Cascadia (Pacific Northwest) Seismicity
William P. Leeman
Keith-Wiess Geological Laboratories
After the 'Great Wave off the coast of Kanagawa', by Hokusai (17th-18th century Japanese painter)
The following document is intended to serve as a example of the type of information and organization we would like to see in the group reports required for Geology 108. For the list of groups and who is in them see project groups.
The Pacific Northwest (northern California to Washington and beyond), also known as 'Cascadia', is associated with one of the world's quietest subduction zones. Despite the presence of a topographically imposing (and scenic) range of volcanic mountains (the Cascades), only recently has it become widely recognized that the region is one of high seismic risk. Because seismicity historically has been relatively low, until as late as the 1980s development around Puget Sound and along coastal areas had proceeded with little awareness of the looming risks posed by local earthquakes. Yet the seismic quietude of the region, coupled with the inexorable underthrusting of the Juan de Fuca and Gorda oceanic plates beneath North America, implies that seismic strain has been accumulating without frequent release - as is the case for more seismically active regions (e.g., southern California or Japan). Consequently, there is increasing concern that a very large earthquake (> M 8.0) could occur in this region. Given locally dense population concentrations (e.g., Seattle, Portland, etc.) and the fact that there has been little emphasis on earthquake-resistant construction until recently, the 'big one' could very well wreak considerable destruction over wide areas along the Cascadia margin. With increased awareness of earthquake hazards (ref 1) and associated tsunami effects (ref 4; also see below) in the past decade, there has been significant activity related to monitoring seismicity, geological investigation of known faults ('paleoseismicity'), risk assessment, and emergency planning (cf. CREW).
This report attempts to draw together many WWW resources concerning earthquake (and related volcanic and tsunami) hazards in Cascadia and nearby regions.
Earthquakes in the Pacific Northwest
Cascadia seismicity clearly is related to oblique convergence of oceanic (Juan de Fuca and Gorda) lthospheric plates with North America, resulting in components of both normal convergence and margin-parallel (roughly N-S) compression. This configuration leads to underthrusting of the oceanic plates as well as dextral strike-slip and E-W fold and thrust deformation within the upper (North America) plate. The Univ. Washington Seismology Lab records roughly 1000 earthquakes per year in Washington and Oregon, few of which cause damage or are even felt. Most occur in the Puget lowlands; many are associated with recognized faults. Click here to get the latest or recent earthquake information for WA-OR. Notable (M > 2) earthquakes in WA-OR and northern California are infrequent, and few have caused significant damage historically. The types of earthquakes observed fall into the following categories:
Visit the WWW sites below for further information about Cascadia earthquakes. For general access to global seismicity resources check out Steve Malone's 'Seismosurfing' site.
- A zone of deep earthquakes associated with the major Cascadia subduction zone thrust fault. The 1949 Olympia (M 7.1) and 1965 Seattle-Tacoma (M 6.5) earthquakes occurred within this zone. These large earthquakes caused significant damage in the greater Seattle-Olympia region. The next big (M > 8.0) earthquake is likely to be of this type; based on paleoseismicity studies (ref 1), such events are likely to recur at intervals of 300 to 1000 years. In contrast, smaller earthquakes (M 6.0) are likely to recur at 10 year intervals.
- Earthquakes that occur along fracture zones in the offshore oceanic plates. Relatively large earthquakes of this type occurred recently off Vancouver Island (M 6.2; 6 Oct 1996) and off the northern California coast near Punta Gorda (M 5.7; 21 Jan 1997). On average there are about 18 events (>M 4) of this type per year; in the past two decades no more than 50 events this large have occurred in any given year. Moment-tensor solutions for these earthquakes commonly indicate dextral strike-slip motion.
- Earthquakes associated with relatively shallow (crustal) deformation, often on strike-slip faults in the upper plate. A recent notable earthquake (M 3.7; 1 Jan 1997) of this type was related to dextral strike-slip motion on a fault near Yakima WA.
