| ||||
Long Valley Caldera, located in eastcentral California, is a 15- by 30-km (9- by 18-miles), elliptical depression at the base of a leftstepping offset in the eastern escarpment of the Sierra Nevada range. The MonoInyo( Craters form a 40-km- (24 mile) long chain of rhyolitic volcanic centers that extends northward from the west-central part of the caldera to the south shore of Mono Lake. The adjacent eastern escarpment of the Sierra Nevada, which is dominated by large, east-dipping normal faults, forms the western margin of the extensional Basin and Range Province (fig. A1; also see figs. 1, 2).
Figure A1. Simplified geologic map for Long Valley Caldera and the Mono-Inyo Craters volcanic chain (based on Bailey, 1989) CD, Casa Diablo; HC Hot Cree; HCF Hiloton Creek Fault; HSF, Hartley Springs Fault; LCF, Laurel Creek Fault; MLF, Mono Lake Fault; SLF, Silver Lake Fault; WCF, Wheeler Crest Fault.
Figure A2. Eruptive history for the Mono-Inyo Craters volcanic chain for the past 5,000 years. Eruption volumes have all been relatively small generally less than 0.1 km3 (0.024mi3) compared with the 0.25 km3 (0.06mi3) for the magmatic component of the May 1980 Mount St. Helens eruption. Several of the small explosive eruptions, however, have produced pyroclastic flows extending as far as 8 km ( 5 miles) from the vent. Phreatic, steam blast produced by superheated ground water; basalt, a hot, fluid lava that solidifies into dark volcanic rock containing 54 to 62 percent silica (typical of the lavas erupted by Hawaiian volcanoes); rhyolite, a viscous, gas-rich lava that solidifies into light-colored rock or obsidian (volcanic glass) with 69 to 80 percent silica (SiO2).
Earthquake and volcanic activity in this part of east-central California and the Owens Valley corridor to the south reflect the long-term interaction between tectonic and magmatic processes in the Earth's crust and upper mantle underlying the Sierra Nevada and the Basin and Range Province to the east. The tectonic processes are driven by a combination of the Pacific Plate sliding northwestward past the North American Plate along the San Andreas Fault System in coastal California and the westward extension of the crust across the Basin and Range Province between the Sierra Nevada and the Wasatch Mountains in central Utah (Dewey and others, 1989). In contrast to most volcanic systems along the Circum-Pacific "Ring of Fire," which are associated with subduction (sinking) of the relatively thin and dense oceanic lithosphere as it is overridden by thicker and less-dense continental lithosphere, magmatic processes and volcanism in eastern California are related to the upwelling of magma into the crust from the underlying mantle as the crust stretches, thins, and occasionally fractures in response to extension across the Basin and Range Province. These tectonic and magmatic interactions are complex, and they remain a focus of active geoscience research.
The region of eastern California that includes Long Valley Caldera has been a persistent source of volcanic activity throughout much of its geologic history (Bailey and others, 1976). The most recent episode of volcanism began about 3 million years (m.y.) ago with widespread eruptions of intermediate and basaltic lavas accompanying the onset of largescale normal faulting and formation of the eastern front of the Sierra Nevada. Beginniqung about 2 m.y. ago, multiple rhyolitic eruptions from vents along the northeast rim of the presentday caldera formed the Glass Mountain complex (fig. A1). Long Valley Caldera was formed about 760,000 years ago by the catastrophic eruption of more than 600 km3 (130 miles3 ) of rhyolitic lavas (the Bishop Tuff), accompanied by subsidence of an elliptically shaped crustal block 1 to 2 km (0.6 to 1.2 miles) into the partially evacuated magma chamber. Smaller eruptions from the residual magma chamber accompanied uplift of the westcentral section of the caldera over the next 100,000 years to form the resurgent dome. Subsequent eruptions of rhyolite lavas occurred around the margin of the resurgent dome at 500,000, 300,000, and 100,000 years ago (Bailey, 1989, 1990; Bailey and others, 1976).
Between about 220,000 and 50,000 years ago, basaltic and rhyodacitic lavas erupted from widespread vents in the west moat of the caldera. During this same interval, repeated rhyodacitic eruptions from a tightly clustered group of vents on the southwestern rim of the caldera produced the domes and flows that form Mammoth Mountain (Bailey, 1989).
