July/August 2006

Shadowed by the Glare of 1906 are Faceless Future Dangers

In the tradition of girding against bygone debacles more than future threats, both due to their searing memory and their ease of study, considerable effort has been directed to reconstructing the 1906 San Francisco earthquake and the 1811–1812 New Madrid earthquakes. The recurrence of these M7 to M8 events may well bring a costly and deadly toll. However, as earthquake cycles alternate between accumulation and release of strain energy, both regions may still be early on their journey to the next earthquake, while considerable and possibly greater danger lies elsewhere.

The Cascadia subduction zone, with its overlying landslide-salted and volcano-peppered mountains cradling the metropolises of Seattle, Portland, and Tacoma, offers a counterpoint. This editorial, I admit, is motivated by the daunting chore for the Pacific Northwest Seismic Network of shadow-boxing threats the most recent appearances of which have not been documented by Hearst newspapers and nanotech gauges. The lessons apparent here may apply to many regions of the world.

The 800-pound catfish of Cascadia geopyrotechnics is a recurrence of the 1700 M9 earthquake, which had slip of 20 m along most of the northern California, Oregon, Washington, and Vancouver Island coasts. This marginally prehistoric great earthquake was first noticed in the 1980s, much later than other comparable North American earthquake risks currently causing concern. Earlier, during the rise of the metropolises, the megathrust was considered most likely aseismic, so the possibility of M9+ events was given short shrift.

On the volcanic front, the 1980 eruption of Mt. St. Helens is firmly in our consciousness and monitoring effort, consistent with our what-have-you-done-to-me-lately hazard mitigation attitude. Less clear in our view are Mt. Hood looming over Portland and Mt. Rainier over Seattle.

The Christmas 2004 Sumatra earthquake is an extreme analog for the recurrence of such an earthquake in the wake of unhurried preparations. While the rate of such megathrust earthquakes may leave previous events out of sight, they are an uncomfortable risk. The estimated average recurrence interval is 500 years, and the Gutenberg-Richter distribution forecasts additional lesser events that can still pack a punch.

The potential for devastation in such an M9 earthquake is poorly understood. This problem is being faced more directly with additional research. We are trying to estimate the extent of the locked zone with geodetic measurements, thermal modeling, analogy with other subduction zones, and perhaps the location of slow earthquakes under the accretionary prism as well as episodic tremor and slip. The spatial pattern of strong shaking may be distorted and fortified by critical reflections off the subducting Moho and reverb in the deep Puget Sound basins. The stratigraphy in offshore canyons may reveal the timing, magnitude, and repeatability of earthquakes across several millennia.

On the volcanic front, the 1980 eruption of Mt. St. Helens is firmly in our consciousness and monitoring effort, consistent with our what-have-you-done-to-me-lately hazard mitigation attitude. Less clear in our view are Mt. Hood looming over Portland and Mt. Rainier over Seattle. Ten of the Cascade volcanoes are listed as “very high threat” in a 2005 report, and all save St. Helens have “limited” or “minimal” monitoring, with lone seismometers as the sentries on a few of them.

Verdant mountains, world-class sedimentary basins, and inland waters breed additional hazards. An M7.2 earthquake broke the Seattle fault in 900 A.D., for example. It shook the bejeebers out of the current location of the heart Seattle and swamped the shores of Puget Sound with a several-meter tsunami. Also, every few centuries, lahars slosh down now well-populated valleys. Although a seismometer-based lahar early warning system is now in place to watch two valleys near Mt. Rainier, urban sprawl places ever more people onto the already congested routes for escape when the alarms sound, if they work.

The dangers of the Seattle fault were calibrated only a few years ago. From new mapping and trenching, the nearby South Whidbey Island fault, which splits the ground between Seattle and Everett to the north, has garnered a similar degree of earthquake hazard. Mapping such regional crustal faults under the dense canopy of trees and through the glacier-scoured landscape has to wait for acquisition of LIDAR swathes, which are yet far from complete.

Also imperative is the timely distribution of information about such geophysical natural events. Recognizing these needs, geophysical monitoring is gaining support. The state of Washington has this year started to invest in regional monitoring, in return for the promise of modernized seismic products such as rapid estimates of shaking, damage, and aftershock probability, and exploration of earthquake early warning systems. The regional network will capture a dozen or more of the broadband USArray Bigfoot seismic sites rotating through the Pacific Northwest before the instruments escape to the east. The EarthScope project, well underway, and the future NEPTUNE project to instrument the offshore Juan de Fuca plate, offer the promise of a deeper view of deformation and subduction plumbing and mechanics, likely with hazard mitigation benefits. On this path, we will be forearmed and, as a bonus, enchanted by the natural discoveries that arise from the added probing of our diverse tectonic landscapes.

Without the buzzing of frequent small earthquakes, which keeps the burnt sepia photos from 1906 embedded in our consciousness, we sometimes take nature too lightly. In the Pacific Northwest, as in many other places around the world, we must learn from the past but also look beyond commemoration of past spectacles to keep an objective eye on the geophysical dangers nearer home.

John E. Vidale
University of Washington

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Posted: 30 June 2006