Historic Seismicity, Neotectonics and Active Faulting in OregonRyan Stanley, Earth and Physical Sciences Department, Western Oregon University, Monmouth, Oregon, 97361email: rstanley06@wou.edu

Abstract

The instrumental period of earthquake monitoring is brief in the context of geologic time. Therefore current earthquake source models (Fig. 2) do not definitively represent Oregon’s seismic hazards. Multiple lines of geologic evidence indicate a successive history of megathrust earthquakes along the Cascadia Subduction Zone, yet the historical record lacks such events. Buried soils in coastal intertidal lowlands suggest a land subsidence event caused by rupture along the CSZ (Atwater, 1987). Locations along the Oregon coast yield evidence for tectonic activity, including tsunami sands, buried peats, and trees killed by salt-water (Wong & Bott, 1995). Surmounting evidence suggests the most recent megathrust earthquake occurred A.D. 1700 with a moment magnitude of 9. From 1841 to 1994, nearly 6,000 earthquakes ranging from magnitude 1 to 6.75 have occurred in Oregon (Wong & Bott, 1995). Until the 1840s, written documentation of earthquakes had not been recorded. In 1906 the first Pacific Northwest seismograph station was installed in Seattle, but Oregon did not receive one until OSU installed an instrument in 1946 (Wong & Bott, 1995). Significant improvements in seismic sensing began in 1979 when UW expanded its network into Oregon.

Introduction

Neotectonics

Historical Earthquakes

Pacific Northwest Seismic Network

Active Faulting

Conclusion

References

The Pacific Northwest experiences seismicity generated from three different sources. Crustal, intraslab, and subduction zone earthquakes (Fig. 1) result from stress associated with the Cascadia Subduction Zone (CSZ). Subduction zone earthquakes are less frequent yet release larger amounts of energy than other seismic sources. Crustal earthquakes result from active faulting in the overriding North American plate, and deep earthquake hypocenters appear in the subducting Juan de Fuca slab. Seismograph networks and other seismic instruments allow real-time tracking of seismicity and active faulting in the Pacific Northwest.

Evidence of active faulting and seismicity in the Pacific Northwest has been documented through analysis of stratigraphic relationships and use of modern instrumentation. Crustal, intraplate, and subduction zone earthquakes represent three types of Cascadia deformation, each with unique seismic characteristics. Earthquake monitoring utilizes specialized instruments that collect information to understand sizes, locations, frequency of occurrence, and types of earthquakes. Written documentation of historical events did not appear in Oregon until the 1840s, and seismograph stations were not established in the Pacific Northwest until 1906. The discovery of multiple buried soils in coastal intertidal lowlands suggests that great megathrust earthquakes have repeatedly occurred within the Cascadia subduction zone. Multiple lines of evidence indicate an average megathrust earthquake recurrence interval of ~500 years, with the most recent occurring A.D. 1700. Since 1841 more than 6,000 earthquakes have occurred in Oregon. Oregon's largest earthquakes include the 1873 Crescent City, 1936 Milton-Freewater, 1962 Portland, 1993 Scotts Mills, and 1993 Klamath Falls earthquakes. This paper provides a synopsis of historic seismicity and seismic sources in Oregon, with implications for hazard planning throughout the state.

Atwater, B.F., 1987, Evidence for great Holocene earthquakes along the outer coast of Washington State: Science, v. 236, p. 942-944.Braunmiller, J., Nabelek, J., Leitner, B., and Qamar, A., 1995, The 1993 Klamath Falls, Oregon, earthquake sequence: Source mechanism from regional data: Geophysical Research Letters, v. 22, p. 105-108.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.Pezzopane, S.K., and Weldon, R.J., 1993, Tectonic role of active faulting in central Oregon: Tectonics, v. 12, p. 1140-1169.Topinka, L., 2000, University of Washington's Pacific Northwest Seismic Network. Retrieved May 23, 2009, from USGS Cascades Volcano Observatory (CVO) Web site: http://vulcan.wr.usgs.gov/Volcanoes/Pacific NW/Seismicity/ uw_seismic_network.html.USGS, 2002, February 6, Lifelines and earthquake hazards in the greater Seattle area. Retrieved May 23, 2009, from Western Earth Surface Processes Team Web site: http://geomaps.wr.usgs.gov/pacnw/pacnweq/index.htmlWang, Y, Wang, Z, & Black, G.L., 1998, Measuring earthquakes in Oregon: Oregon Geology, v. 60, p. 42-44. Wong, I.G., & Bott, J.D.J, 1995, A look back at Oregon's earthquake history, 1841-1994: Oregon Geology, v. 57, p. 125-139.Yelin, T.S., and Patton, H.J., 1991, Seimotectonics of the Portland, Oregon, region: Sisomological Society of America Bulletin, v. 81, o. 109-130.

Figure 3. University of Washington's Pacific Northwest Seismic Network (Topinka, 2000).

