06 March 2011

Earthquakes and the moon: should we worry?

Borrowed from: http://drquigs.com/

I will expand on this section over the next couple days so please stay posted.
Here are a couple points.
1. No one has predicted the recent earthquakes in Canterbury. Vague quotes about dates of 'increased' activity plus or minus several days, without magnitudes, locations, and exact times do not constitute prediction. Consider this: Ken Ring's probability of getting a prediction correct based on perigee/apogee new moon/full moon for 2010 was 63%.  That's 230 out of 365 days that fall on some day that he would argue influences earthquake activity.  For days that combine several factors of new moon/perigee etc. he missed out on several predictions and NOTHING unusual happened on those days.  (i.e. Jan 30th, Feb 14th, Feb 27th, Mar 29, June 14th, Jul 12, Aug 10, and so on for his liberal interpretation of the aftershock sequence). This does not constitute 'prediction'. It is opportunistic and meaningless self promotion during a time of national crisis.
2. Consider your chances of getting a 'prediction' correct given this unscientific defintion of prediction. On average, New Zealand gets ~330 earthquakes of M4-4.9 every year, 26 M5-5.9s / yr, 2 M6-6.9s / yr, and 1 M 7-7.9 every 3 years (see stats on Geonet). If unspecific about magnitude and location, then your chances of 'predicting' an earthquake that is likely to be locally felt and recorded is >90% (based on the simplified method of assuming each earthquake occurs on a different day, which isn't the case, but you get the picture). This of course goes up during the recovery phase following a major earthquake like our 7.1 where the occurrence of large events is high. So far we have had 203 earthquakes > 4 in the Canterbury region close to the 7.1 rupture in the 6 months since Sept 4th. So one's chances of 'prediction' are actually quite high, aren't they?
3. If we had been specifically predicting large earthquakes (M>6) on the faults near Christchurch that ruptured in September 4th and Feb 22nd using the moon over the last several thousand years, we would have been wrong many thousands of times, with a success rate of "zero", even invoking the broad criteria cast by invoking all of the possible moon scenarios listed above.
4. There is no correlation between the largest aftershocks in the Darfield earthquake aftershock sequence and diurnal tides. Some of our largest earthquakes have occurred near high tide and some near low. More statistically robust calculations will appear here in the future.
5. Consider implementation of this 'predictive' strategy. Should we evacuate an area every time the moon is on its closest approach, is full or new, is moving rapidly, is at its maximum declination or is crossing the equator? Imagine the fear and frustration of such an approach, particularly given the unspecified times, locations, and magnitudes of the supposed 'imminent' events. Without a basic understanding of how faults generate earthquakes, where the faults are, at what stage they are at in the seismic cycle, and how they have been affected by prior activity, where should we evacuate and where should we go to? This would require several evacuations a month of 'unspecified areas' to other 'unspecified areas'. This is ludicrous.
*By the way, earthquakes occur when the stress acting on a fault exceeds the yield strength of said fault. It is much easier to break the crust along pre-existing faults than to create new faults in intact bedrock from earthquakes. Consider shards of glass lying in a box, when you push on one side do the shards not slide by each other on pre-existing faults or do they break internally? Sometimes faults are blind (don't reach the surface) and sometimes they break through a younger surface, which gives the appearance of creating a 'new' fault line. This is not almost certainly not a 'new' fault, but rather a previously unidentifed fault beneath the surface.
6. Since humans first looked into the sky and felt the effects of earthquakes, they have wondered if the moon and planets are in some way responsible for major earthquakes. As early as 1897, scientists began to pose hypotheses about moon-earth earthquake connections and test them in honest and rigorous way. After all, the moon still gets earthquakes in the absence of plate tectonics, so perhaps there is some validity to this claim. While some astrologers may feel isolated from the scientific community, this shows a true lack of appreciation for all of those dedicating significant effort to this issue. Many of these findings from studies comparing earthquake catalogues to tides have been published in high quality journals such as Science (e.g., Cochran et al., 2004) and some scientists have argued based on statistical data from global earthquakes for an influence of tides on earthquake activity under certain circumstances, such as beneath the oceans and within active volcanoes. Some scientists have even argued for a small correlation (perhaps an increased earthquake likelihood of 0.5-1%) between smaller, shallower continental earthquakes and 'solid earth tides' (changes in the shape of our planet due to the gravitational pull of the moon) - see http://www.ipgp.fr/~lalmetiv/metivier_etal_epsl2009.pdf. This is peer-reviewed but controversial research; it does not make it so, but it has undergone scrutiny and will continue to do so. This is the scientific process.
Typical earthquake-induced stress changes are about 100 to 1000 times greater than those induced by the tides. Earthquake induced stress increases are also constant, that it until the breaking of the rock alleviates that stress, while tidal stress changes occur in brief intervals related to the moon's elliptical orbit about the earth. There is significant evidence to suggest that the tidal oscillations are too brief and too weak to trigger major earthquakes. So the redistribution of stress related to our 7.1 mainshock and resultant aftershocks is undoubtedly the dominant control on our aftershock sequence, not the moon or planetary alignment. I won't be going anywhere in late March, but I will always be willing to engage in scientific discussion and debate if it appears in an open and honest format.
In the midst of a crisis, however, I feel quite strongly that the time is not yet right.  

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