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

- **Authors:** D. E. Dempsey; R. A. Archer; S. M. Ellis; J. V. Rowland
- **Journal:** Journal of Geophysical Research: Solid Earth, vol. 118(1), pp. 195-211 (2013)
- **Publisher:** American Geophysical Union (AGU)
- **DOI (version of record):** [https://doi.org/10.1029/2012jb009395](https://doi.org/10.1029/2012jb009395)
- **This page:** https://ddempsey.github.io/papers/2013-dempsey-hydrological-effects-dip-slip-fault/
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- **Notice:** Published version, posted under AGU's author self-archiving policy. © 2013 American Geophysical Union. All Rights Reserved. Version of record: https://doi.org/10.1029/2012jb009395
- **Explainer:** all explainers on this site are AI-generated.
- **BibTeX:** https://ddempsey.github.io/papers/2013-dempsey-hydrological-effects-dip-slip-fault/citation.bib
- **Topics:** Induced seismicity & faults; Geothermal systems; Groundwater & hydrology
- **Keywords:** Geology; Fluid dynamics; Petrology; Geothermal gradient; Pore water pressure; Slip (aerodynamics); Fault (geology); Hydrothermal circulation

## In brief

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.

- Before rupture, a fault core 10,000 times less permeable than the rock around it split the plume into two upflows, with 10 MW of heat conducted across the fault between them.
- Horizontal compression of rock near the surface during the modelled earthquake of about magnitude 5.7 increased water outflow from 360 to about 1,100 kg/s.
- This excess outflow halved within 2.6 days and was less than 10% above the natural rate after about 20 days.
- Damage to the fault core raised outflow by 14 to 15 kg/s over the 10 years modelled and let hot water rise across the fault.
- Heat crossing the fault rose from 10 MW to 215 MW at rupture, and upflow moved from the footwall scarp into the hanging wall.

## 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.

<div class="widget" id="w-fault-rupture" role="group" aria-label="Interactive comparison of excess water outflow after the pore-pressure change and heat flow across the fault after damage to the fault core, at the times reported in the paper"></div>

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.

## Cite

Dempsey, D. E., Archer, R. A., Ellis, S. M., Rowland, J. V. (2013). Hydrological effects of dip‐slip fault rupture on a hydrothermal plume. Journal of Geophysical Research: Solid Earth, 118(1), 195-211. https://doi.org/10.1029/2012jb009395

```bibtex
@article{dempsey2013hydrological,
  title = {Hydrological effects of dip‐slip fault rupture on a hydrothermal plume},
  author = {Dempsey, D. E. and Archer, R. A. and Ellis, S. M. and Rowland, J. V.},
  journal = {Journal of Geophysical Research: Solid Earth},
  year = {2013},
  volume = {118},
  number = {1},
  pages = {195-211},
  publisher = {American Geophysical Union (AGU)},
  doi = {10.1029/2012jb009395},
  url = {https://ddempsey.github.io/papers/2013-dempsey-hydrological-effects-dip-slip-fault/}
}
```