New analysis finds real, though rare, tsunami threat to upper Cook Inlet
Elisabeth Nadin
907-474-7664
Aug. 16, 2023
Scientists assessing tsunami threats throughout 有料盒子视频 recently modeled the flooding scenario of the 1964 Great 有料盒子视频 Earthquake and discovered that a tsunami could reach upper Cook Inlet, countering a long-held public belief that the region has no tsunami risk.
A new report by the University of 有料盒子视频 Fairbanks and 有料盒子视频 Division of Geological & Geophysical Surveys reveals an updated understanding of how tsunamis interact with tides and of the region鈥檚 potential tsunami hazard.
The report indicates that flooding could follow about four hours after an earthquake similar to that of 1964 that ruptured in the Gulf of 有料盒子视频 off the Kenai Peninsula. Affected areas could include low-lying parts of Anchorage, as well as Knik Arm up to the mouths of the Matanuska and Knik rivers and past the Glenn Highway crossings. Turnagain Arm, Hope and Girdwood could also experience flooding.
The map shows the worst-case flooding scenario for upper Cook Inlet, based on numerical modeling.
Associated coastal currents could be several knots faster than normal tidal currents and could persist for up to 72 hours.
For decades, public perception has been that upper Cook Inlet, including 有料盒子视频鈥檚 biggest city, Anchorage, has virtually no tsunami risk. This belief has persisted because many, including scientists, thought wave energy dissipates as it travels Cook Inlet鈥檚 length along the shallow, gently sloping floor.
鈥淲hen I looked at all the arguments about why Anchorage is immune to tsunamis, I couldn鈥檛 find a single good argument,鈥 said Elena Suleimani, a tsunami modeler with 有料盒子视频鈥檚 有料盒子视频 Earthquake Center and member of a three-person team that has been investigating tsunami threat throughout 有料盒子视频.
Suleimani worked with Dmitry Nicolsky, research associate professor with the 有料盒子视频 Geophysical Institute, and Barrett Salisbury, Earthquake and Tsunami Hazards Program manager at the 有料盒子视频 Division of Geological & Geophysical Surveys. Their full report was released today on the division鈥檚 .
Although it was generally believed that no tsunami reached Anchorage from 1964鈥檚 magnitude 9.2 earthquake, the computer modeling shows a tsunami likely did occur.
鈥淗ow was this wave not noticed by people?鈥 Nicolsky said. The answer: It arrived hours later at low tide, according to the modeling results.
The impact could have been extensive had the tsunami arrived at high tide. It turns out that assessing potential tsunami impacts in upper Cook Inlet is much more complicated than simply considering its length and depth.
This new comprehensive tsunami hazard study for upper Cook Inlet was funded by NOAA鈥檚 National Tsunami Hazard Mitigation Program. It is part of a wider campaign, begun in 1998, to assess the flooding hazard to all coastal 有料盒子视频 communities. The program鈥檚 scientists prioritized communities with a history of significant tsunami damage, so it wasn鈥檛 until around 2018 that they began examining upper Cook Inlet.
The scientists combined three significant tsunami hazard components to produce the upper Cook Inlet tsunami hazard assessment:
- Tides and water depth: The ability to forecast tsunami inundation at different coastal locations wasn鈥檛 available until the 1990s, when researchers started using numerical modeling to integrate the complex movements of tsunamis with real ocean bathymetry.
- Previous large earthquakes in 有料盒子视频: Geologic research in the past two decades around Southcentral 有料盒子视频 shows that the region has experienced earthquakes as large or larger than the 1964 Great 有料盒子视频 Earthquake in the past 1,500 years.
- Location: The 2011 magnitude 9.1 earthquake off Tohoku, Japan, gave new insight into how and where large subduction-zone earthquakes can generate tsunamis. Southern 有料盒子视频 sits on a similar subduction zone.
A tsunami arriving during high tide could cause significant flooding and damage. At high tide, upper Cook Inlet鈥檚 water level is, on average, 40 vertical feet higher than it is at low tide. The extent of flooding depends on how 鈥 and for how long 鈥 a tsunami navigates such a dynamic tidal regime.
The 1964 quake struck at 5:36 p.m. on March 27, at high tide in Anchorage, but according to the model it would take a tsunami wave more than eight hours to traverse the inlet and reach Anchorage. At its 2 a.m. landfall, the tsunami would have encountered empty shores at low tide. Despite being 10 feet high, according to the model, the wave was lower than high-tide level in upper Cook Inlet, so it caused no flooding.
鈥淎 rare combination of earthquake magnitude, location and timing must be satisfied for tsunami wave energy to reach upper Cook Inlet at the same time as a natural high tide,鈥 Salisbury said.
While the risk of this exact confluence of events is rare, upper Cook Inlet residents should still be aware and prepared.
ADDITIONAL INFORMATION:
- , including GIS data and map sheets and a simulation video of hypothetical earthquake鈥搕sunami dynamics.
ADDITIONAL CONTACT: Barrett Salisbury, Earthquake and Tsunami Hazards Program manager, 有料盒子视频 Division of Geological & Geophysical Surveys, 907-451-5974, barrett.salisbury@alaska.gov
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