This MS Word file contains teacher background information along with the Answer Key for the Pacific Northwest Tectonic Block Model Lesson Plan.
Chris Hedeen, Oregon City High School, developed this classroom activity that features a hands-on model of crustal block motions within the Pacific Northwest active continental margin. (The accompanying PDF by Philip Dinterman, Chris DuRoss, and Ray Wells describes construction of the model itself.) Through this activity, students can investigate the motions of crustal blocks in the Pacific Northwest and relate these to the tectonics of western North America. Ray Wells, a US Geological Survey geologist who is an expert on the tectonics and earthquake hazards of the Pacific Northwest, used paleomagnetic and GPS observations to determine how crustal blocks of this region slide past each other along boundaries marked by earthquake zones and how rocks of some blocks are faulted and folded as they are squeezed between blocks of stronger crust. The resulting earthquakes that occur on crustal faults near or immediately beneath densely populated cities of Oregon, Washington, and British Columbia are a major risk.
Instructions for building the Pacific Northwest Tectonics Block Model are provided in this PDF by Philip Dinterman, Chris DuRoss, and Ray Wells (US Geological Survey, Menlo Park, CA). The model is complex to build because it requires printing maps on a large-format printer, gluing maps to foam-core board, plasticizing maps, and many steps of construction. However, the Pacific Northwest Tectonics Block Model Lesson Plan can be done without building the model.
Roger Groom, Mt Tabor Middle School, developed this activity while working with Becca Walker at UNAVCO in Boulder, Colorado. Through this activity, students can learn about an exciting discovery made possible by invention of high-precision GPS receivers and deployment of these receivers across the Pacific Northwest. This activity can be used to illustrate how invention of new technologies can lead to new scientific discoveries that would have been impossible without the new instruments; a good lesson in how science works.
Some background: The friction between a subducting and an overriding plate of a subduction zone changes with depth. At shallow depths from the surface to 20 km depth, friction is high and the subducting boundary remains locked between very large earthquakes that occur every few decades or centuries. Deeper than about 40 km, friction on the subduction zone is very low and the subducting plate slides into the mantle without major earthquakes on the interface between the two converging plates. In some subduction zones, there is a transitional behavior called “episodic tremor and slip” (ETS) that takes place at intermediate depths of 20 to 40 km. ETS events on the Cascadia subduction zone occur when the Juan de Fuca Plate slips a centimeter or two farther beneath the North American Plate over a time interval from a few days to about two weeks in duration. This “slow slip” is accompanied by release of seismic waves (tremor) that are much longer in duration than seismic waves released by standard earthquakes. Because this process increases the stress on the locked shallow portion of the subduction zone, the probability of a great subduction zone earthquake may be higher during ETS events than at other times so this discovery has important implications for earthquake risk.
This 24″ wide by 36″ tall poster illustrates the violence and distribution of ground shaking produced by three types of Pacific Northwest earthquakes:
1. A magnitude 9 Cascadia subduction zone earthquake;
2. A “deep” earthquake within the Juan de Fuca Plate beneath the southern Puget Sound (e.g. like the 2001 Nisqually earthquake);
3. A magnitude 7.0 earthquake on a local crustal fault (e.g. the Seattle Fault).
Peak ground acceleration and likely building damage is illustrated for these three types of earthquakes.
Roger Groom, Mt Tabor Middle School, developed this activity with assistance from Bob Butler, University of Portland, and Shelley Olds and Becca Walker, UNAVCO. This activity allows students to analyze high-precision differential GPS observations of deformation of the North American Plate near its boundary with the oceanic Juan de Fuca Plate. Students analyze the rates of motion of three GPS receiving stations at different distances from the Cascadia subduction zone boundary. Through this activity, students appreciate how GPS monitors the gradual buildup of elastic energy as the continental margin within the Pacific Northwest is compressed by convergence between the North American and Juan de Fuca plates. This slowly accumulating elastic energy will be released in the next Cascadia megathrust earthquake!
This QuickTime animation was developed by Jenda Johnson. The animation shows the tsunami generated from the great Cascadia subduction zone earthquake of January 26 1700 traveling across the Pacific Ocean. The travel time to northwest Japan is about 9 hours. The arrival of this tsunami is recorded in historic documents of coastal towns in Japan. This tsunami is referred to as an “orphan tsunami” because it was not preceded by felt ground shaking in Japan cuased by a “parent” earthquake.
This QuickTime animation illustrates a scenario for a magnitude-9 earthquake on the shallow portion of the Cascadia subduction zone. Initial rupture starts off the coast of southern Oregon and ruptures northward to Vancouver Island for the full length of the plate boundary. The extent of this rupture would be similar to the rupture that occurred in Sunda Trench off northern Sumatra on December 26, 2004. This animation presents features of the great Cascadia earthquake that occurred at about 9:00 PM on January 26, 1700, although we do not know where rupture started during that great earthquake.
The animation is run in real time (one second on the animation equals one second in real time). Yellow circles radiating outward from the rupture area are P-wave fronts while orange circles show the S-wave fronts. Within several hundred kilometers of an earthquake, S waves and surface waves arrive at essentially the same time. It is important to realize that, for a great earthquake that ruptures several hundred kilometers along a fault, the rupture will take several minutes to propagate along the subduction zone. Seismic waves are being generated for the duration of the rupture process. The vertical axis on the graph at the bottom of the animation shows the violence of ground shaking in Seattle. “Level of Perception” is the level of ground shaking required for people in Seattle to become aware that the ground is shaking. “Level of Damage” is the level of ground shaking required to produce damage to weaker buildings in Seattle. As the shaking level rises farther above the yellow line, more and stronger buildings will be damaged by earthquake ground shaking. Green dots on the map are seismic stations that record ground vibrations and telemeter these observations in real time to the Pacific Northwest Seismic Network and the US Geological Survey.
