AN ALTERNATIVE MODEL OF THE LASSEN HYDROTHERMAL SYSTEM, LASSEN-VOLCANIC-NATIONAL-PARK, CALIFORNIA

We describe an alternative two-dimensional numerical model for the Las sen 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 a t depth. The new model however, also includes a recirculation region b eneath the horizontal channel and a recharge zone. Magmatic heating is explicitly represented by specifying heat flows near the bottom of th e upflow lone. Simulations show that a Periodically appearing vapor-do minated 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 similar to 1 :1 discharge ratio between water and steam and to match estimates on c urrent heat output from the system. The development of the vapor-domin ated zone does not require dramatic-changes in geohydrologic condition s. Our model also shows that both the vapor-dominated zone and the who le system evolve quasiperiodically with a period of similar to 10(3) y ears. These oscillations are related to the unstable two-phase hydroth ermal fluid flow within the vapor-dominated zone. The observed high-fr equency oscillations near the vapor-dominated zone superimposed on low frequency ones probably result from the oscillatory instability intro duced by rapid changes in permeability of the low-permeability caprock . The results indicate that the system is probably still in a transien t state.