Related WWW sites
Volcanism related to subduction zone phenomena
The Cascades Mountains are commonly visualized as the topographically imposing stratovolcanoes that extend from Mt. Lassen in northern California northward to Mt. Garibaldi in British Columbia. In actuality, Cascade volcanism began with initiation of the current subduction regime over 40 million years ago! The range consists of voluminous volcanic deposits similar to those produced at modern 'High Cascades' volcanoes. Familiar examples of the latter include Mt. Shasta (CA), Crater Lake, Mt. Bachelor, and Mt. Hood (OR), Mt. St. Helens, Mt. Rainier, and Mt. Baker (WA). Nearly all of these volcanoes have erupted in the past 100,000 years (some quite recently) and all are considered 'dormant' rather than 'extinct' (see 'preparing for the next eruption' below). Those near population centers pose definite hazards, and Mt. Rainier has been designated as a 'Decade Volcano' for this reason. Recent volcanic activity in the Cascades is monitored by the Cascades Volcano Observatory among others. Seismicity (harmonic tremor) associated with magma movements and related deformation is distinct from that related to tectonic processes in the region and poses little regional threat. Discussion of other volcanic hazards can be found in the WWW sites below.
Related WWW sites
Tsunami events related to subduction zone phenomena
Description: Tsunamis are great sea waves produced by abrupt land movements or explosions (e.g., related to submarine earthquakes or volcanic eruptions). In contrast, the term 'tidal wave' is restricted to the periodic variations of sea level produced by the gravitational attractions of the sun and the moon, or to large storm waves caused by a hurricane wind or a severe gale. These terms commonly are incorrectly interchanged.
Tsunamis have long wavelengths and travel at about 800 km (500 miles) per hour. The waves at sea are something like an hour apart at a given point, are only perhaps 30 centimeters (1 foot) high, and are virtually undetectable. As a wave approaches land, however, its bottom is slowed down by contact with the shallowing sea floor, whereas the top is slowed much less and catches up with the bottom. Where sea-floor topography and orientation are optimal for a tsunami from a given direction, the wave can hump up into a breaking wall of water 9 meters (30 feet) or more high, and rush onto shore to cause enormous destruction. Nearby coastal points, where the bottom configuration is different, may record the same wave only as a rapid surge and withdrawal of water, with much lower height (ref 2). The largest tsunami ever recorded (Lituya Bay, Alaska; July, 1958) resulted from a huge rock and ice fall and sent water surging to a high water mark of 500 meters (1640 feet)!
The most extensive sudden changes in depth of the sea floor, and hence the greatest seismic sea waves, result from shallow subduction-zone earthquakes, such as the Alaska earthquake of 1964. The enormous change in sea-floor configuration during that earthquake produced a train of seismic sea waves which battered Alaskan coastal communities facing the uplifted region (ref 2).
Geological studies at several sites along the Pacific coast of Cascadia have documented the presence of sedimentary debris deposits which are interpreted to be deposited by high-energy tsunami waves (ref 1). Carbon-14 and dendrochronology (tree-ring) studies indicate that there may have been up to ten significant (i.e., regionally destructive) tsunami events in the past 5000 years - roughly one every 400-500 years. It is estimated that the most recent 'great earthquake' event occurred about 300 years ago, in the early 1700s, before the region was settled by Europeans. There were no written records of this event, although it was almost certainly experienced by aboriginal peoples in the area. Careful examination of historic tsunami records from Japan indicates that several coastal villages there were heavily damaged by a tsunami on the night of 27/28 January 1700; careful analysis of these records and physical simulations of the source mechanism suggest that the tsunami likely was triggered by a great earthquake somewhere along the Cascadia margin (refs 3, 4). Obviously, generation of tsunamis by a large (M > 8.0) earthquake potentially could devastate extensive areas along the now heavily developed Cascadia margin.
Check out the WWW sites listed below for more detailed physical descriptions of tsunami effects; they provide further information concerning the types of effects to be expected during tsunami events. In particular, note the animations of tsunami effects, tabulations of wave heights, summary of historic events, and the scenario for a northern California tsunami event.
Related WWW sites
- Atwater BF and 15 others (1995) Summary of coastal geologic evidence for past great earthquakes at the Cascadia subduction zone. Earthquake Spectra, v. 11, p. 1-18.
- Hamilton W (1976) Plate Tectonics and Man. USGS Annual Report, Fiscal Year 1976, p.15-16.
- Kanamori H, and Heaton TH (1996) The wake of a legendary earthquake. Nature, v. 379, p. 203-204.
- Satake K, Shimazaki K, Tsuji Y, and Ueda K (1996) Time and size of a giant earthquake in Cascadia inferred from Japanese tsunami records of January 1700. Nature, v. 379, p. 246-249.
click here to return to Bill's Home Page.
Send comments, queries to:
Updated: 1 Oct 98