PERSPECTIVE

LONG VALLEY – UNREST: THE KNOWNS AND THE UNKNOWNS

David P. Hill1

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This perspective is based largely on my study of the Long Valley Caldera Hwy 395 (California, USA) over the past 40 years. Here, I’ll examine the “knowns” omes Long Valley Caldera SN and the “known unknowns” of the complex tectonic–magmatic system of the Long Valley Caldera volcanic complex. I will also offer a few brief R rgesu t en do em thoughts on the “unknown unknowns” of this system. west east moat moat Basin & THE KNOWNS Range ML The Long Valley Caldera, located along the eastern escarpment of the mountain range, formed 760 ky ago with the erup­ Crowley Lake 3 Mammoth SMSZ tion of ~600 km of we now call the . The caldera Mtn sits between two range-front normal faults: the Hilton Creek Fault HCF to the south and the Hartley Springs Fault to the north (Fig. 1). The Sie 1,300–650-year-old silicic vents forming the Inyo Domes extend into rra Nev Hwy 3 95 the west moat of the caldera. Mammoth Mountain is a 100–50 ka W dacitic, cumulo-volcano surrounded by mafic volcanic vents as young a CF as 8 ka that lies on the southwest rim of Long Valley Caldera (Hildreth da 2004). Mammoth Mountain at 11,000 feet and the town of Mammoth Magnitudes Lakes at its base serve as a year-round resort and one of the largest ski M 6 areas in the USA. M 5 M 4 No notable volcanic activity was documented in the caldera from the M 3 time of the early settlers in the mid-1800s through to early 1980. The M 2 onset of current caldera unrest occurred in May 1980, just a week after the May 18 eruption of Mount St. Helens (Washington, USA), with four 0 1 2 3 4 5 miles magnitude 6 (M 6) earthquakes. Three were located beneath the Sierra Nevada just south of the caldera and the third was located beneath the 0 2 4 6 8 10 km southern margin of the caldera. A leveling survey later that summer Seismicity and structural map of the Long Valley Caldera, Mammoth revealed that the in the center of the caldera had bowed Figure 1 Mountain, and surrounding area (California, USA). Symbols are as upward by 25 cm since the late 1970s, implying magmatic, rather than follows: HCF – Hilton Creek Fault; HSF – Hartley Springs Fault; Hwy – Highway; ML tectonic, processes were at work (Savage and Clark 1982). – Mammoth Lakes; SMFZ – South Moat Fault Zone; WCF – Wheeler Crest Fault; heavy barbed lines are Sierra Nevada range-front faults; colored circles are high- Attempts by US Geological Survey (USGS) geologists to explain the resolution epicenters for magnitude 2–6 earthquakes from 1982–2014; solid black implications of the ongoing unrest were initially greeted with outrage circles are epicenters of the May 1980, magnitude 6 earthquakes; opposing half- and denial in the resort community of Mammoth Lakes—a popula­ arrows indicate the sense of strike-slip motion across faults or fault zones; large open arrows indicate the relative sense of displacement of the Basin and Range to the east tion largely unaware of the long history of volcanism in the area. with respect to the Sierra Nevada block; heavy dashed red line indicates Antagonism toward Earth scientists gradually waned through the 1980s approximate location of the dike feeding the Inyo Domes vents with opposing and early 1990s as the caldera’s unrest continued to produce many arrows indicating the sense of extension across the dike; the orange circle with locally felt earthquakes. The community began to accept the message opposing arrows indicates the best-fit location of the compact inflation source driving tumescence of the resurgent dome. Inset: Simplified kinematics of the SMSZ presented by scientists through frequent public lectures and geological as a “leaky transform fault”. B&R – Basin and Range; LVC – Long Valley Caldera; SN field trips open to the public. Outreach has included USGS support for – Sierra Nevada. Stuart Wilkinson produced the shaded relief and seismicity background. civil authorities from Mammoth Lakes and Mono County to attend th the 10 anniversary meeting on the eruption of Mount St. Helens. crustal, brittle-failure earthquakes, centered at depths of 20–30 km, Seismic unrest of the Long Valley Caldera has continued with recurring occurred from June 2006 to September 2009, each followed by a seis­ earthquake swarms in the south moat seismic zone (SMSZ), accompa­ micity increase in the upper 10 km of the crust beneath Mammoth nied by elevated seismicity in the Sierra Nevada block to the south. Mountain. There was a doubling in uptake of magmatic CO2 in the Inflation of the resurgent dome has continued at rates as high as 20 2009–2012 tree-rings in a large tree near the CO2 tree-kill area (Lewicki cm/y (1980–1982 and 1997–1999), with a relatively stable interval from et al. 2014). This post-1989 seismicity illuminates the crustal roots of 2000–2010 (Hill 2006). Uplift resumed in 2011 at a rate of ~2 cm/y, the magmatic system and the path of CO2-rich magmatic fluids from and continues to this day. The center of the resurgent dome currently the base of the crust (depth ~30 km). This seismicity stands in contrast stands ~80 cm higher than before the onset of inflation in 1979–1980 to the Long Valley Caldera, which has produced no earthquakes deeper (Montgomery-Brown et al. 2015). than 10 km. Both findings limit our ability to access the state of the magmatic system beneath the caldera. Mammoth Mountain has a magmatic system that is distinct from that of the Long Valley Caldera. But it, too, joined in the regional unrest Included with the knowns are the surface geology and structure of the with a nine-month earthquake swarm in 1989–1990. Mid-way through upper 5 km of the crust. this sequence, long-period “volcanic” earthquakes began occurring at mid-crustal depths (10–20 km). By early 1990, diffuse emissions of THE KNOWN UNKNOWNS magmatic CO2 began killing trees in several areas around the mountain, Geophysical studies of the area have sometimes produced conflicting and elevated levels of 3He/4He were detected from a on the results. Just how the results of these studies relate to one another and to upper flank of the mountain. The CO2 emissions have since resulted in the actual physical properties of the crust beneath the caldera are the four fatalities when skiers fell into CO2-rich pits. Swarms of lower- known unknowns. Without a clear image of the deep structure beneath the caldera, a critical known unknown is the process inflating the resur­ 1 US Geological Survey, Menlo Park (California, USA) gent dome. Two views prevail. One holds that inflation isdominated ­

