# Earthquake Correlations - II

In the year 1657, the Dutch scientist Christian Huygens observed that if two pendulum clocks were mounted adjacent to each other on the same support, the subtle and almost imperceptible vibrations of the swinging pendulums would cause those pendulums to become synchronized, swinging in opposite directions.  He referred to this phenomenon as "odd sympathy".

Today we know that nonlinear systems, examples of which are earthquake faults, can become "mode-locked", or synchronized.  Earthquake faults are in fact a kind of "clock".  Faults of a given size are thought to slip in typical, or "characteristic" earthquakes.    If the tectonic forces that drive the plates are applied in a persistently increasing manner, it would be expected that the earthquakes produced by this process would recur more-or-less regularly, if the faults were isolated.  Introducing interactions between the faults, in the form of seismic waves, is akin to mounting pendulum clocks on the same support.  Nonlinear science would expect that the earthquakes on the faults might become synchronized, or correlated.

Examples of synchrony in nonlinear systems are common, and have been modeled as systems of coupled nonlinear oscillators in a paper by Mirollo and Strogatz (Siam. J. Appl. Math, 1990).  A particularly famous example is the "firefly trees" in southeast Asia.  Here one finds whole forests of trees holding many thousands of fireflies.  On warm evenings, observers have documented that the blinking of the fireflies becomes highly synchronized.  In this case, the feedback mechanism is the visual observation by each firefly of all the other insects.

Another example of synchrony in nonlinear systems is the flocking or schooling of birds, fish, and insects.  In my part of Northern California, as in many other places, one can find large flocks of sparrows that behave almost as a single coherent entity.  The flock can be seen to swoop low along the ground as a huge coherent mass, rising suddenly as one group to pass over power lines and other obstacles in their path.  The effect is striking.

Other interesting examples can be found in the book "Sync: The Emerging Science of Spontaneous Order" by Albert-Laszlo Barabasi.   To return to the question of whether the recent earthquake activity represents purely random occurrences, or whether it represents the synchronization or correlation of earthquake events, we have to conclude that correlated activity of these events is plausible.  But the jury is still out.