Effects of the Truckee, California, earthquake of September 12, 1966

INTRODUCTION

The Truckee earthquake of September 12, 1966, occurred at 9:41 a.m. P.d.t. (1641 G.m.t.) and was estimated to have had a Richter magnitude of 5.4 by the U.S. Coast and Geodetic Survey, of 6.25- 6.5 by the California Institute of Technology at Pasadena, and of 6.1 by the University of California at Berkeley.

The earthquake was felt over an area which extended from San Francisco on the west to Salt Lake City on the east and from Bakersfield on the south to Chico on the north (fig. 1). Russel Valley, about 8 miles north-northeast of Truckee, was the only area to show any permanent effects of the earthquake.

About 21 seismic shocks greater than magnitude -l (fig. 2) are reported in the area for the period 193-l-61 by the California Department of Water Resources (1964). None of the 21 epicenters is known to be associated with faulting at the ground surface, although some in the northwestern part of the area can be associated spatially with northwestward-trending faults that are buried in the Truckee area (south of lat 39°30' N.).

Scope of present study

The purpose of the investigation described herP was to locate and document the significant effects of the earthquake and, where possible, to relate them to the geologic environment. The investigation extended over a 3-day period beginning late on September 13, the day following the main shock

Acknowledgments

\Ve received help from numerous people, particularly Mrs. A. L. Scott, Chico, Calif.; Mrs. R. M. Gwinn, Sacramento, Calif.; Mr. Frank Fox and Mr. H. C. ~Johnson, California Division of Highways; Dr. L ..J. Payen, Sacramento, Calif.; Mr. Reuben Alcala, Southern Pacific Lines; Mr. B. H. Crater, U.S. Bureau of Reclamation; and Mr. Francis Longo, Truckee Utility District.

GEOGRAPHIC AND GEOLOGIC SETTING

The Truckee-Russel Valley area is located within the "Truckee Basin" (Birkeland, 1963)-an intermontane basin that, with the Lake Tahoe basin to the south, is flanked by the 9,000-foot-high Sierra Nevada on the west and the 10,000-ft-high Carson Range on the east. The area is underlain by Tertiary lavas and pyroclastics north of lat. 39°30' N. and by Pliocene and Pleistocene basalt and latite flows to the south. West of Donner Lake the faulted east escarpment of the Sierra Nevada is underlain by Mesozoic granitic rocks that intrude Mesozoic metasedimentary and metavolcanic rocks (Burnett and .Jennings, 1962). Except for its oc- <'Urrence in canyon bottoms, bedrock throughout the lower lying parts of the area is buried by deposits of glacial, lanch;lide, <'011nvia1, lake, and al- ·lnvial material.

MAIN SHOCK AND AFTERSHOCKS

Surface effects suggest that the epicenter was located 10 miles west-southwest of Verdi and in the vicinity of Russel Valley. The valley is 9 miles north-northeast of Truckee at lat 39°26" N., long 120°08' W.

There were more than a thousand aftershocks within a week following the earthquake; many aftershocks seem to have been generated in the immediate vicinity of Prosser Reservoir. According to preliminary estimates, the focal depths of the aftershocks were less than 5 km and maybe around 2 km (Roger Greenfelder, oral commun., 1966). The preliminary instrumental locations of some of the aftershocks appear to correspond ~lose ly with the surficial effects of the main shock.

The known effects of the earthquake were entirel.v due to seismic shaking. The permanent awl transitory effects are keyed by locality to the map on plate 1. In addition to localized ground breakage, th{,:' earthquake affected dams, bridges, highways, buildings, railroad, water facilities, pipelines, and a flume.

Seismic motion

The character and duration of the seismic motion throughout the area apparently depended upon the distance from the epicenter and the local geologic environment.

