Abstract
Seismic rate increases often precede eruptions at volcanoes worldwide. However, many eruptions occur without such precursors, limiting their usefulness for forecasting in some cases. Additionally, identifying seismic rate increases near volcanoes with high levels of background seismicity is non-trivial and many periods of elevated seismicity occur without ensuing eruptions. Although these issues are commonly known, efforts to quantify them are limited. In this study, we consistently apply a common statistical tool, the -statistic, to seismically monitored eruptions in Alaska of various styles to determine the overall prevalence of seismic rate anomalies immediately preceding eruptions. We find that 6 out of 14 (43%) Volcanic Explosivity Index (VEI) ≥2 eruptions have statistically significant precursory seismic rate increases. Of these 6 eruptions, 4 of them occur at volcanoes with relatively felsic compositions, repose periods greater than 10 years, and VEI ≥ 3. Overall, our results confirm that seismic rate increases are common prior to larger eruptions at long dormant, ‘closed-system’ volcanoes, but uncommon preceding smaller eruptions at more frequently active, ‘open-system’ volcanoes with more mafic magmas. We also explore the rate of other anomalies not precursory to eruptions and investigate their origins. Some of these non-eruptive anomalies can be explained by aftershocks of regional seismic events, magmatic activity that did not lead to eruption, or unrest at other nearby volcanoes within our search radius. Some open-system volcanoes have high non-eruptive anomaly rates and low pre-eruptive anomaly rates and are thus not amenable to forecasting with this method. In this study, we find that 26% of anomalies lead to eruption, while 74% do not. With continued calibration at more volcanoes, the statistical tool that we apply may be used more broadly to analyze future periods of seismic unrest at other volcanoes, properly placing such episodes into the context of the long-term background rate. These results and this tool may be useful for informing future eruption forecasts around the world.
Highlights
Earthquake swarms are common occurrences at volcanoes worldwide and are often associated with periods of increased volcanic unrest
We find that 6 out of 20 eruptions in Alaska show precursory rate increases, including all 3 eruptions at volcanoes that have been dormant for at least 15 years, and that erupted with a VEI of 3 or greater (Figure 3)
We expect that with more calibration from a wider dataset this tool could prove useful for future eruption forecasting at volcanoes worldwide
Summary
Earthquake swarms are common occurrences at volcanoes worldwide and are often associated with periods of increased volcanic unrest. The onset of earthquake swarms near a volcano does not necessarily herald an oncoming eruption, nor does the lack of seismicity preclude an eruption from occurring. The presence or absence of earthquake swarms at a volcano is but one factor to be considered when forecasting eruptions, and the seismic signature of intrusion can vary widely by volcano. In order to incorporate all events occurring prior to an eruption onset, we specify Ta windows that end at the eruption start time. Ta windows are necessarily defined backward in time based on the eruption onset. This retrospective approach is applied in order to maximize the identified anomalies. Catalogs that likely results from volcano- or intrusion-specific factors such as the local stress field and/or intrusion size and rate
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