Fig. 3. Annotated conceptual model overlaid on cross section A-A′.

Solid blue arrows note the direction of earthquake migration, which corresponds to the direction of inferred fluid motion. Dashed blue lines indicate gaps between swarms where flow may be aseismic, and horizontal barriers (depicted here as sills) inhibit flow. Fluids are presumably sourced from near-Moho depths and propagate along a preexisting fracture zone. Insets denote two possible views along B-B′ for a dike tip model (top) and a fracture mesh model (bottom) to explain the reversed focal mechanisms. The zone between the arms hosts LP earthquakes, likely of a more plastic rheology than the surrounding rock (for example, partial melt and higher temperatures). Volatiles eventually work toward the shallow subsurface where they may be stored (13, 14) before finally escaping to the surface.