from California Geology, September 1980, Vol. 33, No. 9.

MAMMOTH LAKES EARTHQUAKES, MAY 25-27, 1980

MONO COUNTY, CALIFORNIA

By

Richard D. McJunkin, Geologist

Trinda L. Bedrossian, Geologist

California Division of Mines and Geology

All photos by Richard D. McJunkin unless otherwise noted.

On May 25, 1980 at 0933 Pacific Daylight Time (PDT) a magnitude 6.0 earthquake (all magnitudes are from Caltech Seismological Laboratory) occurred approximately 10.5 km east-southeast of Mammoth Lakes, California (figure 1). During the next 16 minutes, four magnitude 4.1 - 5.0 shocks and one 5.5 shock occurred. This seismic activity was the beginning of an earthquake sequence that produced 72 magnitude 4.0 - 4.9 events, six magnitude 5.0 - 5.5 events and three events of magnitude 6.0 - 6.3 during the next 48 hours; thousands of magnitude < 3.9 earthquakes were generated during this same time period. The largest earthquake in the sequence was magnitude 6.3 and occurred at 1245 (PDT) on May 25. Seismic activity after this event was fairly continuous (photo 1) for the next three days; however, most events were less than magnitude 5.0.

SEE PHOTO 1

Photo 1. CDMG seismologist Charles Real calibrating smoke drum recorder the morning of May 26, 1980. This site, near the Mono County Sheriff's substation, was a temporary CDMG sensitive seismograph station instrumented the evening of May 25. Several small-magnitude earthquakes were being recorded every minute. Note the high concentration of events on the smoke drum.

Epicenter locations and focal depths for the largest events (table 1) were determined by California Division of Mines and Geology (CDMG) seismologists Chris Cramer and Tousson Toppozada. Epicenter locations suggest that seismic rupture began north of Convict Creek and propagated south-southeast to the upper reaches of McGee Creek (figure 1). Epicenters of the first and third magnitude 6 earthquakes are separated by 10.5 km.

SEE TABLE 1

Damage from earthquake shaking was most pronounced in the Mammoth Lakes community and surrounding local areas. After the first event on May 25, Mammoth Lakes was without power until noon; during this period vital community services operated from auxiliary power supplies. Most damage to buildings was nonstructural and included broken windows and water mains, cracked plaster, and fallen chimneys. Damage to shelf stock and fixtures was moderate to severe in many stores, restaurants, and motels; in addition, extensive destruction to breakable contents in homes was commonly reported. Hot Creek Fish Hatchery and Mammoth Elementary School, east of U. S. Highway 395, also received considerable nonstructural damage from earthquake shaking. Initial damage losses to schools, other public buildings, and roads in the Mammoth Lakes region was estimated to be $2 million (Cole, 1980, p. 1).

SEE FIGURE 1

Figure 1. Region most affected by the May 25-27, 1980 Mammoth Lakes earthquakes showing epicenters for magnitude > 6.0 events. The Hilton Creek fault, Quaternary faults in Long Valley caldera, and tectonically-induced concordant crack zones mapped as of June 27, 1980 are located. Fault data is modified after Rinehart and Rose (1964), Bailey and Koeppen (1977), and Bryant and others (1980). Crack zones were field mapped by CDMG geologists William Bryant, Gary Taylor, Earl Hart, James Kahle, Trinda Bedrossian, and Richard McJunkin. Epicentral locations are by CDMG seismologists Chris Cramer and Tousson Toppozada.

Landslides and rockfalls were wide spread through the region during the Mammoth Lakes earthquakes. Near the epicentral region in Convict and McGee Canyons, rockfalls were common; in places, rock debris partially or completely covered snowfields (photo 2). A magnitude 5.3 earthquake at 1158 PDT on May 26 generated numerous rockfalls into the McGee Creek drainage and its tributaries. Large dust plumes could be observed over the Sierra Nevada immediately following many magnitude > 4.5 events. Several backcountry roads and trails were buried by debris that locally was more than 30 m thick. Two hikers in Yosemite Valley, approximately 70 km to the west, were severely hurt by a rockfall.

SEE PHOTO 2

Photo 2. View to south from McGee Creek Road of rockfall onto snowfield. Trees in foreground are approximately 4 m high; individual rocks on snowfield average 0.5 m to 1.0 m in diameter. Photo taken May 26, 1980.

