The Thomson ReservoirTriggered Earthquakes

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The Thomson Reservoir-Triggered Earthquakes

• Trevor Allen(1), Gary Gibson(2) and Colin Hill(3)

Monash University(1), Seismology Research
Centre(2) and Melbourne Water Corporation(3)
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Outline

• Reservoir-triggered seismicity
• The Thomson Reservoir
• Thomson seismograph network
• History of seismicity
• The 1996 Thomson earthquake
• Summary

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Reservoir-triggered seismicity

• Elastic stress increase following filling of the
  reservoir
• Increase in pore water pressure in saturated
  rocks (due to the decrease in pore volume
  associated with compaction)
• Changes in pore water pressure associated with
  water migration (Bell and Nur, 1978)

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Melbourne
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The Thomson seismograph network

• Aims of the project
• Study spatial distribution of earthquakes
• Determine earthquake magnitudes and intensities
• Obtain an indication of earthquake focal
  mechanisms
• Monitor the response of the dam and outlet towers
  to vibratory motion of earthquakes
• Predict future patterns of earthquake occurrence

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History of seismicity

• Pre 1976
• Few accurate earthquake locations in the area
  known before reasonable seismograph coverage in
  Victoria commenced in 1960 (operated by BMR and
  ANU)
• In the period between 1891 and 1907, several
  strong ground motions were felt in the
  Walhalla-Moondarra areas.
• No major earthquakes occurred from 1960 to 1976

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History of seismicity

• January 1976 - December 1982
• The SRC has operated and maintained a number of
  microearthquake seismographs throughout Victoria
  since 1976
• With the completion of the Thomson Reservoir
  network, it became clear that the Gippsland area
  possessed a higher level of background seismicity
  than average for Victoria

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History of seismicity

• January 1983 - December 1985
• In 1983, the Thomson River development was
  complete and filling of the reservoir commenced
  later that year
• In November of 1983, a swarm of 37 earthquakes
  occurred 6 km northwest of the dam at a depth of
  about 11 km. Reservoirtriggered or coincidence?
• Apart from the cluster, few events occurred in
  the three years subsequent to filling.

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History of seismicity

• January 1986 - April 1988
• From 1986, shallow earthquakes within 3 km of the
  surface began occurring immediately under the
  reservoir, with magnitudes to ML 3, at rates of
  up to five events per week.
• These earthquakes were almost certainly
  reservoir-triggered resulting from water
  diffusion increasing pore water pressure.

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History of seismicity

• May 1988 - December 1993
• Seismicity migrated to greater depths and away
  from the reservoir, with activity levels
  decreasing
• This can be explained by the increasing pore
  water pressure migrating to greater depths at
  relatively slow rates (depending on rock
  permeability)

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History of seismicity

• January 1994 - December 1998
• From 1994, earthquake activity was consistently
  at depths from 10 to 12 km
• On 25 September 1996 at 0549 pm, an ML 5.0
  earthquake occurred at a depth of 12 km adjacent
  to the reservoir
• Many aftershocks (approximately 200) followed for
  several months subsequent to the main shock.
  Several of these exceeded magnitude ML 2.0
• Abatement of aftershocks continued until no
  earthquakes were recorded for the quarter ending
  December 1998

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The 1996 Thomson Earthquake

• The ML 5.0 Thomson Earthquake was located at
  146.45?E, 37.86?S at a depth of 12 km
• The location suggests that the event was most
  likely on the Yallourn Fault
• The fault plane solution illustrates northwest to
  southeast compression, consistent with reverse
  faulting on the Yallourn Fault
• Focal mechanisms for the aftershocks were more
  complex

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History of seismicity

• Since December 1998
• Relatively few earthquakes have occurred at the
  Thomson Reservoir since the abatement of
  aftershocks
• Water levels at the reservoir have also decreased
  significantly since 1996

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Summary

• There is little doubt that the earthquakes
  occurring about the Thomson Reservoir following
  the commencement of filling were
  reservoir-triggered
• Records acquired provide one of the best examples
  of reservoir-triggered seismicity in the world
• The fault plane solution of the magnitude ML 5.0
  Thomson Earthquake illustrates northwest to
  southeast compression, consistent with reverse
  faulting on the Yallourn Fault
• The area now seems to be approaching the end of
  the period when triggered earthquakes are likely
• How will the reservoir respond to refilling?