EERI SPECIAL EARTHQUAKE REPORT -JUNE 1992

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(Photographs provided by Tom Varner(EERI 1989) and Laura Varner, Dames& Moore.)

Overview. During April 25-26,three earthquakes of magnitude 6or greater occurred in a 24-hourspan in the Petrolia area ofHumboldt County, California. Thefirst, and largest, was magnitude7.0 near Petrolia, and was thestrongest earthquake in Californiasince Loma Prieta in 1989. Thesecond (magnitude 6.0) and third(magnitude 6.5) events were offthe coast of Cape Mendocino.There was building damage toolder wood frame buildings inFerndale, Rio Dell, Fortuna, andScotia .

Damage to a Victorian house in Ferndale~eismology and Geology. Anc instrument at Cape Mendocino

(2.5 miles epicentral distance)recorded a peak acceleration of 2glasting 0.1 to 0.2 seconds duringthe magnitude 7.0 shock. This isthe highest acceleration ever re-corded in California, but it may beattributable to instrument malfunc-

tion or local ground failure. Threefaults meet near Mendocino; theSan Andreas Fault is to the south,the Cascadia subduction zone is tothe north, and the Mendocino Faultis to the west. There is a broadzone of deformation between thefaults, and the resulting structureis quite complex. If the mainshock was on the Cascadia sub-duction zone, it would be the firstto rupture it. Geological evidenceindicates that there have been fiveevents of magnitude 8 or greaterin the past 1700 years 250 kmnorth of the triple junction. Themagnitude 6.5 event produced anunusual amount of energy, wasfelt further away than the mainshock, and produced additionaldamage around the epicenter. Pre-liminary observation by the USGSindicates that there were approxi-mately six to seven miles of theHumboldt Coast that experienced aseismic uplift of one-half to onemeter .

obtained. Of particular interestwas the 2g or greater peak accel-eration recorded at Cape Mendo-cino, discussed earlier, and a0.69g record obtained in Petrolia.Acceleration levels of 1.25g wererecorded at an abutment of thePainter Street Overcrossing, whilethe ground acceleration was0.55g. Details are in CaliforniaDivision of Mines and GeologyCSMIP Report OSMS 92-04.

Building Damage. Most of thebuildings that suffered damage inthis earthquake were wood framehouses and 1 and 2-story woodframe commercial buildings.Homes slid off foundations. Chim-neys collapsed. Stucco wallscracked and unreinforced masonryparapets failed. Buildings designedto meet seismic codes did well.Main Street in Ferndale is a threeblock long historic area, of woodconstruction and western facades.Ferndale suffered more damagethan the other cities; a number ofdamaged historic buildings ofVictorian architecture are to be

Kitchen in the FerndaleVolunteer Fire Station

Stro(1g Motion Records. A number

of strong motion records were

EERI SPECIAL EARTHQUAKE REPORT -JUNE 1992

and communications} performed

well except for a few instances. ..,u.s. Highway 101 remained openwith some temporary closures forbridge inspection and cleanup ofminor landslides. Most of thehighway bridges closest to theepicenters experienced settlementof the fill adjacent to the bridgeabutments and concrete spalling atjoints. Local roads had someclosures for inspection, cleanup,and minor repairs.

The North Coast Railroad operatesbetween Willits and Eureka trans-porting gravel and lumber. Thetwo trains operating on the day ofthe first earthquake were stoppedin accordance with companypolicy. Trains were cleared toproceed to their terminals after thetracks, tunnels and bridges wereinspected. On the next day,Sunday, a normal non-operatingday, a more detailed inspectionwas made. The tracks werecleared of minor loose landslidematerial and were reballasted atone location of minor settlement.There was no settlement at bridgeabutments. Train service returnedto normal on Monday without aloss of service.

Approximately 6" of settlement occurred at the

approach to the Lindley Bridge over the Mat tole River

There was no evidence of damagein 12 school buildings. Tenexperienced minor non-structuraldamage, ranging from stucco andplaster cracks to some minorcracks in structural members.