The most recent eruptions in the region occurred along the MonoInyo Craters volcanic chain. Rhyolitic eruptions began along this chain about 40,000 years ago and have continued through recent times with eruptions along dthe north end of the Mono Craters about 600 years ago (Bursik and Sieh, 1986) and along the south end of the Inyo Craters about 550 years ago (Miller, 1985). In both cases, the eruptions resulted from the intrusion of an 810 kmlong, northstriking feeder dike into the shallow crust that vented several places along strike. Intrusion of a shallow crypto dome beneath Mono Lake 250 years ago uplifted the lake-bottom sediments to form Pahoa Island and vented in a small eruption of andesitic lavas from vents on the north side of the island (Lajoie, 1968; Stine, 1987). As illustrated in figure A2, the eruptive history of the Mono-Inyo volcanic chain over the past 5,000 years includes some 20 small eruptions at intervals ranging from 250 to 700 years. Although small, most of these eruptions have been explosive in nature. Given this 5,000-year eruptive history, the odds of another eruption somewhere along the Mono-Inyo volcanic chain are about one in 200 in any given year (0.05 percent per year).
See Ewert and Harpel (2000) for a bibliography on Long Valley Caldera and the associated volcanic field.
The region south of Long Valley Caldera that includes the eastern Sierra Nevada and Owens Valley has been one of the most persistent sources of moderate to strong earthquakes in California in historical time (dating from the 1860's in eastern California) (Hill, Wallace, and Cockerham, 1985; Ryall and Ryall, 1980). The northern end of the rupture zone of the great (magnitude ~8) Owens Valley earthquake of 1872 extended to within 60 km of the caldera. From 1910 through 1970, some 20 magnitude (M)=5 to 6 earthquakes occurred within 40 km of the south margin of Long Valley Caldera, including a cluster of four M=5 events and one M=6 event in September 1941 near Tom's Place approximately 10 km southeast of the caldera (Cramer and Toppozada, 1980). However, none of the M>5 earthquakes during the 1910-70 interval were located within the caldera. The same appears to be true for M>3 earthquakes as far back as the 1940's when the evolving regional seismic networks operated by the California Institute of Technology and the University of California at Berkeley became capable of detecting and locating M> or =3 earthquakes in eastern California.
The following paragraphs describe the unrest in Long Valley Caldera and vicinity that began in 1978 and persisted through the end of the 20th century. Figures A3, A4, and A5 summarize the activity through this period (1978-99) in terms of (1) the distribution of M> or =2 earthquakes in the greater Long Valley Caldera-Mono Craters region (also see the frontispiece for a map of M> or =3 earthquakes for the same period), (2) the uplift history of the resurgent dome and the cumulative number of M> or =3 earthquakes in the caldera and the Sierra Nevada block, and (3) the dominant seismicity clusters within Long Valley Caldera and the adjacent Sierra Nevada.
Figure A3. Seismicity map for magnitude (M)> or =2.0 earthquakes in the Long Valley Caldera and Mono-Inyo Craters region for the period 1978 through 1999. Circles indicate earthquake epicenters with circle size scaling with earthquake magnitude in fi ve steps from M=2.0 to M=6.0. Dominant earthquake clusters for this period include: (1) recurring swarms in the south moat of Long Valley Caldera and the Sierra Nevada block immediately to the south, (2) the 1984 Round Valley cluster, (3) the 1986 Chalfant Valley cluster, (4) the Mono Lake-Adobe Hill clusters, and (5) the Fish Lake Valley cluster located just east of the northern end of the White Mountains and the north end of the Death Valley-Furnace Creek Fault.
Figure A4. The history of earthquake activity and swelling of Long Valley’s resurgent dome from 1978 through 1999 in terms of (1) the cumulative number of magnitude (M)> or =3 earthquakes within both Long Valley Caldera (Caldera M>3) and the Sierra Nevada block to the south (Sierra Nevada M>3), and (2) deformation of the resurgent dome reconstructed from the uplift history of a benchmark near the center of the resurgent dome (near the intersection of lines 4 and 5 in fig. D5) from leveling surveys through mid-1997 and the extension (thick line) of the 8-km-long (5 mile long) baseline spanning the resurgent dome between the monuments CASA and KRAKATOA since mid-1983 based on frequent measurements with the two-color EDM instrument (see fig. C3).
Figure A5. A, Seismicity patterns in Long Valley Caldera and the adjacent Sierra Nevada block defined by magnitude (M)> or =1.5 earthquakes from 1978 through 1999. B, Dominant seismicity clusters for the same period along with selected landmarks in the area; solid dots are epicenter of the M~6 earthquakes of May 25-27, 1980. Abbreviations for seismic clusters (shaded areas) are: CL-MM, Convict Lake-Mt. Morrison; DLP’s, deep long-period earthquakes (epicenters not plotted in A); HC-DR, Hot Creek-Doe Ridge; LC, Laurel Creek; MC limb, McGee Creek limb; MM, Mammoth Mountain; SMSZ, south moat seismic zone with east (E) and west (W) lobes; SE RD, southeast resurgent dome; SW RD, southwest resurgent dome; TF limb, Tobacco Flat limb. Abbreviations for major, range-front normal faults (heavy lines) are: HCF, Hilton Creek Fault; HSF, Hartly Springs Fault; and RVF, Round Valley Fault. Abbreviations for roads and places are: Hwy 395, Highway 395; ML, Mammoth Lakes; TP, Tom’s Place..
