Jose C. Borrero

In this paper, the well-established approaches of coupling tsunami generation to seismic seafloor motion and the following trans-oceanic wave propagation will be briefly introduced. The focus of the paper will be on the complex transformation of the tsunami as it approaches very shallow water, as well as how these possibly large and fast-moving water waves interact with coastal infrastructure. Examples of coastal impact will be discussed and used to frame the theoretical efforts. The majority of the discussion will focus on tsunami-induced currents in ports and harbors. Tsunamis, or "harbor waves" in Japanese, are so-named due to the common observation of enhanced damage in harbors and ports. However, the dynamic currents induced by these waves, while regularly observed and known to cause significant damage, are poorly understood. We will show that the strongest currents in a port are governed by horizontally sheared and rotational shallow flow with imbedded turbulent coherent structures. Without proper representation of the physics associated with these phenomena, predictive models may provide drag force estimates that are an order of magnitude or more in error. Such an error can mean the difference between an unaffected port and one in which vessels 300 meters in length drift and spin chaotically through billions of dollars of infrastructure. Here, we present example simulation results of a numerical modeling study aimed at providing the California Geological Survey (CGS) and the California Emergency Management Agency (CalEMA) quantitative guidance on maritime tsunami hazards in California ports and harbors. The study focuses on tsunami-induced currents and seeks to define the relative hazard in specific ports and harbors as a result of these currents.

We model tsunami runup and inundation along the Northern California Coast caused by hypothetical earthquakes on the Cascadia Subduction Zone (CSZ). The CSZ forms the boundary between the Pacific and North American tectonic plates and extends from Northern California to Southern British Columbia and is believed capable of producing great earthquakes with moment magnitudes (M_w) 9.0 or greater. A suite of CSZ rupture scenarios based on the geological characteristics of the southern part of the CSZ were simulated with the tsunami propagation and inundation model MOST. CSZ inundation projections were compared and related to historical accounts and modeled data for two teletsunamis at Crescent City: the tsunami of 1 April 1964 that caused 12 deaths and more than 17 million in damages to Northern California, and the 15 November 2006 Kuril Islands tsunami that caused nearly 10 million in damages to the Crescent City harbor. Our modeling suggests that near field tsunamis from the CSZ present a hazard greater than the 1964 event for the northernmost part of the State. However, south of Shelter Cove tsunamis generated by great earthquakes along the Alaska — Aleutians subduction zone are potentially more damaging.

A long section of the Sunda megathrust south of the great tsunamigenic earthquakes of 2004 and 2005 is well advanced in its seismic cycle and a plausible candidate for rupture in the next few decades. Our computations of tsunami propagation and inundation yield model flow depths and inundations consistent with sparse historical accounts for the last great earthquakes there, in 1797 and 1833. Numerical model results from plausible future ruptures produce flow depths of several meters and inundation up to several kilometers inland near the most populous coastal cities. Our models of historical and future tsunamis confirm a substantial exposure of coastal Sumatran communities to tsunami surges. Potential losses could be as great as those that occurred in Aceh in 2004.

After the 26 December 2004 earthquake and tsunami, field data on the extent of the inundation in Banda Aceh, Sumatra, were combined with satellite imagery to quantify the tsunami effects. Flow depths along the shores of Banda Aceh exceeded 9 meters, with inundation reaching 3 to 4 kilometers inland. To the southwest, at Lhoknga, flow depths were more than 15 meters at the shoreline and runup exceeded 25 meters. Erosion and subsidence moved the shoreline of Banda Aceh inland up to 1.5 kilometers, and 65 square kilometers of land between Banda Aceh and Lhoknga were flooded.