EERI SPECIAL EARTHQUAKE REPORT -JUNE 1992 (0 (Photographs provided by Tom Varner (EERI 1989) and Laura Varner, Dames & Moore.) Overview. During April 25-26, three earthquakes of magnitude 6 or greater occurred in a 24-hour span in the Petrolia area of Humboldt County, California. The first, and largest, was magnitude 7.0 near Petrolia, and was the strongest earthquake in California since Loma Prieta in 1989. The second (magnitude 6.0) and third (magnitude 6.5) events were off the coast of Cape Mendocino. There was building damage to older wood frame buildings in Ferndale, Rio Dell, Fortuna, and Scotia . Damage to a Victorian house in Ferndale ~eismologyc instrument atand Cape Geology. Mendocino An tion or local ground failure. Three obtained. Of particular interest (2.5 miles epicentral distance) faults meet near Mendocino; the was the 2g or greater peak accel- recorded a peak acceleration of 2g San Andreas Fault is to the south, eration recorded at Cape Mendo- lasting 0.1 to 0.2 seconds during the Cascadia subduction zone is to cino, discussed earlier, and a the magnitude 7.0 shock. This is the north, and the Mendocino Fault 0.69g record obtained in Petrolia. the highest acceleration ever re- is to the west. There is a broad Acceleration levels of 1.25g were corded in California, but it may be zone of deformation between the recorded at an abutment of the attributable to instrument malfunc- faults, and the resulting structure Painter Street Overcrossing, while is quite complex. If the main the ground acceleration was shock was on the Cascadia sub- 0.55g. Details are in California duction zone, it would be the first Division of Mines and Geology to rupture it. Geological evidence CSMIP Report OSMS 92-04. indicates that there have been five events of magnitude 8 or greater Building Damage. Most of the in the past 1700 years 250 km buildings that suffered damage in north of the triple junction. The this earthquake were wood frame magnitude 6.5 event produced an houses and 1 and 2-story wood unusual amount of energy, was frame commercial buildings. felt further away than the main Homes slid off foundations. Chim- shock, and produced additional neys collapsed. Stucco walls damage around the epicenter. Pre- cracked and unreinforced masonry liminary observation by the USGS parapets failed. Buildings designed indicates that there were approxi- to meet seismic codes did well. mately six to seven miles of the Main Street in Ferndale is a three Humboldt Coast that experienced a block long historic area, of wood seismic uplift of one-half to one construction and western facades. meter . Ferndale suffered more damage than the other cities; a number of Kitchen in the Ferndale Stro(1g Motion Records. A number damaged historic buildings of Volunteer Fire Station of strong motion records were Victorian architecture are to be EERI SPECIAL EARTHQUAKE REPORT -JUNE 1992 and communications} performed well except for a few instances. .., u.s. Highway 101 remained open with some temporary closures for bridge inspection and cleanup of minor landslides. Most of the highway bridges closest to the epicenters experienced settlement of the fill adjacent to the bridge abutments and concrete spalling at joints. Local roads had some closures for inspection, cleanup, and minor repairs. The North Coast Railroad operates between Willits and Eureka trans- porting gravel and lumber. The two trains operating on the day of the first earthquake were stopped Approximately 6" of settlement occurred at the in accordance with company approach to the Lindley Bridge over the Mat tole River policy. Trains were cleared to There was no evidence of damage proceed to their terminals after the in 12 school buildings. Ten tracks, tunnels and bridges were experienced minor non-structural inspected. On the next day, damage, ranging from stucco and Sunday, a normal non-operating plaster cracks to some minor day, a more detailed inspection cracks in structural members. was made. The tracks were cleared of minor loose landslide CAl TRANS Bridges. Over 50 material and were reballasted at bridges were inspected. A few one location of minor settlement. were closed temporarily, but none There was no settlement at bridge remained closed after inspection. abutments. Train service returned Many bridges showed signs of to normal on Monday without a movement. The Van Duzen River loss of service. Bridge, an 800 foot steel girder bridge. had minor cracks in its The most significant loss of water piers. North Van Duzen, a 750 service occurred in the city of Rio foot concrete arch, developed Dell when their 8-inch water main cracks in a few columns connect- broke at the riser at the abutment ing the arch to the superstructure. of the southbound Eel River Bridge. The water line to Rio Dell failed at The break caused the supply tanks Soil movement at the its transition onto the southbound to drain, leaving the city without a