Thousands of small earthquakes, detected for the first time by a machine learning process, reveal the distinct, razor-sharp edge to the Yakutat microplate as it subducts beneath the North America plate.
The Yakutat oceanic plateau is caught in the middle of a tectonic traffic jam with the Pacific plate as it subducts beneath the North American plate. The position and structure of the plates in this congested zone play a significant role in the earthquake and volcanic landscape of south-central Alaska.
The research published by Meghan Miller of Australian National University and her colleagues in The Seismic Record now shows the edge and extent of the Yakutat plate in astonishing detail.
Using data collected by permanent and temporary seismic stations, including a temporary array deployed by the researchers from 2018 to 2021, Miller and colleagues used a machine learning workflow to develop an expanded earthquake catalog for the region.
Their analysis revealed a 250-kilometer-long linear cluster of about 1750 earthquakes running northwest to southeast — a cluster that had never been identified in previous studies.
Using ambient seismic noise data to further map the region at depth, the researchers conclude the line of earthquakes marks the edge of the Yakutat microplate as it slips shallowly and directly under the North American plate, without an intervening mantle wedge as often occurs in subduction zones.
The new extent of the Yakutat microplate places it directly below the apex of curvature of the Alaska range and the Denali fault, the major continental fault system in south-central Alaska.
In their paper, Miller and colleagues propose that seismic stress caused by the collision could propagate through the overriding North American plate up to the Denali fault, and may have been the initial cause of the 2002 magnitude 7.9 Denali Fault earthquake.
Miller said the newly defined Yakutat edge fits well with an earlier study that used a different seismic signal, called tectonic tremor, to suggest the Yakutat plate extended farther eastward than previous estimates.
"This linear feature, that no one has seen before, basically lines up exactly where the end of this tremor signal," Miller said. "It was putting all of these different pieces together that I think makes a really convincing argument to suggest that this is the edge of the Yakutat plate."
The combination of tremor and earthquakes could mean that the composition of the Yakutat plate differs along its extent, the researchers noted.
Tremor west of the "razor edge" indicates a rock composition that allows slow, continuous slipping where stress can't build up to create an earthquake. The edge of the plate defined by the earthquakes suggests a different composition in that part of the Yakutat plate "that allows brittle failure," Miller said.
The new extent of the microplate also matches with the alignment of small volcanic cones around its northern and northeastern margins, suggesting that the missing mantle wedge between the Yakutat and North American plates may have begun to reestablish itself about 1 million years ago.
The next step for researchers will be to look further back in time, prior to 2018, to identify and locate more earthquakes along the Yakutat edge, and to also examine the configuration of the congested tectonic zone farther to the south, closer to the Alaskan coast.
Miller says the machine learning aspect of the study was essential to uncovering the Yakutat edge. "There's a lot of information hidden in the data that we're now able to extract out that we weren't able to see as easily with more traditional methods."
