Hydrological effects of dip‐slip fault rupture on a hydrothermal plume

Dempsey, D. E.; Archer, R. A.; Ellis, S. M.; Rowland, J. V.

doi:10.1029/2012jb009395

We modelled a hot-water plume beside a normal fault in New Zealand's Taupō Volcanic Zone. A modelled magnitude 5.7 earthquake tripled water outflow, but within 20 days the excess fell below 10% of natural outflow. Fault damage caused a lasting rise in flow and moved upflow away from the fault.

The problem

In a geothermal field, hot water transports heat that can be used to generate electricity. Earthquakes change how water moves underground: water levels in wells rise or fall (Quilty and Roeloffs, 1997), streams flow faster (Muir-Wood and King, 1993), and geysers erupt more or less often (Ingebritsen and Rojstaczer, 1996). Compression or extension of rock during an earthquake changes the pressure of the water in its pores. An earthquake can also fracture rock and change its permeability.

The Te Kopia geothermal field in New Zealand's Taupō Volcanic Zone (TVZ) lies on the Paeroa Fault. Its fumaroles and mud volcanoes occur almost entirely on the uplifted side of the fault and on its 220 m high scarp (Bignall and Browne, 1994). An earthquake larger than magnitude 6 on this fault is credible (Berryman et al., 2008). Our goal in this study was to model (1) how a fault that blocks flow shapes a rising plume of hot water, and (2) how rupture of that fault changes the flow of heat and water through the field.

What we did

We used FEHM (Zyvoloski, 2007), a computer code that simulates the flow of heat and water through rock, to model a block of crust 15 km square and 8 km deep, heated from below at its centre. An inclined fault crosses the block. Its 2 m thick core is four orders of magnitude (10,000 times) less permeable than the rock around it.

A separate mechanical model gave the change in rock stress during one earthquake on a normal fault dipping at 65°. The fault slipped 0.83 m, equivalent to a magnitude of about 5.7. We converted the stress change to a change in pore pressure (Skempton, 1954), on the assumption that the water has no time to flow during the rupture. In a second set of models, we represented damage to the fault core by a random, patchy rise in its permeability, on average 2.3 orders of magnitude. We ran 100 simulations with different patterns of damage and followed each for 10 years.

What we found

We find that, before the earthquake, the fault split the plume in two. Hot water rising in the footwall was deflected up along the underside of the fault core to the scarp. The footwall upflow transported about 115 MW and the hanging wall about 100 MW. Between them, 10 MW of heat was conducted across the fault. The total lies within the 125 to 250 MW estimated for Te Kopia (Bibby et al., 1995). However, the model has low surface heat flow in the hanging wall, where Te Kopia has only a few small thermal springs. This suggests the heat source at Te Kopia does not extend as far beneath the hanging wall as it does in our model.

Although the two fault blocks moved apart during the earthquake, the mechanical model showed increased horizontal compression near the surface. Pore pressure rose there and water was expelled. Water outflow from the field rose from 360 kg/s to about 1,100 kg/s, roughly a tonne of water every second and three times the natural rate. The excess halved within 2.6 days. After about 20 days it was less than 10% above the natural rate and probably too small to detect. After three large normal-fault earthquakes, streamflow stayed high for 6 to 8 months (Muir-Wood and King, 1993). Our modelled decline was faster. We suggest those longer responses reflect lasting changes in permeability.

Averaged over the 100 simulations, damage to the fault core raised outflow by 17 kg/s at rupture, less than the pore-pressure change did. Outflow reached 20 kg/s above the natural rate after 1 to 3 weeks, and then remained 14 to 15 kg/s above it, about 4%, for the 10 years modelled. Heat crossing the fault rose from 10 MW to 215 MW, about as much as the whole plume transported upwards before the earthquake. Hot water from the footwall now rose straight across the fault. As a result, the upflow moved from the footwall scarp into the hanging wall and away from the fault. However, in a simulation that also included the pore-pressure change, heat flow across the fault fell to 80 MW after a week and 50 MW after two months.

Limitations

Our model holds water and rock at the same temperature. Because rock stores a large amount of heat for each degree of warming, it damps any change in water temperature. The model therefore cannot reproduce the 5 to 10°C changes measured at mid-ocean ridge vents after earthquakes (Johnson et al., 2000; Dziak et al., 2003). We did not model sealing of the damaged fault core by mineral deposits. If sealing is rapid, we expect the migration of upflow into the hanging wall to stop and reverse. The model also omits topography and the gradual lowering of the water table at Te Kopia (Bignall and Browne, 1994). The streamflow responses (Muir-Wood and King, 1993) followed earthquakes of magnitude 6.9 to 7.3. Their excess flows were about an order of magnitude larger than ours, probably because of their larger pore-pressure changes.

Finally, changes in pore pressure in our model do not alter the stress on the fault. Including that effect would let a model test whether a flow change caused by one rupture could trigger failure on other faults, as the spread of pore pressure after an earthquake may do for aftershocks (Miller et al., 2004).

Glossary

The Taupō Volcanic Zone (TVZ) is the region of active volcanism and geothermal activity in New Zealand's central North Island.

Permeability is a measure of how easily water flows through a rock.

Pore pressure is the pressure of the water held in the pores and fractures of a rock.

A plume is the column of hot, buoyant water rising from depth beneath a geothermal field.

Upflow is hot water rising towards the surface, and an upflow is a zone where it rises.

A fumarole is a vent in the ground that releases steam and volcanic gas.

A mud volcano is a mound or pool of hot mud formed where steam and gas rise through clay-rich ground.

The dip of a fault is its angle below the horizontal.

Topography is the shape and height of the land surface.

A normal fault is an inclined fault on which the upper block slides down as the crust stretches.

The footwall is the block beneath an inclined fault, and the hanging wall is the block above it.

The scarp is the step in the ground surface formed by repeated movement on a fault.

The fault core is the central layer of crushed, clay-rich rock (gouge) in a fault.

An order of magnitude is a factor of ten.

A megawatt (MW) is a million watts, a unit of power.

Magnitude is a measure of the size of an earthquake, based on fault slip and rupture area.

Abstract

Earthquakes cause a variety of hydrological effects, including changes in well levels, streamflow, hot‐spring temperatures, and geyser periodicity. These may be produced by changes in pore‐fluid pressure or by changes in permeability. We investigate near‐field effects of normal earthquakes on fault‐controlled hydrothermal plumes, e.g., at the Te Kopia geothermal system on the Paeroa Fault, New Zealand. A numerical model is detailed that addresses the effects of coseismic pore‐pressure and permeability perturbations on geothermal upflow along a dip‐slip fault. Fluid flow is modeled using the heat and mass transfer code Finite Element Heat and Mass transfer. The 2 m wide fault is impermeable and thus behaves as a partition between fluid flow in the adjacent fault blocks, as well as a structure for upflow to localize upon. A realistic distribution for the coseismic change in mean rock stress is obtained from a plane‐strain mechanical model for normal fault rupture in an elastic‐plastic‐viscous crust. These stress changes are imported into the fluid flow model as pore‐fluid pressure changes. In addition, we consider the effects of fault core fracture and damage by introducing an increase in permeability at the fault plane. Results show that short‐term effects are dominated by pore‐pressure changes, which cause increased heat and mass fluxes at the surface for a period of several weeks. Longer‐term, an elevated mass flux remains due to the increase in subsurface permeability. Upflow migrates from the footwall scarp into the hanging wall and away from the fault, because rising fluids are no longer deflected by the fault.