The most significant loss of waterservice occurred in the city of RioDell when their 8-inch water mainbroke at the riser at the abutmentof the southbound Eel River Bridge.The break caused the supply tanksto drain, leaving the city without awater supply. An emergencypotable supply was provided bythe American Red Cross, NationalGuard, and Anheuser Busch, Inc.Fire protection was supplied bycontractor tank trucks. Watersupply to the city was restoredfour days later on April 29th.

Soil movement at the

lindley Bridge abutment

restored, based on the provisionsof the California State HistoricalBuilding Code. Many owners wereinterested in strengthening, notonly repairing, their buildings. Inthe Historical Building Code, lifesafety is the minimum level re-quired for repair and strengthening .Non-compliant materials may beused for resisting elements.

CAl TRANS Bridges. Over 50bridges were inspected. A fewwere closed temporarily, but noneremained closed after inspection.Many bridges showed signs ofmovement. The Van Duzen RiverBridge, an 800 foot steel girderbridge. had minor cracks in itspiers. North Van Duzen, a 750foot concrete arch, developedcracks in a few columns connect-ing the arch to the superstructure.The water line to Rio Dell failed atits transition onto the southboundEel River Bridge. The northboundEel River Bridge was base isolated;although there were signs ofsubstantial movement, the systemperformed well.

tfj

rScotia, across the Eel River fromRio Dell, has separate water supplysystems for domestic and fireprotection. The fire protectionsystem was damaged and was

(Ron Hamburger, Sam Swan,David Cocke, Andy Michael,James Gates, and Patrick Campbellcontributed material for thepreceding sections of this report.)

Public Schools. Twenty-two publicschools in the area were inspected.

01;her Lifelines. Lifelines (trans-

portation, water, electric power'lit

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EERI SPECIAL EARTHQUAKE REPORT -JUNE 1992

inoperable. A fire destroyedfour-store shopping center

after the second earthquake. TheCalifornia Office of EmergencyServices provided the town with aportable piping system and pumpwhich was installed by the ScotiaVolunteer Fire Department. It wasavailable for service on April 2ath.There was no reported damage tothe domestic water system.

The water and sewer systems inFerndale continued to function. InFortuna, there was a power loss toa sewer pumping plant, but therewere no reported sewer spills. Thecommunities and cities in theEureka area are served bya re-gional water agency which storesrunoff in Ruth Lake Reservoir onthe Mad River. The dam and smallhydroelectric plant (2 megawatts)continued to perform satisfactorily,as did other portions of the distri-bution system.

Painter Street Bridge -1.25g recorded at top deck

pressure main from the Sacra- capacity of the system. Themento Valley and a very small company initiated load control,supply from a local gas field. restricting incoming calls toFerndale and other smaller com- Humboldt County for a few hoursmunities rely on bottled gas. Gas after the earthquakes.mains responded well in bridgestructures even though the bridgeswere subject to slight displace- I

ment. There were no undergroundgas leaks in the distributionsystem; however, there were somesmall leaks at meter risers due tocorrosion or falling debris. In RioDell and Fortuna approximately 50service connections were shut offdue to structural damage or leaks.

There are six AM/FM radio stationsand three television stationsserving the area. Most lost theirsignals when they lost powersupply. Some of the stations hadto obtain emergency generators torestore service. The radiofrequency shared by severalemergency organizations becamecongested. The organizationsinvolved are considering usingseparate frequencies in the future.

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As in past earthquakes, the mostsignificant interruption of gasservice was the unnecessary turn-off of gas. There were 1190 ser-vices turned off unnecessarily inScotia, Rio Dell, and Fortuna. Theregional gas agency crews, withassistance of personnel from theSan Francisco Bay area, went fromhouse to house to relight theservices.

