust earthquake.