The 1700 Cascadia Earthquake and the Implications

for the Next Big Event

Robert Bourque

Abstract

The runs for 1000 km off of the west coast of North

America, from 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 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 .

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 , 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 down to , 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 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 and , 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. The stories themselves come from a wide range of tribes, with details varying from the more descriptive to more fantastical, as the stories were passed down with the belief that the event was caused by higher beings. 32 independent sources were taken along the west coast, from tribes on Vancouver Island down to Northern California, along the length of the entire fault, to hear what stories have been passed down through generations.

In terms of the more realistic details, the most consistent ones are shaking and flooding, both of which resulted in the destruction of homes, and the latter resulting in the loss of lives.

These details are frequently accompanied by supernatural beings that exist within Native

American culture. The beings in question are the Thunderbird and the Whale; with the two of them in some sort of combat that result in the ground shaking and floods occurring. These fights result in the Thunderbird overpowering or killing the Whale, as the whale drags the bird to the bottom of the ocean, which is capable of creating thunder as the name implies.

Subsidence

One of the more important questions in regards to the 1700 Cascadia event is how much subduction actually occurred. This is a difficult task to make as several variables have a wide range of possibilities to them and provide a range of uncertainties. Most of what we know of the subsidence is through a combination of observed data from the previously mentioned sedimentary sequence and through modeling with different assumptions on the amount of slip or time between events. In regards to the sediments, the amount of sedimentation that occurred is variable between sites, which are already one issue (see Figure 3 below). Another issue is that the sedimentation rate may not necessarily have been constant at all sites, and depending on local conditions could have been slower or quicker. Furthermore, the local environments could have been subjugated to different levels of subsidence, as it is unlikely that the amount of subsidence that occurred along all 1000 km of the fault was uniform. In regards to the modeling, the aforementioned uniform slip is one issue that presents itself, as earlier papers assumed uniform slip simply due to the lack of more precise instruments that are present now. Another issue is deciding the parameters to influence, given how no direct measurements are known on the event.

Using either the range of time between the 1700 event and the prior event, the amount of slippage to have occurred, or the direction of the dip were all factors that needed to be tested in models that would then need to be compared to the sedimentary observations.

Figure 3. Stratigraphic sections from Vancouver Island, after Clague (1994).

One of the earlier subsidence tests assumed uniform slippage along the entire fault and relied primarily on the measurement of sediments that were deposited from the 1700 event as a comparison to test their models against. However, data for comparison was rather limited, as the only area that was extensively covered was most of Oregon and the southern half of Washington, with only a few data points from California and Vancouver Island. The models looked at either changing the timing between events or the amount of displacement that occurred during the slip, with the time variables having more consistent results with the observations. As for the results of subduction, northern Oregon and southern Washington experienced the most subsidence, at 1 to

2 m, while all the other areas experienced at most 1 m of subsidence. Figure 4 bellow demonstrates the results that were obtained in terms of the amount of subsidence that occurred.

Figure 4. Measure of coseismic subsidence, taking postglacial rebound into account, after

Leonard (2004).

A more recent paper looked at the level of subduction once more, only this time emphasizing the possibility of heterogeneous slip having occurred along the fault plane. The model was also more complex as they focussed on heterogeneous slip both along the strike and along the dip, providing a wider range of possibilities. Little information regarding dip slip was available though, so the best models that they came up are those that emphasized differences in slip along the strike of the fault. As with the previous study, it found that there were peaks in subsidence within Oregon and Washington, though it wasn’t continuous and was broken up by regions of lower subsidence, with the data from California having the largest margin of error in regards to the amount of subsidence that occurred. One particular area that experienced very little subsidence was near Alsea Bay in Oregon, which the researchers suspected was due to a seamount that is subducting underneath the North American plate, acting as a cushion to weaken the amount of subsidence that would occur during a slip event.

Future Warnings

As stated earlier, current estimates for the reoccurrence interval for the Cascadia subduction zone is between 500 and 800 years, though evidence from previous events suggests it can be as low as 300 years, which is a fair warning that Cascadia can go off at any moment, and we are grossly unprepared for what it can do. As the existence of earthquakes along this zone was only discovered in 1994, the public and many officials are unaware of the possibility of a magnitude 9.0 earthquake occurring along such a huge stretch of land. For cities and towns right along the coast, there is almost no hope for them as it is, with waves that are estimated to be anywhere from 7 to 33 meters in height. Models show that it won’t be just one wave coming in, as waves will continue to hit the coast for hours after the slip event occurs, assuming that the entire fault even goes off at once. It will be especially bad in the bay between Vancouver Island and the mainland, as the narrow passage funnels waves into the bay and amplifies them.

