ANCHORAGE (KTUU) — Friday’s big earthquake happened deep within the Earth in the zone where a slab of a Pacific tectonic plate is being pulled under another — and then cracked.
It was the cracking that created the earthquake, said Peter Haeussler, a research geologist with the U.S. Geological Survey in Anchorage.
Partly because it was so deep — about 27 miles — it wasn’t as strong as it could have been, he said.
According to Michigan Technological University in Houghton, Michigan, a 7.0 earthquake is considered to be “major” with “serious” damage. About 20 occur around the world each year, the university said.
“This earthquake was part of the many earthquakes that occur around the so-called Ring of Fire,” Haeussler, of the USGS, said in an interview. “In southern Alaska, offshore, we have what’s called the Pacific Plate and it slides underneath the southern Alaska margin and it essentially gets recycled into the mantle down below. At the shallow part, where that plate slides past, you make the really big earthquakes, like the 1964 earthquake, or globally, like the one in Japan in 2011, or the great Chile earthquake in 1960.”
Then there are earthquakes related to bending and pulling of this slab as it goes down into the mantle, Haeussler said. “This earthquake occurred because that slab, as it’s getting pulled down into the mantle, is pulled apart a little bit.”
Haeussler said that Friday’s 7.0 Cook Inlet quake probably had similar forces of motion as the 9.2-magnitude 1964 Good Friday Earthquake in Alaska. “It just went on for something like nine times longer,” he said of the ’64 quake.
“As a friend of mine said who went through both, this one was scary —‘64 was terrifying.”
The aftershocks from the 7.0 earthquake could continue for two years, Haeussler said, though their frequency and strength will likely diminish over time.
“There’s always a statistical possibility that there could be an aftershock that’s equal in size or even larger to the main shock, but the percentage is quite small,” he said.
Repeating a phrase often used by seismologists, Haeussler said, “Earthquakes don’t kill people, buildings do.” Even worse are the components of a building that could become loose in a quake, he said.
“Water heaters, things inside buildings that are not part of the actual frame construction of the building often cause some of the biggest damage in earthquakes,” Haeussler said.
“There’s no doubt that since the 1964 earthquake that building codes for making earthquake resistant and resilient buildings have improved a lot in Southcentral Alaska,” Haeussler said. “There’s no doubt that there’s been a relatively progressive geotechnical and engineering community in southern Alaska that has strongly advocated for better building standards within this region and I think that we can definitely give thanks to those people that things were not far worse in this earthquake.”