Earthquakes of M=3 to 4 began occurring intermittently within the south moat of the caldera in the months following a M=5.8 earthquake on October 4, 1978, located beneath Wheeler Crest midway between Mammoth Lakes and Bishop, some 15 km southeast of the caldera (see figure A6; Van Wormer and Ryall, 1980). This activity culminated in an intense earthquake sequence that began in late May 1980 and then gradually slowed through the summer. The late-May activity included four M ~ 6 earthquakes. Three occurred on May 25-the first located just west of Convict Lake near the south margin of the caldera (the CL-MM cluster in fig. A5), the second beneath the south moat (within the west lobe of the south-moat seismic zone (SMSZ) of fig. A4), and the third within the Sierra Nevada block about 5 km south of the caldera (midway along the central limb of the Sierra Nevada cluster of fig. A5). Aftershocks continued to shake the region, and on May 27 the Director of the USGS announced a Hazard Watch (see appendix B) for additional potentially damaging earthquakes in the region. A fourth M~6 earthquake on May 27 struck an area about 10 km south of the caldera (the southern section of the central limb in the Sierra Nevada cluster of fig. A3) later that day (Hill, Bailey, and Ryall 1985; Hill, Wallace, and Cockerham, 1985).
Figure A6. Seismicity patterns in the Long Valley region for the six time intervals identifi ed in fi gure A4. Names identifi ed in A: ML, Mammoth Lakes; TP, Tom’s Place; HCF, Hilton Creek Fault; HSF, Hartley Springs Fault; WC-RVF, Wheeler Crest-RoundValley Fault. The stars in A are the epicenters of the four M=6 earthquakes of May 25-27, 1980. (Click for large images of Figure A6A, Figure A6B, Figure A6C, Figure A6D, Figure A6E, Figure A6F.)
The focal mechanisms of the May 1980 M~6 earthquakes all showed dominantly strike-slip solutions with T-axes (extension directions) having a northeast-southwest orientation generally consistent with right-lateral slip within the west-northwest-trending SMSZ and left-lateral slip in the north-northeast trending seismicity lineations in the adjacent Sierra Nevada block. This basic kinematic pattern with a northeast-southwest extension direction prevailed for earthquakes in the caldera and the adjacent Sierra Nevada block at least through the end of the 20th century. Two of the May 1980, M=6 earthquakes (the northernmost and southernmost in the Sierra Nevada block), as well as the M=5.8 earthquake of October 1978, also showed significant nondouble-couple components consistent with either fluid (magma) injection or simultaneous shear failure on fault segments at oblique angels to one another (Chouet and Julian, 1985; Julian and Sipkin, 1985; Wallace and others, 1982). Similar nondouble-couple patterns have appeared in the focal mechanisms of a number of earthquakes over the subsequent 20 years (see, for example, Dreger and others, 2000).
Leveling and trilateration measurements completed in 1980 showed that the central section of the caldera (the resurgent dome) had developed a 25-cm domical uplift between the fall of 1979 and the summer of 1980 (fig. A4, Savage and Clark, 1982). Earthquakes continued to occur in the Sierra Nevada block south of the caldera, including a M=5.9 event on September 30, 1981 located beneath the west end of the Convict Lake-Mt. Morrison cluster (fig. A5B) and just 2 to 3 km west of the initial M=6.1 earthquake of May 1980. Earthquake swarm activity within the caldera, which commonly included M=3 to 4 earthquakes, continued in the SMSZ through 1982 (fig. A4, A6A). Concerns raised by these persistent earthquake swarms and inflation of the resurgent dome, together with new fumarolic activity in the Casa Diablo area at the southwestern margin of the resurgent dome in January 1982, prompted the Director of the USGS to issue a Notice of Potential Volcanic Hazards (appendix B) on May 25, 1982. Strong reaction to this Hazards Notice within the local community, the news media, federal, state, and local agencies overshadowed the relatively modest earthquake swarm activity that persisted within the caldera through remainder of 1982 (Hill, 1998; Hill, Bailey, and Ryall, 1985; Mader, and others, 1987).