Along with the immediate damage from the earthquake and tsunamis, rebuilding after the damage is done will not only be a tremendously long task, but an incredibly expensive one.

Homes will collapse, foundations will break down, and entire power grids will fail. After the chaos from the shaking and waves is over though, rebuilding will take an extensive amount of time and investment, with the complete rebuilding of the west coast taking at least 2 years of work. The reconstruction will not be the worst of it though, as communities that will become submerged from the subduction will be left homeless, and many of the other regions that will be affected will leave people homeless or without basic necessities for months.

Despite the tragedy that this event can bring, there are measures that can be taken to help mitigate the damage. Reinforcing existing structures to make them more stable during earthquakes is one of the major factors, as the shaking alone from the earthquake is likely to cause a lot of damage in areas where the floods will not reach. Preparing evacuation routes and emergency centers are about the best that can be done for trying to save people when the event occurs. However, the most powerful tool for protecting people from the tragedy is public awareness. People need to become aware of the possibility of this event occurring, become aware of what will occur and how to avoid it. Entire evacuation of the inundation zone will not be possible, but having people become aware of it will ensure that a good proportion of the population will know what to do when it occurs. Only by doing this can we mitigate the incredible destructive power that this event will bring.

Conclusion

As with all high-magnitude earthquakes, it is evident that Cascadia will be incredibly destructive, but what make it even scarier is how recently its existence is known, and how much damage is possible. While it is unclear how soon the earthquake will occur, it is clear that we are grossly unprepared for the events that are to come. Only estimates can be made as there is no directly recorded data on the event, but they are strong enough to tell us of the extent of damage that will be brought along the west coast.

References

Clague, J.J., and Bobrowsky, P.T., 1994, Evidence for a Large Earthquake and Tsunami 100-400 Years Ago on Western Vancouver Island, British Columbia: Quaternary Research, v. 41, p. 176-184. Satke, K., Shimazaki, K., Tsuji, Y., and Ueda, K., 1996, Time and size of giant earthquake in Cascadia inferred from Japanese tsunami records of January 1700: Nature, v. 379, p. 246- 249. Jacoby, G.C., Bunker, D.E., and Benson, B.E., 1997, Tree-ring evidence for an A.D. 1700 Cascadia earthquake in Washington and northern Oregon: Geology, v. 25, no. 11, p. 999- 1002. Ludwin, R.S., Dennis, R., Carver, D., McMillan, A.D., Losey, R., Clague, J., Jonientz-Trisler, C., Bowechop, J., Wray, J., and James, K., 2005, Dating the 1700 Cascadia Earthquake: Great Coastal Earthquakes in Native Stories: Seismological Research Letters, v. 76, no. 2, 140-148. Leonard, L.J., Hyndman, R.D., and Mazzotti, S., 2004, Coseismic subsidence in the 1700 great Cascadia earthquake: Coastal estimates versus elastic dislocation models: GSA Bulletin, v. 116, no. 5/6, p. 655-670. Wang, P.L., Engelhart, S.E., Wang, K., Hawkes, A.D., Horton, B.P., Nelson, A.R., Witter, R.C., 2013, Heterogeneous rupture in the great Cascadia earthquake of 1700 inferred from coastal subsidence estimates: Journal of Geophysical Research: Solid Earth, v. 118, p. 2460-2473. Schulz, Kathryn, “The Really Big One”, The New Yorker, Annals of Seismology (2015): 1-11. Ludwin, 1. Dating the 1700 Cascadia Earthquake: Great Coastal Earthquakes in Native Stories, 2005. Leonard, 2. Coseismic subsidence in the 1700 great Cascadia earthquake: Coastal estimates versus elastic dislocation models, 2004. Clague, 3. Evidence for a Large Earthquake and Tsunami 100-400 Years Ago on Western Vancouver Island, 1994. Leonard, 4. Coseismic subsidence in the 1700 great Cascadia earthquake: Coastal estimates versus elastic dislocation models, 2004.