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Rates of Rock Fracturing as a Tool for Forecasting Eruptions at Andesitic-Dacitic Stratovolcanoes

Rosanna Smith

Increasing rates of volcano-tectonic (VT) seismicity, produced by rock fracture and slip along faults, are a common precursor to eruptions. Precursory VT seismicity can thus be related to how fractures develop within and below a volcano before it erupts. This study combines field and experimental data to define constraints on theoretical models for accelerating VT seismicity. VT seismicity before all 17 episodes of lava-dome growth at Mount St Helens between 1980 and 1986 and before eruptions at selected andesitic-dacitic volcanoes following more than a century of repose was analysed. VT precursors developed within three weeks of lava-dome growth episodes, with stronger patterns before eruptions through larger domes, suggesting that the domes themselves inhibited magma ascent. The accelerations fluctuated about mean trends between exponential and hyperbolic, suggesting that individual sequences evolved under different boundary conditions, including applied stress and rock temperature, that determine the rates of seismogenic fracturing. Before the first eruption after a long repose interval, a hyperbolic acceleration in VT seismicity developed over approximately ten days when the eruption was dome building rather than phreatomagmatic.

To investigate the effect of changing boundary conditions, samples of andesite were deformed in compression at temperatures up to 900°C and confining pressures up to 50 MPa with acoustic emissions (AE) recorded as analogues to VT events. There was no temperature or pressure dependence in mechanical properties nor AE precursors to sample failure at temperatures up to 600 C for all confining pressures tested. This supports the use of models of precursory VT seismicity based on brittle fracture for volcanoes erupting after long repose intervals, where a new magmatic pathway must be formed. High AE rates for samples tested at 900 C, which is within the brittle-ductile transition, indicate that VT events can originate from hot material within the lava dome and at magma conduit margins.