Last week’s magnitude-7.6 Alaska quake raises questions about the nature of aftershocks.
By Elisabeth Nadin, Ph.D., Associate Professor, University of Alaska Fairbanks
Citation: Nadin, E., 2020, Alaska’s double quakes exceed expectations, Temblor, http://doi.org/10.32858/temblor.130
It is difficult to think of a magnitude-7.6 earthquake as an aftershock, but the one that struck southern Alaska on Oct. 19 appears to be exactly that. It came three months after the magnitude-7.8 Simeonof earthquake that partly “closed” the so-called Shumagin Gap, and was large enough to generate a tsunami warning for the region.
An aftershock is any earthquake of smaller magnitude that follows a main shock. These are distinguished from foreshocks, which come before the main event. Both are critical to deciphering the dynamics of a fault zone that is capable of large, destructive quakes. However, aftershocks don’t always follow the rules that scientists erect for them. One such rule, called Båth’s Law, states that the largest aftershock is expected to be one full magnitude lower than the mainshock.
“If you go by this rule, [the magnitude-7.6 on Oct. 19] is a larger aftershock than anyone expected,” says Natalia Ruppert, senior scientist with the Alaska Earthquake Center. “But it’s still an aftershock.”
Some people might take issue with this definition, notes Alaska Earthquake Center director Michael West. But, he says, “this earthquake probably wouldn’t have happened on [Oct. 19] had it not been for the earthquake that happened in July. Therefore, we call it an aftershock.”
An uniquely large aftershock
Seismic activity has been steady in the region since the July 22 Simeonof quake, with about 2,400 aftershocks above magnitude-1.6 filling a 50-mile-wide (80 kilometers) region between the mainshock and last week’s magnitude-7.6 aftershock. The largest of these — until now, that is — was a magnitude-6.1 a few days after Simeonof. All aftershocks have fallen within the approximately 60 x 120 mile (100 x 200 kilometer) region that ruptured in July. The Oct. 19 event was within this aftershock zone. In addition to the quake’s notably high magnitude, its location and sense of motion are also intriguing.
July’s mainshock ruptured the Pacific–North American plate interface in a thrusting motion. This recent aftershock was deeper — likely in the middle of the thick Pacific oceanic slab that is descending beneath North America. Its fault-motion “solution,” called a focal mechanism, indicates that the fault plane was oriented north–south, perpendicular to the fault that ruptured in July.
The aftershocks don’t match
Because of how different the earthquake types were — one occurred on a thrust fault and the other had strike-slip motion — it’s worth asking if there was any buildup to the more recent event. “The thinking is that there is some delayed stress triggering involved,” says Ruppert. In other words, the July earthquake relieved stress on the particular fault that ruptured, but that event loaded stress onto a different fault, which subsequently ruptured in a different way.
Following a large earthquake, the U.S. Geological Survey (USGS) constructs a model of how much a fault slipped in the event, which can shed light on regions of potential future ruptures. In the case of the July quake — a thrust event — the majority of aftershocks were consistent with thrust faulting. The few that didn’t match raised no eyebrows. But, interestingly, the USGS model for that quake did not match the aftershock distribution, says Ruppert. Last week’s magnitude-7.6 is also inconsistent with the model, which has left some open questions.
The particularly large October aftershock is an object of special focus because it suggests that, contrary to prevailing thought, a singular large quake doesn’t necessarily mean stress disappears from the area. This aftershock highlights the importance of understanding the full network of faults in an area, and how stress can be passed from one to another.
“We don’t know enough about [last week’s quake] now to make any more definite interpretations. We’d like to wait a little bit to relocate more aftershocks to see where they fall,” says Ruppert. The Oct. 19 quake has triggered its own well-defined region of aftershocks, whose locations will shed further light on the dynamics of what has ruptured where in this complex region of faulting.