On January 7, 1983, an intense earthquake swarm, which included two M=5.3 earthquakes and a multitude of smaller events, began in the west lobe of the SMSZ (near the epicenter of the second M~6 earthquake of May 25, 1980). This swarm was accompanied by an additional 7 cm uplift of the resurgent dome, the possible intrusion of a dike to within 4 km of the surface beneath the south moat, and by roughly 20 cm of right-lateral slip along the SMSZ (Savage and Cockerham, 1984). The January 1983 swarm, which involved the entire SMSZ, gradually subsided in intensity over the next several months and was followed by occasional smaller swarms through the remainder of 1983 and the first half of 1984. The Notice of Potential Volcanic Hazard issued in May 1982 was withdrawn de facto on September 30, 1983, when the USGS changed its formal hazard notification terminology from the threelevel Notice-Watch-Warning system to a singlelevel Hazard Warning system (appendix B; Mader, and others, 1987).st
Following an earthquake swarm in the west lobe of the SMSZ in the last half of July 1984, which included M=3.6 and M=3.2 earthquakes, activity within the caldera declined to a relatively low level that persisted through early 1989 (figs. A4, A6B). Strong earthquake activity continued in the vicinity of the caldera, however, with a M = 5.8 earthquake in Round Valley 20 km southeast of the caldera on November 23, 1984 (fig. A3; Priestley and others, 1988) and a M=6.4 earthquake in Chalfant Valley, 30 km east of the caldera, on July 21, 1986 (Cockerham and Corbett, 1987; Smith and Priestley, 1988). Both these earthquakes produced felt shaking throughout the region, and both were followed by prolonged aftershock sequences. The Chalfant earthquake sequence was particularly intense. It was preceded by an energetic foreshock sequence that increased in intensity during the month prior to the M=6.4 mainshock and included a M=5.9 earthquake 24 hours before the mainshock. The protracted aftershock sequence included four M>5 earthquakes, the largest of which was a M=5.8 event on July 31.
Deformation measurements (fig. A4) showed continued but slowing inflation of the resurgent dome with the uplift rate dropping below 1 cm per year from 1984 through late 1989. The cumulative uplift over the central part of the resurgent dome, with respect to its pre-1980 level, exceeded 50 cm by mid-1989 (Langbein, 1989; Savage, 1988).
In early May 1989, an 8-month-long swarm of small earthquakes began under Mammoth Mountain on the southwest rim of the caldera (fig. A6B; the MM cluster in fig. A5B) and persisted to the end of that year. This swarm appears to have been associated with a dikelike (tabular shape with a vertical orientation) intrusion of magma to depths as shallow as 3 km beneath Mammoth Mountain (Hhill and others, 1990; Langbein and others, 1993). Although the swarm was not particularly energetic, it was prolonged. It was accompanied by minor deformation (approximately 1 cm of uplift) and included only four M~3 earthquakes in addition to thousands of smaller earthquakes and frequent spasmodic bursts, the latter of which were likely associated with very-long-period (VLP) earthquakes (see appendix C). This swarm marked the onset of (1) a continuing series of deep, long-period (LP) volcanic earthquakes centered at depths of 10 to 25 km (6 to 15 miles) beneath the southwest flank of Mammoth Mountain and the Devils Postpile (see DLP's in fig. A5), and (2) the diffuse emission of cold, magmatic CO2 in the soil in several areas around the flanks of Mammoth Mountain (Farrar and others, 1995; Hill, 1996). Both the deep LP earthquakes and CO2 emissions have continued into 2001, and both appear to be related to the presence of basaltic magma at mid-crustal depths (10 to 25 km or 6 to 15 miles) beneath the southwest flank of Mammoth Mountain and Devils Postpile (Gerlach and others, 1999; Sorey and others, 1998).
Efforts to keep local civil authorities appraised of the significance of the evolving Mammoth Mountain swarm activity led to the development of a five-level, alphabetic status scheme (E through A) for relating activity levels to response actions. This alphabetic status scheme was formally adopted for the Long Valley Caldera-Mono Craters region with the publication of USGS Open File Report 91-270 in June 1991 (Hill and others, 1991).
Beginning in late September 1989, measurements with a twocolor Electronic Distance Meter (EDM) showed that the extension rate across the resurgent dome had increased abruptly from less than 1 cm (0.4 inches) per year toc more than 7 cm (2.8 inches) per year, heralding a return of unrest within the caldera (fig. A4). Three months later (early January 1990), earthquake swarm activity resumed in the south moat (Langbein and others, 1993). At about the same time, the 7-cm/yr (2.8 in/yr) extension rate began slowing, and by late March 1990, the extension rate had slowed to 2 to 3 cm (0.8 to 1.2 in) per year. This rate persisted with only minor variations through 1995 (fig. A4). The renewed earthquake swarm activity began in the western lobe of the SMSZ (fig. A5B). By the end of 1990, it involved both the east and west lobes, as well as much of the southern section of the resurgent dome. The strongest swarm during this 1990-95 period occurred during March 24-27, 1991. It was located in the west lobe of the SMSZ and included more than 1,000 detected events with twenty-two M>3 earthquakes, two of which had magnitudes of M=3.7.