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David P. Hill worked as a staff seismologist at the USGS Hawaiian Volcano Observatory (1964–1966) and as Scientist-in-Charge of the USGS Long Valley Observatory (1982–2009). His responsibilities in the latter role included coordinating monitoring of the unrest in Long Valley Caldera and Mammoth Mountain and explaining the hazard implications to civil authorities and the Long Valley– region (Chen et al. 2014) represents one the public. In 2002, David received the Mineralogical Society of America's step in this direction. The above known unknowns illustrate the limits Distinguished Service Medal for his work with the public and civil authori- of current understanding of the state of magmatic systems for both the ties at Mammoth Lakes. He continues to pursue research on Long Valley/ caldera and Mammoth Mountain and their proximity to criticality or Mammoth Mountain unrest as Scientist Emeritus with the USGS. tipping points. Moreover, these known unknowns present a challenge in communicating to the local residents and authorities the significance of the ongoing unrest, potential volcanic hazards, and reliable erup­ by activation of hydrothermal fluids, while the other involves renewed tion forecasts. intrusion of and associated volatiles into the upper crust. Careful mapping and analysis of the eruptive history of the Long Valley Caldera THE UNKNOWN UNKNOWNS led Hildreth (2004) to suggest that current inflation is driven by hydrous volatiles from secondary boiling of the final stages of a moribund, 760 Unknown unknowns further complicate the challenge of making socially ka Bishop magma chamber. Others suggest that inflation is driven by useful eruption forecasts. Even the most successful models developed advection of a melt-fraction into the upper crust. The simplest model for Earth’s structure and active processes are simplified versions of providing a good fit to the deformation data is a volume increase in a reality. The gap between models and reality is potentially a rich source compact magma body centered at a depth of 7 km beneath the center of unknown unknowns. Similarly, unrecognized regional strain perturba­ of the resurgent dome (Montgomery-Brown et al. 2015). tions in an evolving tectonic–magmatic environment can compromise long-to-intermediate-term eruption forecasts based either on models The difference between these two views carries important implica­ or on probabilistic analysis of the recurrence history of past eruptions. tions for hazard assessment. If the volume of low seismic wave-speeds Earthquake-triggered eruptions represent an extreme example of the 10–15 km beneath the caldera inferred by Weiland et al. (1995) and challenge in making short-term eruption forecasts. On a more positive Seccia et al. (2011) holds up to further testing, does it correspond to a note, a number of successful short-term (days to weeks) eruption fore­ zone of secondary boiling in a moribund Bishop magma chamber or casts have been based on on-site experience and pattern recognition to recent (last ~10,000 years) emplacement of a melt fraction at mid- during accelerating unrest episodes leading to eruptions (White and crustal depths resulting from, say, basalt underplating and collapse of McClausland 2016). a lower-crustal crystal mush? Geothermal fluids upwelling beneath the Inyo Domes flow eastward REFERENCES down the hydrologic gradient within the postcaldera fill (2–3 km deep). Brown ST, Kennedy BM, DePaolo DJ, ring radiocarbon and seismicity Magmatic CO2 carried by this thermal water is apparently derived Hurwitz S, Evans WC (2013) Ca, at Mammoth Mountain, CA, USA. from a basaltic reservoir somewhere beneath the Inyo Domes (Brown Sr, O, and D isotope approach to Earth and Planetary Science Letters et al. 2013). The elevated 3He/4He ratios in thermal springs in the defining the chemical evolution of 390: 52-58 east moat have been attributed to fluids ascending from upper-mantle hydrothermal fluids: example from Long Valley, CA, USA. Geochimica Montgomery-Brown EK and 7 coau­ sources along an extension of the Hilton Creek Fault into the caldera et Cosmochimica Acta 122: thors (2015) Renewed inflation of (Suemnicht et al. 2015). At issue here is lack of evidence for postcaldera 209-225 Long