REPORT:
Historical Unrest at Large Calderas of the World

Principal Conclusions

1. The most common processes causing seismic swarms, uplift, and other elements of caldera unrest are regional tectonic strain, magmatic intrusion and processes within magma reservoirs, and hydrologic processes, as listed in the section on "Processes of Caldera Unrest".

2. Unrest resembling that at calderas occurs in noncaldera and even nonvolcanic settings, calling special attention to the role of regional tectonic and hydrothermal processes as agents of unrest.

3. Most unrest is a hybrid of several processes acting simultaneously or in sequence, and in many instances interacting with each other. Because of feedback mechanisms between processes, it is rarely possible or meaningful to classify specific unrest as purely tectonic, magmatic, or hydrothermal.

4. Calderas are dynamic and delicately balanced systems, and they can be disturbed by even small stimuli. Most unrest at silicic calderas occurs in response to routine, relatively minor tectonic strain or small-volume basaltic underplating of silicic reservoirs.

5. Different processes with different implications for hazards produce similar symptoms of unrest. Even a well-understood hybrid of processes can lead to many different outcomes.

6. The most common types of unrest (for example, seismic swarms and thermal, fumarolic, and hydrologic changes) are not reliable indicators of impending eruptions. Changes that are more diagnostic of impending eruptions are harmonic tremor; an exponentially escalating rate of seismic energy release or a sudden, pronounced drop in seismic energy release; a sudden, pronounced increase in the rate of uplift or sudden deflation after an extended period of inflation; opening of, and intense fumarolic emissions from, new fissures; or a sudden, sharp increase in hot-spring discharge.

7. Although large calderas hold a greater potential for catastrophic eruptions than small centers, most eruptions at calderas are relatively small.

8. Caldera unrest is likely to persist for months to years, sometimes even for decades to centuries, and to be intermittent rather than to show a systematic increase until culmination.

9. Much unrest does not culminate in an eruption, but eruptions are likely enough that many persons will choose to take precautionary measures. The choice of such measures is complicated by the long periods over which unrest can persist and the wide range of scales of potential eruptions, from minor to colossal eruptions that change the course of human history.

10. The outcome of a specific episode of unrest cannot and should not be forecast solely on the basis of patterns of unrest at other calderas, given the uncertainties in generalizeing from one caldera to another or from this entire compilation to a single restless caldera. Rather, we suggest that this compilation can
    1. show what is ordinary and extraordinary (qualitatively and in a few instances semiquantitatively),
    2. suggest multiple interpretations of unrest, and
    3. suggest new investigations to constrain interpretations of unrest.

11. The delicate equilibrium of calderas may cause them to become restless if regional stress increases, as in advance of major regional earthquakes. Calderas should be monitored for unrest that might forewarn of strong regional earthquakes.

There is no simple solution to the interpretation and forecasting of complex events at calderas. However, the rich record of historical caldera unrest is important for basic research and crisis response alike. Processes that last millennia and longer are recorded in the geological record, and day-to-day or year-to-year processes are recorded by modern monitoring. Many of the processes in large magma reservoirs and hydrothermal systems, as seen at large calderas, operate over periods of months to millennia. The historical records in this study are a bridge between classical geology and modern volcano-tectonic monitoring. We recommend that fascinating walk across this bridge, before and during future crises at calderas.

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