Production of Seismic Radiation By Top Down Excitation of a Magmastatic Column: The Role of Non-Equilibrium Degassing and Complex Conduit Geometry

Abstract:

Strong variation of multiphase fluid properties and conduit geometry with depth are defining characteristics of magmatic conduits. This background stratification has important implications for the excitation and propagation of waves through conduits, and the coupling of these fluid motions to the surroundings to produce seismic and infrasonic radiation. We develop a spectral and time domain numerical method to study perturbations to a basaltic magmastatic column, including non-equilibrium degassing and conduit width changes with depth, with a free top surface to model an active lava lake. We show that resonant modes of such a conduit to first order reflect the total magma water content, which sets the depth of volatile exsolution. Above the exsolution depth, wave propagation through the multiphase fluid is strongly dispersive, due to gas exsolution effects at periods greater than the time scale for exsolution and gravitational restoring forces at wavelengths greater than the scale height. We also find a class of nonpropagating and highly attenuated modes. Due to background stratification, wave modes are localized spatially around the depth of exsolution. Below this depth, waves propagate at the pure fluid sound speed, which may be nearly two orders of magnitude faster than the multiphase sound speed.

We have also extended our model to couple waves in conduits to dikes and sills branching off of the conduit. This coupled model matches fluid pressure at the fracture mouth and accounts for mass exchange between the fracture and the conduit. It can be used to investigate how waves propagating down the conduit can excite resonant oscillation of dikes and sills as has been inferred in, for example, the shallow plumbing system at Kilauea, HI, where Very Long Period (VLP) seismic signals have been commonly although episodically observed since 2007. These signals commonly have source depths around 1 km depths, where source mechanisms suggest a sill intersects the conduit, but likely are triggered near the surface from some combination of degassing or rockfall that excites conduit resonant modes. We will explore the importance both of this coupled sill-conduit geometry, and of conduit modes localized at the water exsolution depth, on the seismic wave field generated by near surface fluid excitation of a magmastatic column.