EERI SPECIAL EARTHQUAKE REPORT -MA y 1993

to resolve this issue. quake swarm beneath Woodburn, epicenter. The instrument located at the north end of the fault. at the downstream toe of the dam Geology recorded a peak acceleration of There was no observed surface O.O6g while an instrument located There are no known faults that co- rupture of the fault. This is as in a gallery within the dam incide directly with the epicenter would be expected for an earth- recorded a peak acceleration of of the Scotts Mills earthquake. quake of this magnitude. No evi- O.18g. The closest (and at this point only) dence of landslides or liquefaction candidate for a causative structure triggered by the earthquake has The other three instruments for is the Mt. Angel fault. This NW- been observed. Given the magni- which records have been pro- trending structure was first tude of the earthquake and the cessed are in the Portland, Oregon- mapped by Hampton (1972) and type of unconsolidated sediments Vancouver I Washington area. A later extended by Yeats and others in the epicentral area, the absence recently installed digital instrument (1992) on the basis of water well of liquefaction is not surprising. A in Portland recorded a peak accel- data and seismic reflection lines. few minor rock falls in road cuts eration of 0.03g while older analog However, the mapped extent of near the epicenter seem to have instruments at Portland State Uni- the Mt. Angel fault ends at least 5 resulted from the earthquake. versity and at the Vancouver V A miles (8 km) west of the Scotts Hospital both recorded peak mo- Mills epicenter. A geologic map of Strong Motion Record tions of 0.02g. The stations in the epicentral area by ¥iller and Portland are approximately 35 Orr ( 1984) shows no significant To date, strong motion recordings miles (56 km) north of the epicen- faulting. Yeats and others (1992) of the Scotts Mills earthquake ter and the V A hospital is approxi- suggested that the Mt. Angel fault have been processed for four sites. mately 44 miles (70 km) north. showed right lateral strike-slip The strongest motions were re- offset with some north dipping corded by three instruments at the All of these measured motions are reverse motion, and postulated US Army Corps of Engineers De- consistent with the intensities that the Mt. Angel fault might be troit Dam located approximately ~bserved ne~r the respective loca- A active on the basis of an earth- 23 miles (36 km) southeast of the tlons of the Instruments. The Portv land-Vancouver recordings were

;,[I soil sites. The record for a

!; bedrock site in Portland is still ,i 1.000 being processed. Figure 2 shows the recorded peak accelerations I I. .~ (soil sites) plotted against distance. =tm from Scotts Mills Ea~qU8~ A ground motion attenuation relationship for rock has been superimposed for comparison 0.100 purposes.

1'Oi I 1 Intensity and Building Damage . If GI "ii The Scotts Mills earthquake u u la caused significant structural

0.010 damage to a number of unrein- forced masonry (URM) buildings in and around the epicentral area, but -+ left most wood framed houses and I 1111 buildings unscathed. In Molalla, the 9 unreinforced masonry high school 1trt' suffered significant damage and 0.001 ~~ Molalla Saleml Portland remains closed. In Mt. Angel, 1000 1 10 Newberg 100 unreinforced masonry buildings at Distance (km) ~he Benedictine conv~nt. and train- a) Ing center, the Benedictine Abbey, ~ and St. Mary's church and school Figure 2-Ground motion attenuation with distance for the magnitude 5.6 Scotts Mills earthquake. Solid line shows bedrock ground motion attenuation relationship were significantly damaged. developed by Joyner and Boore (1982) for comparison. Numerous URM commercial build-

2 EERI SPECIAL EARTHQUAKE REPORT. MA y 1993

ings in downtown Woodburn were ~~ignificantJy damaged and remain ,i closed. The Oregon State Capitol in Salem suffered cracking of the inner walls of the rotunda, and other minor damage to beams sup- porting the ceilings of the legislative chambers. Numerous chimneys toppled in Scotts Mills, Woodburn, Mt. Angel, and Molalla.