A south-moat earthquake swarm on June 28 to 30, 1992, is particularly noteworthy, not because of its intensity but because of its timing. This swarm began within seconds after the S-wave from the M=7.3 Landers earthquake of June 28, 1982, passed through the caldera. (The epicenter of the Landers earthquake was centered in the Mojave Desert some 400 km south of the caldera.) As it turned out, many areas across the western United States showed an abrupt increase in local seismicity rates following the Landers earthquake, providing the first clearly documented case of remotely triggered seismicity by a large, distant earthquake (Hill and others, 1993). At Long Valley Caldera, the shear waves from the Landers earthquake also triggered a transient, caldera-wide uplift that reached a peak strain of 0.3 parts per million (ppm) 5 to 6 days after the Landers event (Hill, and others, 1995; Johnston, and others, 1995). (Note: the 0.3-ppm peak strain corresponds to a peak uplift of roughly 0.5 cm or 0.2 inches.) The June 1992 swarm itself included more than 250 located events distributed throughout the SMSZ, the largest of which was M=3.4 earthquake in the west lobe. Triggered seismic activity in the Sierra Nevada block immediately south of the caldera included a M=3.7 earthquake.
Caldera earthquake activity and inflation of the resurgent dome gradually slowed from early 1994 through early 1996 (fig. A4). Earthquake activity in the Sierra Nevada block south of the caldera, however, continued at a relatively steady rate (fig. A6C). Most of this 1990-95 Sierra Nevada activity was concentrated in elongated clusters forming the western, central, and eastern limbs of the north-northeast trending seismicity lineations south of the caldera (figs. A3, A5, A6C). The most intense of this Sierra Nevada seismic activity was centered beneath Red Slate Mountain in the southern cluster of the west limb (fig. A5B) during August 10 to 15, 1993. This swarm sequence included more than 400 M>1 earthquakes, the largest of which had a magnitude of M=4.5.
In contrast to the general tendency of caldera earthquake swarm activity to track the uplift rate of the resurgent dome (fig. A4), one of the strongest earthquake swarms in the caldera occurred in March and April of 1996 as inflation of the resurgent dome was slowing. This seismic sequence began as a series of small earthquake swarms in the east lobe of the SMSZ in early 1996 (figs. A5B, A6D). Activity gradually escalated in intensity through February and early March, culminating in late March and early April with what at the time was the most energetic earthquake swarm within the caldera since the January 1983 swarm. This activity included more than 24 earthquakes of M=3.0 or greater, all located within the east lobe of the SMSZ. The three largest events included a pair of M=4.0 events on March 30 and a M=4.3 event on April 1. Altogether, this swarm included more than 1,600 events located by a real-time computer system (M>0.5), and it had a cumulative seismic moment of roughly 5x1022 dyne-cm, or the equivalent of a single M=4.8 earthquake. The deformation monitoring networks showed no significant ground deformation associated with this swarm activity, and deformation of the resurgent dome indeed continued to slow through 1996 and well into the spring of 1997 (fig. A4). Subsequent seismic activity within the caldera included a series of three minor swarms in the west lobe of the SMSZ in June 1996, followed by nearly ten months of relative quiescence.
The fluctuating unrest within the caldera from 1991 through 1996 frequently activated the lower levels of the alphabetic status ranking of the 1991 response plan. A number of the swarms during this period triggered E- and D- STATUS notifications, and the relatively strong March-April swarm of 1996 triggered a C-STATUS notification. Such communications worked well enough with civil authorities who had been involved in establishing the system, but they were widely misunderstood by the media and the public. Relatively minor swarms with a single M>3 earthquake (E-STATUS), for example, were apt to be reported as "E-level volcano alerts" by the media, producing widely varying levels of unnecessary concern with the public. In an effort to come up with a scheme that would be more intuitively meaningful to the public and the media, USGS scientists worked with local and state civil authorities to develop the four-level color code described in this document. In June 1997, the four-level color code in table 1 was officially adopted to replace the five-level alphabetic status ranking that had been in place since 1991.
By the end of April 1997, declining extension rates across the resurgent dome had dropped to less than 1 cm per year (fig. A4). The most notable activity during the first six months of 1997 involved M=4.2 and M=4.1 earthquakes on February 10 and 24, respectively, both located in the Sierra Nevada 4 km south of the caldera and 2 km south of Convict Lake (in the CL-MM cluster, fig. A5B, fig A6E). By coincidence, the timing of these minor earthquakes caught the imagination of the media. The first occurred three days after the opening of the movie "Dante's Peak," and the second occurred the day following the prime-time airing of ABC's movie for TV, "Volcano, Fire on the Mountain," which was based on the fictitious eruption of "Angel Peak," a thinly veiled take off on Mammoth Mountain.
