An alternative model of the Lassen hydrothermal system, Lassen Volcanic National Park, California

Abstract

We describe an alternative two‐dimensional numerical model for the Lassen hydrothermal system. As in earlier models, the new model consists of a vertical upflow zone with a low permeability caprock near its top and a permeable horizontal channel that connects to the upflow zone at depth. The new model, however, also includes a recirculation region beneath the horizontal channel and a recharge zone. Magmatic heating is explicitly represented by specifying heat flows near the bottom of the upflow zone. Simulations show that a periodically appearing vapor‐dominated zone, which sometimes consists of pure‐steam, develops beneath the caprock and feeds steam‐type discharges at high elevations, while the horizontal channel feeds the hot‐spring discharge region at lower elevations. This model is able to reproduce the inferred ∼1:1 discharge ratio between water and steam and to match estimates on current heat output from the system. The development of the vapor‐dominated zone does not require dramatic changes in geohydrologic conditions. Our model also shows that both the vapor‐dominated zone and the whole system evolve quasiperiodically with a period of ∼103 years. These oscillations are related to the unstable two‐phase hydrothermal fluid flow within the vapor‐dominated zone. The observed high‐frequency oscillations near the vapor‐dominated zone superimposed on low frequency ones probably result from the oscillatory instability introduced by rapid changes in permeability of the low‐permeability caprock. The results indicate that the system is probably still in a transient state.