There were widespread reports of minor damage, such as cracked plaster and foundations, from as far away as the Portland metro- politan area. Surprisingly, at least 90 buildings located 28 miles (45 km) from the epicenter, in the town of Newberg, were damaged. Possible explanations for the damage are still being investigated (See following report for more Figure 3-Fal!en gables over entrances to Molal!a Union High School. information on structural response. ) future earthquakes. An intensity approximately 100 feet (30 m) questionnaire published in several high and suffered a broken cross Damage assessments were made Oregon newspapers has resulted in brace in the highest level of three /\ by the Federal Emergency Man- approximately 4,000 intensity cross brace sets. The broken (\. jagement Agency (FEMA), assisted reports from the public which have brace had compromised the tank by the Oregon Emergency Manage- yet to be compiled. A preliminary and further shaking could have ment Division. FEMA reported 16 intensity map derived from these caused collapse. residences and 54 businesses with data is shown on page 4. major damage, 186 residences and Local electric power disruption 94 businesses with minor damage, lifelines occurred within parts of the and over a thousand buildings with intensity VII zone. No damage has cosmetic damage. The estimated The State Highway 18 bridge been reported to buried lifelines or dollar loss for these buildings is across the Yamhill River near communications lifelines. over $2 million. The estimate for Dayton was closed for 4 days by damage to public facilities is over damage to the supports for the $12 million. That figure does not roller bearing at one end of a steel include the $850,000 in damage beam span. Differential movement to the state capitol rotunda, the caused by shaking appears to have largest single earthquake-damaged .caused the bearing to drop off its state building. The figure also support. The support was crushed does not include damages to in- when the bearing moved back sured properties, such as the $1 5 towards its original position. The million in damage to the school, bridge had not been equipped with nursing home, and church at the any seismic restraints. A passing The publication and distribution of Benedictine Abbey complex. motorist had all four tires blown this report was funded by National out when he hit the resulting 6-8 Science Foundation grant #BCS- The distribution of damage and felt inch ( 15-20 cm) ledge in the road 9215158. effects is being compiled at the surface. Oregon Department of Geology .Complete copies of reconnaissance ((( \3nd Mineral Industries (DOGAMI) The Benedictine Abbey at Mt. reports submitted to EERI are 'v to aid in production of an intensity Angel has two elevated water archived at the EERC/NISEE library. map. This map will help identify storage tanks, of 60,000 and Call EERC at 510-231-9401 for areas like Newberg that may be 80,000 gallons capacity (230 m3 further information. more prone to severe damage in and 300 m3) .The larger tank is

3 EERI SPECIAL EARTHQUAKE REPORT -MA y 1993

4 EERI SPECIAL EARTHQUAKE REPORT -MA y 1993

This report, with accompanying gable end walls, tall story heights, Collapse of brick parapets was ob- figures and photographs, interaction with adjacent buildings served at several locations in contrl"buted by David Bonnevli'le, of different height, and high para- Newberg and Woodburn (Figure 2). Anna Dal Pino and Dominic Kelly pets all were factors. Downtown Signs of separation between roofs of H. J. Degenkolb Associates, areas in most of the affected and walls also led to building Engineers. Figure 3 of bridge towns have many unreinforced ma- closures. damage by Oregon Department of sonry structures. Most of these Transportation. Strong-motion are regular buildings and experi- Building Inspections records from USGS. enced little or no damage. Posteartbquake evaluation is an Introduction Two gable end walls and a chim- uncommon duty for local officials ney collapsed at the Molalla Union in Oregon. Indeed, earthquakes Structural damage in the Scotts High School. Molalla students are historically infrequent in the Mills, Oregon, earthquake was luckily escaped tragedy, since the area. People were somewhat very limited, affecting only earthquake occurred early in the puzzled as to the reasons some buildings in the highest category of morning during spring break. The damage occurred. Postings risk. The earthquake again potential for loss of life was high, offered no description of the confirmed the susceptibility of since the damage occurred over damage or hazardous conditions unreinforced masonry buildings to the building's entrances where that led to closure. Evaluations severe damage, even in a minor students usually congregate. The appeared to be assessed on local r::.arthquake. In additio~, it. demon- masonry gable end walls, \Vtrated the need for seIsmic unanchored to the roof 1 preparation in Oregon to mitigate framing, fell and shattered - I the hazard in both existing build- at the school steps. See I ings and bridges, and the need to Figure 1 . ! mathrespond of aeffectively larger event. in theThe after- One of Mount Angel's following is a summary of struc- architectural treasures, St. tural damage found during a recon- Mary's Church, was naissance trip to the towns of damaged. The masonry Molalla, Newberg, Woodburn, Mt. structure is a national Angel, and Dayton, which lie historic landmark, con- within 40 km of the epicenter . structed in 1912. Some cracks in the masonry Building Damage walls, extending from the tops of the arched window Building damage was scattered but openings, were visible significant in the affected, mostly from the exterior. The I rural area. The structural and life- transept arches reportedly threatening, nonstructural building spread and the bell tower damage occurred to unreinforced was cracked, though no masonry structures, again confirm- local collapses occurred. ing past experience that buildings The structural damage of this type of construction are the appeared to be repairable most likely to suffer in a minor to and shoring was about to J moderate earthquake. begin the day after the earthquake. The nonstruc- cOparticularly unusual conditions or tural damage to plaster bad construction were generally wall finishes and statues Figure l-Damage to unreinforced masonry gable prerequisites to significant damage was more dramatic. over entrance of Molalla high school. in this earthquake. Unanchored

