Deep landslides triggered by the 1964 Alaska earthquake were not re-activated in the 2018 Anchorage earthquake

According to a new study, published in Seismological Research Letters, the deep landslides that were triggered by the massive M 9.2 Alaska earthquake in 1964 were not re-activated by the M 7.1 earthquake that hit Anchorage on November 30, 2018. 

About a year ago, Alaska's largest city was struck by a massive earthquake that caused significant infrastructure damage but, fortunately, no casualties. It was the strongest tremblor the region has experienced since the 1964 incident, the most powerful ever recorded in the United States. 

According to Dr. Randall Jibson, researcher at the U.S. Geological Survey (USGS) and lead author of the study, some cracks appeared on the regions where landslides occurred back in 1964 but the shaking was not intense enough to cause any significant movement. The research team visited the affected region some days after the 2018 shock to record the ground failures including cracks, landslides and liquefaction phenomena. 

Researchers concluded that the deep landslides triggered by the 1964 tremblor were not re-activated as the recent seismic shock had a lower duration and higher frequency. 

By examining this phenomenon, scientists came to an interesting conclusion about the 2018 earthquake. According to prediction modeling and the magnitude of the earthquake, a larger number of landslides was anticipated.

The researchers point out a major difference between the focal mechanisms of the massive tremblors that led to this fact. While the 1964 Alaska earthquake resulted from a massive rupture along the subducting boundary between the Pacific and North American plates, the 2018 earthquake occurred at a larger depth in a normal fault within a tectonic plate. “We feel pretty strongly that one of the reasons the Anchorage earthquake didn’t trigger so many landslides is because it was an intraslab event,” Dr. Jibson said.

Nevertheless, those findings do not imply that ground failures caused by the 2018 earthquake were minor. On the contrary, severe damage to infrastructure was caused especially as cracks opened between artificial and natural slopes. 

The new evidence aids in understanding the complexity of predicting an earthquake's effect.  “For the past 30 years we’ve made comparisons based on magnitude alone. But now we have seen enough earthquakes to know that it’s not just magnitude that affects landslides, it’s also focal mechanisms and tectonic settings and the frequency of earthquake waves,” Dr. Jibson concluded.