The new, four-level color code (table 1) had been in effect for less than a month when, after nearly a year of relative quiescence within the caldera, unrest gradually resumed in mid-1997. The onset of renewed unrest initially appeared in the two-color EDM deformation data as gradually accelerating extension across the resurgent dome in May and June, followed by the onset of minor earthquake swarm activity in the west lobe of the SMSZ (figs. A5B, A6E) in early July (fig. A4). The rates of resurgent dome tumescence and earthquake swarm activity (both event rate and seismic moment rate) continued to increase through the summer and early fall, with peak rates of 2 mm/day and 1,000 M>1.2 events/day, respectively, on November 22 and an average extension rate of 1 mm/day (0.04 inches/day) from mid November through early December (fig. A4). The earthquake swarm activity was concentrated at depths between 3 and 8 km beneath a broad, 15-km-long (9 mile long) zone spanning the entire SMSZ and the southern margin of the resurgent dome. It included more than 12,000 M>1.2, 120 M>3.0, and eight M>4.0 earthquakes during the seven-month period through mid-January, with a cumulative seismic moment of 3.3x1024 dyne-cm (the equivalent of a single M=5.4 earthquake).
The larger (M " 4) of these earthquakes involved dominantly right-lateral slip along a WNW-trending fault zone within the south moat. The great majority of earthquakes had the broadband character of brittle, double-couple events (tectonic or volcano-tectonic earthquakes), although a few events had energy concentrated in the 1- to 3-Hz band typical of shallow, LP volcanic earthquakes or nondouble-couple focal mechanisms admitting the possibility of a significant dilatational (opening) component in the source process (Dreger and others, 2000). Mammoth Mountain on the southwest margin of the caldera was largely immune from the shallow (<10 km or 6 miles) swarm activity, although numerous LP earthquakes occurred deep (10 to 25 km or 6 to 15 miles) beneath the southwest flank of the mountain through early April. Continuous gas monitoring showed a marked increase in CO2 soil gas concentrations around the flanks of the mountain that persisted from September through December. Both the earthquake swarm activity and inflation of the resurgent dome declined to background levels through March. By the end of March, inflation of the resurgent dome had essentially stopped, at which point the center of the resurgent dome stood roughly 10 cm (4 inches) higher than in the spring of 1997 (and nearly 80 cm higher than the pre-1980 profile; see fig. A4).
The 1997 to mid-1998 earthquake activity was exceeded in intensity only by the January 1983 swarm with two M=5.3 earthquakes and the May 1980 sequence with two M~6 earthquakes within or immediately adjacent to the caldera. Although the condition remained GREEN throughout the 1997-98 unrest, the activity levels closely approached the criteria for condition YELLOW on November 22 and again at the end of that month (see tables 1, 2).
By mid-spring of 1998, unrest
within the caldera had declined to negligible levels. Deformation data showed
that swelling of the resurgent dome had essentially stopped and seismic activity
within the caldera involved just a few small (M<3) earthquakes per day. The
caldera remained quiet with virtually no additional deformation through 1999.
Meanwhile, the focus of seismic activity shifted to the Sierra Nevada block
south of the caldera with M=5.1 earthquakes on June 8 and July 14, 1998, and
a M=5.6 earthquake on May 15, 1999 (figs. A4, A6F).
The M=5.1 earthquakes of June 8 (10:24 PM, Pacific Daylight Time (PDT)) and
July 14 (8:53 PM, PDT), 1998, and the M=5.6 earthquake of May 15, 1999 (6:22
AM PDT), all occurred within the footwall block of the east-dipping Hilton Creek
Fault with epicenters located 1.5, 4.2, and 8.0 km south of the caldera boundary,
respectively (fig. A6F). The rich aftershock sequences to these M> or =5.1
earthquakes define an orthogonal pattern with the apex pointing eastward toward
Lake Crowley (the TF and MC limbs in fig. A5B) and the south-southwest MC (McGee
Creek) limb extends some 14 km (9 miles) into the Sierra Nevada south of the
caldera. The southeast trend of aftershocks to the June 8 mainshock coincides
with the right-lateral slip plane of the strike-slip focal mechanism for this
event, and the south-southwest trend defined by the aftershocks to the July
14, 1998, and May 15, 1999, events coincides with the left-lateral plane of
the dominantly strike-slip focal mechanism for the M=5.6 event of May 15. The
focal mechanism for the July 14 mainshock was dominantly normal with a northerly
strike. All three focal mechanisms have the direction of maximum extension (T-axis)
oriented to the east-northeast. These earthquakes were sufficiently close to
the caldera boundary that they initially raised concerns over a transition to
condition YELLOW. All, however, had the mainshock-aftershock behavior typical
of "tectonic" earthquakes, and, more importantly, none was followed
by an increase in seismicity or deformation within the caldera. Accordingly,
the condition remained GREEN (no immediate risk of volcanic activity).
