1964 Alaska Earthquake: Subsidence at Palmer Hay Flats
Welcome to an earthcache, where there is no container to find or logbook to sign. Instead, your task is to locate answers on-site to four questions about the geology of this location, and to email or message your answers to the cache owner to demonstrate what you’ve learned here during your visit.
The Palmer Hay Flats are part of the estuary zone located where the Knik River and Matanuska River enter Upper Cook Inlet. These marshlands carry within their peat layers a stratigraphic record of a cycle of subsidence and subsequent silt deposition. Studies of peat stratigraphy in the upper Cook Inlet region at Goose Bay, Kenai and Kasilof River flats, Portage Flats, Girdwood Flats, and Chickaloon Bay match evidence seen here at Palmer Hay Flats. In general, the peat layers show that about every 600-800 years the plant materials in the peat have been subjected to rapid immersion by water-borne silt deposits, in events which correlate with other records of significant seismic activity. In every case, ‘rebound’ has occurred, where the land rises in elevation following the co-seismic subsidence event and subsequent silt deposition.
The Palmer Hay Flats experience a secondary source of deposition from glacial outburst flooding events occurring on the Knik River. Nearly annual glacial outburst floods are recorded from 1948 to 1966, and these account for additional volumes of deposited materials on the Palmer Hay Flats. These glacial outburst deposition events accelerated the conversion of much of the area to land later found suitable for farming. As part of the Matanuska Colony Project during the Great Depression, farmers and their families moved to the Palmer region to establish new farms where no agricultural efforts had ever been undertaken. Much of the Palmer Hay Flats were high enough before the 1964 earthquake to be suitable for hay production and grazing of domestic farm animals, leading to the name applied to the area by colonists.
During the 1964 Alaska Earthquake the Palmer Hay Flats experienced 2 feet of subsidence – not enough to completely flood the Flats (as has occurred in the past according to peat stratigraphics), but enough to move the land into wetland status and make it unsuitable for the agricultural uses which had given the region its modern-era name. Today 28,800 acres of this landscape are managed as the Palmer Hay Flats State Wildlife Refuge. By comparison, subsidence in the Portage and Girdwood Flats area during the 1964 Earthquake measured more than 8 feet – enough to kill entire stands of coastal forest, whose skeletons stand today as stark reminders of that seismic event.
The Reflections Lake Trail brings you to a sign kiosk explaining how Tectonic Plate Subduction has worked to create the area known as the Palmer Hay Flats and its surroundings. Subsidence is a geologic process where one tectonic plate slides under another at a boundary zone due to differences in density. Knowing that peat stratigraphy has confirmed a cycle of regular seismic activity in this region, your task is to review the information presented on the sign kiosk and answer these questions designed to help you understand current geologic theory about how the landscape around you came into existence.
Email or message your answers to the cache owner – do not place the answers in your on-line log or it will be deleted. Please help build a photographic record of how this area looks across the seasons by providing photographs in your cache log which document its appearance during your visit.
- What is the direction of travel and annual speed of travel of the Pacific Plate relative to the North American Plate?
- How are the Kenai and Chugach Mountains forming, and what is this process called?
- Subduction zones are noted for their high rates of volcanism, seismic activity, and mountain building. What is the name of the nearest volcano, and how far away is it located?
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- Look at your location relative to the surface of the lake. Given that the lake surface is about 10 feet above average high tide, how much subsidence might need to occur to kill currently-established trees here such as happened at Girdwood and Portage during the 1964 Earthquake?
Sources:
Division of Geological & Geophysical Surveys, PUBLIC DATA FILE 93-84
INVESTIGATION OF PEAT STRATIGRAPHY IN TIDAL MARSHES ALONG COOK INLET, ALASKA,
TO DETERMINE THE FREQUENCY OF 1964-STYLE GREAT EARTHQUAKES IN THE ANCHORAGE REGION
by R.A. Combellick
Alaska Dept of Fish & Game - Palmer Hay Flats State Game Refuge Area Overview