JVIrs. R. l\II. Gwinn of Sacramento reported that shaking lasted 20-30 seconds and was -in a southeast-northwest direction. At Chico, Mrs. A. l.J. Scott was having a telephone conversation with a friend 15 miles (24 km) east of Chico and fe~t the earthquake 4-5 sec after the person to whom she was speaking felt the motion. Mrs. Scott reported two distinct shocks a few seconds apart in which the motion was rolling-east-west in direction of vibration-and lasted for about 15-20 sec.

Oral reports of witnesses indicate that the direction of motion was northeast-southwest at Truckee and Tahoe City, north-south at Verdi, and east- \Vest at Sierraville and Loyalton. Sierraville and l.Joyalton are built on alluvium; residents of these areas reported a rolling motion that lasted for about 30 sec. At Truckee, Mr. Richard Keylor \vas on bedrock and stated that the motion was sharp and lasted for about 10 sec. However, in the alluviated area of the Truckee Airport, about 2 miles southwest of Truckee, Mr. Lester Michael reported that the seismic motion was of a rolling nature and lasted for about 30 sec. At Tahoe City, located on the northwest shore of Lake Tahoe approximate])' 20 miles southwest of Russel Valley, the motion was rolling and was felt for about 15 sec; Tahoe City is built on a thin veneer of sediments overlying bedrock.

Observers on unconsolidated sediment or at a distancr from the epicenter felt a rolling motion, whereas those on bedrock or close to the epicenter felt a jarring motion.

Effects on terrain

Broken ground, slumping and fissuring of natural and artificial fill, and rock falls were found along a zone trending north-northeast about 5 miles wide and 10 miles long centered in the Russrl Yalle~r area (fig. 2).

Minor but extensive ground breakage occurred in a zone about 10 miles long that trends about N. 30° E.; the zone ranges in width from about 75 ft near Prosser Reservoir and Hoke Valley to about 1 mile in Russel Valley. Ground breakage was confined to low elongated areas of unconsolidated natural fill, commonly subparallel to the zone. The breakage is expressed locally as cracks, slightly sinnons small ridges, wedges, and mounds of freshly disturbed soil and turf; the linear features are commonly oriented subparallel to the zone and arc as much as 20 ft long. Pebbles were locally overturned. All these effects are attributed entirely to shaking but are in no case associated with lateral spreading of the ground toward an unconfined face. They may be, however, associated with the effects of topography on the passage of gravitational waves. (See Coulter and Migliaccio, 1966, p. 20.)

Alinement of the zone of ground breakage with the trend of a fault exposed at the Stampede damsite (fig. 2) suggests an association with the fault; however, a thin veneer of undisturbed soil overlying the fault trace indicates that movement did not occur here during the earthquake.

Dry natural slopes as low as 20° underlain by semiconsolidated alluvial and colluvial deposits showed minor slumping at two localities along the Little Truckee River north of Russel Valley. Headwall fissures opened as much as 1 inch and the ground surface dropped as much as 3 in. across these fissures; a buried telephone cable was damaged by one of the slumps. A small slide occurred just above water level on the \·est side of Prosser Reservoir (fig. 3) .

Effects on structures

Three dams were studied-two earthfill and one concrete. Both the Prosser and Boca earthfill dams were damaged, whereas the concrete structure, located at Tahoe City 20 miles south of the epicentral area, sustained slight damage.

Prosser Dam is located about 3 miles southeast of Russel Valley and within a mile of the zone of ground breakage (fig. 2). The dam rests upon lake sediments and apparently received severe seismie shaking. Two axial cracks, one on the downstream side and one on the upstream side of the dam, developed along the northeastward-trending crest and extended over most of its length (figs. 4 and 5). A third less conspicuous crack was confined to the center of the crest. Drill holes by the Burean of Reclamation indicate that the cracks do not extend into the impervious core of the dam (Brent Crater, oral commun., 1966). Maximum downslope displacement of the well-graded selected compacted fill of 2 in. vertically and 3 in. horizontally occurred in the vicinity of the spillway (fig. 6).