At the end of 1999, the aftershocks to the M>5 earthquakes south of the caldera continued to wane, and the caldera itself had remained quiet for more than a year with little seismicity and virtually no further deformation.
The following is a brief summary of the current, and still incomplete, understanding of the proximal sources driving the 1978-99 unrest, based on analysis of the data collected to monitor the unrest. Figure A7 is a schematic representation of these sources.
Dominant sources contributing to the ground deformation and earthquake activity include:
A7. Map showing locations of recognized sources (in red) contributing to the 1978-99 unrest in Long Valley Caldera and vicinity. Red circles with radial arrows indicate pressure centers of infl ating magma bodies. Heavy dashed red lines indicate near-vertical fault zones (small, single-barbed arrows indicate sense of slip across individual faults zones). The larger double-barbed arrows indicate an average sense of slip across all the fault systems. Large open arrows schematically indicate regional, east-northeast extension direction. Thin dot-dashed red line indicates the dike intruded beneath Mammoth Mountain in 1989 with small arrows indicating opening direction. Heavy dot-dashed blue line is the dike that fed the Inyo Dome eruptions 500 to 600 years ago. Major range-front normal faults indicated by solid black lines with the ball on the down-dropped block. HSF, Hartley Springs Fault; HCF, Hilton Creek Fault; WCF, Wheeler Crest Fault; ML, town of Mammoth Lakes. (Click for large version of Figure A7.)
The dominant source driving deformation within the caldera is an inflating magma reservoir beneath the resurgent dome centered at a depth of 7 to 12 km (4 to 7.5 miles). Although the size and shape of this magma body remain poorly resolved, the cumulative, 80-cm ( 2.6 foot) uplift of the resurgent dome since the late 1970's (see fig. A4) implies a volume increase for this magma body of roughly 0.3 km3 (0.07 mi3) reflecting an infusion of additional magma from greater depths (Battaglia and others, 1999; Langbein and others, 1995). Such a volume increase is comparable to the volume of magma erupted during the May 1980 eruption of Mount St. Helens. USGS scientists conclude that the earthquake activity within the caldera occurs in response to the more fundamental, magmatic process associated with intrusion of magma into the crust beneath the resurgent dome because (1) the deformation associated with inflation of this magma body is significantly larger than can be accounted for by the earthquakes within and immediately adjacent to the caldera, and (2) accelerated episodes of deformation (magma-body inflation) generally precede increases in earthquake activity by several weeks (Langbein and others, 1990).
Most of the earthquakes in the area (particularly those with magnitudes M>4) are generated by slip on a series of near-vertical, west-northwest striking faults within the south moat of the caldera (the SMSZ) and north-northeast striking faults in the Sierra Nevada block south of the caldera (see figs. A3, A5, and A6). Focal mechanisms for these earthquakes indicate that the sense of slip on along these faults is dominantly right-lateral in the SMSZ and left-lateral in the Sierra Nevada block (fig. A7). Cramer and Topozada (1980) recognized that this fault geometry is consistent with local east-northeast extension such that the crustal block including the caldera north of the SMSZ moving to the east-northeast with respect to the corner of the Sierra Nevada south and west of the caldera. Kinematically, this geometry requires an opening (extensional) mode within one or both of these fault zones. Indeed, focal mechanisms for a subset of earthquakes in both fault zones involve significant oblique-normal displacement components, and, in a few cases, significant nondouble-couple components consistent with a local volume increase (Dreger and others, 2000; Julian, 1983). In principle, these are "leaky" strike-slip fault zones providing potential pathways for magmatic fluids to migrate into the upper 10 km (6 miles) of the crust from mid- to lower-crustal depths. The high-resolution analysis of seismicity patterns and focal mechanisms in the Sierra Nevada block by Prejean and others (2000), however, suggests that the nondouble-couple mechanisms for earthquakes within the north-northeast striking fault system reflect simultaneous slip along obliquely aligned faults rather than fluid injection. As an aside, it is worth noting that the "faults" depicted in figure A7 have no clear surface expression, which is not unusual for faults associated with M~6 or smaller earthquakes; they very likely consist of a series of sub-parallel fault segments rather than a single, through-going fault plane.
Interestingly, the Hilton Creek Fault and the other large, range-front normal faults defining the eastern escarpment of the Sierra Nevada (solid lines in fig. A7) have not been involved in the 1978-99 activity in any significant way. Nevertheless, the overall extension direction associated with the recent earthquake activity is essentially perpendicular to the north-northwest strike of these large, range-front, normal faults, and thus consistent with the geologic evidence for pure dip-slip displacement on these faults.
A second, relatively deep (depth, z>10 km or 6 miles) inflation source (magma body?) beneath the south moat seems to be required to account for local deviations from the deformation dominated by inflation of the shallower magma body beneath the resurgent dome (Langbein, and others, 1995). This deeper but smaller inflation source may serve as a temporary way station for magma as it migrates from even greater depths to the shallower magma body beneath the resurgent dome, and it may be the magma source for small intrusions into the south moat that apparently accompanied the 1983 and 1997 south moat earthquake swarms (Malin, and others, 1998; Savage and Cockerham, 1984). Alternatively, it may represent a distributed source of elevated fluid pressures in the form of magmatic "brine" that permeates the SMSZ to facilitate recurring brittle failure (earthquake activity) within the south moat in preference to the seismically quiescent zones elsewhere around the resurgent dome with comparable strains. We have yet, however, to see independent evidence in the form of deep LP earthquakes or harmonic tremor beneath the south moat or resurgent dome that might be associated with active fluid (magma or magmatic brine) transport at depths of 10 km (6 miles) or greater.
The north-northeast orientation of the small dike intruded to depths as shallow as 3 to 4 km (1.8 to 2.5 miles) beneath Mammoth Mountain during the 1989 earthquake swarm (Hill and others, 1990; Langbein, and others, 1993), together with focal mechanisms of individual swarm earthquakes, indicates that the extension direction in the vicinity of Mammoth Mountain has a west-northwest orientation. The nearly north-south strike of the dike that fed the Inyo Dome eruptions 500 to 600 years ago suggests an east-west extension direction in the vicinity of the Inyo volcanic chain and the west moat of the caldera. Together, these intrusive sources emphasize the spatially heterogeneous nature of active deformation in the vicinity of Long Valley Caldera.
Finally, the deep LP earthquakes beneath the southwest flank of Mammoth Mountain, which became active at the time of the 1989 Mammoth Mountain swarm, appear to coincide with a volume of distributed basaltic magma moving through a plexus of small dikes and sills (Pitt and Hill, 1994). This volume of distributed basaltic magma also is the likely source of the carbon dioxide (CO2) that began emerging through the soil at several sites around Mammoth Mountain in early 1990 (Farrar and others, 1995; Hill, 1996), and it may be the source of the volatiles producing the VLP earthquakes detected beneath Mammoth Mountain in 1996 and 2000 (see appendix C).
Future eruptions in the Long Valley-Mono Craters region are most likely to consist of the types and scales of eruptive activities that have occurred in the past. Eruptions within the last 50,000 years in the region include explosive eruptions of rhyolitic and rhyodacitc (silicic) lavas like those that formed the Mono Craters and Inyo Domes 500 to 600 years ago (Bailey, 1989; Miller, 1985). Such eruptions produced ashfalls, pyroclastic flows and surges of small to large volume, and relatively nonexplosive eruption of silicic lava domes and flows (Miller, 1989; Miller and others, 1982). Some other relatively nonexplosive eruptions within the last 50,000 years have also produced lava flows and cinder cones of basaltic (mafic) composition such as the severalthousandyearold Red Cones south of Mammoth Mountain. In individual future eruptions, scientists expect to see onehe or the other of these eruptive types but likely not both. The geologic record suggests that silicic eruptions are somewhat more probable than mafic ones. Both silicic and mafic eruptions are likely to be preceded by phreatic (steam blast) eruptions similar to those that formed the Inyo Craters. Such eruptions often occur when magma comes sufficiently close to the surface to interact explosively with shallow ground water, without the magma itself necessarily reaching the surface.
Specific effects of future eruptions in the Long Valley-Mono Craters region will depend upon the composition and volume of magma erupted as well as the location(s) of eruptive vent(s). Although patterns of seismic activity and ground deformation will likely provide strong clues to the locations of the eruptive vents shortly before magma reaches the surface, we cannot yet reliably interpret monitoring data in terms of the likely composition or volume of an impending eruption.
Lava fountains typical of basaltic eruptions would scatter ash and coarser material over the region and spawn lava flows that would flow downhill from the vent at relatively slow speeds (tens of m/hr to a few km/hr). Significant accumulations of tephra could develop within 10 km (6 miles) of the vent. These accumulations would be thickest near and directly downwind from the vent. Basaltic flows could extend several km down slope from their vents.
See Miller and others (1982) and Miller (1989) for a more extensive discussion of potential hazards from future eruptions, details about the nature and effects of hazardous volcanic processes, and volcano-hazard zonation maps for the Long Valley-Mono Craters region.
URL: https://pubs.usgs.gov/bul/b2185/appenda.html
Maintained by: Michael Diggles
Last modified: May 18, 2005
