Rates of Volcano-tectonic Seismicity Before Basaltic Flank Eruptions

We present the results of an analysis of pre-eruptive seismicity at the basaltic volcanoes Kilauea and Mauna Loa, Hawaii, and Mt. Etna, Sicily. Seven out of the twenty-eight flank eruptions investigated are preceded by periods of elevated rates of VT seismicity exceeding 4 weeks in duration. Although the daily earthquake rate during these periods of unrest fluctuates strongly, the sequences are underlain by longer-term systematic accelerations in earthquake rate, most clearly observed as accelerations in the cumulative number of earthquakes as a function of time, N(t). These accelerations are well characterised by an exponential function, N(t) = k.exp((t-t0)/τ), where τ and k are constants that vary for each sequence; the parameter τ defines a characteristic timescale for the acceleration. The total duration of each acceleration, T, is directly dependent on τ. In addition, both τ and k are related to the time-dependent background rate of VT seismicity, nbr, according to an inverse power-law, such that τ = 17(nbr)^{-0.71} and k = 42(nbr)^{-0.53}. Most of the remaining twenty-one pre-eruptive sequences occur at times of high background rates of VT seismicity, and so the duration of these accelerations is expected to be short, explaining why there is often limited warning times for many basaltic flank eruptions. These observations can be attributed to the response of a volcanic edifice to increases in magma pressure and provide the basis for (1) improved forecasts of eruptions and (2) identifying practical time-limits for initiating responses (e.g., evacuations) to a volcanic emergency.