Boca Dam, which also lies 3 miles southeast of Russel Valley, but about 3 miles from the zone of breakage (fig 2) and three-fourths of a mjle south of the instrumental epicenter, was less .severel:.r damaged than Prosser Dam. Damage to Boca Dam was restricted to small cracks in the earthfill crest of the dam at its junction with the concrete spillway structure.

The concrete dam on the Truckee River at Tahoe City controls the outflow from Lake Tahoe. This structure is reported to have cracked slightly during the earthquake, but the only cracks noted by the writers during a brief examination of the dam were along construction joints, particularly in the counterfort buttresses, and all appeared to be preearthquake.

Most of the damage to high·way bridges occurred on Interstate Highway 80 between Donner Lake and Floristan, but one bridge along Highway 89 north of Truckee was damaged slightly. The ensuing discussion on bridge damage is based on information collected by Mr. H. C. Johnson, California Division of Highways, and by R. Kachadoorian :md R. F. Yerkes.

The two bridges of Interstate Highway 80 at Union Mills, about 5 miles northeast of Truckee, sustained minor but widespread damage (figs. 7 and 8). Damage to the bridge on the eastbound lane included a westward shifting of the east concrete pedestal relative to the abutment and a consequent cracking of the upper part of the abutment during lateral spreading of the abutment fill. r~ateral spreading of fill also caused vertical displacement of the approaches. Steel rockers on piers 2, 3, 4, and 5 were out of plumb. Slight spalling occurred at the concrete deck expansion joints in 1·esponse to ramming of the spans against one another during the earthquake. All three concrete pedestals at the west end of the bridge were cracked vertically due to stresses that developed during the earthquake.

Damage to the westbound lane bridge at Union l\iills was less severe than that to the eastbound lane bridge. The concrete pedestals of the westbound lane bridge were displaced as much as 11j8 in. relative to the shoe plate by which the stringers are attached to the pedestal, and some lateral displacement downslope of the abutments and pedestals of both approaches occurred (fig. 9).

Minor damage was sustained by the eastbound lane of the Donner Park Exchange bridge on Interstate Highway 80, llh miles west of Truckee. Mr. H. C. Johnson reported that at the south abutment, where the girders rest on steel rockers, the bolts holding the keeper plates were sheared from two of the plates from each of the five bearings (written commun., 1966). The top and bottom keeper plates were sheared off along the easternmost girder, and the rocker moved laterally about 2 in. Johnson also reported a small spall of concrete in the right curb at the south abutment and that the left curb was cracked about 2 ft in from the expansion joint. Damage to this bridge is attributed to lateral displacement of the superstructure relative to the abutments.

At the Donner Lake Interchange bridge of Interstate Highway 80, about 5 miles west of Truckee, artificial fill of the east abutment of the westbound lane bridge failed by shJmping. This fill consists of decomposed granite that was displaced about 31h ft southward at the surface; the depth of displacement extended to at least 4 ft.

Little or no damage occurred to the bridges from Union Mills east to Floristan. The approaches to the bridges at Floristan and between Union Mills and Boca, however, settled.

The south abutment of the Prosser Creek bridge on Highway 89 just west of Prosser Reservoir was damaged slightly. This damage consisted primarily of spalling of the concrete, probable displacement of the south abutment about half an inch to the west, and cracks in the west curb. .:

Artificial fill along a 3-mile section of Interstate Highway 80 southeast of Boca Reservoir locally slumped during the earthquake. The longest of these slumps was a 1,100-ft section near Hinton that failed along the cut-fill contact and resulted in displacements of as much as 6 in. horizontally and 10 in. vertically (figs. 10 and 11). The several smaller failures along the 3-mile section southeast of Boca Reservoir were chiefly in the eastbound or outer lanes or in filled sections of the highway. The failures occurred at the cut-fill contact.

Additional damage along Interstate Highway 80 was caused by boulders as large as 10 by 10 by 8 ft that fell along cuts especially where the highwa~r passes over or through talus deposits.

