Exploring the Causes of Distal Volcano-Tectonic (dVT) Seismicity Using Hydrothermal Modeling
Abstract
Based on observation of 111 volcanic eruptions, White and McCausland (JVGR, 2016) found that distal volcano-tectonic (dVT) seismicity typically preceded eruption at long-dormant volcanoes by days to years. They hypothesized that precursory dVT seismicity reflects magma-induced fluid-pressure pulses that intersect critically stressed faults. We explored this idea using the USGS HYDROTHERM model, an open-source magmatic-hydrothermal code that simulates multiphase fluid and heat transport over the temperature range 0 to 1200oC. We examined fluid pressure changes caused by a small (0.04 km3) intrusion into host rock, and explored the effects of coordinate systems (Cartesian vs. radial), magma devolatilization rates (0-15 kg/s), and intrusion depths (5 and 7.5 km, above and below the brittle-ductile transition). Magma and host-rock permeabilities were key controlling parameters and we tested a wide range of permeability (k) and permeability anisotropies (kh/kv), including k constant, k(depth), k(T), and k(depth,T,P) distributions. We examined a total of 1500 realizations to explore the parameter space. Propagation of pressure changes (ΔP≥0.1 bars) to the mean dVT location (6 km lateral distance, 6 km depth) was favored by focused fluid flow (i.e. Cartesian geometries), high devolatilization rates, and permeabilities similar to those found in geothermal reservoirs (k 10-16 to 10-13 m2). In Cartesian coordinates we found cases of ΔP ≥ 0.1 bars for every permeability in the range 10-16 to 10-13 m2, whereas in radial coordinates with no devolatilization, ΔP < 0.1 bars occurred for all permeabilities. Changes in distal fluid pressure transpired before proximal changes given modest anisotropies (kh/kv 10-100) typical of layered volcanic rocks. Invoking k(depth,T,P) and high, sustained devolatilization rates caused large dynamic fluctuations in k and P in the near-magma environment but had little effect on pressure changes at the distal dVT location. Intrusion below the BDT damps and delays pressure transmission to the dVT site.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.T43A3013I
- Keywords:
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- 1034 Hydrothermal systems;
- GEOCHEMISTRYDE: 1859 Rocks: physical properties;
- HYDROLOGYDE: 8045 Role of fluids;
- STRUCTURAL GEOLOGYDE: 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS