Cascadia Great Earthquake and Tsunami Suite
I The Great Sumatran Earthquake and
Tsunami of December 2004
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II Is the stage being set for a Great
Cascadian Earthquake and Tsunami?
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III The Search for Great Cascadian
Earthquakes and Tsunamis in the Past
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IV Impact of a Great Cascadian Earthquake
and Tsunami on one Coastal Community
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V Rupture on the Seattle Fault: A Case
Study
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Natural Hazard Case Studies
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Unit III. Evidence for Great Cascadian Earthquakes and Tsunami in the Past
Whereas Unit 2 is concerned primarily with geophysical evidence of modern-day deformations within Cascadia that may presage a future great earthquake, Unit 3 involves the use of stratigraphic evidence of such events in the past.
The unit begins with an excerpt from a transcription of Native oral history that tells the story of a great earthquake, accompanied by a devastating tsunami. That such stories are common to Native groups throughout coastal Cascadia means that such an event must have occurred in the not-too-distant past; yet no such events are recorded in written histories since European settlement of Cascadia began in the late 18th century. With that background, students begin an investigation of when the event from Native oral traditions occurred and whether it was unique.
Students review the characteristics of the three greatest subduction zone earthquakes ever recorded—the 1960 M9.6 Chilean earthquake, the 2004 M9.3 Sumatran earthquake, and the 1964 M9.2 Alaskan earthquake—and note their common attributes, in particular a characteristic co-seismic inversion of topography in a buckled upper plate, causing a sudden down-drop of elevated coastal regions to sea level. The result can be seen in drowned coastal forests in Alaska following the 1964 earthquake. Students are then introduced to the “Ghost Forests” of coastal Cascadia—stands of red cedar dropped to sea level and killed by sea water—evidently an analogous event in the relatively recent past.
Students then learn about the characteristic stratigraphy of estuaries all up and down coastal Cascadia that clearly points to down-drop of marshland containing the cedar trees, deposition of a tsunami sand layer, and further deposition of an overlying layer of estuarian muds. Students interpret this stratigraphy as a logical result of topographic inversion accompanying a great earthquake and tsunami in the relatively recent past.
Students then review radiocarbon data from sedimentary organic material and from the dead cedars, along with tree ring chronology, and progressively narrow down the time window of the event; they are ultimately able to deduce its year, date, and hour. The connection is made to the event told of in the Native oral traditions.
They then consider the question of whether the event was unique. They learn about the nature of turbidity currents and turbidite deposits in the deep ocean basins, and then investigate the turbidite data set from the deep Cascadia Basin. There are18 turbidite layers, correlative the full length of the Cascadian margin, present in sediment cores from the basin. Students are able to infer that these represent turbidity currents generated by great earthquakes, each possibly of magnitude 9+. From bracket dates on the turbidites, they compute recurrence intervals.
In localities onshore, the stratigraphy also points to multiple events, the dates for which have been bracketed. Students also compute recurrence intervals for these multiple events, and are asked to consider what this data can reasonably tell us about when the next great event is likely to occur.
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ArcGIS version
MyWorld GIS version
This material is based upon work supported by the National Science Foundation under Grant Number DUE-0521936. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
Whereas Unit 2 is concerned primarily with geophysical evidence of modern-day deformations within Cascadia that may presage a future great earthquake, Unit 3 involves the use of stratigraphic evidence of such events in the past.
The unit begins with an excerpt from a transcription of Native oral history that tells the story of a great earthquake, accompanied by a devastating tsunami. That such stories are common to Native groups throughout coastal Cascadia means that such an event must have occurred in the not-too-distant past; yet no such events are recorded in written histories since European settlement of Cascadia began in the late 18th century. With that background, students begin an investigation of when the event from Native oral traditions occurred and whether it was unique.
Students review the characteristics of the three greatest subduction zone earthquakes ever recorded—the 1960 M9.6 Chilean earthquake, the 2004 M9.3 Sumatran earthquake, and the 1964 M9.2 Alaskan earthquake—and note their common attributes, in particular a characteristic co-seismic inversion of topography in a buckled upper plate, causing a sudden down-drop of elevated coastal regions to sea level. The result can be seen in drowned coastal forests in Alaska following the 1964 earthquake. Students are then introduced to the “Ghost Forests” of coastal Cascadia—stands of red cedar dropped to sea level and killed by sea water—evidently an analogous event in the relatively recent past.
Students then learn about the characteristic stratigraphy of estuaries all up and down coastal Cascadia that clearly points to down-drop of marshland containing the cedar trees, deposition of a tsunami sand layer, and further deposition of an overlying layer of estuarian muds. Students interpret this stratigraphy as a logical result of topographic inversion accompanying a great earthquake and tsunami in the relatively recent past.
Students then review radiocarbon data from sedimentary organic material and from the dead cedars, along with tree ring chronology, and progressively narrow down the time window of the event; they are ultimately able to deduce its year, date, and hour. The connection is made to the event told of in the Native oral traditions.
They then consider the question of whether the event was unique. They learn about the nature of turbidity currents and turbidite deposits in the deep ocean basins, and then investigate the turbidite data set from the deep Cascadia Basin. There are18 turbidite layers, correlative the full length of the Cascadian margin, present in sediment cores from the basin. Students are able to infer that these represent turbidity currents generated by great earthquakes, each possibly of magnitude 9+. From bracket dates on the turbidites, they compute recurrence intervals.
In localities onshore, the stratigraphy also points to multiple events, the dates for which have been bracketed. Students also compute recurrence intervals for these multiple events, and are asked to consider what this data can reasonably tell us about when the next great event is likely to occur.
Download project files now...
ArcGIS version
MyWorld GIS version
This material is based upon work supported by the National Science Foundation under Grant Number DUE-0521936. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.