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Earth shattering: mining induced seismic activity


 
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John Polglase delves further into the 'substrata’, exploring why earthquakes occur, and what happens when human activity makes changes to the sedimentary rocks and to the loads above and below them. The layers of sedimentary material beneath the earth’s surface are 'flawed' and in slow flux. Sedimentary strata, including coal seams from which we extract coal seam gas, naturally exhibit planes of weakness; jointing (stationary cracks caused by de-watering and loading); and sub-lateral load and relaxation cracks due to mass loading (eg surface sedimentation, increasing groundwater) or unloading (eg surface weathering and erosion).
Added to these genetic 'flaws' over time, are fractures and fissures from gentle folding to fault systems and sheer zones caused by catastrophic folding and material failure, and earthquake activity. All such natural 'flaws' in sedimentary rocks are potential 'highways' for fluid and gas flow. Changes to groundwater regimes, such as by the construction and filling of large dams, can alter both the stresses on, and the strains within, the rocks beneath, and the pressures of the fluids and gases contained within them.

 

 
Mining-induced seismic activity also is an often-observed and well-documented phenomenon, with instances from all over the world of small to medium sized earthquakes. Such quakes can be caused by either removing or injecting 'mass' into the 'flawed' subsurface strata.    
One example of mass injection is Denver, Colorado, which experienced an unprecedented series of earthquakes of similar magnitude after a waste fluid injection programme into the nearby Rocky Mountains strata, which injection altered the mass balance, rock strains and fluid pressures. An example of mass extraction is the city of Galveston, Texas, which over-pumped its aquifers to the degree that the city sunk some nine feet, revealing previously unknown faults. When subterranean voids are created in 'flawed', heterogeneous material, Nature must rebalance the low-pressure and low-strain domains and fill them with fluid and/or rock, hence Galveston's subsidence. Sedimentary strata will often physically and/or chemically respond to the imbalances induced by either mass extraction or mass injection, and this will invariably impact upon the groundwater regime.

 
As most people know, the Richter scale indicates the severity of an earthquake via a scale of 1 to 10, with each number on the exponential scale indicating ten times more energy than the previous number.     
Regions like the Hunter Valley in NSW have recorded many instances of earthquakes on the scale of Richter 2, as a direct result of mining activity, and these low level quakes can be shown to mirror the mines and long-walling on seismic maps.
While earthquakes of such magnitude may not impact human activity, they do impact the groundwater pressure and flow regimes. The current escalation of mining and/or re-injection activities may be accompanied by an escalating impact on groundwater regimes. 

 
However, this cannot be confirmed until an automated and strategically located network of appropriate seismometers is in operation, and any groundwater impacts are recorded, monitored and traced.     
Within the Hunter alone there are 20 of the world’s largest open cut coal mines operating almost continuously, contiguously and concurrently, extracting and relocating massive amounts of material and groundwater. Perhaps more disturbingly, is that the strata are experiencing altered rock strain and fluid pressure regimes, and these imbalances are having a 'knock-on', off-site impact, through the Valley. The Newcastle earthquake (5.6 on the Richter scale) is a stark reminder of what happens when earthquakes occur in areas of high human density and infrastructure.

 
Christian Klose, a German geophysicist, postulated that the Newcastle quake was the result of 100+ years of mass unloading and relocating of coal, groundwater and inter- /over-burden above the sub-horizontal Hunter-Mooki fault system.    
This fault system is 'locked' at about 10 and 12 kilometres depth, with the load of material above helping to keep it in place. Klose's thesis is that mass removal caused the fault system to shift, resulting in the earthquake. The results of such rebalancing events are never uniform and depend upon, amongst other things, the relative strengths and coherence of the individual members of the rock formations, such that without bearing triangulation from appropriate seismic sensors, the origin and impacts of shock events cannot be accurately determined. There is still much investment, research and data required in this scientific pursuit.

 

 
The book, "Predicting the Unpredictable" by Susan Hough, a US seismologist, is recommended reading.
She not only relates the history and origins of seismology in an entertaining and interesting way, her book reinforces the fact that earthquakes are, in fact, unpredictable. It is not surprising then to realise that the effects of mining activity can be earth shattering.

John Polglase was interviewed by Ruby Vincent for A Question of Balance. Images provided by John Polglase. Summary text by Victor Barry, May 2012

Images are of John Polglase's seismometers. He uses a Teledyne Broadband Seismometer (BB-13) that senses seismic acceleration, and a Mark Products L-15B tri-axial velocity geophone.  The final image are of some overlapping earthquake range circles recorded by two seismic stations, in his GIS (Manifold).

 
How many earthquakes occurred around the world in 2011?  How big and where.
To find out, go to the following link and turn up the computer sound - better than New Year's fireworks!

http://www.youtube.com/watch?v=2a--NC4Nong&feature=relmfu

The intense activity during March is regarded as due to the influence of the moon.

    


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