Five cities of Puget Sound are likely to feel a strong jolt in the massive earthquake

By: Ian Stone, Ph.D. Washington University Candidate (NoisyRocks)

Cascadia has experienced major earthquakes in the past. If something else happens today, the cities of the region will feel a tremor – some more than others.

Quote: Stone, I., 2020, Five Cities of Puget Sound are likely to feel a strong tremor in a massive earthquake, Tumblr, http://doi.org/10.32858/temblor.094

Although Puget Sound vibrates in Washington state every few decades, the damage in the region’s largest cities, including Seattle, Tacoma and Olympia, has been absolutely modest.

However, there is a huge threat lurking off the Pacific Northwest coast. Here, Juan de Foca tectonic plate is jammed under the North American plate at a rate of a few centimeters per year. The façade of the panels is the Cascadia subduction zone, an error that generated earthquakes of magnitude + 8.0 over periods of several centuries. The last major earthquake in the region was in 1700, when an earthquake of magnitude 9.0 shook the full length of the error, which runs from the northern California coast to Vancouver Island in Canada.

The Cascadia Subduction Zone is located off the northwestern Pacific coast. Red shading indicates areas of probable slip error during a magnitude 9.0 earthquake. Credit: USGS

The repetition of the 1700 earthquake today could cause massive damage throughout the Northwest Pacific. The risk is particularly high in Puget Sound, where major population centers grow directly in harm’s way.

A geological puzzle has been solved

An earthquake occurred in 1700 centuries before the seismometers were invented and widely installed. So we do not have direct measurements of the magnitude of the earthquake. Records of judges from eastern Japan and the oral history of Native America from the Pacific Northwest indicate that a large wave of water swept these regions around this time. It was not until the twentieth century that researchers compared these records with coastal geology where the source of this tsunami was identified: a 9.0 magnitude earthquake in the Cascadia subduction zone (Atwater et al., 2005; Sataki et al., 2003; Ludwin et al., Et al., 2005).

A disaster in the making

Since the beginning of European settlement in the late nineteenth century, the northwestern Pacific population has grown to millions, and the region is currently experiencing one of the largest population booms in the country. Within Puget Sound, Washington state, urbanization has turned muddy tidal apartments into the bustling coastal cities of Seattle, Tacoma and Olympia, which have rapidly grown into huge technical and manufacturing centers. Much of this urbanization occurred before realizing the threat posed by a potential major earthquake.

Downtown Seattle was damaged by the 6.9-magnitude earthquake in 2001. Credit: Seattle Municipality Archives

Knowing how the 1700 event shook the region will provide valuable information about areas that need further preparation before the next Great Cascadia earthquake. While there are no seismic records of the last Cascadia earthquake, seismologists can simulate shaking the Earth using error information and observations from other earthquakes in the subduction zone. With these results, we can see which parts of Puget’s sound will experience the strongest vibration during a possible repeat of the 1700 earthquake.

Estimated shaking in Puget’s voice

Estimated peak Earth velocities (PGV) in the Puget Sound region from an earthquake of magnitude 9.0 on the Cascadia subduction zone. The results represent the maximum average Earth speed of thirty possible rupture scenarios. Data from Frankel et al., 2018, and Wirth et al., 2018.

Earth’s peak simulation velocities of a magnitude 9.0 earthquake over the Cascadia subduction zone (taken from the results of Frankel et al., 2018 and Worth et al., 2018) highlight differences in the impact of such an earthquake throughout the Puget Sound area. The Earth’s Peak Speed ​​describes the maximum speed at which Earth moves during an earthquake and is usually well associated with perceived vibration intensity and – and damage – during large earthquakes. As a point of comparison, the Earth’s peak velocity of 1.1 mph (0.5 m / s) was strong enough to catch train cars during the 7.0 magnitude 1906 earthquake in San Francisco (Veeraraghavan et al., 2019).

Since we do not know where the center of the hypocenter or the precise sliding distribution of the 1700 earthquake was, what appears on the map is the average Earth’s peak velocity of thirty different rupture scenarios, each thinking of a different hypocenter and sliding distribution. While the Earth’s peak velocity of a city may change from one rupture scenario to another, the relative response between cities is often the same, as vibration is strongly influenced by the geological structure and location of the error. Using this data, we can estimate cities with a population of more than 50,000 in the Puget Sound region that is expected to experience the strongest shaking.

These results may not be immediately obvious. After all, it is estimated that Olympia, the closest city to the big hit error, is facing the least sharp shaking; Meanwhile, many cities on the east side of Puget Sound are seeing – out of error – a much stronger vibration. This stranger can be attributed to the influence of sedimentary basins, which trap and amplify seismic waves. The coastline is located near the middle of the Seattle Basin, which is why it is expected to experience the strongest shaking, with a higher ground speed of about 1.6 mph (0.7m / s). The cities with the least estimated shaking are either far from sedimentary basins or further east. This list is topped by Olympia, Everett and Samamesh, all of whom estimate the maximum Earth speed of about 0.7 mph (0.3m / s).

