ToeQuest

The earthquake trigger mechanism is actually a very simple process that can be easily explained with basic High School physics. However, the most important prerequisite for really understanding this process is common sense coupled with an open mind without prejudices. Common sense dictates, for example, that this process cannot be caused by some vague supernatural forces that never really do occur in some form in everyday life. On the other hand, an open mind implies that one cannot always insist on a certain rigid view of the Universe one had once learned in school without making allowances for all the new discoveries that were made in the meantime.

Forces Involved

In order to satisfy the common sense requirement, the simplest analogy of an everyday occurrence to the earthquake trigger mechanism is perhaps a fast car traveling on a winding mountain road. If the car accelerates rapidly, the people in the car are forced backwards into their seats and if the car decelerates rapidly, then the people will be propelled forward — if not restrained by seatbelts. Similarly, if the car makes a fast, tight turn to the left, people seem to get pushed to the right side of the car as their momentum will want them to keep moving in a straight line. And if the car makes a fast, tight turn to the right, they are naturally pushed to the left. But not so a hypothetical ant on the back seat. Because of its negligible momentum resulting from its negligible mass, it barely experiences any “push” and stays put even without seatbelt.

In the case of earthquakes, the phenomenon is similar: As we all know by now from the study of Plate Tectonics (it wasn’t known during my time at university 50 years ago), the crust of the Earth consists of a number of plates which, on their borders where they slide past as well as over or under their neighbouring plates, disintegrate into a jumble of large and small fault blocks (in geology, a "fault" is a break in the rock surface of the solid crust of the Earth), which are separated from each other by differently oriented and earthquake-producing fault planes. Given the same velocity changes (i.e. acceleration, deceleration and “curves”, to use our car analogy), very large masses, such as the just described fault blocks with their huge volumes of rock, are affected to a much higher degree than small masses, like people (or the ant in the car analogy), who can usually barely feel earthquakes of smaller magnitude. This is, of course, a result of the different magnitude of momentum they acquire (or lose) due to their different masses.


Spiral Movements

All astrophysicists agree that a predominant portion of observed galaxies form spirals, so most astrophysicists would agree that this is the predominant path form a star takes when traveling around the center of a galaxy and with it through space. Based on geological and other evidence in the case of our Sun, this round trip is most probably shaped like a Kepler ellipse and lasts between 200 and 300 million years, depending on the different interpretations of individual geologists and astrophysicists.

Some astrophysicists have noticed that there is also an up and down motion, so that the star appears to travel sometimes above the galactic disk and sometimes below it. This implies that superimposed on the spiral path around the galaxy there is also a smaller-amplitude spiral motion around the higher-density center of the galactic arm the Sun belongs to. Again, according to geological and other evidence from differing interpretations, this superimposed motion lasts somewhere between 30 and 70 million years in the case of our Sun. The latest figure from geological evidence seems to point to 62 million years.


Nowadays even elementary school children learn that the planets travel around the Sun in recurring elliptical orbits which, of course, result in even smaller-amplitude spiral paths superimposed on the spiral path of the Sun and lasting, in the case of the Earth, a year. Now add to this path of three superimposed elliptical spirals, that any point on the surface of the Earth travels around the galaxy, the irregularities that are caused by the daily rotation and the seasonal inclinations of the axis of the Earth, and you can see that we all, people and fault blocks alike, are traveling at constantly changing speeds and with constantly changing directions on our journey through space. Just like in a car on a winding mountain road, we all are thus continuously exposed to the various forces these changing speeds and directions produce (with gravity acting as our “seatbelts”) — but presumably we feel these forces only if their magnitudes exceed a certain threshold. Perhaps this is how the unusual animal behaviour before earthquakes can be explained as we know that some life forms have more highly developed sensitivities for particular conditions than others.

Let’s now look at some of those conditions during specific earthquakes: Fig] shows, for example, the probably familiar solar time view of the relationship between the Earth and the Sun, but with a certain twist in it which, incidentally, needs an “open mind” to appreciate. The sketch represents a cross-section through the Earth at the latitude of the largest earthquake ever recorded (magnitude 9.5, Chile, 1960) and also indicates the approximate position of the epicenter at the particular solar time when the earthquake occurred (as shown in blue). The red rectangle indicates therefore the particular region of the crust of the Earth which needs to be examined more closely — as done in Fig.2. The red lines, labeled Fl to F4, are just some of the forces emanating from the Sun and acting on the whole of the Earth.