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Perspectives

Mammoth Lakes Earthquakes and Ground Uplift: Precursor to Possible Volcanic Activity?

By Roy A. Bailey

Prior to the eruption of Mount St. Helens, few Americans were aware or could have been persuaded that volcanoes posed a serious hazard to life and property in this country. And understandably so, for as far back as most people can remember, most of the volcanoes have been deceptively quiet. Even since the tragedy of Mount St. Helens, a common misconception seems to be that the danger of eruptions in the near future, both from Mount St. Helens and other Cascade volcanoes, is now past. A careful look at the historic record for the Cascade Range volcanoes suggests otherwise.

During the mid-1800's, when Mount St. Helens was last active, several other Cascade volcanoes were also active. Historic accounts indicate that, between 1830 and 1880, ash or steam eruptions occurred repeatedly at Mount Baker, Mount Rainier, Mount Hood, and possibly Mount Shasta, as well as at Mount St. Helens; and, in 1851, a cinder cone and lava flow, not unlike that of Paricutin Volcano which erupted through a Mexican cornfield in 1945, erupted in what is now Lassen Volcanic National Park. On more than one occasion in the mid-1800 period, as many as three volcanoes were active during a single year. That these eruptions caused little public concern is due to the sparse population and the undeveloped state of the region at that same time. Similar eruptions today would probably have considerably greater social and economic impact.

For the past 100 years, the Cascade volcanoes have been unusually quiet, but, since 1975, seismicity and apparent precursory volcanic activity in the Cascade Range have increased significantly enough to suggest that we may be on the threshold of another period of more frequent eruptions similar to that of the mid-1800's.

In 1975, Mount Baker suddenly began steaming vigorously and still continues, although considerably abated. In 1978, earthquake swarms accompanied by unusual ground rifting occurred in volcanic terrain

east of Mount Shasta. In 1980, 3 months after Mount St. Helens burst to life, earthquake swarms shook the slopes of Mount Hood. In 1981, swarms were again recorded east of Mount Shasta. And, of course, Mount St. Helens continues its restless course, pumping new lobes of lava onto its crater floor every few months. In 1982, additional signs of restlessness have been observed and (or) felt further south in California. Unfelt, but instrumentally recorded, seismic activity at Lassen Volcanic National Park increased significantly during early 1982, and a similar increase in seismicity has been noted beneath the Coso Range volcanoes in the southern Owens Valley. However, the most unsettling activity of 1982 has been that in the vicinity of Mammoth Lakes, California, where apprehensions akin to those that preceded the Mount St. Helens outburst have been rekindled.

The Mammoth Lakes Earthquake Sequence of 1980

The activity at Mammoth Lakes began in 1980, but its implications were not fully understood until 2 years later in early 1982. On May 25, 1980, coincidently on the same day that Mount St. Helens erupted explosively for the second time, Mammoth Lakes, a resort town nestled at the eastern foot of the Sierra Nevada, was shaken by an historically unprecedented sequence of earthquakes. During a 48-hour period, the area was jolted by four magnitude 6 earthquakes, tens of magnitude 4 and 5 earthquakes, and literally hundreds of earthquakes of lesser intensity. Avalanches and rockfalls cascaded down slopes in the surrounding Sierras; the ground over an extensive area fissured and buckled; foundations, walls, and chimneys cracked; and the contents of cupboards and grocery store shelves tumbled to floors. Fortunately, most of the damage was superficial, and few serious injuries were reported. As a consequence of

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that earthquake sequence and on the basis of knowledge of the historic pattern of seismicity in the area, the U.S. Geological Survey issued an earthquake hazard watch on May 27, 1980, warning of possible continued severe earthquakes in the area. That earthquake hazard watch has remained in effect to the present time.

The town of Mammoth Lakes, which has a permanent population of about 4,000 and a transient residency of as many as 20,000 during the height of the winter ski season, lies on the active fault zone bounding the eastern front of the Sierra Nevada, a region long noted for its earthquake activity. The nearby Owens Valley was rocked by a magnitude 8.2 earthquake in 1872, and sporadically, every decade or so the region experiences magnitude 5 and 6 earthquakes. Geologists generally agree that this earthquake activity has been predominantly, if not entirely, tectonic in nature-probably related to the continuing uplift of the Sierra Nevada and possibly to regional extension at the western edge of the Basin and Range province, which is fundamentally caused by interaction between the Pacific and North American crustal plates that abut and grind together along the San Andreas rift 300 miles west of Mammoth Lakes. However, over the past 2 years, continuing events in the Mammoth Lakes area and recent

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VOLCANIC CENTERS MAMMOTH LAKES AREA, CALIFORNIA

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Molt Domes

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discoveries concerning the nature of the unusually prolonged aftershock activity following the May 1980 earthquake sequence have raised concern that other natural forces of a different nature may be stirring in the area. This likelihood has prompted the Geological Survey to add a notice of potential volcanic hazard to the 1980 earthquake hazard watch and to mount a major monitoring effort in the area.

