By Dan Oancea - Twitter

 

The 1960s Rocky Mountain Arsenal Case represents an illustrious case of injection induced earthquake. The U.S. Army’s Arsenal tried to dispose some of its contaminated fluids by injecting them into an almost 4 km deep well. A few months later thousands of earthquakes were recorded, some strong enough (> 5.0 magnitude) to create millions in material damage.

A geologist noticed that:

“… the volume of liquid being injected into the Arsenal disposal well correlated with the number of earthquakes occurring in the area; the greater the volume of injected liquid,   the higher the number of earthquakes. “

The U.S. Army and USGS tried to prove him wrong, but after research work was done they found that David Evans was actually right. Hail the little guy who found himself opposed to two Goliath-sized organizations and won the dispute.

The Basel case of injection induced earthquakes caused by a quest for geothermal energy will be detailed in a subsequent article.

Anyways, it is not just injection that could trigger earthquakes, but oil and gas extraction as well. In the case of California’s Wilmington Oil Field  an elliptically shaped subsidence bowl has been produced by the extensive withdrawal of the underlying oil triggering damaging earthquakes from 1940s to 1960s.

Seismicity associated with old mining works collapse is well documented. It was believed that it would never cause sustained damages because the collapse usually creates low magnitude earthquakes.

And then came the day of December 28, 1989. The 300,000 people city of Newcastle in New South Wales, Australia was hit by a 5.6 Richter magnitude earthquake. Thirteen people died and 3.5 billion dollars in damage were recorded.

Geoscience Australia writes that:

“The epicentre (the point on the Earth’s surface immediately above the focus which is the point of initiation of rock fracture) was some 15 km southeast of the Newcastle Central Business District (CBD). Characteristic depth phases on distant seismograms confirm that the focus itself was 10 to 12 km deep within the crust under Boolaroo an outer suburb of Lake Macquarie.”

And,

“…This cocktail of old vulnerable buildings on poor foundations at Newcastle was an accident waiting to happen, as is obvious in retrospect. Could this disaster have been prevented?”

No earthquake building regulations and a forgotten lesson of past seismic events were to blame for the loss. But, was that all that came into equation? Was that indeed a natural occurring earthquake or something else could be blamed too?

In January 2007, Christian D. Klose of Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York announced a different theory: the earthquake was triggered by changes in tectonic forces caused by underground coal mining, which was accompanied by massive removal of underground water. Many don’t agree, but it is again another case of thinking out of the box. And it might be right, too. An interesting aspect is the fact that the total monetary value of the mined coal is less than the monetary value of damages produced by the earthquake.

A rock burst is defined as:

“A sudden and often violent breaking of a mass of rock from the walls of a tunnel, mine, or deep quarry, caused by failure of highly stressed rock and the rapid or instantaneous release of accumulated strain energy. It may result in closure of a mine opening, or projection of broken rock into it, accompanied by ground tremors, rockfalls, and air concussions.”

As the mine goes deeper, chances of a massive blow caused by a rock burst increase. That’s why these events are a common hazard in South African mines. The size of the excavation as well as the presence of faults or joints could influence and trigger rock bursts.

A DREF paper details what was later considered to be another case of a mining induced earthquake:

“On 9 March 2005, an earthquake measuring 5.3 on the Richter scale occurred at a mining area in Stilfontein as well as two surrounding towns of Klerksdorp and Orkney, some 200 km from Johannesburg, South Africa. Some 3,200 gold miners working in the mine in Stilfontein— who were initially trapped— were evacuated by the disaster management (DM) teams of the mining company and the local municipality.”

Just think about the sheer size of this one – some 3,200 miners were trapped underground ... That's why in South Africa they built a Natural Earthquake Laboratory in their deep underground mines.

There is yet another type of man-made earthquakes, an unexpected category: reservoir-induced earthquakes.

The first reported case came with the 1936 creation of the Hoover Dam (U.S.A.). An increased seismicity rate followed. Then things slowed down.

December 10, 1967 – some 200 km south of Bombay, Maharashtra State, India. A magnitude 7.0 earthquake hit the sparse populated region surrounding the large Konya Dam. During the rest of the month hundreds of aftershocks shook the region. As a result of the earthquake some 200 people lost their lives, thousands got injured and were left homeless.

Soon after the 1962 impounding of the Konya reservoir, a long string of tremors were noticed in the area. Sometimes they were accompanied by blast sounds. Seismological observatories built close to the dam reported that the hypocenters were found to be at shallow depths beneath the reservoir. At that time the region was considered to be seismically inactive; subsequent geological studies found that the area is being traversed by a significant fault which was reactivated by the 1967 event.

It was the weight of the recently added water – the impounding of the reservoir - that triggered the release of the natural tectonic strain by means of an earthquake (the slip on the fault). Read the ‘Reservoir-Induced Earthquakes and Engineering Policy’ paper to further your knowledge on this subject.

To be continued …