OREGON GEOLOGYIN THIS ISSUE: The Portland Hills faultSeismic rehabilitation at the Oregon State Library in SalemResults of a new method of Fourier grain-shape analysisOregon Department of Geology and Mineral Industries Volume 63, Number 2, Spring 2001OREGON GEOLOGY, VOLUME 63, NUMBER 2, SPRING 2001 39The Portland Hills fault: An earthquake generator or justanother old fault?by Ivan G. Wong, Seismic Hazards Group, URS Corporation, 500 12th Street, Suite 200, Oakland, CA 94607; MarkA. Hemphill-Haley, Seismic Hazards Group, URS Corporation and Department of Geological Sciences, University ofOregon, Eugene, Oregon 97403; Lee M. Liberty, Center for Geophysical Investigation of the Shallow Subsurface,Boise State University, 1910 University Drive, Boise, ID 83725; and Ian P. Madin, Oregon Department of Geology andMineral Industries, 800 NE Oregon Street, Suite 28, Portland, OR 97232ABSTRACTSeveral lines of indirect evidenceand preliminary interpretations of re-cently collected seismic reflectiondata have led to the conclusion thatthe Portland Hills fault at the easternbase of the Portland Hills appears tobe capable of generating large-mag-nitude earthquakes. Although nohistorical earthquake can be associ-ated with the Portland Hills fault,small-magnitude seismicity in thepast 20 years in the vicinity of thePortland Hills fault zone, which in-cludes the Oatfield and East Bankfaults, suggests that one or all ofthese structures may be seismogenic.The Portland Hills fault may be40–60 km long, probably dips to thesouthwest beneath the PortlandHills, and may slip in a reverse-oblique sense. Limited observationssuggest that, on average, the inter-vals between large earthquakes area few thousand to more than10,000 years. Given its location inthe midst of the Portland metropoli-tan area, rupture of the PortlandHills fault resulting in a large earth-quake could be devastating. Futurestudies are required to characterizethe earthquake potential of the faultin a more definitive manner and toprovide an improved basis for pre-dicting the hazards that would resultfrom such a large earthquake.INTRODUCTIONSince the mid-1960s, it has beensuggested that a fault is located atthe eastern base of the northwest-trending, westward-sloping PortlandHills and within the Portland metro-politan area which has a populationof 1.4 million (Figure 1). Little con-sideration was given to the possibili-ty that the “Portland Hills” fault wasactive1(or seismogenic, i.e., capableof generating earthquakes). This wasconsistent with the prevailing viewheld until the past decade that Ore-gon was not particularly seismicallyactive. However, as speculationturned to recognition in the PacificNorthwest that the Cascadia sub-duction zone megathrust was seis-mogenic, attention began to shiftalso to evaluating the earthquake po-tential of inland crustal faults, partic-ularly those located in or near urbanareas. For example, significant ef-forts have been focused on the Seat-tle fault since the suggestion that itruptured in a large earthquake(>MW[moment magnitude] 7)1,100 years ago (Bucknam and oth-ers, 1992). If urban crustal faultssuch as the Seattle and the PortlandHills faults are seismogenic, theycould generate large, disastrousearthquakes similar to the MW 6.9earthquake in Kobe, Japan, in 1995.In the following paper, we reviewwhat is known about the geology ofthe Portland Hills fault, particularlyits characteristics, which might helpquantify its earthquake potential.Recent studies, which have focusedon this quantification, are discussed.Last year, Wong and others (2000a;2000b) released scenario earthquakeground shaking hazard maps for thePortland metropolitan area. They as-sumed a MW 6.8 earthquake ruptur-ing 30 km of the Portland Hills faultat the base of the Portland Hills (Fig-ure 2). The potential ground shakingfrom such an event would greatlyexceed the ground motions from aMW9 Cascadia subduction zonemegathrust event in the Portlandarea. This paper elaborates on theEditor’s NNoteDuring tthe ffinal sstages oof ppreparing tthis aarticle ffor ppublication, eevidence wwas ddiscovered tthat tthrows nnewlight oon tthe qquestion iin tthe ttitle, wwithout iinvalidating tthe eessence oof tthe ppaper. IIn aa ssidebar oon ppages 447–49,Ian PP. MMadin ggives aa ppreliminary aassessment oof tthe nnew-ffound eevidence ffor aactivity oon tthe PPortland HHills ffaultin ttimes mmore rrecent tthan hhad bbeen kknown bbefore. AA mmore eextensive ddescription aand ddiscussion wwill bbe ppub-lished bby MMadin iin Oregon GGeology at aa llater ddate.1We consider a fault in the Pacific Northwestto be “active” if it has moved (been dis-placed) at least once during late Quaternarytime (the past 780,000 years).40 OREGON GEOLOGY, VOLUME 63, NUMBER 2, SPRING 2001basis for selecting a MW6.8 sce-nario, although the evidence at thattime was not compelling that thePortland Hills fault was seismogenic.HISTORICAL SEISMICITY The Portland area has exhibited alow to moderate level of historicalseismicity (Figure 2), compared toother areas in the Pacific Northwest.The area is not as seismically activeas the Puget Sound region to thenorth but may be the most activearea in Oregon for events of MW≥3.0 based on the historical record(Wong and Bott, 1995). Detaileddiscussions of the historical seismici-ty can be found in Bott and Wong(1993) and of instrumentally record-ed seismicity since 1982 in Yelin andPatton (1991) and Blakely and oth-ers (1995). Based on the historicalearthquake record, seven felt earth-quakes (Richter magnitude [ML]>3.5) have occurred in the vicinity ofPortland since 1850 (Bott andWong, 1993).The first significant earthquake tostrike the Portland area occurred onOctober 12, 1877. It was felt intowns around Portland, but its maxi-mum intensity was in the city (Modi-fied Mercalli [MM] VII). This eventof approximate magnitude ML5¼damaged chimneys and was feltover an area of 41,000 km2. OnFebruary 3, 1892, a “severe” earth-quake of estimated ML5 causedbrick buildings to sway and windowsto rattle in Portland, terrifying its oc-cupants. A ML4½ event on Decem-ber 29, 1941, shook an area ofabout 9,000 km2, including thetowns of Portland, Hillsboro, Sher-wood, and Yamhill in northwestOregon and Vancouver and Wood-land in southwest Washington. Ef-fects included shattered windows,cracked plaster, and overturned ob-jects. Another ML4½ earthquakeoccurred somewhere between Port-land and Vancouver on December15, 1953.