Extreme runup from the 17 July 2006 Java tsunami

Geophysical Research Letters (Impact Factor: 3.98). 06/2007; 34(12):L12602. DOI: 10.1029/2007GL029404

ABSTRACT The 17 July 2006 magnitude Mw 7.8 earthquake off the south coast of western Java, Indonesia, generated a tsunami that effected over 300 km of coastline and killed more than 600 people, with locally focused runup heights exceeding 20 m. This slow earthquake was hardly felt on Java, and wind waves breaking masked any preceding withdrawal of the water from the shoreline, making this tsunami difficult to detect before impact. An International Tsunami Survey Team was deployed within one week and the investigation covered more than 600 km of coastline. Measured tsunami heights and run-up distributions were uniform at 5 to 7 m along 200 km of coast; however there was a pronounced peak on the south coast of Nusa Kambangan, where the tsunami impact carved a sharp trimline in a forest at elevations up to 21 m and 1 km inland. Local flow depth exceeded 8 m along the elevated coastal plain between the beach and the hill slope. We infer that the focused tsunami and runup heights on the island suggest a possible local submarine slump or mass movement.

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    ABSTRACT: In the present study we propose a modified version of the nonlinear shallow water (Saint-Venant) equations for irrotational surface waves in the case when the bottom undergoes some significant variations in space and time. The model is derived from a variational principle by choosing an appropriate shallow water ansatz and imposing some constraints. Our derivation procedure does not explicitly involve any small parameter and is straightforward. The novel system is a non-dispersive non-hydrostatic extension of the classical Saint-Venant equations. We also propose a finite volume discretization of the obtained hyperbolic system. Several test-cases are presented to highlight the added value of the new model. Some implications to tsunami wave modeling are also discussed.
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    ABSTRACT: The M-w 7.8 October 2010 Mentawai, Indonesia, earthquake was a "tsunami earthquake," a rare type of earthquake that generates a tsunami much larger than expected based on the seismic magnitude. It produced a locally devastating tsunami, with runup commonly in excess of 6 m. We examine this event using a combination of high-rate GPS data, from instruments located on the nearby islands, and a tsunami field survey. The GPS displacement time series are deficient in high-frequency energy, and show small coseismic displacements (<22 cm horizontal and <4 cm subsidence). The field survey shows that maximum tsunami runup was >16 m. Our modeling results show that the combination of the small GPS displacements and large tsunami can only be explained by high fault slip at very shallow depths, far from the islands and close to the oceanic trench. Inelastic uplift of trench sediments likely contributed to the size of the tsunami. Recent results for the 2011 M-w 9.0 Tohoko-Oki earthquake have also shown shallow fault slip, but the results from our study, which involves a smaller earthquake, provide much stronger constraints on how shallow the rupture can be, with the majority of slip for the Mentawai earthquake occurring at depths of <6 km. This result challenges the conventional wisdom that the shallow tips of subduction megathrusts are aseismic, and therefore raises important questions both about the mechanical properties of the shallow fault zone and the potential seismic and tsunami hazard of this shallow region.
    Journal of Geophysical Research 06/2012; 117(B06402). · 3.17 Impact Factor
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    ABSTRACT: The initial free-surface displacement generated by a submarine earthquake has a dipolar nature, which is computed analytically by Okada's solution [1] and is finite crested. The resulting leading long wave has an N-wave shape as noted by Tadepalli & Synolakis [2, 3]. Here, we present a simple analytical solution of the linear shallow-water wave equations over a constant depth to study the propagation of a finite strip source. We show the existence of focusing points of dipolar initial displacements, i.e. points where wave amplification may be observed, due to the directional focusing of three waves, namely a positive wave from the center of elevation part and two positive waves from the sides of depression. N-wave focusing is not restricted to linear non-dispersive wave theory, but can also be observed using nonlinear shallow-water wave theory and dispersive theory. The location of the focusing point depends on the strip length. The focusing mechanism is an inherent property of the initial waveform and thus is not caused by bathymetric lenses, which can have a significant combined effect on the evolution of earthquake-generated tsunamis. Using the 1998 Papua New Guinea, 2006 Java and 2011 Japan tsunamis as examples, we discuss the geophysical implications of the focusing and how this can be related to abnormal high run-up values observed during these events, which were insufficiently explained so far. [1] Okada, Y. 1985 Surface deformation due to shear and tensile faults in a half-space. Bull. Seism. Soc. Am. 75, 1135-1154. [2] Tadepalli, S. & Synolakis, C. E. 1994 The run-up of N-waves on sloping beaches. Proc. R. Soc. Lond. A 445, 99-112. [3] Tadepalli, S. & Synolakis, C. E. 1996 Model for the leading waves of tsunamis. Phys. Rev. Lett. 77, 2141-2144.


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