The high crustal seismicity of the Puget Sound region is a clue that there should be additional active faults. The search for these faults faces two problems. First, much of the region is covered by dense forest and underbrush so that from the air, one cannot see small landforms like fault scarps as one could in semi-arid eastern Washington. Second, the region was buried by glacial ice similar to that found today in Greenland as recently as 14,000 years ago. During advance and retreat of the glacier, deposition and erosion erased subtle tectonic features, removing any evidence for active faulting older than latest Pleistocene.
However, in the 1970s, prior to the discovery of the Seattle Fault, Joseph Wilson, a graduate student at North Carolina State University at Raleigh, and Bob Carson of Whitman College in Walla Walla, Washington, found evidence for four faults in the southeastern Olympic Peninsula between Lake Cushman and Hood Canal with evidence for Late Quaternary displacement. Radiocarbon dating shows that the latest movement on one of them, the Saddle Mountain East Fault, took place around 1,240 years ago.
The general tectonic outline of the Puget Sound region worked out in the 1980s by Howard Gower and James Yount of the USGS (Figure 6-1) showed other faults in addition to the one marking the boundary between an uplifted area in south Seattle and south Bainbridge Island and a thick basin of young sediments to the north, which would later be called the Seattle Fault and Seattle Basin. In the 1990s, Sam Johnson, Tom Brocher, Rick Blakely, and their USGS colleagues described other basins: the Tacoma Basin to the south and the Everett and Port Townsend basins to the north. They proposed that several of these basins were bounded by active faults (Figure 6-1).
To convince skeptics like myself who thought that, except for the Seattle Fault, the case for active faulting had not been made, more evidence was needed. It was necessary to part the obscuring veil of dense second-growth forest that covered subtle fault scarps that might have formed since the glaciers melted away.
The solution came from LiDAR (Light Detection And Ranging), a new method of imaging the ground using a laser beam reflected by a spinning mirror in a light airplane to penetrate the tree canopy. A LiDAR flight had been commissioned by the Kitsap Public Utility District to study groundwater infiltration and runoff on Bainbridge Island. North of Toe Jam Hill, on the south end of the island, the survey imagery found an unexpected surprise: an east-west-trending fault scarp that became known as the Toe Jam Hill Fault (Figure 6-11). Backhoe trenches across the fault scarp (Figures 6-12, 6-13) revealed evidence for not just one earthquake around eleven hundred years ago, but three and possibly four earthquakes between twenty-five hundred and a thousand years ago. The most recent earthquake is probably the one that raised Restoration Point in A.D. 900-930. The scarp of the Toe Jam Hill Fault faces south, in the opposite direction from the blind Seattle Fault to the north, suggesting that the Toe Jam Hill Fault intersects the Seattle fault at a shallow depth and is secondary to it (Figure 6-10).
More recent LiDAR surveys have found additional post-glacial fault scarps at Toe Jam Hill (Figure 6-11), on a fault south of the Darrington-Devils Mountain Faults, on the Tacoma Fault, and in the southeastern Olympic Peninsula, where Wilson and Carson had worked (Figure 6-1). Post-glacial fault scarps were also found on the northern margin of the Olympic Mountains, and west of the Toe Jam Hill Fault on the Kitsap Peninsula. The Southern Whidbey Island Fault (SWIF, Figure 6-1), which comes ashore south of Everett, underwent displacement of one to two meters on Whidbey Island three thousand years ago. This fault may be the longest active fault in the Puget Sound region, possibly capable of an earthquake even larger than that proposed for the Seattle fault. It is traced southeastward into the Cascade foothills, and it may influence the location of Snoqualmie Falls. A regional wastewater treatment plant has been built by Seattle Metro across the fault in southern Snohomish County north of Woodinville, despite protests from homeowners living nearby. The slip rate is likely to be low in comparison with the subduction zone, thereby reducing the likelihood that it will rupture during the life of the treatment plant.
In the Snohomish River delta near Everett, Joanne Bourgeois of the University of Washington and Sam Johnson of the USGS found evidence for at least three earthquakes and one tsunami. The tsunami deposit appears to be related to the earthquake on the Seattle Fault in A.D. 900-930. The most recent earthquake is dated between A.D. 1430 and 1640, younger than any other earthquakes in the Puget Lowland identified by paleoseismology and not too much earlier than the historical record. Vented sand found by Steve Obermeier of the USGS in overbank deposits of rivers near Centralia, Washington, appears to be related to a crustal earthquake south of Puget Sound, but the surface fault source for this earthquake has not been identified.
Puget Sound has been the target of focused studies of active faults in part because that’s where the people live who are at risk. Are we likely to find a similar concentration of active faults elsewhere? The high crustal seismicity of the Puget Sound and south Georgia Strait region suggests that this region is special. The confirmation comes from the GPS network, which had already confirmed the earlier land-based geodetic survey data that most of the compression is north-south. Stephan Mazzotti of the Pacific Geoscience Centre found that there is active crustal shortening between the south end of Puget Sound near Olympia and the Strait of Juan de Fuca as far north as Victoria. If one takes out the elastic deformation marking the buildup toward the next Cascadia Subduction Zone earthquake, this region is being squeezed together at about a quarter-inch (six millimeters) per year. So the answer is: yes, there is something special about the weaker crust of northwestern Washington.
When will the next big crustal earthquake strike? The answer to this question is unknown, but some questions may now be asked. First, the time around A.D. 900 must have been characterized by many violent crustal earthquakes, perhaps leading to the horned serpent stories of Native Americans. Was there only one huge earthquake extending from Copalis River to Lake Sammamish, or was there a cluster of earthquakes? The earthquakes recorded on the Toe Jam Hill Fault and in the Snohomish River delta suggest a cluster of earthquakes. (An example of a historical cluster is found in western Nevada, where large earthquakes struck in 1903, 1915, 1932, 1934, and four in 1954, although the recurrence interval on any given fault there is measured in thousands of years.)
Does the active faulting of a millennium ago mean that more crustal faulting is due soon? We don’t know.
Now and then, a moderate-size crustal earthquake strikes the Puget Sound region. On April 14, 1990, an earthquake of magnitude 5.2 struck near the town of Deming, east of Bellingham. On May 2, 1996, a magnitude 5.3 earthquake had its epicenter a few miles east of the small town of Duvall, in the foothills of the Cascades northeast of Seattle. It resulted in only minor damage, and its main claim to fame was that it caused the evacuation of the Kingdome during a Seattle Mariners baseball game. The previous year, on January 28, a magnitude 5 earthquake struck the southern Puget Sound region north of Tacoma. Earthquakes such as these are likely to occur anywhere west of the Cascades, although they are more likely in the Puget Sound region. They rate a newspaper story for a day or so, a story which usually gives off a whiff of impending doom, but earthquakes like these do little damage. It is impossible to assign them to a specific fault.
We will revisit the problem of forecasting the next crustal earthquake in the following chapter.