This morning a 8.9 magnitude earthquake struck near the east coast of Honshu Japan sending a tsunami around the Pacific Ocean. While watching the news, it can be easily said that Japan has felt the greatest impact of the earthquake as well as the after effects of the earthquake generated tsunami. My heart goes out to all that have been affected by the earthquake and tsunami. Recently, I had the pleasure of visiting Nagoya and Kyoto Japan for my best friend’s wedding – thankfully her family and friends are safe – so this earthquake and tsunami feels alot closer to home after having an emotional connection with the country.
Living in the Puget Sound area is it possible for a Pacific Ocean generated tsunami to reach the inland areas of Mukilteo, Everett, and Tulalip coast lines? More than likely not. A tsunami generated in the Pacific Ocean would have difficulty working its way through the Strait of Juan de Fuca, Admiralty Inlet, Deception Pass, and around the southern point of Whidbey Island to reach Snohomish County. The channel and many islands in the Puget Sound would help shield inland residents from the destructive force of raging water though water levels could rise. The Washington Peninsula is a different story. Recent reports by King5.com have shown the first set of waves striking the Washington coast line.
The probability of casualties from the Honshu tsunami are unlikely but there is a chance for coastal damage within in the harbors and ports. One casualty has been reported when 5 people where swept out to sea while taking pictures on the California coast which was impacted more greatly than in Washington.
Tsunamis are triggered by earthquakes, under water landslides, and coastal landslides which displace large volumes of water. The amount of energy released during these events is massive and that energy is then transferred into the water where is takes the form of a wave. The waves generated from these waves can range from a few inches to hundreds of feet tall. Still fresh in the World’s memory is Indian Ocean earthquake of 2004 that killed 200,000+ people.
Can a Tsunami Happen in the Puget Sound?
Yes. In the Puget Sound the two major tsunami threats stem from earthquakes and land slides. If a major earthquake happens in the Seattle area and it is violent enough to trigger a Puget Sound tsunami it won’t be like the one’s unfolding today. Imagine a bowl filled with water and what happens when that bowl is shaken from right to left? The water splashes about surging up the sides of the bowl. The effect would be very similar in the Puget Sound with it taking a while for the waters to calm down due to the waves bouncing off the shorelines and back into the deeper water to then bounce back towards the shore.
Q: Where are the major faults in the Pacific Northwest?
A: There are many faults in the Pacific Northwest that can produce damaging earthquakes, including hard-to-identify faults that exist entirely underground and have not been identified at the earth’s surface. At the same time, some mapped faults have been located that have not generated earthquakes in recent geologic time. New faults continue to be discovered as more field observations and earthquake data are collected.
There are three different sources for damaging earthquakes in the Pacific Northwest. The first of these is the “Cascadia Subduction Zone”, a 1000 km long thrust fault which is the convergent boundary between the Juan de Fuca and North American plates and is the most extensive fault in the Pacific Northwest area. It surfaces about 50 miles offshore along the coasts of British Columbia, Washington, Oregon and northern California. No historic earthquakes have been directly recorded from this source zone. According to recent research, an earthquake estimated to be as large as 8.0 to 9.0 occurred in this zone in January of 1700.
The second source for damaging earthquakes is the Benioff Zone. This zone is the continuation of the extensive faulting that results as the subducting plate is forced into the upper mantle. The Benioff Zone can probably produce earthquakes with magnitudes as large as 7.5. Benioff Zone earthquakes are deeper than 30km.
The third source consists of shallow crustal earthquake activity (depths of 0 to 20 km) within the North American continental plate where faulting is extensive. Past earthquakes have revealed many shallow fault structures, including the Western Rainier Seismic Zone and the Mt. St. Helens Seismic Zone. Our best known crustal fault, the Seattle Fault, runs east-west through Seattle from Issaquah to Bremerton. This fault generated a very large earthquake approximately 1100 years ago. Other crustal faults have been located in the Puget Basin region. Click Here for a map of major faults located in the Puget Basin. [SOURCE]
A landslide would have similar long term effects but the initial strike would be much different. Any boater can tell you about the massive cliffs of Whidbey, Camano, and Hat Island. If a large piece of one of these islands collapsed into the Puget Sound the residents directly across from it would see a hundred feet high traveling a few hundred miles an hour rising up their banks. Landslide generated tsunamis are not as uncommon as one would think in Washington and as a recent as 1949 the Tacoma Narrows area was affected.
One of the main factors contributing to landslides in the bluffs bordering Puget Sound is a contact between two separate geologic units within the bluffs, the Esperance Sand and underlying Lawton Clay (Galster and LaPrade 1991). The Esperance Sand layer is more permeable to water, relative to the Lawton Clay, and the plentiful rainfall in the area readily percolates through the sand (Downing 1983). Water does not tend to penetrate the more impermeable clay, but instead builds up in the overlying sand unit. Eventually a critical mass is attained and the slope fails. Landslide scars, where the Esperance Sand and other overlying units have moved downslope exposing the underlying Lawton Clay, are visible in many locations in the Puget Lowland (Downing 1983).
Seismically induced landslides in the Puget Lowland are discussed by Jacoby et al. (1992) and Logan and Walsh (1995). Martin (1995) has also described a landslide located in the central Puget Sound area near Mukilteo which transported a large volume of material, approximately 1.18 x 107 m3, from the nearshore bluffs into the deep waters of Puget Sound. The inferred age of this landslide correlates closely to the time of the event on the Seattle Fault approximately 1000 ybp and may have been triggered by that seismic event. If the landslide wasn‟t triggered by the seismic event itself, it is possible that the tsunami generated by that event eroded the toe of the bluff, causing the slope to over-steepen and fail (Martin 1995). This
landslide transported material in a fluid manner and even though the result was submarine deposition, it probably did not result in a tsunami (Martin 1995). Bill Whiteaker 2008: [SOURCE].
Tsunamis are a real threat and ultimately unavoidable to the Puget Sound region. So much so, that Snohomish County planners have a “whole chapter dedicated to the nightmarish scenario in the county’s 2010 Natural Hazard Mitigation Plan“. That is a new piece of information and thanks to the Everett Herald for bringing it to the public’s attention.
Snohomish County Tsunami Map and Impact Areas
Tsunami Information on Around the Web
- National Oceanic and Atmospheric Administration (NOAA) – http://www.noaa.gov/
- Pacific Tsunami Warning Center – Pacific Regrion – http://ptwc.weather.gov/?region=1
- Twitter #PrayForJapan
- West Coast and Alaska Tsunami Warning Center – http://www.tsunami.gov/