Research shows that there is a pattern of quakes that range from 8. 7 to 9. 2 that occur every 240 years or so, and we know this because Oregon State University has done a 13 year study on the margin stretching from southern Vancouver Island to the Oregon-California border. The last mega-earthquake in the Pacific-Northwest being on January 26, 1700; We know this because written records in Japan document how a tsunami destroyed that year’s rice crop stored in warehouses.
We also know from documentation of data that scientists have collected from when there are off shore earthquakes, these quakes will cause mud and sand to “slide” down the continental margins into undersea canyons. These coarse sediments called turbides stand out from the fine particulate that accumulates normally. By dating these particles (using Carbon-14 analysis) researchers can estimate with some degree of accuracy that major quakes have occurred periodically over the last 10,000 years.
Going any further back would be difficult because the sea level was lower and rivers emptied directly into the undersea canyons causing the samples to be contaminated with river sediments. Collecting Data In a study from Oregon State University Chris Goldfinger states, “The turbidite data matches up almost perfectly with the tsunami record that goes back about 3,500 years” (Fig. 1 map of sample locations)(1). He goes on to say that Tsunamis won’t always leave a signature and that the ones that do will deposit coastal substances or marsh deposits, and this will coincide with earthquakes.
Documented data shows that there is a much higher recurrence of major earthquakes at the southern margin of Cascadia than in the north. Goldfinger, a professor in OSU’s College of Earth, Ocean, and Atmospheric Sciences and lead author of the study thinks that it is overdue for a major quake. Further Evidence In 1990 Adams looked at the discovery of the first buried marsh sequences on land; he analyzed the core samples for evidence of the Holocene earthquake history in Cascadia zone. The samples were from Cascadia, Astoria, and the Rogue channels and their associated canyons and tributaries.
The Cascadia cores nearly all contain evidence of an ash layer from Mt. Mazama, (now Crater Lake caldera); this ash was deposited in the channel system through the drainage basins of rivers. He also discovered that the Cascadia channel has 13 turbidites and these correlate along the channel. Resulting evidence demonstrates the time line of these events and establishes the “pattern” of mega-earthquakes. Some data revealed that Cascadia may also record of mix of storm and earthquake “signal” in their early Holocene sections due to low sea levels.
There is also a lack of turbidite “triggering” in the Cascadia Basin by storm (El Nino) and flood events such as the 1964 Alaskan earthquake and Tsunami(2). The Cascadia Turbidite Record Figure 1: Cascadia margin turbidite canyons, channels and 1999-2002 core locations are marked on map above. Major canyon/channel systems are outlined in blue. Bathymetric grid constructed from newly collected multibeam data in 1999, Gorda Plate swath bathymetry collected in 1997 (Dziak et al. , 2001), and archive data available from NGDC. Primary core sites shown with yellow rim, all other 19992002 cores are grey.
White core numbers preceded with cruise number “M9907”, collected on the RV Melville, Yellow symbols are preceded with the cruise designation “RR0207”, collected on the R/V Revelle. Core EW950416PC shown in red. Earlier OSU cores are shown in grey. “PC” = Piston Core? “BC” = Box Core? “KC = Kasten core? “GC” = Gravity core? “TC” = Trigger core. Trigger cores omitted for clarity. Inset of Effingham Inlet shows collection site of Pacific Geoscience Centre (PGC) collected piston cores (http://activetectonics. coas. oregonstate. edu/Cascadia_turbs. htm) (3).
Analyzing the Data Looking back at the Japanese tsunami evidence from the Jan. 6, 1700 (it being the youngest great earthquake on the Cascadia subduction zone) and comparing between the onshore and offshore paleoseismic records along the Cascadia margin, as well as with the high resolution AMS radiocarbon dates to validate the turbidite event record and establish a good method for dating long term paleoseismic history. Onshore and offshore paleoseismic radiocarbon dating and available AMS radiocarbon dates core for each turbidite event show that the average interval for full margin paleoseismic events to be about 500,000 years, give or take 200 – 1,200 years.
The Holocene event record shows a probability for full margin rupture within the next 50 years. The turbidite mass found along separate margin sites suggesting that the turbidites may crudely demonstrate earthquake magnitude. There have been four Holocene cycles of two to five quakes with long intervals of around 7,000,000 years found in the seismic record. The ruptures along the northern and central Cascadia margin show thick sediments supporting this data. This data indicates that the next likely event should be along the southern margin (3).
Figure 2: Space time diagram for the Cascadia margin shows Holocene marine radiocarbon data and stratigraphic correlations. Filled symbols are marine 14C ages; smaller filled symbols are hemipelagic calculated ages. Marine data plotted as 2s midpoints and 2s ranges. Dashed lines show stratigraphic correlation of the turbidite data, which show deviations from the preferred age range where correlation overrules an individual 14C age. Up arrows show the marine data where site-wide erosion suggests a maximum age.
Marine error ranges are RMS 2s propagated errors. Smaller southern Cascadia events are shown with thinner dashed lines. Green bars are best fitting offshore/onshore age trends for Cascadia earthquakes. Land data plotted as published, with some sites revised as discussed in text. Preference among land sites given to recent publications using well constrained ages. Down arrows indicate minimum ages as published (land only). The two sided arrows show where maximum and minimum ages averaged (land sites only) (3).
Figure3: Summary core logs and digital photographs of all sections of Cascadia Channel core M9907-25Pc Turbidite numbers shown on ovals (3). More Current Research Research at the northern margin of the Cascadia subduction zone has revealed slow slip on the deeper interface of the zone. Tremor-like seismic signals found correlate with slip events identified from the crustal motion data from a six year study. This episodic tremor phenomenon can be used as a “real-time” indicator of the stress on the Cascadia megathrust earthquake zone.
Slip events have been detected deeper (25 – 45 km) in the northern part of the zone by observation of transient surface deformation using Global Positioning system (4). Can We Minimize the Damage? Scientists met in Seattle in April of 2014 to discuss an expected magnitude nine subduction zone earthquake along the coast of British Columbia, Washington, Oregon, and northern California. The last quake that size was over 314 years ago and earlier data has shown that there is a 240 year cycle for this type of earthquake. In this meeting they discussed what steps could be taken to prepare and recover from a quake of that magnitude.
Using data gathered from tsunami reports from Japan and local reports of a “giant wave” along with evidence from rings in old trees killed when marsh land along the Washington coast dropped several feet allowing sea water to envelope their roots (this supports the report from natives) scientists estimated the possible damage. They called this initiative M9 (5). Conclusion Researchers have worked tirelessly to collect data to piece together the history of the Cascadia Subduction Zone; for me at least it has made this a fascinating subject to read about.
Data collection comes from many sources and many hours spent in the field. I have learned the different methods that scientists use to determine the time line and events of these natural occurrences. In human terms the events would be disastrous but are only part of our natural world. Unfortunately the mountainous amount of data demonstrates that we overdue for a major disaster. In their quest scientists are discovering the answers to what has happened and what we can expect in the future.