The danger of rockfalls prompted the closure of many wilderness and backcountry areas. The steep-walled Hot Creek drainage was also closed primarily because of rockfall danger; in addition, a potential for sudden increases in water temperature and volume of discharge of the hot springs was a severe threat to bathers. A rhyolite boulder the size of a one car garage was dislodged by earthquake shaking from a cliff 1.5 km northwest of Mammoth Elementary School and rolled approximately 500 m to lower slopes below (photo 3).

SEE PHOTO 3

Photo 3. Rhyolite boulder that toppled and rolled from cliff above into an unnamed canyon approximately 1.5 km north-northwest of Mammoth Elementary School. View is to northeast. The moving boulder knocked down all trees in its path and formed a trail of impact craters 1.5 m to 2.0 m deep in lower-slope alluvial debris. Note person standing at left of boulder for scale. Photo taken May 30, 1980 by Charles Real.

Ground cracks were abundant in the Mammoth Lakes region after the earthquakes. Many lurch cracks formed in fill along paved and dirt roads (photo 4). Earth materials throughout the region for the most part are unconsolidated, particularly in the Long Valley caldera. Unconsolidated and locally moist soils contributed to ground failures from lurching, downslope movement, settlement, and liquefaction.

SEE PHOTO 4

Photo 4. Lurch and slump cracks in roadbase of Little Antelope Valley Road at south end of Little Antelope Valley. Note rock hammer (right center) for scale. Photo taken May 30, 1980.

REGIONAL GEOLOGY

Rock Types

The oldest rocks in the Mammoth Lakes region occur in the Paleozoic Mt. Morrison roof pendant. Rocks in this roof pendant originally formed in a miogeosynclinal environment (Russell and Nokelberg, 1977) and include calc-silicate rocks, marbles, schists, hornfels, quartzites, and metavolcanic rocks. Fossils are relatively abundant in the Mt. Morrison pendant (Rinehart and others, 1959) and provide relative age dates of the prebatholithic terrane.

The Mt. Morrison roof pendant is intruded by Mesozoic granitoid type plutonic rock of the Sierra Nevada batholith. Individual plutonic bodies include Wheeler Crest Quartz Monzonite, Round Valley Peak Granodiorite and granodiorite of Deep Canyon (Bateman and others, 1963; Pakiser and others, 1964; Rinehart and Ross, 1964).

Cenozoic rocks in the earthquake region include rhyolitic tuffs, glacial deposits, fanglomerate, and valley alluvium. Late Pleistocene lake beds, including diatomaceous earth (Cleveland, 1961), underlie upland portions of Long Valley. Terrace deposits are exposed locally in valley portions of the region. Glacial moraines and outwash underlie most drainages from the eastern Sierra Nevada. The thickest accumulations of glacial deposits in the Mammoth Lakes region occur along drainages of Mammoth, Laurel, Convict, and McGee Creeks.

Volcanic rocks are the most common Cenozoic rock types in the Mammoth Lakes region. The oldest volcanic rocks are late Tertiary rhyolites locally exposed several kilometers north of Crowley Lake (Bailey and others, 1976). Pleistocene eruptions from the Long Valley caldera, a resurgent cauldron (Smith and Bailey, 1968) 15 km wide by 30 km long, produced the greatest amount of volcanic rock in the area. Bishop Tuff, a rhyolitic ash flow (Gilbert, 1938), is the most voluminous of these acid-rich rocks.

Bishop Tuff is dated by K-Ar (Dalrymple and others, 1965) and fission-track (Izett and Naesen, 1976) methods at about 700,000 years Eruption of the tuff from a magma chamber beneath Long Valley included approximately 600 km3 of pyroclastic debris and generated the onset of collapse that formed the caldera (Bailey and others, 1976). Post-collapse eruptions have deposited several hundred meters of volcanic debris in the caldera bottom. The youngest volcanic rocks within the caldera form craters at Inyo domes, the youngest of which is less than 720 years old (Bailey and others, 1976).

Structural Setting

The epicentral region of the Mammoth Lakes earthquakes is on the western margin of the Basin and Range Province and occupies the steep frontal escarpment of the Sierra Nevada. Normal displacement of faults in the Sierra Nevada frontal system is activated by extension as in other portions of the Basin and Range Province.

Block faulting in areas of the eastern Sierra Nevada began in late Miocen-early Pliocene time (Pakiser and others 1964). Normal faulting was renewed in late Pliocene and early Pleistocene time (Hudson, 1960; Axelrod, 1962; Bachman, 1978); subsequent uplifts produced the steep eastern escarpment of the present Sierra Nevada.

