At Cape Mendocino, California, tectonic plate activity abruptly changes from the vertical motion of the Pacific Plate diving beneath the North American Plate to the horizontal motion that characterizes the San Andreas Fault. These changes, as well as other local and regional conditions, will dramatically influence the amount of sea level increase that is likely to be experienced by different areas of the West Coast over the course of the next few decades. U.S. Geological Survey
A National Research Council report published last week reveals that regional and local factors—including earthquakes—will affect sea level rises along the West Coast for many decades to come.
January 8, 2013—Sea level increases along the 1,600 mi of the United States’s West Coast—encompassing the states of California, Oregon, and Washington—will differ significantly to the north and south of Cape Mendocino, the westernmost point of the contiguous 48 states, a new report reveals. Located in Humboldt County, north of the San Francisco Bay Area, Cape Mendocino is the point at which tectonic plate activity abruptly changes and the Cascadia Subduction Zone (characterized by vertical movement as the ocean plate dives beneath the North American Plate) gives way to the San Andreas Fault (characterized by horizontal movement, as the plates slide past one another). A new National Research Council report, published last week, details this—and other—regional and local factors along the West Coast shoreline that are likely to have profound effects on sea level increases along the coastline, and offers new projections of what those increases will be by 2030, 2050, and 2100.
The report was commissioned in 2008 by then governor Arnold Schwarzenegger as part of California’s effort to aid state agencies in their planning for the impact of sea level increases on the coastline and on coastal communities and infrastructure. The states of Washington and Oregon, the U.S. Army Corps of Engineers, the National Oceanic and Atmospheric Administration, and the U.S. Geological Survey joined California in sponsoring the study after the order was issued.
State agencies in California “were operating under temporary guidance, and so the purpose of the report was to give them some hard numbers to work with,” said Robert A. Dalrymple, Ph.D., P.E., D.CE, Dist. M.ASCE, a professor of civil engineering at Johns Hopkins University, in Baltimore, Maryland, and the chair of the National Research Council’s Committee on Sea Level Rise in California, Oregon, and Washington, the 13-member committee that authored the report.
“The biggest surprise out of the report was the fact that of all the sea level rise that’s going to occur on the West Coast, we found that in Oregon and Washington, it was less than in California because of the fact that the land is rising [there],” Dalrymple says. Counterintuitively, the fact that the land is rising north of Cape Mendocino is not good news: The cause is the Cascadia Subduction Zone, and “sometime in the foreseeable future, that will rupture and there will be a catastrophic earthquake and all of that rising of the land will disappear,” Dalrymple explains.
The coastal plate is bending and flexing upward as the ocean plate dives beneath it at the subduction zone, Dalrymple says, and when the seismic stresses that this creates finally result in a major temblor, “it will release all of that warping out of the coastal plate, and [the plate] will just drop and flatten out.” The drop could be as a significant as 1 to 2 m, and will occur—as these things do—abruptly during the seismic event.
The last major earthquake to occur in this region as a result of this type of plate activity took place on January 26, 1700, and the U.S. Geological Survey estimates it was roughly a magnitude 9 on the Richter scale. Earthquakes of this type are judged to occur between a few hundred to a thousand years apart, but the quake will definitely happen—it’s just a question of when, according to Dalrymple. “And of course, [when] you have a catastrophic earthquake, you have a catastrophic associated tsunami, and it is not good,” he said.
In creating its sea level rise projections for the West Coast, the committee examined such regional and local factors as plate tectonics and the extraction of underground water and natural gas, which can lower local surface elevations by tens of centimeters annually. The committee also looked at weather patterns. The multiyear, cyclical El Niño-Southern Oscillation climate pattern, for example, can increase local sea levels up to 10 to 30 cm during the warm phases known as El Niño, and lower sea levels during the cool phases known as La Niña.
The committee also took into account factors impacting sea levels that can be traced to climate change, including the warming of ocean waters and the melting of large masses of glaciers and ice sheets. In addition to being a source of melt water, creating a direct rise in sea level, large ice masses create a gravitational pull that draws ocean water closer, according to the report. However, once the ice begins to melt, not only does that gravitational pull lessen, but the land and ocean basins previously held down under the weight of all that ice deform as the weight disappears. This phenomenon is referred to as a “sea level fingerprint,” according to the report, and can cause land levels to rise or sink, depending on where the ice had been located.
Using recorded sea levels from the year 2000 as a baseline, the committee predicts that sea levels south of Cape Mendocino will increase between 4 and 30 cm by 2030, between 12 and 61 cm by 2050, and between 42 and167 cm by 2100. The report notes that these numbers are close to current global sea level rise predictions.
North of Cape Mendocino, the committee found a high level of potential variability, predicting that sea levels along the coasts of California, Oregon, and Washington will change between -4 cm and +23 cm by 2030, -3 cm and +48 cm by 2050, and +10 to +143 cm by 2100. A major earthquake—magnitude 8 on the Richter scale or higher—could increase those estimates by 1 to2 m, the report notes.
Of course, these projections are not the final word on the matter. The United Nation’s Intergovernmental Panel on Climate Change (IPCC) releases projections of global sea level rise every five to six years, the most recent publication dating to 2007. A new report is expected to be released in sections from 2013 through 2014.
“As time advances, there are more measurements [and] more observations, so we have more certainty about what is happening in the future,” Dalrymple says. “So it will be interesting to see how this thing develops.”
The full National Research Council report, Sea-Level Rise for the Coasts of California, Oregon and Washington: Past Present, and Future, can be downloaded from the website for the National Academies Press.