What is the evidence for megathrust earthquakes in the Pacific Northwest?
This simplified animation illustrates both the subduction-zone processes that lead to a "ghost forest" as well as the evidence that scientists collected to determine that the Pacific Northwest has had many great earthquakes and tsunamis in the past, and will again in the future. This is based on the work of Brian Atwater who published his findings in the book "The Orphan Tsunami of 1700" (USGS Professional Paper 1707). On January 26, 1700 at 9:00 pm a great earthquake (M8.7?9.2) struck the Pacific Northwest shaking mountains, dropping coastal forests, and causing a tsunami that wiped away entire villages (http://walrus.wr.usgs.gov/tsunami/NAlegends.html). (This occurred before written history. Oral history of Native Americans, interviewed in 1868, told of their ancestors relating these events.) Nine hours later, in Japan, a mysterious tsunami arrived without warning flooding fields and washing away houses along the coastline from north to south. Samurai, merchants, and villagers recorded the event, but nearly three centuries would pass before scientific discoveries in North America revealed the tsunami's source. Evidence supporting the occurrence of the 1700 earthquake is detailed in the book The Orphan Tsunami of 1700 (geologist Brian Atwater and others, 2005; http://pubs.usgs.gov/pp/pp1707/). Recent findings conclude that the Cascadia Subduction zone is more complex and volatile than previously believed. Geologists predict a 37 percent chance of a M8.2+ event in the next 50 years, and a 10 to 15 percent chance that the entire Cascadia Subduction will rupture with a M9+ event within the same time frame (http://oregonstate.edu/ua/ncs/node/13426). Sumatra, Chile, and Japan provide vivid examples of what could happen.
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Evidence for past megathrust earthquakes in the Pacific Northwest
The subduction zone iswhere two tectonic (lithospheric) plates come together, one subducting (diving) beneath the other. The plates are locked together and periodically overcome the friction causing the leading edge of the overlying plate to surge back, lifting a wall of water producting a tsunami.
Oblique view of a highly generalized animation of a subduction zone where an oceanic plate is subducting beneath a continental plate. (See sketch below for parts.) This scenario can happen repeatedly on a 100-500 year cycle. The process which produces a mega-thrust earthquake would generate a tsunami, not depicted here.
Subduction zones show that there are 3 distinct areas of movement in the overlying plate:
GPS can record the movement of the leading edge of the overlying continental plate in a subduction zone. The plates are locked and the overlying plate is forced back. When friction is overcome and strain is released, the GPS receiver will snap back toward its original position.
This animation by UNAVCO shows us how an earthquake warning system uses existing seismic networks to detect moderate to large earthquakes. Computers, communications technology, and alarms are devised to notify the public while an earthquake is in progress.
Subduction-zone megathrust earthquakes, the most powerful earthquakes in the world, can produce tsunamis through a variety of structures that are missed by simple models including: fault boundary rupture, deformation of overlying plate, splay faults and landslides. From a hazards viewpoint, it is critical to remember that tsunamis are multiple waves that often arrive on shore for many hours after the initial wave.
It is common knowledge that the Pacific Northwest can expect a subduction-zone megathrust earthquake in the future. But did you know that there are other types of damaging earthquakes. This animation uses analogies and cartoon block diagrams to teach about the three types of earthquakes.
Students work in small groups to analyze and interpret Global Positioning System (GPS) and seismic data related to “mysterious ground motions” first along the northern California coastline, and then in British Columbia. This activity emphasizes the analysis and synthesis of multiple types of data and introduces a mode of fault behavior known as Episodic Tremor and Slip (ETS)