The 1700 Cascadia Earthquake and the Implications for the Next Big Event

The 1700 Cascadia Earthquake and the Implications for the Next Big Event Robert Bourque Abstract The Cascadia subduction zone runs for 1000 km off of the west coast of North America, from Vancouver Island down to California. Though quiet for over 300 years, there is the possibility that the fault can go off at any moment. Everything that is currently known on the subduction zone has largely been found through sediment records, Japanese tsunami records, and tree-ring data from trees that experienced the event. Estimates on the amount of subsidence that occurred have been inferred from a combination of sediment records and 3D modeling of the region based on what little is known about the region. All of these point towards a truly powerful event that, if it were to occur tomorrow, would cause unprecedented damage to the Pacific Northwest. Introduction Earthquakes have been a major force on the planet ever since the formation of the continents some 4 billion years ago, and have been a constant presence ever since. We as humans have only been studying for a relatively short time, giving us an abysmally small record to work with, and in some cases, non-existent records. One particular region of the Ring of Fire, the most seismically active region on the planet, has been dormant over the past few hundred years, leaving us without any direct evidence of what a potential earthquake in the area would be capable; this region of course being the Cascadia subduction zone, as shown in figure 1. Figure 1. The Cascadia subduction zone, after Ludwin (2005). Stretching for over 1000 km from Vancouver Island down to northern California, this single fault plane is the last connection of the Explorer, Juan de Fuca, and Gorda plates with the North American plate, as the former plates subduct underneath the larger plate in an eastward fashion. The rate of subduction varies, though subduction is fastest at the northern end of the subduction zone at around 39 mm/year, and slowest at the southern end at around 25 mm/year. Furthermore, the rupture area of the subduction zone is approximately 1700 km2, with varying degrees of slippage occurring across the fault. The most recent estimates suggest that the timing interval between earthquakes is between 500 and 800 years, though these estimates are still up for debate. How this much information was obtained considering there has never been an earthquake that occurred along the Cascadia fault that was directly measured is an interesting combination of different fields which help to paint a picture of what has occurred in the past, as well as to fear what is yet to come. Sedimentation Evidence The first evidence of an earthquake to have occurred along the west coast that was not associated with the San Andreas Fault came from sedimentation records on Vancouver Island in 1994, British Columbia. Various sediment records found within the peat marshes on Vancouver Island showed very quick changes in sediment type, with the previous peat records being quickly overlain by sand. These sand beds are then slowly replaced by muds going upwards in the sequence, and finally are overlain by the modern day peat marsh conditions. This sequence is shown below on figure 2. This change in sedimentation is very uncharacteristic of the region, and lower beds do not show the same sequence as what occurred between the two peat beds, suggesting that an event caused the change in depositional environment. Figure 2. Formation of sand and mud layers as a result of rapid subsidence, after Leonard (2004). Further clues on the true origin of the event came in the form of foraminifera, unicellular organisms that leave behind their shells when they die, and are exclusively marine. The species found in the Vancouver Island sediments match with modern day species that live offshore from where the specimens were taken, suggesting that they were washed inland, though a tidal wave alone was unlikely as it took time for the sediments to be deposited. The deposition of sands and then muds, along with the presence of foraminifera, suggests that subsidence occurred from a tectonic event in the region, setting the region underwater for some time before the region was once again uplifted. This was evidence to show that the region was likely a result of a powerful earthquake due to the Cascadia subduction zone, the only likely candidate for subduction of this magnitude. While estimates for earthquake magnitude were not possible with the evidence available, the presence of foraminifera and vascular plants allowed the team to carbon date them in an attempt to put a time constraint on the event. Carbon dating suggested the event took place between 100 and 400 years ago, so between 1600 and 1900, which is a rather large gap in time. Japanese Records While European settlers did not yet reach the western coast of North America at the time of the earthquake to record it, the Japanese have been recording the dates and scales of earthquakes and tsunamis that hit its coastline long before the events of Cascadia. That is why, in 1996, a team of researchers looked into the Japanese records for any possible tsunamis that could have been correlated to the mysterious event. One particular tsunami was recorded to have hit three separate areas of the Japanese coast, causing minor damage to housings and crops. The arrival time of the tsunami started at midnight on January 27th, Japanese time, in 1700. In two of the recorded areas, the waves reached a height of approximately 3 meters, while the southernmost area that was hit only experienced a wave of 1 meter in height. The records also state how the tsunamis were unrelated to any local earthquake event as no shaking was experienced, just the waves themselves. The researchers also knew that the tsunamis were not the result of weather conditions out in the Pacific as it was the wrong time of year for them to occur, with the tsunamis occurring in the middle of winter while strong enough storms to generate powerful waves occur in summer. However, while the evidence suggested the waves came from a separate earthquake event out in the Pacific, there were several other candidates other than Cascadia that could have generated it. The first candidate was the Aleutian Trench running along the southern margins of Alaska. This subduction zone could not have worked though for the waves that reached Japan, as the trench is perpendicular to the coast of Japan, and waves generated by earthquakes are far weaker when going perpendicular from the zone of slippage, making it impossible for waves of that size to have reached Japan. Another possibility was the subduction zone along the western coast of South America. While this subduction zone could have generated the waves observed in 1700, European settlers did colonize the coast and there were no records of earthquakes at that time. The last other possibility was Kamchatka, a subduction zone off of the coast of Russia, and the closest one to Japan. Similar to South America, Russian settlers arrived in the region before the earthquake occurred, and the earliest record of an earthquake in the region was in 1737, too late to match up with the Japanese records. This left Cascadia as the only culprit for the 1700 earthquake. Along with the origin of the earthquake, the records of the tsunami can aid in estimating the magnitude of the earthquake and the timing of when the earthquake occurred. By comparing the height of the tsunami with the height of tsunamis that hit the west coast of North America from earthquakes that originated in Japan, one can work backwards to assume the magnitudes would have been roughly the same. Simulations showed that magnitude 8.0 earthquakes would produce waves of only 0.3 meters, which is consistent with earthquakes of similar magnitude occurring off the coast of Japan, which suggests that simulations of 9.0 are the most consistent with the observed data. Similar simulations were used to work back when the earthquake occurred, with the estimated time being 9:00 PM, January 26th in local time. Tree-ring data About a year and a half after the paper was published on the Japanese records of the 1700 Cascadia earthquake, another paper was published with more findings on the topic. This time more direct evidence from the earthquake along the coast of North America. The team of researchers explored Oregon and Washington, finding areas which would have likely been subducted bellow the water level and subjected to saline conditions. The team found a number of mostly dead, and a few living, trees from which they took samples of. Tree ring data was taken from all of the specimens and compared to each other in order to date them. As the trees are in a northern climate, they only grow during half of the year, making yearly dating an easy task to do. By comparing the living trees with dead ones, they were able to constrain the even to between the growing seasons of 1699 and 1700, which lined up perfectly with the Japanese tsunami records which were inferred to have been connected to this event. Along with a date, the tree ring data from trees that survived the event typically experienced stunted growth for several years after the event, along with reaction wood, a type of growth which is the result of a traumatic event, and can be identified from thicker tree rings. Native American Stories While not as accurate as some of the previous lines of evidence, the stories that have been passed down through generations by the Native Americans provide the only human accounts of what directly occurred in the Pacific North West at the time of the event.

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