Geomorphic and lithologic controls suggest that extensional deformation has occurred in the Mammoth Lakes region during the last several hundred thousand years. Depositional and erosional surfaces overlying the Bishop Tuff are deformed and rotated west indicating post-eruptive deformation. However, the tuff is confined, for the most part, to the region between the Sierra Nevada on the west and White-Inyo Mountains on the east, suggesting that onset of Sierra Nevada range-front faulting and uplift predates eruption. Small erosional remnants of Bishop Tuff are exposed along the walls of the middle Fork of the San Joaquin River (Huber and Rinehart, 1967) indicating that some eruptive debris crossed the ancestral Sierra Nevada crest.

Holocene fault activity along the eastern Sierra Nevada escarpment is indicated by disturbed glacial moraines at the base of the range front. Tiogan-age (Lipshie, 1976) lateral moraines at McGee Creek are cut by the Hilton Creek fault and downdropped on the east approximately 15 m (photo 5).

SEE PHOTO 5

Photo 5. View south-southeast from McGee Creek Road of Hilton Creek fault. The fault cuts glacial lateral moraines on south side of McGee Canyon. Two fault breaks are defined by aligned snowfields which connect at top of moraine. Eastern side of lateral moraine is downdropped approximately 15 m by prehistoric fault movements. Photo taken May 26, 1980.

Strike-slip components of some normal faults in the Sierran frontal system are described by several investigators (Whitney, 1872; Gianella, 1959; Bonilla, 1968). The cumulative lateral displacement of these faults is interpreted to be minor (Christensen, 1966). This interpretation is further supported by studies of basement rock on opposite sides of the Owens Valley which suggest minimal lateral offset along faults between the Sierra Nevada and White-Inyo Mountains (Moore and Hopson, 1961; Ross, 1962).

Total displacements of major faults in the Mammoth Lakes region can be inferred by geomorphology. Vertical displacement of the Hilton Creek fault at McGee Mountain is about 1600 m (Rinehart and Ross, 1964). An unnamed fault located 11 km east of the Hilton Creek fault (Rinehart and Ross, 1964; Bateman, 1965; Jennings and others, 1975) can be traced in an arcuate pattern from the Lake Crowley region for 30 km to the south. Cumulative displacement at the northern end of this fault is as much as 2100 m (Bateman and Wahrhaftig, 1966). Relief in other areas of the northern Sierra Nevada indicates that the range has been uplifted by faulting 2100 m to 2700 m.

Numerous historical earthquakes in the eastern Sierra Nevada (Coffman and vonHake, 1973; Real and others, 1978; Toppozada and others, 1979) indicate active deformation and uplift. Future displacements of Sierra Nevada frontal faults will produce earthquakes that are equal or greater in magnitude than those generated during the May 1980 events.

SEISMIC HISTORY

The largest historic earthquake along the Sierra Nevada frontal fault system was magnitude > 8.0 on March 26, 1872. This earthquake was centered in the Owens Valley of California and generated up to 7 m of vertical displacement along 70 km of the frontal fault system (Oakeshott and others, 1972).

Most of the post-1900 magnitude > 5.0 earthquakes that have originated along the eastern Sierra Nevada between Ridgecrest and Bridgeport, California, a distance of 300 km, have been in the Bishop-Mammoth Lakes area (Real and others, 1978). Between 1900 and May, 1980 the largest earthquakes within this region were two magnitude 6.0 events in September of 1927 and 1941.

Seismic activity in the Mammoth Lakes area has increased since the October 4, 1978 Bishop earthquake. The 1978 Bishop earthquake was magnitude 5.8 and generated some surface cracking (Fuis and others, 1979); magnitude 3.0 -4.9 earthquakes have occurred in the region almost every month since this event.

In June 1979 small- to moderate-magnitude swarm activity began in a area southeast of Mammoth Lakes along the boundary of the Long Valley caldera. Seismic activity in the swarm area included only events of magnitude < 4.5 until the initial earthquake of the Mammoth Lakes sequence on May 25 (Chris Cramer, CDMG, personal communication, 1980).

