Seismic properties of the uppermost igneous crust of the oceans are dominated by porosity effects, that is, the size, concentration, and shape of void spaces. Porosity is initially determined by the physics of extrusion (does an eruption form breccia, pillows, or massive flows?) but is very rapidly modified by alteration and hydrothermal deposition. Laboratory data provide insight into compressional wave velocity‐porosity behavior of basalts at a hand sample scale, while well logs provide data at outcrop scale. Relating observations at all scales to porosity structure and extrapolating to seismic scale requires application of rock physics theory. Using information from ophiolites and deep ocean cores, we have defined rock physics parameters for two simple models of upper oceanic crust. The models approximate different levels of void filling by alteration products by differing in the amount of crack (low aspect ratio) porosity they contain. From the models we compute theoretical compressional wave velocity and porosity profiles. Calculated profiles agree well with both well logs and seismic data and illustrate that the increase in seismic velocities measured seismically in the upper crust need not be accompanied by large changes in total porosity.
Abstract. Case studies of landslide tsunamis require integration of marine geology data and interpretations into numerical simulations of tsunami attack. Many landslide tsunami generation and propagation models have been proposed in recent time, further motivated by the 1998 Papua New Guinea event. However, few of these models have proven capable of integrating the best available marine geology data and interpretations into successful case studies that reproduce all available tsunami observations and records. We show that nonlinear and dispersive tsunami propagation models may be necessary for many landslide tsunami case studies. GEOWAVE is a comprehensive tsunami simulation model formed in part by combining the Tsunami Open and Progressive Initial Conditions System (TOPICS) with the fully nonlinear Boussinesq water wave model FUNWAVE. TOPICS uses curve fits of numerical results from a fully nonlinear potential flow model to provide approximate landslide tsunami sources for tsunami propagation models, based on marine geology data and interpretations. In this work, we validate GE-OWAVE with successful case studies of the 1946 Unimak, Alaska, the 1994 Skagway, Alaska, and the 1998 Papua New Guinea events. GEOWAVE simulates accurate runup and inundation at the same time, with no additional user interference or effort, using a slot technique. Wave breaking, if it occurs during shoaling or runup, is also accounted for with a dissipative breaking model acting on the wave front. The success of our case studies depends on the combination of accurate tsunami sources and an advanced tsunami propagation and inundation model.
Based on numerical simulations presented in Part I, we derive predictive empirical equations describing tsunami generation by submarine mass failure ͑SMF͒ that are only valid in the vicinity of the tsunami sources. We give equations for slides and slumps, along with some cautions about their appropriate use. We further discuss results obtained here and in Part I and their practical application to case studies. We show that initial acceleration is the primary parameter describing SMF center of mass motion during tsunami generation. We explain an apparent paradox, raised in Part I, in slump center of mass motion, whereby the distance traveled is proportional to shear strength along the failure plane. We stress that the usefulness of predictive equations depends on the quality of the parameters they rely on. Parameter ranges are discussed in the paper, and we propose a method to estimate slump motion and shear strength and discuss SMF thickness to length values, for case studies. We derive the analytical tools needed to characterize SMF tsunami sources in propagation models. Specifically, we quantify three-dimensional ͑3D͒ effects on tsunami characteristic amplitude, and we propose an analytical method to specify initial 3D tsunami elevations, shortly after tsunami generation, in long wave tsunami propagation models. This corresponds to treating SMF tsunami sources like coseismic displacement tsunami sources. We conduct four case studies of SMF tsunamis and show that our predictive equations can provide rapid rough estimates of overall tsunami observations that might be useful in crisis situations, when time is too short to run propagation models. Thus, for each case, we show that the characteristic tsunami amplitude is a reasonable predictor of maximum runup in actual 3D geometry. We refer to the latter observation as the correspondence principle, which we propose to apply for rapid tsunami hazard assessment, in combination with the predictive tsunami amplitude equations.
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