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A M~ = 5.8 earthquake on October 4, 1978, located beneath Wheeler Crest 14 km southeast of Long Valley caldera (roughly midway between Bishop and Mammoth Lakes), marked the onset of the extended episode of unrest in the caldera and vicinity that continues today. Over the next year and a half, seismic activity in the form ofM > 3 and occasional M > 4 earthquakes gradually migrated toward the northwest and the southern margin of the caldera.
Then, on May 25, 1980, just seven days after the catastrophic eruption of Mount St. Helens, three M-6 earthquakes shook the southern margin of the caldera, accompanied by a rich aftershock sequence.
By the morning of May 27, the USGS had released a formal "Hazard Watch" noting the possibility of additional M-6 earthquakes in the area. Just hours later, a fourth M-6 earthquake shook the area--a successful "shortterm" forecast!
Jim Savage and Malcolm Clark promptly made arrangements to have a section of Highway 395 through the area releveled to document the coseismic displacement expected from this series of four M-6 earthquakes. What they found instead was a broad, dome-shaped uplift of the caldera floor (the resurgent dome). Monuments near the center of the resurgent dome were 25 cm higher in the summer of 1980 than they had been in 1975. Measurements of a trilateration network spanning the area made in 1979 suggested that most, if not all, of this deformation developed sometime between the summer of 1979 and the summer of 1980. With fresh images of Mount St. Helens in mind, it required no great leap to recognize a volcanic signature in this combination of strong earthquake swarm activity and ground deformation. Still, if the activity had died away as with most aftershock sequences, I suspect our collective attention would have soon focused elsewhere.
Earthquake activity continued, however, with frequent swarms that included locally felt earthquakes (M-3--4 events), rapid-fire bursts of small earthquakes reminiscent of "spasmodic tremor" seen at some volcanoes, and evidence that focal depths were getting shallower with time. In discussions including a number of us in the USGS, the California Division of Mines and Geology (CDMG), and Alan Ryall at
the University of Nevada, Reno over the winter of 1981-82, a consensus developed that we had an obligation to inform local civil authorities of our concerns about the volcanic nature of this activity. A formal draft of such a statement went to the USGS Director's office, followed by an extended
dialogue between the Director's office and the Governor's office in Sacramento over the precise wording of this announcement and how and when it should be released. Officials for Mono and Inyo Counties remained in the dark.
Meanwhile, George Alexander, then a science writer for the Los Angeles Times, got wind that something was up. His article announcing that the USGS was about to release a "Notice of Potential Volcanic Hazards" for the area appeared in the Los Angeles Times on the morning of May 24, 1982 (the Monday before Memorial Day weekend and just one day before the second anniversary of the May 1980 M -6
earthquakes). This, of course, caught both local civil authorities and the citizens of Mono County completely by surprise and short-circuited the official "Notice of Potential Volcanic Hazards" (the lowest level in the Notice/Watch/Warning hazard terminology in use by the USGS at the time), which
was released the next day. The local response was one of outrage, anger, and disbelief ("What volcano?!"), exacerbated by inflammatory headlines and news stories about "brewing lava eruptions" and a town in denial (the Jaws syndrome).
Geologists, and USGS geologists in particular, immediately became personae non gratae in Mammoth Lakes and Mono County, an attitude that only gradually mellowed over the years. Under the best of circumstances, communicating information on a newly recognized hazard is tricky business; this stands as an outstanding example of how to start out on the wrong foot.
As low-level earthquake swarm activity continued through the summer and fall of 1982, a series of public meetings did little to mitigate the simmering anger. Then, on the afternoon of January 7, 1983, activity abruptly resumed with an intense earthquake swarm in the south moat of the caldera that included two M = 5.3 earthquakes accompanied by nearly constant felt shaking from frequent
M-3-4 earthquakes over the next several weeks. This was an El Nino winter, and the snow was piled high along the roads within Mammoth Lakes and along the only paved road connecting the town to Highway 395. At his own initiative, Mike Jenks, then Chairman of the Mono County Board of Supervisors, ordered a second (dirt) road plowed. He also initiated steps to have this road widened and paved to provide
an alternate way out of town. This was not a popular decision, in part because it carried an implicit acknowledgment that there might actually be a volcanic hazard. Setting some sort of record from inception to completion, the newly paved "escape route" was formally dedicated in October of
1993 as the "Mammoth Scenic Loop."
Meanwhile the Chairman of the Board of Supervisors and a second member of the board who had been proactive in support of the new road and other mitigation issues (including monthly public updates by the USGS on the evolving caldera unrest) were recalled in a special election over the summer.
Releveling of Highway 395 during the summer of 1983 showed that the resurgent dome had been uplifted by an additional 7 cm over the winter. Following the intense January 1983 swarm, however, both the earthquake activity and deformation rates within the caldera activity began a gradual
decline that persisted through the remainder of the 1980s.
