Using state-of-the-art tree ring and radiocarbon dating methods, researchers found the most recent major earthquake on the Seattle Fault wasn’t a solo act. The Saddle Mountain Fault, which slices across the Olympic Peninsula near Lake Cushman, ruptured at about the same time.
Their results were published Wednesday in the journal Science Advances.
The project, which spanned more than five years and included divers with underwater chain saws to sample trees drowned by the quakes, is a scientific tour de force nearly unprecedented in seismology, said Harold Tobin, director of the Pacific Northwest Seismic Network at the University of Washington. To help nail down the date, the team used a new approach that detects traces of ancient solar storms captured in tree rings.
But the findings add a new worst-case possibility to the seismic threats facing a region that’s home to 4 million people, said Tobin, who did not participate in the study. A seismic double whammy would be much more damaging than any single quake, especially to old brick and concrete buildings, and vulnerable bridges and infrastructure. It’s a scenario that hasn’t been factored into hazard maps, building codes and emergency planning — but it needs to be, Tobin added.
“The chance in any given year is not high, and there’s no reason to freak out because of this study,” Tobin said. “But it underscores that these are things that we need to be prepared for.”
A 2005 analysis estimated a relatively modest magnitude 6.7 quake on the Seattle Fault could kill 1,600 people, destroy nearly 10,000 buildings and cause up to $50 billion in economic losses. If the Seattle and Saddle Mountain faults ruptured simultaneously, the new study estimates, the resulting quake would clock in at magnitude 7.8 — nearly 40 times more powerful — and affect a much bigger area. (...)
Seismologists used to categorize most clustered quakes as aftershocks, and assumed they occurred on the same fault, said John Cassidy, a senior seismologist for Natural Resources Canada who was not involved in the project. But over the past several years, more has been learned about how a slip on one fault can put stress on others. February’s devastation in Turkey was caused by a powerful quake that triggered the rupture of an adjacent fault nine hours later.
Shallow quakes near urban areas can be especially nasty because they are so close to the surface. Shaking would be far more intense than from Washington’s most recent big earthquake — the magnitude 6.8 Nisqually event in 2001, which originated more than 20 miles underground.
“An earthquake on the Seattle Fault is … likely to be much, much more catastrophic,” Tobin said.
Tree rings can yield much more precise dates than radiocarbon, Black explained. But the samples have to be in good condition, with bark and extensive ring sequences.
One of the most daunting tasks was rounding up dozens of cores and wood slices collected over the years by other researchers. Eventually, Black corralled usable samples from 47 Douglas firs at six sites where trees were drowned by the earthquakes. Some of the samples recorded nearly three centuries of history.
The team had to collect new samples from Price Lake, where stumps of the quake-killed Douglas firs remain visible. USGS divers struggled in the murky water to slice usable samples with intact bark from the bases of the trees. “It would take up to an hour to cut a single wedge from a tree, because they were working with almost zero visibility,” said Black, lead author of the study.
To anchor the tree rings in time, he used a reference sequence from 1,300-year-old firs collected on Vancouver Island in the early 1990s — so the date of the outermost ring is known.
Black sanded each sample multiple times, including with micron-level diamond grit sheets called lapping films. “It’s sanded to the point where you can see the individual cells within the wood under a microscope,” he said.
As he lined up his samples and compared them with the reference trees, the answer was unmistakable.
The validity of the date was confirmed through radiocarbon analysis of individual tree rings, looking for evidence of cosmic timestamps called Miyake events. Named for the Japanese physicist who discovered them, the events represent spikes of radiation from solar flares or exploding stars centuries or millennia ago. Luckily for dendrochronology, spikes in cosmic radiation generate spikes in atmospheric levels of carbon-14, the isotope whose slow decay is the clock that anchors radiocarbon dating.
Even luckier for Black and his colleagues, a Miyake event occurred in the year 774 A.D. and showed up in their Douglas fir samples, providing an absolute benchmark from which to count.
“Boom,” he said. “Everything came together, and we saw that these trees all died with the last completely formed ring being the year 923.”
by Sandi Doughton, Seattle Times | Read more:
Images: Stephani Gordon / Oregon Public Broadcasting; ESRI, Brian Black, Univ. of Arizona; Mark Nowlin
[ed. Science.]
