Field observations of the 17 July 2006 Tsunami in Java

International audienceThe 17 July 2006, a tsunami struck the southern coast of Java, Indonesia, causing over 730 casualties. The triggering earthquake located 225 km off the coast of Pangandaran (9.222° S, 107.320° E), occurred at 15:19 LT (UTC +7) with a 7.7 magnitude on the Richter scale (Harward Center and CEA/DAM). In order to calibrate numerical models and understand the phenomenon, we conducted a 6-weeks field survey in July and August 2006 from Cimerak district in West Java to Gunung Kidul district in Central Java. Data collection involved measurements of wave height before its breaking, flow depth, run-up height, inundation depth, flow directions and a detailed chronology of the tsunami. Eyewitnesses accounted for three main waves. The maximum height of the second wave ranged from 4.2 to 8.6 m before its breaking. Maximum flow depth after the wave's breaking reached 5 m, and maximum runup heights reached 15.7 m. Our run-up values are about 1.5 higher than those obtained by the other field surveys carried out until present. They are also higher than the values computed through preliminary models. The 17 July 2006 tsunami has been generated by a "tsunami earthquake", i.e. an earthquake of low or medium scale that triggers a tsunami of high magnitude. The run-up heights progressively decreased eastwards, which is consistent with a tsunami triggered by fault dislocation, as the one that hit the Nicaragua's coast with similar run-up heights on the 2 September 1992. An earthquake with associated landslides could also have generated the 17 July 2006 tsunami, as ever observed in Papua-New-Guinea in 1998