1. 30 seconds after the earthquake begins:
P waves begin to arrive at multiple seismic stations. These P wave observations can be quickly analyzed to provide a “Preliminary Warning” that a large earthquake has begun on the Cascadia subduction zone. A preliminary location could also be determined at this time.
2. 45 seconds after the earthquake begins:
S waves have arrived at multiple seismic stations. These observations would be analyzed to provide a “Warning Confirmed” status that the Cascadia subduction zone earthquake in progress is indeed a very large event. In Japan and in some areas of California, earthquake-warning systems have been developed. These systems use the fast-velocity P waves as a warning that larger-amplitude S waves and surface waves will soon arrive in nearby cities. Critical infrastructure like natural gas lines, high-speed trains, and highway overpasses and bridges can be shut down in the interval between the earthquake warning and the arrival of the potentially damaging S waves and surface waves.
3. 1 minute after the earthquake begins:
P waves begin to arrive at Portland where they would be felt as a jolt of high-frequency ground shaking. P waves travelling from the point of initial rupture probably would not be felt in Seattle.
4. 2 minutes after the earthquake begins:
S waves and surface waves begin to arrive at Portland where they are felt as strong ground shaking with periods ranging from a few seconds to about 20 seconds. Houses built to modern seismic codes would sustain minimal damage. However, poorly built structures such an unreinforced masonry buildings would be heavily damaged. Some “long and tall” structures would sustain major damage because of the minutes-long duration and long periods (20 – 30 second period) of ground shaking.
4. 2 minutes and 50 seconds after the earthquake begins:
S waves and surface waves begin to arrive at Seattle and the earthquake is “Felt in Seattle”. Notice that there is a two-minute interval between the “Warning Confirmed” and “Felt in Seattle” times. If an earthquake-warning system could be developed for the Pacific Northwest, this could minimize the damage to the built environment across the region and mitigate the human and economic consequences of the next great Cascadia earthquake.
5. 4 minutes after the earthquake begins:
“Minor damage in Seattle” begins to occur after about 1 minute of ground shaking by S waves and surface waves. Most of this damage would be to weak multistory buildings like unreinforced masonry structures. Notice that rupture is now occurring adjacent to the Olympic Peninsula immediately west of Seattle. Because of the proximity to Seattle, seismic waves generated by rupture of this segment of the subduction zone will be very strongly felt in Seattle. At this time, the ground will have been shaking strongly in Portland for over two minutes!
6. 5 minutes after the earthquake begins:
“Major Damage” begins to occur in Seattle as the strongest seismic waves arrive and buildings already weakened by one minute of less violent ground shaking start to fail. The rate of damage production in Seattle peaks about 5 minutes and 30 seconds after the beginning of the earthquake.
7. 6 minutes after the earthquake begins:
Rupture is finally complete so generation of seismic waves has ceased although waves from previously ruptured segments of the subduction zone continue to arrive in Seattle and Portland. Residents and visitors to coastal areas now have about 20 minutes to evacuate to high ground before the tsunami produced by this great Cascadia earthquake will arrive on the coast.
Permission to use this animation from Steve Malone, Pacific Northwest Seismic Network, who created the model.
Steve notes “…keep in mind that the level of perception and damage is quite qualitative. In fact the damage from such an event is highly debatable. It may be extensive for some types of structures and not very much at all for others. The small stiff structures that one usually thinks about being damaged (concrete) may do fine here while the large flexible structures may have lots of dramatic problems because of the low-frequency and long-duration of shaking. The purpose of the animation was to show how long a really large earthquake takes to occur and the implications for the time it takes damaging waves to reach a distant site and how “early-warning” might be applicable. I think it is fine for you to include this on a general web page with suitable caveats about its qualitative nature.”
The graphics/animation were done by the “Center for Environmental Visualization, University of Washington” based on Steve Malone’s estimates “of rupture velocity, P- and S-wave velocities and even wilder guesses for amplitudes of surface waves following the S-waves. Since it runs in real time it gives a good feeling for how much time you do or don’t have to warn people and do something about it. It also illustrates directivity as a side benefit.”
QuickTime animation developed by Jenda Johnson to illustrate how sudden release of stored elastic energy (elastic rebound) in a subduction zone causes the leading edge of the over-riding plate to jump seaward and uplift while the near-shore land area subsides. The sudden displacement of the ocean floor generates a tsunami. The tsunami that arrives onshore near the subduction zone is the “local tsunami” that arrives 20 – 30 minutes after the displacement of the ocean floor by the earthqauke. The tsunami that travels into the open ocean will arrive hours later on distant shores. Notice that the near-shore area uplifts as elastic energy is slowly stored by deformation of the plates that are locked by friction along the plate interface. When the earthquake releases the stored energy, the near-shore area suddenly drops by a meter or more. This causes near-shore areas that were near sealevel before the earthquake to drop into the intertidal zone. This “co-seismic subsidence” kills trees in near-shore forests and results in a ghost forest of dead trees in a tidal marsh.
QuickTime animation of earthquakes and episodic tremor and slip (ETS) events that occurred in the Pacific Northwest from 2006 through 2009. Most of these earthquakes are shallow (less than 20 km depth) with the on-shore events occurring within the North American continental crust and the off-shore events occurring within the upper portion of the oceanic lithosphere. The ETS events are “slow slip” between the base of the North American Plate and the top of the subducting Juan de Fuca Plate in the 20 km to 40 km depth range. ETS events usually take place over time intervals of 10 days to several weeks during which the area of slip migrates along the plate boundary. This animation was developed by Michael Brudzinski, seismologist in the Department of Geology at Miami University of Ohio.