Seismic Monitoring and Eruption Forecasting of Volcanoes: A Review of the State-of-the-Art and Case Histories

Abstract

Seismology is an important and effective tool for monitoring volcanoes and forecasting eruptions. In the last two decades, improvements in computerized acquisition and analysis systems, and more precise and higher quality data, have lead to significant scientific advances as well as a number of successful forecasts. Well-monitored volcanoes have six or more local seismic stations (1–15 km) and several regional stations (30–200 km), which are able to detect volcanic earthquakes of M = 0 to 1 and larger. Recording of several years of background seismicity is important to establish a baseline for evaluation of possible precursors. Four main types of events are recorded: high-frequency earthquakes, low-frequency earthquakes, explosion earthquakes, and volcanic tremor, although terminology varies considerably and there is no consensus about physical processes in all cases. Source mechanisms may include shear failure, tensile failure, and active and passive fluid involvement (e.g., resonance) in both linear and nonlinear processes. Volcanic earthquakes generally (1) occur in swarms as opposed to tectonic mainshock-aftershock sequences; (2) have smaller maximum sizes (typically M < 5) than earthquakes associated with tectonic structures; (3) include many events with similar waveforms; (4) have high b-values; (5) increase in numbers before eruptions; and (6) occur beneath or near the site of eruption. Studies of eight case histories (Mount St. Helens 1980, Izu-Oshima 1986, Ito-oki 1989; Redoubt 1989–1990, Unzen 1991, Pinatubo 1991, Spurr 1992, and Galeras 1993) and summary information on several hundred other eruptions reveal a generic volcanic earthquake swarm consisting of some or all of the following components: (1) background; (2) swarms of high-frequency events; (3) relative quiescence after the peak rate of seismicity; (4) low-frequency events; (5) volcanic tremor; (6) eruption; and (7) deep earthquakes following eruption. This sequence reflects systematic changes in locations and dominant physical processes. Moderate and large earthquakes (M > 5) occur at large volcanic structures such as calderas or in association with large deformations such as sector collapses; some of these earthquakes are large enough to produce substantial shaking hazards nearby. Swarms of volcanic earthquakes have log-normally distributed durations with a mean of 5 days. Swarms of high-frequency earthquakes last longer and have a higher false alarm rate, whereas swarms of low-frequency events and tremor are shorter, and more often lead to eruptions. The seismic expressions of intrusions and extrusions are similar, so false alarms may never be completely eliminated. Intensive monitoring for 3 to 6 months or more after an eruption is recommended because additional eruptions may occur with little or no additional precursors.