Estimating earthquake frequency and patterns in the Puget Lowland

Jul 17, 2014
Figure 1. Credit: Nelson et al.

The hazard posed by large earthquakes is difficult to estimate because they often occur hundreds to thousands of years apart. Because written records for the Puget Lowland of northwestern Washington cover less than 170 years, the size and frequency of the largest and oldest earthquakes on the Seattle and Tacoma faults are unknown. Past earthquakes can only be estimated through geologic studies of sediments and landforms that are created when faults break the ground surface.

Along the Cascadia margin, the cities of Seattle and Tacoma occupy the Puget Lowland, a structurally complex, seismically active forearc trough between the Coast Range and the Cascade volcanic arc. Deformation of the lowland is caused by the clockwise motion of western Oregon, pushing northward and compressing western Washington against slower moving Canada. Much of the north- to northeast-directed strain in the southern Puget Lowland is accommodated by deformation in the Seattle fault zone, the Tacoma fault zone, the Saddle Mountain deformation zone, and along the Olympia fault.

In their paper for GEOSPHERE, published online 25 June 2014, Alan R. Nelson and colleagues write that although in 1992 a very large earthquake on the Seattle fault was dated to a thousand years ago, the size and frequency of earlier earthquakes on active faults of the southern Puget Lowland remains uncertain. Through a synthesis of earlier geologic data with new field evidence of faulting at nine sites on the Seattle and Tacoma faults, Nelson and colleagues show that four or five earthquakes large enough to break the ground surface occurred on the Seattle fault or related faults during the past 3,500 years.

Explore further: Magnitude of quake scales with maturity of fault, new study suggests

More information: "Diverse rupture modes for surface-deforming upper plate earthquakes in the southern Puget Lowland of Washington State." Alan R. Nelson et al., Geologic Hazards Science Center, U.S. Geological Survey, MS 966, PO Box 25046, Denver, Colorado 80225, USA. Published online 25 June 2014; dx.doi.org/10.1130/GES00967.1

add to favorites email to friend print save as pdf

Related Stories

Helping to forecast earthquakes in Salt Lake Valley

Apr 17, 2013

Salt Lake Valley, home to the Salt Lake City segment of the Wasatch fault zone and the West Valley fault zone, has been the site of repeated surface-faulting earthquakes (of about magnitude 6.5 to 7). New research trenches ...

California faults moved quietly after Baja quake

May 01, 2014

(Phys.org) —A new NASA study finds that a major 2010 earthquake in northern Mexico triggered quiet, non-shaking motions on several Southern California faults that released as much energy as a magnitude ...

Study links earthquake faults to slow-moving depths

Nov 08, 2013

(Phys.org) —Most earthquakes erupt suddenly from faults near Earth's surface, and the big ones can topple cities. But miles below, rocks heated to the consistency of wax moving over thousands to millions ...

Recommended for you

Improving forecasts for rain-on-snow flooding

8 hours ago

Many of the worst West Coast winter floods pack a double punch. Heavy rains and melting snow wash down the mountains together to breach riverbanks, wash out roads and flood buildings.

The Greenland Ice Sheet: Now in HD

8 hours ago

The Greenland Ice Sheet is ready for its close-up. The highest-resolution satellite images ever taken of that region are making their debut. And while each individual pixel represents only one moment in time, ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.