GROUND RUPTURE

Surface rupture from the Mammoth Lakes earthquake activity was first observed on the evening of May 25 by CDMG seismologist Chris Cramer. This ground rupture (photo 6; figure 1), exhibited up to 10 cm of vertical displacement (east side down). Glacial moraine deposits were offset where the road crosses the Hilton Creek fault on the north side of McGee Canyon. Cracking along the Hilton Creek fault north of the road was traced during later field investigations for approximately 5.5 km in a N10-20W trend (photo 7). This trend projects into the Long Valley caldera. The same crack zone extended a short distance south of the road along the fault but was not traceable beyond the lateral moraine on the south side of McGee Canyon (Malcolm Clark, U. S. Geological Survey, written communication, 1980).

SEE PHOTO 6

Photo 6. Ground rupture in McGee Creek Road where road crosses Hilton Creek fault. Maximum displacement (east side down) in the road is 10 cm. View is to north. Note rock hammer (foreground) for scale. Ground cracks cross the lateral moraine in background and continue to north. Top of lateral moraine (on skyline) is downdropped on the east (right) from prehistoric movement on the Hilton Creek fault. Photo taken May 26, 1980.

Individual cracks along the Hilton Creek fault commonly exhibited 5-10 cm of vertical displacement (east side down). Maximum offsets were approximately 27 cm; however, slumping may be partially involved in producing this amount of slip. Along the crack zones, prehistoric movements on the Hilton Creek fault have produced 15-20 m high, well defined scarps.

Field examination of areas north of U. S. Highway 395 in the Long Valley caldera revealed numerous crack zones that formed along Quaternary faults (figure 1). These crack zones mostly developed along faults trending N20-50W (photo 8) that have a subparallel trend with the Hilton Creek fault to the south; faults and concordant cracks trending N40-45W were most common. Pre-existing faults in the caldera with northeast trends are not abundant. Field examination of northeast trending faults revealed very few ground cracks; northeast trending cracks, where observed, were discontinuous.

SEE PHOTO 7

Photo 7. Ground cracking along the Hilton Creek fault on northeast flank of McGee Mountain. View is to north. Cracks at this location cut colluvial slope wash and have a maximum displacement of 15 cm.

Several aspects of observed surface rupture and focal plane analyses of faulting in the Mammoth Lakes region are difficult to explain. First motion analyses of earthquakes along the trend of magnitude > 6.0 events indicate that the earthquakes occurred by either of two mechanisms:

(1) displacement along a north-striking fault from left lateral movement, or (2) displacement along an east-striking fault from right lateral movement (Chris Cramer and Tousson Toppozada, CDMG, personal communication, 1980). Observed Hilton Creek fault rupture and surface cracks in the caldera suggest regional east-west to northeast-southwest extension. Focal plane analyses of earthquakes, located east and west from the south-southeast trending region of magnitude > 6.0 events (figure 1), indicate that the earthquakes are caused by extension; this confirms surface observations.

Normal displacement on the Hilton Creek fault in response to regional east-west to northeast-southwest extension is one interpretation for generating the Mammoth Lakes earthquake activity. This interpretive tectonic-fault model is complicated by the magnitude > 6.0 epicenters which plot west of the east-dipping Hilton Creek fault. Preliminary epicentral locations suggest that if a normal fault is responsible for May 1980 seismic activity, its surface trace must be west of the Hilton Creek fault. A second interpretation for generating the earthquakes, which possibly could be combined with extensional tectonic forces in the first model, involves induced seismicity from fluid movement (magma or hot water) at depth. Fluid movement could easily generate the lateral slip motion as calculated from focal plane analyses.

Fault mechanisms derived from seismological calculations and mapped surface rupture suggest that complex tectonic processes are deforming the region. This is further substantiated in the Mammoth Lakes region by locally thick accumulations of volcanic rock and an irregular mountain front topography. In contrast, south of Bishop, volcanic rocks are less abundant and the Sierra Nevada fault zone creates an almost linear mountain flank bordering the Owens Valley.

SEE PHOTO 8

Photo 8. Resistant rhyolite bed cut by northwest-trending Quaternary fault. View is to south from Little Hot Creek area. East portion of rhyolite bed is downdropped approximately 17 m by prehistoric faulting. Ground cracking from Mammoth Lakes earthquake activity occurred discontinuously along this fault trace in the Long Valley Caldera. Photo taken May 30, 1980.

EARTHQUAKE RESPONSE

Seismology Group

CDMG seismologists have monitored the Mammoth Lakes area with a network of three sensitive seismograph instruments since late September 1979. These instruments were initially installed in response to an increase in seismicity that began during June 1979. The equipment was in operation when the May 1980 Mammoth Lakes earthquake sequence began.