lindley Bridge abutment Eel River Bridge. The northbound water supply. An emergency restored, based on the provisions Eel River Bridge was base isolated; potable supply was provided by of the California State Historical although there were signs of the American Red Cross, National Building Code. Many owners were substantial movement, the system Guard, and Anheuser Busch, Inc. interested in strengthening, not performed well. Fire protection was supplied by only repairing, their buildings. In contractor tank trucks. Water the Historical Building Code, life (Ron Hamburger, Sam Swan, supply to the city was restored safety is the minimum level re- David Cocke, Andy Michael, four days later on April 29th. tfj quired for repair and strengthening . James Gates, and Patrick Campbell Non-compliant materials may be contributed material for the Scotia, across the Eel River from used for resisting elements. preceding sections of this report.) Rio Dell, has separate water supply systems for domestic and fire Public Schools. Twenty-two public 01;her Lifelines. Lifelines (trans- protection. The fire protection schools in the area were inspected. portation, water, electric power system was damaged and was 'lit r 6 EERI SPECIAL EARTHQUAKE REPORT -JUNE 1992 inoperable. A fire destroyed four-store shopping center after the second earthquake. The California Office of Emergency Services provided the town with a portable piping system and pump which was installed by the Scotia Volunteer Fire Department. It was available for service on April 2ath. There was no reported damage to the domestic water system. The water and sewer systems in Ferndale continued to function. In Fortuna, there was a power loss to a sewer pumping plant, but there were no reported sewer spills. The communities and cities in the Eureka area are served bya re- gional water agency which stores Painter Street Bridge -1.25g recorded at top deck runoff in Ruth Lake Reservoir on the Mad River. The dam and small pressure main from the Sacra- capacity of the system. The hydroelectric plant (2 megawatts) mento Valley and a very small company initiated load control, continued to perform satisfactorily, supply from a local gas field. restricting incoming calls to as did other portions of the distri- Ferndale and other smaller com- Humboldt County for a few hours bution system. munities rely on bottled gas. Gas after the earthquakes. i OElectric power is supplied by a mains responded well in bridge structures even though the bridges There are six AM/FM radio stations were subject to slight displace- I regional entity which had power and three television stations outages ranging from seconds to ment. There were no underground serving the area. Most lost their several hours. Sources of power gas leaks in the distribution signals when they lost power are a fossil fuel generating station, system; however, there were some supply. Some of the stations had three transmission lines from small leaks at meter risers due to to obtain emergency generators to I outside the area, and several co- corrosion or falling debris. In Rio restore service. The radio generation facilities in the area. Dell and Fortuna approximately 50 frequency shared by several The peaking unit operating at the service connections were shut off emergency organizations became time of the first event tripped off due to structural damage or leaks. congested. The organizations and could not be started again due involved are considering using to condenser tube leaks and low As in past earthquakes, the most separate frequencies in the future. water levels in the steam drum. significant interruption of gas The other peaking unit was "hot" service was the unnecessary turn- The Humboldt County Emergency .and took 6 hours to reach oper- off of gas. There were 1190 ser- Operation Center in Eureka was ating output. It then tripped during vices turned off unnecessarily in activated and other cities and the second event. Despite these Scotia, Rio Dell, and Fortuna. The agencies activated their EOC's. events, there was adequate power regional gas agency crews, with There was a separate public supply because of the availability assistance of personnel from the service answering point (PSAP), of the outside sources. Local out- San Francisco Bay area, went from 911 service, for the county ages were caused by transformer house to house to relight the unincorporated areas and the city. fires, wires welded together. wires services. Amateur radio operators were used slapping together, and wires by several agencies. burning down. Most lifelines and At the regional telephone c "essential services had emergency company, there was no damage to (Lifelines information excerpted \(~enerator back-up power supplies central offices or telephone from a report by Le Val Lund and for their critical facilities. equipment, except for the loss of Anshel Schiff, prepared for the two 25-pair cables.