Earthquake Precursors: Signs of a Coming Tremor?


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Animals reportedly fled before the 1959 Montana earthquake. Image credit: USGS

Earthquake prediction is the ultimate goal of seismologists. Being able to predict when and where an earthquake will occur could save thousands, if not hundreds of thousands, of lives, over the years. Even after decades of study, earthquake forecasting remains notoriously difficult, however. So what are the signs which occur before an earthquake – earthquake precursors – and how useful are they?

Foreshocks: Sign of a Quake to Come

Generally speaking, earthquakes occur in clusters, rather than just a single tremor. The mainshock is, by definition, the largest earthquake in a series – and as such it may be the first (followed by a series of aftershocks) the last (preceded by a series of foreshocks) or somewhere in between.

The problem is that there’s no way of knowing, until the sequence is complete, which is the mainshock. “What marks them out as foreshocks is the fact that a big main shock happens afterwards,” remarks seismologist Roger Musson in The Million Death Quake – and that’s something which we can only know with hindsight.

Foreshocks have been used as a successful predictor of earthquakes in the past – most famously in the Chinese city of Haicheng in 1975, when the city was evacuated prior to an M7.0 tremor. Although 2,000 people, died the casualties were considerably lower than might have been expected.  The following year, in the same area, a second major earthquake struck, killing over a quarter of a million people. This earthquake, however, had no foreshocks and seismologists had no idea it was on its way.

Geological Precursors to Earthquakes

Using detailed instrumentation, it’s possible to identify and measure minute changes in the immediate area of a fault which may occur before an earthquake. These changes are many and varied, including emission of radon gas, alterations in the local magnetic field, emission of radio waves, changes in levels of groundwater and observed changes on the ground.

As with foreshocks, the problem here is twofold. In a summary of the literature on the subject Cicerone et al remark that “while each of these phenomena has been observed prior to certain earthquakes, such observations have been serendipitous in nature.” In other words, sometimes the signs will occur and sometimes they won’t.

In one of the best known prediction experiments, at Parkfield in California, when an earthquake eventually occurred, researchers observed none of the precursors  – despite extensive scientific scrutiny of the area.

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