by Allison Sayer for Seward City News-
Last week, a 7.9 magnitude earthquake shook the state. The prolonged shaking prompted many coastal residents to immediately seek higher ground. A tsunami warning was issued for Southeast and Southcentral Alaska, the Alaska Peninsula, the Aleutians, and British Columbia. Due to the size of the earthquake, tsunami watches were issued as far south as California.
Potential tsunami impact time in Seward was 1:55 am.
Seward residents knew to take this earthquake extremely seriously. A USGS report on the 1964 Earthquake describes the devastation in Seward: “Thirteen people were killed and five were injured as a result of the earthquake. Eighty-six houses were totally destroyed and 260 were heavily damaged. The harbor facilities were almost completely destroyed, and the entire economic base of the town was wiped out.”
And then… nothing much. The earthquake did technically generate a tsunami, defined by NOAA as a series of extremely long waves caused by a large and sudden displacement of the ocean. However, the waves were too small, and too far away from the coast, to create any havoc when they reached the shore. The sea level increased less than one foot.
By 3:12 am, the Tsunami Warning Center had downgraded Southcentral Alaska and the Alaska Peninsula to “advisory” status. Alerts were cancelled for all other areas.
Why Wasn’t the Tsunami Bigger?
There is a big difference between the tsunami risk from a “strike-slip” earthquake and an earthquake at a subduction zone. The reason for that is that a tsunami is generated by the displacement of water at the site of the earthquake.
A strike-slip earthquake occurs when:
1. Two pieces of earth’s crust are moving past each other laterally
2. Stress builds up, perhaps from one piece getting a little bit “stuck.”
3. The stress releases, and a bigger movement occurs all at once.
This can move quite a bit of water around during a large earthquake. However, it typically does not displace as much water as an earthquake that occurs in a subduction zone because most of the movement is from side to side, not up and down.
Most large tsunamis are generated by earthquakes in subduction zones, where one plate is diving underneath another. The same basic process occurs: The plates move slowly until stress builds up, and then move quickly (for plates), when the stress is suddenly released.
During these earthquakes, there is a lot of up and down movement at the seafloor, which displaces a lot of water. The depth of the sea floor also influences how much water is displaced.
Imagine all of the water between the sea floor and the surface being displaced upwards or downwards like a piston. That is why more water is displaced if the earthquake occurs in deeper water. It was this type of massive water displacement that caused the devastating tsunamis in Japan in 2011.
Huge Waves Can Occur Locally During Strike-Slip Earthquakes
Strike-slip earthquakes can cause damaging waves. However, these waves are usually not generated far out at sea. Rather, they are the result of landslides or undersea collapses at the local level that can be created by any type of large earthquake.
Landslide-generated waves are dangerous because they occur much sooner after the earthquake than tsunamis generated at the epicenter. However, they are typically localized just to the area surrounding the landslide. Other areas are not jeopardized by them.
Much of the devastation in Seward in 1964 was caused by waves, as opposed to the ground shaking. Some of these waves were generated at the earthquake epicenter. However, the aforementioned USGS report states that “much of the wave damage was caused when a huge strip of land 50–400 feet wide along the Seward waterfront, together with docks and other harbor facilities, slid into Resurrection Bay as a result of large-scale submarine landsliding.”
A Good Wake Up Call
Scientists currently can not predict where or when the next big earthquake will occur, aside from the fact that it is very likely to be somewhere on a fault line. We don’t know when the earthquake will come that can create real damage to human infrastructure in Alaska.
Hopefully the most recent earthquake helped residents to troubleshoot their own preparedness for an emergency in the future.
If you would like to learn more about tsunamis, including how to receive tsunami messages and tips on tsunami safety, the tsunami.gov FAQ page is a great place to start.