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Cascadia subduction zone & 1700 Cascadia earthquake

History

In the 1960s, underground fractures were uncovered by oil companies in Puget Sound. These were believed to be inactive through the 1990s.

In the 1980s, geophysicists Tom Heaton and Hiroo Kanamori had begun to compare the generally quiet Cascadia to more active subduction zones elsewhere in the Ring of Fire. They found similarities to faults in Chile, Alaska, and Japan's Nankai Trough, locations known for megathrust earthquakes, a conclusion that was met with skepticism from other geophysicists at the time.

Oral history

At the time of the 1700 earthquake, there were no written records of the event in Cascadia. Orally-transmitted legends from the Olympic Peninsula area talk of an epic battle between a thunderbird and a whale. Therefore, in a 2005 study, seismologist Ruth Ludwin set out to collect and analyze anecdotes from various First Nations groups. Reports from the Huu-ay-aht, Makah, Hoh, Quileute, Yurok, and Duwamish peoples referred to earthquakes and saltwater floods. This collection of data allowed her team to come up with an estimated date range for the event, whose midpoint fell in the year 1701.

Ghost forests

During low tide one day in March 1986, paleogeologist Brian Atwater dug along Neah Bay using a nejiri gama, a small hand hoe. Underneath the top layer of sand, he uncovered a distinct plant—arrowgrass—that had grown in a layer of marsh soil. This was proof that the ground had suddenly sunken under sea level, causing saltwater to kill the vegetation. The events had happened so quickly as to cause the top layer of sand to seal away any air, thus preserving the centuries-old plants.

In 1987, Atwater mounted another expedition paddling up the Copalis River with Dr. David Yamaguchi, who was then studying the eruptions of Mount St. Helens. The pair happened upon a section of "ghost forest," so-called due to the dead, gray stumps left standing after a sudden inundation of salt water had killed them hundreds of years ago. Originally thought to have died slowly due to a gradual rise in sea level, closer inspection yielded a different story: the land plummeted up to two meters during an earthquake. Having initially tested spruce using tree-ring dating, they found that the stumps were too rotted to count all the outer rings. However, upon having examined those of the western red cedar and comparing them to the living specimens meters away from the banks, they were able to approximate their year of death. There were rings up until the year 1690, indicating that the incident had occurred shortly thereafter. Root samples confirmed their conclusion, narrowing the time frame to the winter of 1699 to 1700.

As with the arrowgrass site, the banks of the Copalis River are lined with a layer of marsh followed by a layer of sand. Jody Bourgeois and her team went on to demonstrate that the sand cover had originated with a tsunami surge rather than a storm surge.

In 1995, an international team led by Alan Nelson of the USGS further corroborated these findings with 85 new samples from the rest of the Pacific Northwest. All along British Columbia, Washington State, and Oregon, the coast had fallen due to a violent earthquake and been covered by sand from the subsequent tsunami.

Yet another ghost forest was identified by Gordon Jacoby, a dendrochronologist from Columbia University, 60 feet (18 m) underwater in Lake Washington. Unlike the other trees, these ones suffered from a landslide rather than a dip in the fault during a separate event around 900 CE.