5

EERI SPECIAL EARTHQUAKE REPORT -MA y 1993

The upper bearing plate was con- hazardous, structures. Oregon '~ected to the girder with square now needs to move forward in fads welded along their length to mitigating building hazards and the plate top side and embedded begin preparation for the next only 112 inch into the concrete earthquake. girder. As the plate was not well connected to the girder I the plate was torn from the girder as the corner of the beam spalled.

The discontinuity failure is similar to those observed in many other bridges in past earthquakes. This failure further emphasizes the need The publication and distrl'bution of to design new bearing conditions this report was funded b y National and retrofit existing ones for this Science Foundation grant #BCS- common failure mode. 9215158.

Conclusion Complete copies of reconnaissance reports submitted to EERI are The Northwest Oregon earthquake archived at the EERC/NISEE library. was a warning. A more severe Call EERC at 510-231-9401 for Figure 4-Six-to-eight-inch ledge on earthquake would almost certainly further information. Yamhil! River bridge surface, caused by not spare the more regular, but support failure below.

/ c ~ .'/ / () 1/2" BAR "/ //, / /// ~- / STEEL GIRDER ~~, » I f :~, ~ <:= "/ => SECTION 8-8 /

6 112" DROP~

SPALLED END -~~ e.-~--: ~,' '// UPPER BEARING PLATE TOPPLED STEEL ~~I -I~' I~I PEDESTAL ! A II CRUSHED Ii III SECTION A-A CONCRETE PEDESTAL

1

II r Il \~-= II Ii

U"=,,, \(~

Figure 5 -Detail of bearing at failure, Highway t 8 bridge, Yamhill River.

7 EERISPECIAL EARTHQUA-KEREPORT -MA y 1993

NATIONAL !;TRONG-MOTTON PROGRAM DIRECTiIQN CONSTANTS MAX.ACCELERATION

Station No.2133 44.72N, 122.25w 198. Sens. = 4.00 cm/g 0.06 9 Freq. -17.2 Hz Detroit Dam, Oregon Damp. = 0.59 crit

SMA No.950 (ACOE) Downstream Up Sens. = 4.00 cm/g 0.05 Freq. -17.9 Hz Earthquake of Damp. = 0.59 crit

25 March 1993, 1334:36.5 G.m.t. lOSt Sens. 34.00 cm/g 0.06 Freq. m 16.7 Hz Damp. = 0.59 crit

Epicentral distance -43 km Film speed 3 1 cm/sec

SENSITIVITY IS TWICE OTHER RECOROS ---0--- --"-"-"-"-"-"-"-"-" "-"-"-"-"-"-. ---"- " ---"-"-"-"- "- "-" -"- "-" " " "-"-"-

NA'T'TI'\NAT. ~'T'RI'\Nr:-MI'\'T'TI'\N PROGRAM DIRECT~ON CONSTANTS MAX A~~ELERATION

Station No.2133 44.72N, 122.25W 198. Sens. = 1.85 cm/9 0.15 9 Freq. = 25.7 Hz

Detroit Dam, Oregon Damp. -0.59 crit Sens. = 1.95 cm/9 SMA No.954 (ACOE) Gallery Level .7 Up 0.07 Freq. = 25.2 Hz Earthquake of Damp. -0.59 crit

25 March 1993, 1334:36.5 G.m.t. 08" Sens. = 2.00 cm/9 0.18 Freq. -26.2 Hz Damp. -0.59 crit

Epicentral distance -43 km Film speed -1 cm/sec

~n-,~'--":..~:,\I( ~

I~ Figure 6 -Five strong-motion accelerographs operated by the National Strong-Motion Program were triggered. Peak accelerations occurred at Detroit Dam 43 km southeast of the epicenter, .06 g at the downstream site, and. 18 g in the upper gallery.

8

~