It is important to emphasize that when it comes to earthquake damage, the estimated shaking described above will only be part of the equation. Factors such as build quality and local soil conditions play an important role in how damaged the structure. For example, a house built well on hard rock in a strong vibration zone may be better than a poorly built house on sand or an industrial filling in a weak vibration area. The results shown here do not take into account topical soil conditions, so it may vibrate in a specific location to be stronger (or weaker) than shown.

It is also important to note that the city that vibrates the “weakest” in this analysis (Olympia) still estimates the Earth’s top speed near 0.7 mph (0.3 m / s), which is strong enough to be widespread felt and causing local damage Humble.

Seattle skyline. Credit: Communist

Prepare for the next earthquake

When the subduction zone is torn again, only the earthquake will be the start of the disaster. A recurrence of the 1700 earthquake is expected to paralyze infrastructure throughout the entire Pacific Northwest, and for many weeks after the disaster, citizens of the region will have to learn to live without access to fully functioning roads, water, electricity and sanitation, according to a group report. Earthquakes in the Cascadia region of 2013. This applies twice as often to remote areas in western Washington and Oregon.

The estimated probability of a 9.0 magnitude earthquake in the Cascadia subduction zone over the next fifty years is around 15% (Goldfinger et al., 2017). The probability of an earthquake of magnitude 8.0 or greater in the same time period, which will partially tear off the error and still cause a great deal of damage, is about 40% (Goldfinger et al., 2012). Also keep in mind that a 9.0 magnitude shock can produce an aftershock with a strength of approximately -0.8, as occurred 30 minutes after an earthquake of 9.0 magnitude Tohoku, Japan, in 2011.

You can use the Temblor app to look at the impact of a 9.0 magnitude earthquake on your home, in terms of cost and repair time, in the Temblor app. Temblor finds your site and takes information about your home from public databases and uses USGS, FEMA and Washington Geological Survey data and forms to estimate your losses.

It is important that those who live in the Pacific Northwest have a personal disaster plan. Emergency managers recommend getting a cache of water and non-perishable food that will support you and your family for up to fourteen days. Other suggestions for building an emergency plan can be found on the Washington Emergency Management website.

This article is the first in a series on earthquake hazards in Puget Sound. Stay tuned for detailed analyzes of the mentioned areas.

Want to know your earthquake risk? Check it out on Tumblr.

In-depth reading

Atwater, B. F., Musumi-Rokkaku, S., Satake, K., Tsuji, Y., Ueda, K., & Yamaguchi, D. K. (2005). 1700 Orphan Tsunami – Japanese Evidence for the North American Mother Earthquake: USGS Professional Paper 1707. https://pubs.er.usgs.gov/publication/pp1707

Frankel, A. D., Wirth, E. A., Marafi, N., Vidale, J., & Stephenson, W.J. (2018). Synthetic broadband seismic diagrams of 9 large earthquakes on Cascadia based on 3D simulation and random synthetic materials, Part 1: Methodology and overall results. Bulletin of the American Seismic Society, 108 (5), 2347-2369. https://doi.org/10.1785/0120180034

Goldfinger, C., Galer, S., Beeson, J., Hamilton, T., Black, B., Romsos, C., Patton, J., Nelson, CH, Hausmann, R., & Morey, A. ( 2017). The importance of site selection, sediment supply, and hydrodynamics: a case study of submarine paleontology on the margins of North Cascadia, Washington, USA. Marine Geology, 384, 4–46. https://doi.org/10.1016/j.margeo.2016.06.008

Goldfinger, C., Nelson, HC, Morey, AE, Johnson, JE, Patton, JR, Karabanov, E., Gutierrez-Pastor, J., Eriksson, AT, Gracia, E., Dunhill, G., Enkin, RJ , Dallimore, A., & Vallier, T. (2012). History of adverse events – methods and effects of Holocene dissolution in the subduction zone Cascadia: USGS Professional Paper 1661-F. https://pubs.usgs.gov/pp/pp1661f/

Ludwin, RS, Denis, R. Carver, D. , Macmillan, M, Lucy, R. , Clage, c. , Junetz-Tresler, C. (2005). Cascadia Earthquake Dating 1700: Great coastal earthquakes in original stories. Earthquake Research Letters, 76 (2), 140-148. https://doi.org/10.1785/gssrl.76.2.140

Satake, K., Wang, K., and Atwater, B. F. (2003). An error slip and seismic moment of the 1700 Cascadia earthquake inferred from the descriptions of the Japanese tsunami. Journal of Geophysical Research, 108 (B11, 2535). https://doi.org/10.1029/2003JB002521

Wirth, a. A., Frankel, A. Dr.. , My knowledge, n. , Vidal, J. a. , Stephenson, c. (2018). Synthetic large-scale seismic diagrams of 9 large earthquakes on Cascadia on the basis of three-dimensional simulations and random synthetic materials, Part 2: Rupture and Variation parameters. Bulletin of the American Seismic Society, 108 (5), 2370-2388. https://doi.org/10.1785/0120180029

Veeraraghavan, S., Heaton, T. H., & Krishnan, S. (2019). Less limits on ground movement at Point Reyes during the 1906 San Francisco earthquake from analyzing train trains. Seismic Research Letters, 90 (2A), 683-691. https://doi.org/10.1785/0220180327

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