The Southern Pacific Lines suffered both physiral damage and downtime because of the earthquake. Between Boca and the California-Nevada boundary boulders fell onto the rails and halted the westbound traffic. Impact of some of the boulders locally displaced the tracks about 2 in. horizontally. Boulders also reportedly fell on a 10- hy 12-ft snowshed and damaged the structure; the exact location of the structure is unknown but was reported by Southern Pacific personnel to be in the Truckee Canyon between Boca and the California-Nevada boundary.

The raihvay roadbed was displaced both vertically and horizontally where it traverses loose filJ over the stream flowing from Boca Reservoir. The tracks here required both realinement and repeated regrading. Mr. Reuben Acala reported that the fill was subsiding on September 14, 1966; it was brought to grade three times on September 13 and nt least once on September 14.

Small slumps in the railroad grade occurred where the main line passes beneath the Interstate Highway 80 bridge at Union Mills. These slumps extended for about 200 yards bnt did not include the r ails.

Effects on buildings consisted chiefly of toppling or minor damage to chimneys, cracking of plaster, and twisting of rafter beams during swaying. Building damage was confined chiefly to alluviateJ areas and occurred as far away as Carson City (26 miles southeast of Russel Valley) and Loyalton (18 miles north-northeast) . The watermaster's house at Boca Dam, which was built on uncompacted fill consisting of soil and boulders, was severely racked, whereas a nearby garage on cut ground was undamaged. Two chimneys toppled at the roofline, and vibration of the frame stressed the sill anchor bolts, rending and cracking the foundation (fig.

At the Feather River Lumber Co. in Loyalton, a 75- by 400-ft shed, made of sheet metal on a wooden frame, oriented with its length trending about N. 45° W., and built on alluvium, was severely racked and tilted. Damage was so severe that the building was deliberately collapsed.

Exposed piplines to the lumber mill at Loyalton were ruptured and separated. Several buried pipelines were ruptured at the watermaster's house at Boca Dam. A power generator at the Drum powerhouse on the Bear River Canal about 4 miles northeast of Auburn (about 60 miles southwest of Russel Valley) was reported to have been out of commission during the earthquake (Nevada State .Journ al, Srpt. 13, 1966, p. 7 ). Although communications were not disrupted, there was a break in the sheath of the transcontin en tal trlephone cable nra1· the Littlr Truckee River.

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FIGURE 9.-West abutment af westbound lane bridge, Interstate Highway 80 at Union Mills; arrows indicate displacement af pedestal relative ta span.

FIGURE 10.-Sauthward view af repaired crack near Hinton an In terstate Highway 80; left lanes far westbound traffic and right lanes far eastbound traffic; artificial fill slumped along cut-fill contact and toward open face at right.

n-ateway-Truckee area (10 miles south-southwest). In the several instances investigated, a dominant east-west component of swaying was indicated-a direction which agrees with the observed effects of swaying on huildings throughout the area.

EFFECTS ON WATER REGIMEN

The impact of the earthquake on the water resour·crs of the region consisted principally of water-level fluctuations in response to seismic shock, some increases in the flow of streams near the epicenter, the roiling of water in wells, and small increases in the flow of springs in the Truckee-Russel Valley area in the vicinity of Truckee.

Seismic effects were shown by water-stage recorders at many gaging stations on streams in California and Nevada east of the Sierra Nevada and in the Central Valley of California. The effects, raused largely by wave motion induced in the stilling well of a gage by ground motion associated with the seismic shock, caused movement of the recorder float and consequent vertical tracings on the recorder graph. The rise and fall of the tracings constitute an approximate measure of the se-Yerity of the earth shock.

F. C. Craig, a Geological Survey hydrologist, had completed inspection of the water-stage recorder on the Sacramento River at Freeport, 10 miles south of Sacramento, on September 12 just prior to the earthquake. He reported as follows:

Shock effects east of the Sierra Nevada were noted at gaging stations over an area extending from Lovelock and Winnemucca, Nev., to Bridgeport, Calif. \Vest of the Sierra Nevada these effects were recorded principally in the lower valley areas extending from Red Bluff to Fresno.

