1. Name: Matthew Moore Organization: Western Oregon University Email: MatthewTMs@Gmail.com Phone: 503-851-6252 Intraplate crustal faulting is a result of active tectonic motion in the Pacific Northwest. Seismic analysis indicates that fault depths can extend 15 km, or deeper, into the earth’s crust. Unlike Cascadia megathrust slip events, these types of structures rupture quite frequently, usually with low magnitudes (M 1 to 2) that are only observed by seismographs. Despite the low magnitudes of the average crustal fault earthquake, they periodically produce sizeable events capable of causing considerable shaking and damage. The Scotts Mills Earthquake of 1962 (M 5.5), the Spring Break Earthquake of 1993 (M 5.6), and late Holocene evidence of a M-7.0 event along the Tacoma Fault Zone are examples. Different types of crustal faults are associated with varying stress regimes across the Pacific Northwest. The region extending from northwestern Oregon to Puget Sound, is characterized by northwest-striking reverse faults. In contrast, southeast and central Oregon is associated with Late Cenozoic normal faults that are generally oriented to the north- northeast. Due to extensive surficial cover and vegetation in the western portions of Washington and Oregon, many surface faults have remained undetected. Recent advances with LIDAR (Light Distance and Ranging) technology have permitted identification of previously hidden scarps, giving a more detailed perspective on intraplate faulting in the Pacific Northwest. INTRAPLATE CRUSTAL FAULTING AND EARTHQUAKE SOURCES IN THE PACIFIC NORTHWEST Presented by: Matthew T. Moore, ES473 Environmental Geology, Spring 2009 INSERT YOUR ORGANIZATION’S LOGO HERE Intraplate crustal faulting is ultimately caused by the subduction of the Juan de Fuca plate beneath the North American plate. Crustal faulting from this process occurs in offshore and nearshore coastal regions with reverse faults striking to the Northwest. Southeastern Oregon’s faults are characterized by normal faulting and are the result of the spreading of the basin and range province. Fracture zones can rupture creating earthquake foci at depths anywhere between the Earth’s surface and 15 meters below. Crustal faults produce frequent seismic activity with magnitude 1 to 2 earthquakes occurring every day. Magnitude 5 earthquakes occur less frequently. An example of this is the Scotts Mills earthquake of 1993. Magnitude 7 earthquakes from crustal faults occur very infrequently. The last one occurred around Seattle between 900 and 930 AD. Structures developed on unconsolidated soil are susceptible to amplification of seismic velocity. Structures built upon solid bedrock resist and dampen the effects of seismic waves. Many intraplate crustal faults are difficult to identify due to unconformities caused by glacial erosion and deposits, as well as vegetative cover. LIDAR “Light Distance and Ranging” makes identification of fault scarps previously unidentified by surveyors identifiable up to 15 cm. LIDAR can detect fault scarps as small as 14m in length. Although interpolate crustal faulting generates earthquakes with maximum magnitudes between 5 and 7 on the Richter scale, they can still cause considerable damage. In efforts to reduce destruction, programs to augment public awareness are being implemented. For future construction, building materials and geological aspects of ground on which construction occurs are being more heavily regulated. Although the effects of intraplate crustal earthquakes may not be as devastating as a Cascadia megathrust earthquake, they still disrupt and cause great burdens for many Pacific Northwesterners. Recent advances in identification of possible hazard zones have greatly assisted in informing builders of the safety of their area. With public education informing people about the effects specific to intraplate crustal faulting and increased awareness of specific danger zones, the Northwest’s inhabitants can better prepare for the inevitable. Intraplate crustal faulting is evident all throughout the pacific coasts of Oregon, Washington, and Northern California. Earthquakes associated with these faults occur frequently, but on average, produce seismic activity below Richter magnitudes of 2. Nonetheless, they are capable of causing considerable harm. INTRODUCTIONDETECTION AND MITIGATION Hill, John, Lee Graham, Robert Henry. "Wide-Area Topogarphic Mapping and Applications Using Airborne Light Detection and Ranging (LIDAR) Technology."(2000): 908-915. Madin, I.P., & Mabey, M.A., 1996, Earthquake Hazard Maps for Oregon: Department of Geology and Mineral Industries. Niewendorp, C.A., & Neuhaus, M.E., 2003, Map of Selected Earthquakes for Oregon, 1841 through 2002: Department of Geology and Mineral Industries. Sherrod, Brian, Thomas Brocher, Craig Weaver, Robert Bucknam, Richard Blakely, Harvey Kelsey, Alan Nelson, Ralph Haugerud. "Holocene Fault Scarps Near Tacoma, Washington, USA." (2004): 9-12. Wang, Yumei. "Environmental, Groundwater and Engineering Geology: Applications from Oregon." (1998): 325-342. CONCLUSIONS EFFECTS STRUCTURAL CHARACTERISTICS REFERENCES Figure 3. Faulting Types. Figure 1. Oregon Earthquake Faults. Figure 2. Map of Selected Earthquakes for Oregon. ABSTRACT CONTACT