Long Valley Magma Chamber, Potential Source for Eruptions

Mammoth Lakes, in addition to being along the tectonically active eastern Sierran front, also lies in the southwestern corner of Long Valley Caldera, a large oval volcanic depression, 20 miles long and 10 miles wide, which formed about 700,000 years ago during colossal pyroclastic eruptions that spread volcanic ash over most of the Western United States. Since that time, lesser eruptions have occurred repeatedly within the caldera. The last major eruption occurred about 100,000 years ago, but smaller eruptions have occurred as recently as 50,000 years ago. Although these eruptions are sufficiently old as to not cause great concern about imminent future eruptions, geophysical and seismological studies undertaken between 1973 and 1974 as part of the

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Geological Survey’s Geothermal Research Program indicate that Long Valley Caldera is still underlain by a large residual magma chamber (a subterranean pool of molten rock). Its existence alone poses a special hazard, particularly in a region frequently shaken by large earthquakes.

Since the colossal eruptions of 700,000 years ago, the Long Valley magma chamber has been slowly cooling and solidifying inward from its margins; initially 12 miles in diameter and about 3 miles below the surface, it is now about 6 miles in diameter and 5 miles deep. Although diminished in size, it is still a large molten mass within the crust, potentially capable of producing future volcanic eruptions. Recent developments accompanying and following the Mammoth Lakes earthquake sequence of May 1980 suggest that the magma is indeed moving and that the potential for an eruption from it may be greater than the rather long past eruption intervals might otherwise suggest.

Ground Uplift and Puzzling
Seismicity

In September 1980, 3 months after the
May 1980 earthquake sequence, a re leveling
survey along U.S. Route 395, which
parallels the Sierran front east of Mammoth
Lakes, showed that the ground surface

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along the 22-mile segment between Tom's
Place and Crestview had bulged upward as
much as 10 inches since the previous survey
in 1975. At first, as with the earthquakes,
this ground deformation was thought to be
related to tectonic mountain-building forces,
but further analysis in the summer of 1981
showed that the center of this bulge coin-
cided with the center of the known residual
magma chamber beneath Long Valley
Caldera, as well as with the center of an
older bulge formed as a result of magma
pressure 650,000 years ago, which was
shortly after the formation of the caldera.
This coincidence suggested that recently
renewed magma pressure rather than tec-
tonic forces was the cause of this present
bulge. During summer 1981, Geological
Survey volcanologists also began to notice
puzzling similarities between seismograms of
the aftershock activity near Mammoth Lakes
and those of the continuing volcanic activity
at Mount St. Helens. These observations
suggested that magma movement

was involved IN the ground deformation
associated with the May 1980 earthquake
sequence and even may have triggered the
quakes themselves. To confirm this possi-
bility, the Geological Survey made plans
during winter 1981-82 to reoccupy the
geodimeter and leveling networks in summer
1982; they had been established in 1975

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of geodimeter lines measured

to detect horizontal changes in distance between stations

on and around the resurgent

dome.

continental scientific deep drilling organized by the Department of Energy and Sandia National Laboratory and serendipitously held in Mammoth Lakes on May 4 to 7, 1982. Attended by about 60 scientists with diverse backgrounds, the workshop had little to do with hazards; rather, it was focused primarily on understanding the nature of magma chambers and their associated hydrothermal systems, which have applications in harnessing geothermal energy and exploration for volcanogenic ore deposits.

At this workshop, the University of Nevada Seismological Laboratory revealed that (1) during the past 2 years since the Mammoth Lakes earthquake swarm of May 1980, earthquakes had become increasingly frequent within the south margin of Long Valley Caldera, (2) one particular epicentral area about 2 miles east of Mammoth Lakes was persistently more active than other areas, and (3) the depth of shallowest earthquakes in this area had crept closer to the surface from about 5 miles in June 1980 to about 3 miles in October 1981. In addition, it was reported that during winter 1981-82, new fumaroles had broken out at Casa Diablo, a nearby solfaric area. Furthermore, they noted that the earthquakes in this particular area were accompanied by

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