This classroom activity is an enhancement of the Cascadia GPS Gumdrop activity. It is intended for teachers who wish to provide their students with a more complete spatial picture of deformation of the Pacific Northwest continental margin. The big-picture is that coastal stations of Oregon and Washington are being pushed toward the northeast at about ½ the velocity of Juan de Fuca Plate motion with respect to interior North America. Urban corridor stations in the Puget Lowlands and Willamette Valley are being pushed toward the northeast at about ¼ of that velocity. Stations on the east sides of Oregon and Washington are not moving at all. The clear implication is that coastal areas are moving towards eastern areas as the active continental margin is compressed in SW – NE direction. Strain is building within the Pacific Northwest margin of the North American Plate as the Juan de Fuca Plate pushes the North American Plate margin toward the northeast. This accumulating strain will eventually be released in the next great earthquake on the Cascadia Subduction Zone. The plate boundary regions of the Juan de Fuca and North American plates are “locked and loading”.
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.
This QuickTime animation shows the motions of crustal blocks in the Pacific Northwest. The animation was developed by Jenda Johnson and is narrated by Bob Butler. Coastal blocks are being pushed northward by the Sierra Nevada block while the thick and strong crust of southern British Columbia acts as a back stop to this northward motion. A result of significance to earthquake risk is that much of the resulting north-south compression is accommodated by thrust faults in the Puget Lowlands such as the Seattle Fault that is capable of magnitude 7 earthquakes.
This QuickTime animation was developed and is narrated by Jenda Johnson. The animation shows the motion of a GPS station on the leading edge of a continent near a subduction zone during the earthquake cycle. The station is slowly pushed inland as elastic energy is stored in the overriding plate of the subduction zone then rapidly jumps seaward during a great earthquake that also generates a tsunami. This pattern of deformation has been observed during recent great subduction zone earthquakes like the February 2010 magnitude 8.8 Chile earthquake.
This QuickTime animation was developed by Jenda Johnson with assistance from Bob Butler and John Lahr. The animation shows the motion of a GPS station in the region that experiences episodic tremor and slip. The “three steps forward then one step back” pattern of motion is shown. The resulting graph of position versus time has the “sawtooth” pattern of motion that is now recognized as the signature of episodic tremor and slip. Seismic tremor vibrations are also illustrated occurring during the “back step” of the GPS station.
This QuickTime animation was developed by Jenda Johnson with assistance from Bob Butler and Roger Groom. The narration is provided by Roger Groom. The animation shows how three regions of a “locked and loading” subduction zone respond as elastic energy is stored in the overriding plate. A GPS station and grid are shown for: 1. the area near the locked subduction zone boundary; 2. the area farther inland (but still outboard of the volcanic arc) that experiences episodic tremor and slip; and 3. the area inboard of the volcanic arc that overlies the deeper portion of the subducting plate where it is freely slipping into the mantle. The grids deform to illustrate the distribution of deformation and graphs are drawn to illustrate the motions of the GPS stations. The characteristic “sawtooth” pattern of motion is observed in the area that experiences episodic tremor and slip.
This PowerPoint presentation covers basics of earthquake hazards, damage to structures due to earthquakes, and building designs that can make structures earthquake resistant.