Table 1. This list outlines the most significant historical earthquake events in Oregon (Wong & Bott, 1995).

Figure 4. The map shows all faults in Oregon capable of producing earthquakes. Faults are divided into three categories based upon the age of recent activity. Earthquakes may occur along a fault and not break the surface (map by Madin & Mabey, 1996).

A primary goal of earthquake monitoring is to detect and locate sources with specialized instruments and acquire enhanced understanding of potential sources. Seismic instruments collect information to understand sizes, locations, frequency of occurrence, and types of earthquakes and their effects. The Pacific Northwest Seismic Network (PNSN), connected through University of Washington and the USGS, covers a large part of the Pacific Northwest (Fig. 3). The Network provides real-time information on earthquakes and maintains strong motion, short-period, and broadband seismic instruments (Wang et al., 1998). Stations provide instantaneous calculation of magnitude and location. Three types of seismic instruments specialize in the recording of earthquakes. Strong motion instruments measure strong levels of shaking over a wide range of frequencies. Such strong motion seismographs record shaking that might severely impact engineered structures. Short-period instruments measure low levels of shaking, but only at higher frequencies. Broadband instruments measure low levels of shaking over a wide range of frequencies. A real-time monitoring network called Rapid Alert of Cascadia Earthquakes (RACE) automatically telemeters earthquake data to facilities in Seattle where an algorithm calculates epicenter location (Wang et al., 1998). The goal is to relay information to nearby communities before onset of damaging shaking. DOGAMI’s instrumentation program focuses on improving the regional seismic network.

Six earthquakes larger than magnitude 5 have occurred in Oregon’s recorded history (Table 1). On December 2, 1841, the first earthquake in Oregon’s record was felt near Fort Vancouver with an intensity of MM III (Wong & Bott, 1995). An 1893 Umatilla earthquake that destroyed a stone building’s wall might be Oregon’s largest event, but little is known about the episode. The Portland region is the most seismically active area in Oregon, having experienced a total of 17 earthquakes magnitude 4 and greater. Such activity may be explained by the Portland pull-apart basin that involves two fault zones (Yelin & Patton, 1991). The 1962 Portland earthquake was felt over an area of 70,000 km2 in northwestern Oregon, causing chimneys, windows, and tile ceilings to break (Wong & Bott, 1995). The first deaths from an Oregon earthquake occurred in Klamath Falls, where two moderate

Geologic and historical records both document Oregon’s seismic activity. Modern seismic instrumentation, such as that employed by the PNSN, now allows real-time tracking of earthquakes in the Pacific Northwest. Primarily crustal earthquakes account for seismic moments, although the 1873 6.75-magnitude earthquake was possibly from an intraplate source. Paleoseismic data from the Oregon coast points to massive subduction zone earthquakes. Additionally, Late Quaternary faults in eastern Oregon reveal the occurrence of magnitude 7 earthquakes (Pezzopane & Weldon, 1993). Increased seismic monitoring and paleoseismic studies will quantify Oregon’s earthquake potential, thereby improving upon the state’s incomplete historical record.

Earthquakes result from sudden displacement along a fault. The area of a rupture dictates the amount of seismic energy released. Some events, such as the 1993 Scotts Mills earthquake, rupture along blind faults. Oregon’s climate and geography ultimately inhibit fault visibility through increased erosion, weathering, and vegetation. Known Oregon faults have been mapped (Fig. 4), but few historical events are associated with known active faults, notably the Milton-Freewater, Scotts Mills, and Klamath Falls earthquakes. Those three episodes may have been associated with the Wallula or Hite faults, Mount Angel fault, and Lake of the Woods fault zone, respectively. Seismicity in the Portland region is attributed to the right lateral, strike-slip Portland Hills and Frontal fault zones.

Figure 2. This map shows historical seismicity, active faults, and counties of Oregon. Over 14,000 known earthquakes from 1841 to 2002 are included. Darkened areas on the map are concentrations of many symbols. This clustering is a result of earthquake activity that occurs in swarms (map from Niewendorp & Neuhaus, 2003).

sized earthquakes caused $7.5 million in damage. Aftershock distribution and focal mechanisms indicate the Lake of the Woods fault zone as a primary source (Braunmiller et. al., 1995). A magnitude 4.0 event occurred at Mount Hood in 1974, demonstrating the possibility of relatively shallow earthquakes associated with Cascade volcanism. In eastern Oregon, the Pine Valley graben-Cuddy Mountain area is the most seismically active (Wong & Bott, 1995). The region’s first recorded earthquake in 1913 was assigned an intensity of MM VI and damaged furniture and windows in Landore, Idaho.

Figure 1. This USGS model of the Cascadia Subduction Zone displays three primary earthquake sources affecting the Pacific Northwest. Crustal and intra-slab earthquakes are more frequent, yet significantly smaller in magnitude than subduction zone earthquakes (USGS, 2002).