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Earthquakes and Solar Activity

A strict test thoroughly rules out an easy connection

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Earthquake prediction has resisted the efforts of geoscientists for generations. Perhaps it's not a coincidence that the topic has attracted all sorts of cranks and lay people who mistake hunches for certainty and vigorous assertion for evidence. Even working scientists are prone to this temptation. A pair of researchers has put the kibosh on one attractive line of argument that earthquakes are related to the sun's activity.

It's an attractive idea that as an earthquake fault teeters on the edge of rupturing, even a tiny stress could push the rocks into failure. All kinds of tiny stresses have been suggested: lunar tides, weather fronts, changes in the water table, nuclear bombs and dams. What about sunspots and other changes in the sun?

Solar Changes

The sun affects the Earth in several different ways. Gravity is obvious: Earth's orbit brings it about 2 percent closer to the sun in January and 2 percent farther in June. But earthquakes are not seasonal.

The sun exerts subtler influences on the Earth through its vigorous electromagnetic activity. The solar wind is a flow of hot, electrically conducting plasma that carries magnetic field lines out into the planetary zone. These sweep into the Earth's geomagnetic field and exert complex forces on the Earth system starting in the uppermost atmosphere (causing auroras). The sun goes through an 11-year cycle of activity, arising from deep in the sun's core, that is determined by counting sunspots. This cycle, called the Schwabe cycle, is actually half of the 22-year Hale cycle during which the sun reverses its magnetic field twice.

On top of that grand 22-year sunspot cycle (and less-established longer cycles), the sun has bursts of energetic activity that show themselves as various kinds of space weather: geomagnetic storms, radio blackouts and radiation storms.

Any of these events send physical forces into the Earth that might conceivably influence the occurrence of earthquakes. No one has a coherent theory, but there are speculations: the Earth's rotation rate might change, magnetic storms might push electric currents along faults and heat them, stresses inside mineral crystals might respond to electric stimulation, and so on.

Testing the Solar Influence

The only way to proceed is to start with the brute-force approach of taking earthquake catalogs and matching them against records of solar activities. In a March 2013 paper in Geophysical Research Letters, Jeffrey Love and Jeremy Thomas reviewed previous studies and found little consistency: Some studies found connections that others denied, others reviewed only selected solar data or earthquakes in specific regions. Despite the spotty results (or perhaps because of them), studies continue to come out, and the public keeps asking people like me and the U.S. Geological Survey about the subject.

Love and Thomas set out to do the job right. First they noted that even scientists can fall prey to "selection bias," in which some unusual coincidence can grab their attention and influence their study design. Then they pointed out that to judge whether a relationship is real—that is, statistically significant—it needs to be tested against a separate, independent set of data. They set out to meet both of these concerns.

Using annual averages in all cases, they tested the global catalog of large quakes (magnitude 7.5 and larger, with aftershocks removed) from 1900 to 2012 against three records of solar data: sunspot number, solar wind velocity and geomagnetic activity (the AA index). They found no relationship between the earthquakes and any of the three datasets.

Statistical Tests

To allow a robust assessment of the statistics, Love and Thomas split the datasets into independent halves: quakes were checked against all solar values below average and all values above average, a bias-free way to divide the data. Their results, expressed in probability values by the standard chi-square and Student's t tests, showed no statistically significant link. They repeated the analyses for monthly averages and daily averages with the same result. They looked at different earthquake magnitudes—7.5, 8.0, 8.5 and 9—with the same result.

To conclude their meticulous test, Love and Thomas used the formal language of scientific logic that is taught in school but usually glossed over by working scientists: "From retrospective analysis of historical data, we cannot confidently resolve a statistically significant relationship between solar-terrestrial variables and earthquake occurrence. Therefore, we cannot confidently reject the null hypothesis of no solar-terrestrial triggering of earthquakes." This precision of terminology extended to the paper's title: "Insignificant Solar-Terrestrial Triggering of Earthquakes." That's how the science of earthquake prediction needs to be done.

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