... While activity remained low within the caldera following the January 1983 swarm, this was not the case for seismic activity outside the caldera. On November 23, 1984, the M L 6.1 Round Valley earthquake and its many aftershocks (located midway between the southeastern caldera boundary
and Bishop) shook the region. The area again was repeatedly shaken from late July through mid-August 1986 by the rich foreshock and aftershock sequence associated with the M w 6.4 Chalfant Valley mainshock of July 21, 1986, located 20 km southeast of the caldera. Of course, each of these earthquake
sequences generated a flurry of news stories on seismic activity and volcanic unrest in the Mammoth Lakes area, further aggravating the local business leaders over negative publicity.
Activity returned to the vicinity of the caldera with the onset of a persistent earthquake swarm beneath Mammoth Mountain that began in late April 1989 and continued through the end of the year. Although only a handful of earthquakes in this swarm had magnitudes as large as M = 3, the swarm had a "juicy" character with considerable evidence suggesting that it was associated with an intrusion of magma
into the shallow crust beneath Mammoth Mountain. In an effort to keep local civil authorities apprised of the situation, I called the City Manager for Mammoth Lakes several times a week with updates on evolving activity. At one point in a conversation with Steve McNutt (then my counterpart at
CDMG on the Long Valley beat), he asked in apparent exasperation if we couldn't provide him with some sort of written criteria for how seriously he should regard the varying levels of activity. His request led to our developing the "Response Plan for Volcanic Hazards in Long Valley Caldera and the
Mono Craters Area, California" (USGS Open File Report 91-270) that included an alphabetic scheme of five "alert levels" (E through A in ascending order of concern) modeled after that used for the Parkfield earthquake prediction experiment. This response plan was published in June 1991.
In an effort to help local civil authorities better appreciate the issues involved with volcanic hazards, we invited Martin Strelnick (Mono County Sheriff), Boe Turner (Mono County Emergency Services Coordinator), Andrea Mead Lawerence (still a member of the Mono County Board of Supervisors), and Paul Marangella (Mammoth Lakes Town Manager) to attend a conference commemorating the
tenth anniversary of the May 1980 Mount St. Helens eruption. This conference, which was held adjacent to Mount St. Helens in Kelso, Washington, on May 17-19, 1990, included presentations by civil authorities as well as scientists who were directly involved in responding to this catastrophic
eruption. The occasion turned out to be enormously productive for all concerned.
Meanwhile, John Langbein's frequent trilateration measurements with the two-color geodimeter showed that the deformation rate across the resurgent dome began to increase substantially in October of 1989. Three months later, earthquake swarm activity resumed in the south moat of the caldera and has continued to wax and wane to the present, accompanied by relatively steady uplift of the resurgent
dome at a rate of 2-3 cm/year. Other developments that have persisted through the 1990s involve high concentrations ofmagmatic CO 2 in the soil around Mammoth Mountain and a growing number of long-period (LP) volcanic earthquakes occurring at depths of 10-20 km beneath the
southwest flank of Mammoth Mountain. Both of these phenomena began at the time of the 1989 Mammoth Mountain swarm. They serve as a reminder that this 11,000-foot-high volcano, which last erupted 50,000 years ago, is not extinct.
...In April-May of 1997, John Langbein's two-color geodimeter data again showed hints of an increasing deformation rate across the resurgent dome. By early July, earthquake swarm activity in the caldera picked up with an increasing frequency and intensity that persisted through the remainder of the year and into January 1998. The peak in this activity from mid-November through early January 1998 included
nine earthquakes with magnitudes of _M r = 4.0 or greater accompanied by thousands of smaller events. The three largest earthquakes had magnitudes of A//= 4.8-4.9. Resurgent dome deformation escalated through the second half of 1997, reaching a peak rate of over 20 cm/year in mid-November. By the end of the year, the center of the resurgent dome was approximately 7 cm higher than in early May.
...During particularly strong swarm sequences on November 22 and 30, however, we came extremely close to meeting the guidelines for a condition yellow (my feeling was rather "chartreuse" both times). The Long Valley caldera web site, on which we post most of the monitoring data in realtime together with frequent updates on the current condition, was receiving tens of thousands of"hits" a day. It also
attracted lots of e-mail, with messages ranging from "Why aren't we at yellow yet? ... you're a pawn of the Realtors ... what are you covering up? I'm moving to Maine!" to "Thanks for an excellent job of keeping us informed and keeping things in perspective."
The saga of Long Valley caldera isn't over yet, so stay tuned.
Earthquake swarms, including spasmodic bursts, occur periodically beneath Mammoth Mountain. The current swarm is notable, however, because it includes the largest magnitude event (M3.0) observed in ~15 years.