The Humboldt County EmergencyOperation Center in Eureka wasactivated and other cities andagencies activated their EOC's.There was a separate publicservice answering point (PSAP),911 service, for the countyunincorporated areas and the city.Amateur radio operators were usedby several agencies.

i OElectric power is supplied by a

regional entity which had poweroutages ranging from seconds toseveral hours. Sources of powerare a fossil fuel generating station,three transmission lines fromoutside the area, and several co-generation facilities in the area.The peaking unit operating at thetime of the first event tripped offand could not be started again dueto condenser tube leaks and lowwater levels in the steam drum.The other peaking unit was "hot"

.and took 6 hours to reach oper-ating output. It then tripped duringthe second event. Despite theseevents, there was adequate powersupply because of the availabilityof the outside sources. Local out-ages were caused by transformerfires, wires welded together. wiresslapping together, and wiresburning down. Most lifelines and

c "essential services had emergency

\(~enerator back-up power suppliesfor their critical facilities.

(Lifelines information excerpted

from a report by Le Val Lund and

Anshel Schiff, prepared for the

ASCE TCLEE Earthquake

Investigation Committee.)

At the regional telephonecompany, there was no damage tocentral offices or telephoneequipment, except for the loss oftwo 25-pair cables. There was theus~al telephone congestion whenthe number of calls exceeded the

Natural gas supply is provided by a

regional entity via a 12-inch high-

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EERI SPECIAL EARTHQUAKE REPORT -JUNE 1992

Philippine Islands,May 18-19, 1992

Two strong earthquakes,magnitudes 7.3 and 6.8, struckjust offshore of the PhilippineIslands approximately 600 milessoutheast of Manilla on May 18and 19. At the time of preparationof this newsletter, there were nocasualty, damage, or tsunamireports. This was the strongestearthquake in the area since amagnitude 7.3 earthquake onMarch 15, 1934.

Landslide on the Mat tole Road between Petrolia and Ferndalecaused tree to fall on overhead lines

It is not too soon to make a few observations on lessons learned relating toearthquake resistant design and construction.

First, the California earthquakes are testimony to the advances in seismicdesign and construction. Buildings and bridges designed to current practiceperformed very well.

Second, where current standards are not followed or obvious problems,such as unreinforced masonry parapet walls, are not corrected, failures arelikely to occur .

Finally, retrofit can be successfully accomplished. Especiallyencouragingwas the observation that retrofitting using base isolation technology wassuccessful for buildings as well as bridges. Although the methods were notseverely tested in these earthquakes, these trial runs were certainly

satisfactory.

The earthquake engineering professions can pause for a moment and takepride in their accomplishments to date. As the old ad went, "You've comea long way, baby!"

(Joshua Tree, cont. from page 3)CAL TRANS Bridges. Of 48 bridgesexamined after the earthquake, 13showed signs of movement, with9 suffering some minor damage.None of the bridges suffered dam-age significant enough to affect itscapacity to carry traffic. Bridgeswith monolithic details performed ,~best. Steel girder and precast c

concrete I-girder bridges were

capable of moving and did,

resulting in minor damage. The

most significant damage was to

the Jackson Street Overcrossing

and Overhead in Indio, which lost

both abutment backwalls from thesteel girders banging against themduring the earthquake. The PalmSprings Overhead abutments hadconcrete spalls. Many bridgeswere retrofitted after the 1 968Palm Springs earthquake and theyperformed well. Among them wasthe Colton Interchange, about 60miles west of the earthquake, aprestressed concrete box girder. Itwas heavily instrumented by theCalifornia Division of Mines andGeology and a peak acceleration of0.20g was recorded. Some soilseffects were observed: settlementbehind abutments, soil pushedaway in front of abutments, air ...spaces between soil and columns. ~

James Letter, Project Manager, EERI Learning From Earthquakes

-INSF Grant #CES-8822367 fundedthe publication and distribution ofthis report. (Condensed from a report prepared

by Mark Yashinsky and submitted

to EERI by James Gates.)

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