Our paleotsunami surveys of the southern Java coast led to the discovery of five imbricate coastal boulder fields near Pacitan, Indonesia that may date to the mid-to-late 19th century or prior and two similar fields at Pantai Papuma and Pantai Pasir Putih that were tsunami-emplaced during the 1994 7.9 Mw event in East Java. Estimated ages for the fields near Pacitan are based on historical records and radiocarbon analyses of coral boulders. The largest imbricated boulders in fields near Pacitan and in East Java are similar in size (approximately 3 m^3) and are primarily composed of platy beachrock dislodged from the intertidal platform during one or several unusually powerful wave impactions. Hydrodynamic wave height reconstructions of the accumulations near Pacitan indicate the boulders were likely tsunami rather than storm-wave emplaced, as the size of the storm waves needed to do so is not viable. We evaluate the boulders as an inverse problem, using their reconstructed wave heights and ComMIT tsunami modeling to suggest a minimum 8.4 Mw earthquake necessary to dislodge and emplace the largest boulders near Pacitan assuming they were all deposited during the same tsunami event and that the rupture source was located along the Java Trench south of Pacitan. A combined analysis of historical records of Java earthquakes and plate motion measurements indicates a seismic gap with >25 m of slip deficit along the Java Trench. A 1000-1500 km rupture along the subduction interface of this segment is capable of producing a 9.0-9.3 Mw megathrust earthquake and a giant tsunami. However, evidence for past megathrust earthquake events along the this trench remains elusive. We use epicenter independent tsunami modelling to estimate wave heights and inundation along East Java in the event that the trench were to fully rupture. By translocating ComMIT slip parameters of Japan's 2011 9.1 Mw event along the trench offshore East Java, we demonstrate possible wave heights in excess of 20 m at various locations along its southern coasts. Approximately 300,000-500,000 people in low-lying coastal communities on the southern coasts of East Java could be directly affected. We recommend at-risk communities practice the "20/20/20 principle" of tsunami hazard awareness and evacuation.

Maps and models of seismic and tsunami risk are constructed from a variety of measurements taken in Indonesia, which have the potential to reduce loss of life and infrastructure. The first study uses the multichannel analysis of surface waves (MASW) method to calculate the time-averaged shear wave velocity to 30 m depth (Vs30). These measurements were taken at 58 sites in the city of Pacitan, Java and on the islands of Lombok, Ambon, and the Banda Islands. Vs30 calculations are compared with local geologic maps to extrapolate site class for locations not measured directly. Site class maps are then compared with Modified Mercalli Intensity (MMI) observations for three earthquake events that impacted Lombok and Ambon to identify regions where the MMI and Vs30 do and do not corroborate one another. Consistent with other Vs30 studies, the lowest values are observed on coastal alluvial plains and the highest values on steeper hillsides underlain by volcanic deposits. The second study focuses on a potential sector collapse of the volcano Banda Api within the Banda Islands. A field survey of its summit identified a steeply dipping normal fault striking NNE-SSW. This, along with the fissure geometry of the volcano's most recent eruption, reveals a failure plane along which a future sector collapse could occur. The numerical model Tsunami Squares (TS) predicts that the tsunami produced by this landslide would inundate an estimated 63% of buildings on the Banda Islands with waves as high as 82 m. These findings highlight the importance of installing a GPS receiver array on Banda Api to monitor the motion of its slopes. The third study analyzes sediment from trenches on the Banda Islands and Ambon to test if historical tsunamis that have impacted the area are preserved in the geological record. Potential tsunami deposits were identified by the presence of marine sand and larger clasts of marine carbonate in an environment which otherwise lacks large storms to bring such material onshore. Several dating methods constrain the ages of at least seven candidate tsunami deposits found in trenches onshore. One of these historical tsunamis (the event of November 26, 1852) is described in significant detail from several locations across the Banda Sea, which enables modeling of the event using a Bayesian statistical approach. The posterior of this model predicts the most likely epicenter was SW of Seram with a mean magnitude of Mw 8.8. It also makes other predictions about fault parameters. The region exhibits a marked slip deficit based on instrumental records of earthquakes in the area.