Owing to propagation effects, however, there is a greater than linear relationship between the amplitude of the tsunami near shore and the amount of average slip. That is, if average slip for one earthquake (EQ 1) is twice that for another earthquake (EQ 2), the seafloor offset and the initial tsunami will be different also by a factor of two. The relationship between fault slip and the permanent offset of the seafloor after earthquake rupture is linear. The distance the two sides move averaged over the rupture area is what we will call average slip. Here we show how the following parameters affect local tsunamis: Amount of Average Slipĭuring an earthquake, one side of the fault moves vertically and/or horizontally with respect to the other side. The nature of earthquake rupture is defined by many different parameters that can vary spatially and with time. What Aspects of Earthquake Rupture Affect Tsunamis? This study was designed to calculate how local tsunamis are affected by variations in earthquake source parameters specific to faulting along the Cascadia subduction zone. For local tsunamis, however, more knowledge than the magnitude is needed to calculate the eventual runup of the tsunami. For distant tsunamis that have traveled far from the origin of the earthquake, the magnitude of the earthquake is a good measure of the size of the tsunami. The relationship between the parameters that define earthquake rupture and local tsunamis is complex. Earthquake Source Parameters and Potential Local Tsunamis in CascadiaĮffect on local tsunamis from source parameters describing uniform rupture Overview Phase 3: Local Tsunami Hazards in the Pacific Northwest from Cascadia Subduction Zone Earthquakes (USGS Professional Paper 1661-B) #1. Phase 2: A Stochastic Model for Potential Tsunamis in the Pacific Northwest - Effect on local tsunamis from spatial variations of slip during rupture Phase 1: Earthquake Source Parameters and Potential Local Tsunamis in Cascadia - Effect on local tsunamis from source parameters describing uniform rupture Under the Wave off Kanagawa, also known as The Great Wave, from the series Thirty-six Views of Mount Fuji, by artist Katsushika Hokusai, Tokyo (Edo), 1760–1849 How do various parameters that describe an earthquake influence the resulting local tsunami? Such USGS research progressed in three phases. Some Native American tribal nations in the Pacific Northwest have legends of large waves striking the coast. Therefore, the possibility exists that local tsunamis may someday accompany a major earthquake along the Cascadia megathrust. Rather, geologic evidence is accumulating that the Cascadia subduction zone is poised between major earthquakes. Does this mean that the two plates are sliding past each other freely without generating earthquakes? This would make the Cascadia subduction zone unlike most other subduction zones around the world. Earthquakes along the fault that is the contact between the two plates, termed the interplate thrust or megathrust, may generate significant local tsunamis in the Pacific Northwest.Įxcept for the M=7.2 1992 Cape Mendocino earthquake at the southernmost part of the subduction zone, there have been no major earthquakes on the megathrust in historic time. The Pacific Northwest is the site of the Cascadia subduction zone, where an oceanic tectonic plate (the Juan de Fuca plate) is being pulled and driven (i.e., subducted) beneath a continental plate (the North American plate). Block diagram of a subduction zone when two oceanic plates converge.
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