Fluctuations recorded at selected gaging stations are listed in table 1; the location of these stations is shown in figure 14. Lesser effects were indicated by the charts from many other stations in the valley and lower foothill areas. Stations in the Sierra Nevada and upper foothill areas on the west slope generally showed little or no effect. For example, the record for Haypress Creek, a headwater tributary of the North Yuba River about 15 miles east of DmmieYille near Sierra City, Calif., and about 20 mil es \Yest of the epicenter, showed no noticeable fluctuation. The record for Onion Creek tributary 2, which is 15 miles southwest of the epicenter and within the headwaters of the North Fork American RiYer, however, indicated water-levei fluctuations as much as 0.035 ft. Table 1.-Fluctuations and net changes in water level recorded at gaging stations, September 12, 1966

SACRAMENTO RIVER BASIN 4 _____ Sacramento River near Red 0.03 0.00 Bluff. 5 _____ Stony Creek below Black Butte .04 .00 Dam, near Orland. 6 _____ Sacramento River at Butte .03 .00 City. 7_____ .00 Station about 20 miles west of Haypress Creek near Sierra .00 epicenter. City. 8 _____ Onion Creek tributary 2 near .035 .00 Soda Springs. 9 _____ Feather River at Nicolaus ____ .00 .07 10_____ Yolo bypass near Woodland ___ .035 .00 11 _____ American River at Fair Oaks__ .00 .02 12 _____ Sacramento River at Freeport_ .10 .00 Approximate .

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lake and that the ensuing violent waves caused him to make for shore. This report might describe the result of a subaqueous landslide in that part of the lake. However, the watch on duty September 12, 1966, at the north-shore station of the U.S. Coast and Geodetic Survey reported that before the earthquake a strong south wind had caused \·hitecaps on the northern part of the lake and that a 31;2-ft swell was running. The same witness remarked that no unusual effects occurred on the lake during the earthquake. The swell was not unusually high for the climatic conditions, and it would probably have masked small seiches (periodic water-level oscillations).

The record for Lake Tahoe at Tahoe City (fig. 15), obtained at a pier 1,000 ft east of the dam at . the outlet near the north end of the lake, indicates a surge in the gage well of 0.40 ft. The stilling well for. this gage is 24 in. in diap1eter and is. equipp€d with a 1;4 -in.-diametm~ inlet to damp local wave action. The stage in the well thus does not respond fully to rapid stage changes ;in the lake, and the complete range of transitory waves, such as swells and seiches, may not be recorded.

U ngaged water-supply wells, springs, and distribution systems in the Truckee-Russel Valley area north of Truckee also were affected by the earthquake.

The town of Truckee obtains its water supply from springs and wells at three localities. The water from these sources was muddied following the quake and did not completely clear up until September 16 or 4 days after the quake. In addition, a 6-in. main supply line was broken at Truckee. Mr. Francis Longo, manager of the Truckee Utility District, reported that there was a similar occurrence after an earthquake in the autumn of 1961.

Springs in Hoke Valley, 41/2 miles northeast o:f Russel Valley, and in Sardine Valley, 3 miles . north of Russel Valley, showed increases in flow, and the water was murky after the earthquake. On September 15 the flow in these springs had not returned to the preearthquake levels, although the water in most had cleared up.

Dr. I_Jouis J. Payen, a rancher in Sardine Valley stated that before the earthquake he could drive anywhere in the valley without difficulty, whereas after the earthquake such travel was difficult because of the damp ground. Local accumulations of standing water were noted at the north end of the valley on September 14, 1966.

A water well being drilled in a development south of Prosser Reservoir and about 3 miles north of Truckee was dry and open to a depth of 570 ft on September 11; when it was reentered on September 13, the hole was obstructed at 300 ft and was wet. The well is on or near the southwesterly continuation of the ground breakage which trends N. 30° E.

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