By road, boat, plane and helicopter: Creating a remote seismic measurement network

Access to SALMON station SALA in western Cook Inlet, showing challenging conditions. At high tide, all the rocks in view are underwater. Here a three-wheeler pulling a trailer is being driven onto the boat at a falling tide. Note the typical bluffs in the background; these bluffs posed challenging access to sites on top.(Credit C.Tape)

-by Sarah Derouin

The Cook Inlet in Alaska is a remote and geologically fascinating place. The Inlet lies in the southern Alaska subduction zone, sits in front of a volcanic arc, and contains a sedimentary basin atop hard bedrock. Seismic waves travel at different speeds between hard rock and soft sediment, and in the bowl-shaped basin of Cook Inlet, the waves can slosh around like water in a bathtub.

To better understand the subsurface structure in the region and assess its seismic hazards, we installed 28 posthole seismometers in remote areas across the Cook Inlet. We dubbed the network SALMON: the Southern Alaska Lithosphere and Mantle Observation Network. Some of these stations were placed in very rough terrain only accessible by helicopter or boat, including two on an island in Cook Inlet.

Typically, seismometers are placed on hard, solid bedrock to get the most accurate data to locate an earthquake. However, we placed the stations throughout the basin and also in a line that slices through the subduction zone. The goal was not to locate an earthquake but instead to get an idea of how the basin is shaped and where the strongest shaking will occur during an earthquake. Early results reveal that both small-magnitude rumbles and seismic noise from the regularly creaking and groaning Earth are amplified in Cook Inlet basin.