Valley Caldera, California (760 ka) displacement along the Hilton Creek Fault into the caldera (2011 to 2014). Geophysical Chen R, Branum D, Wills CJ, Hill DP Research Letters 42: 5250-5257 (Hildreth 2004; Hill and Montgomery-Brown 2015). This leaves a ques­ (2014) Scenario earthquake hazards 3 4 Savage JC, Clark MM (1982) Magma tion of whether the magmatic CO2 and the elevated He/ He in the for the Long Valley Caldera-Mono Lake area, East-central California. resurgence in Long Valley Caldera, thermal water in the east moat might be, in part, derived from a recent California: possible cause of the melt intrusion beneath the resurgent dome. USGS Open File Report 2014-1045/ California Geological Survey 1980 Mammoth Lakes earthquakes. Science 217: 531-533 The temporal correlation between the onset of seismicity in the Sierra Special Report 233, 91 pp Nevada south of the caldera and caldera unrest suggests a tectonic–mag­ Hildreth W (2004) Volcanological Suemnicht GA, Kennedy BM, Evans WC (2015) Helium isotope sys­ matic interaction. A related issue is the possibility of a local tectonic perspective on Long Valley, Mammoth Mountain, and Mono tematics of Long Valley Caldera, earthquake triggering the onset of eruptive activity in a magmatic Craters: several contiguous but California. Proceeding World system that has reached a tipping point in its evolution. Indeed, Hildreth discrete magmatic systems. Journal Geothermal Congress 2015, (2004) points to the possibility that a major earthquake on the Hilton of Volcanology and Geothermal Melbourne, Australia, 19-25 April 2015. 6 pp Creek Fault may have triggered the onset of the 760 ka caldera­ -forming Research 136: 169-198 eruption of Bishop Tuff. In mapping eruptive deposits of the Bishop Hill DP (2006) Unrest in Long Valley Seccia D, Chiarabba C, De Gori P, Caldera, California, 1978–2004. Bianchi I, Hill DP (2011) Evidence Tuff, he found that onset of the eruption began near the point where for the contemporary magmatic the Hilton Creek Fault intersects the southeastern margin of the ring In: Troise C, De Natale G, Kilburn CRJ (eds) Mechanisms of Activity system beneath Long Valley fracture system. Modern examples of proximal triggering include the and Unrest at Large . Caldera from local earthquake summit eruption of Kilauea volcano (, USA), which began half Geological Society, London, Special tomography and receiver function Publications 269: pp 1-14 analysis. Journal of Geophysical an hour after the 29 November 1975, M 7.5 Kalapana earthquake, and Research 116: B12314, doi: the M 5 earthquake that triggered the onset of the catastrophic 18 Hill DP, Montgomery-Brown EK 10.1029/2011JB008471 May 1980 eruption of Mount St. Helens. In many cases, seismic waves (2015) Long Valley Caldera and the Weiland CM, Steck LK, Dawson PB, (dynamic stresses) from large regional earthquakes and major (M > 7.5) UCERF depiction of Sierra Nevada range-front faults. Bulletin of the Korneev VA (1995) Nonlinear tele­ earthquakes at global distances have triggered increased seismicity at Seismological Society of America seismic tomography at Long Valley volcanic and geothermal sites around the globe, including Long Valley 105: 3189-3195 Caldera, using three-dimensional Caldera and Mammoth Mountain. In a few cases, this remote dynamic minimum travel time ray tracing. Hill DP, Prejean SG (2015) Dynamic Journal of Geophysical Research: triggering may have accelerated onset of eruptive activity in magmatic triggering. In: Schubert G (ed) Solid Earth 100: 20,379-20,390 systems already in a near-critical state (Hill and Prejean 2015). Treatise on Geophysics, 2nd edition, Volume 4: Earthquake Seismology. White R, McClausland W (2016) These examples underscore the importance of considering tectonic– Elsevier, pp 273-304 Volcano-tectonic earthquakes: a new tool for estimating intrusive magmatic interactions in parallel with processes within an internally Lewicki JL and 6 coauthors (2014) volumes and forecasting erup­ evolving magmatic system when forecasting eruptions or making vol­ Crustal migration of CO2-rich tions. Journal of Volcanology and canic hazard assessments. A recent report on earthquake hazards in magmatic fluids recorded by tree- Geothermal Research 309: 139-155

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