Five hours after the first May 25, 1980 quake, CDMG Sacramento-based

seismologists with seismograph equipment were enroute to the Mammoth Lakes area. Within 24 hours, the three-station sensitive seismograph network was upgraded to nine stations. The expanded network facilitates more accurate locations of continuing seismic events and provides for better focal mechanism analyses.

Strong-Motion Group

The Strong Motion Instrumentation Program (SMIP) of CDMG is responsible for instrumenting most of the State with accelerographs. These instruments accurately measure strong motion generated by earthquakes up to accelerations of 1.0 g (100% of gravity).

Several accelerograph stations are maintained by SMIP in the Mammoth Lakes earthquake region. After seismicity began on the morning of May 25, it was necessary to quickly retrieve film records and reload accelerographs with fresh film to monitor continuing activity. Three SMIP personnel, including two seismological technicians and a geologist, were enroute to the Mammoth Lakes region less than six hours after the first earthquake occurred. Aside from servicing existing equipment in the area, two temporary freefield stations were installed at locations where collected data would augment records from existing stations. Temporary SMIP stations are maintained until seismic activity decreases.

Fault Mapping Group

One team each of CDMG geologists were dispatched from the Sacramento and Los Angeles District Offices to investigate tectonic surface rupture and identify associated land instability and geologic hazards. On May 27 these teams were joined by CDMG geologists from the San Francisco District office whose purpose was to evaluate fault rupture data for the Alquist-Priolo Special Studies Zones (APSSZ) Program (Hart, 1980). Geological mapping and investigations were conducted through May 31 and will continue periodically as part of APSSZ fault evaluation program.

Clearinghouse

A clearinghouse was established on May 26 at the Mammoth Lakes Fire Station. Clearinghouse operations, supervised by CDMG Seismological Program Manager Roger Sherburne, were maintained for three days. During this time, geologic and seismologic information was organized and disseminated to scientists, local government officials, news media, and concerned citizens in the community. A detailed summary of earthquake information collected by the clearinghouse will be released in the near future as a CDMG Special Publication.

Response by Other Organizations

Professional groups in addition to CDMG that responded with earthquake studies in the Mammoth Lakes region include California Institute of Technology, University of Nevada-Reno, University of Southern California, University of California-Berkeley, University of California - Santa Barbara, Santa Barbara City College, and U. S. Geological Survey.

STRONG-MOTION DATA

Fourteen SMIP accelerograph stations throughout central California were triggered by the Mammoth Lakes earthquake activity. Ten stations are freefield to measure strong ground motion and four are structures instrumented to measure their response to shaking; two of the structural stations are dams and two are buildings.

The first magnitude 6.0 event on May 25 generated some of the highest ground and structural accelerations recorded during the earthquake sequence. The Convict Creek freefield station is located 1.7 km from the epicenter of this event and recorded peak ground accelerations of 0.46 g and 0.43 g on the horizontal axes and 0.43 g on the vertical axis (figure 2). Mammoth Lakes High School Gymnasium, a structural SMIP station located 11.0 km from the epicenter of the same event, is instrumented with a central recording system that monitors 10 channels of individual accelerometers located throughout the building. Peak accelerations of the gym building foundation for this event are 0.33 g and 0.24 g for the horizontal axes and 0.26 g for the vertical axis; the highest structural acceleration was 0.98 g horizontally at roof level on the north side of the building (figure 3).

SEE FIGURE 2

Figure 2. Strong-motion record from CDMG Convict Creek freefield station. Each trace is arranged numerically and lists the orientation for positive motion (upward deflection) on left (beginning of trace) and measured peak acceleration on right (end of trace).

SEE FIGURE 3

Figure 3. Strong-motion record from CDMG from Mammoth Lakes high School Gym structural station. Each trace is arranged numerically and lists the structural reference orientation for positive motion (upward deflection) on left and measured peak acceleration on right.

ACKNOWLEDGMENTS

Appreciation is extended to the individuals, agencies, and organizations that permitted installation of seismic equipment on their property. Mammoth Lakes Fire Department was especially cooperative by providing fire station space and access to telephones for the clearinghouse. The State of California Office of Emergency Services in cooperation with the California Air National Guard provided helicopter service to examine otherwise inaccessible mountainous terrain for ground rupture. Eddie Leivas and Gary Taylor reviewed the manuscript and provided helpful suggestions.

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