Evidence of large ancient earthquakes of the Cascadia fault

The coastal salt marshes of the Cascadia subduction zone (CSZ) preserve evidence from several large earthquakes, between 8 and 9 in magnitude, according to paleoseismologists. So far, the scientists have been using only the stratigraphic data records to gain insight into the mechanism of ancient earthquakes, however, the microfossils from the area have been discovered to contain more records on these intense events, the Geological Society of America (GSA) announced on November 1, 2015.

Now, for the first time the scientists have used both stratigraphic data and records from the microfossils to observe three CSZ earthquake zones at Jacoby Creek, Arcata Bay, California.

Stratigraphic evidence collected over the last 30 years provided evidence of the subsidence that occurred during earthquakes beneath the salt marshes of Humbolt Bay, California, at the southern end of the CSZ. The researchers from the University of Rhode Island now use the foraminera fossil to estimate this subsidence north of Humboldt Bay, at Arcata Bay.

"Previous investigations were able to provide estimates of subsidence with large errors, which are only so helpful to the modelers," J. Scott Padgett, University of Rhode Island, noted.

The new, improved, analysis on the microfossil data in Oregon allowed the scientists to estimate the subsidence with significantly smaller errors. This provided the seismologists with much more accurate data to enable producing earthquake models more consistent with observed subsidence measurements seen in today's instrumented earthquakes.

A research similar to this one is also taking place at a small coastal drainage, Jacoby Creek, flowing into northern Arcata Bay. Three sharp contacts between salt marsh peat and intertidal mud, dating over 2 000 years back in the past have been uncovered in the area. Plant macrofossils at the top of buried peats have been estimated to be 195, 1 280 and 1 710 years old, according to radiocarbon analysis. These, newly estimated, ages have provided tighter constraints on the timing of past earthquakes and subsidence at the southern end of the CSZ.

Strong evidence has been provided to support the results of the research conducted at Jacoby Creek: "These include the sharp mud-over-peat contacts that are laterally continuous over 5 kilometers, changes in fossil foraminifera assemblages across the buried peat contacts, long-lasting submergence also derived from fossil foraminifera records, and radiocarbon ages of plant macrofossils taken from buried peat deposits that are consistent with other southern Cascadia earthquake chronologies derived from buried peat and tsunami deposits."

Plate tectonics along the Pacific Northwest region of North America. Image credit: Google / Stephen Hicks

The stratigraphic records in combination with the microfossils from the CSZ area are expected to allow the researchers to learn more about potential major hazards in store for the area, as well as provide more information about the large ancient earthquakes in general.

Reference:

• "Stratigraphic and microfossil evidence for three Cascadia subduction zone earthquakes at Jacoby Creek, Arcata Bay, California, USA", J. Scott Padgett, Simon E. Engelhart, Eileen Hemphill-Haley, Harvey M. Kelsey, Robert C. Witter – 2015 GSA Annual Meeting in Baltimore, Maryland US

Featured image: Cascadia Subduction Zone. Imagery ©2015 Google.