Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Ethan Siegel is an astrophysicist who is now making a living as science blogger. There is a post titled: Where does an Earthquake's energy come from?

Ethan Siegel points out that massive earthquake events are accompanied by a measurable change in length-of-day, in the order of microseconds. (A very large event was the 2004 Sumatra quake, causing a 6.8 microsecond shortening of length-of-day.) Obviously, to speed up the Earth must contract. That is, what must be happening is that the rearrangement of tectonic plates during the quake allows the Earth to shrink a bit.

As I interpret Ethan Siegel's post he is arguing that the energy of the seismic waves of such a quake come directly from the contraction of the Earth during the event.

In my own words: as I understand Ethan Siegel he is arguing that the Earth contraction during a quake is effectively an implosion event, causing seismic waves directly.

Quoting Ethan Siegel:

Plate tectonics tells you where earthquakes are most likely to occur, and the geophysics of the Earth’s crust tells you the different types of faults that cause these quakes, but neither of these tells you where the energy for these quakes comes from.

Please read Ethan Siegel's original post to see if you agree with my interpretation of his words.

In comment #5 on that blog post I argued against Ethan's theory.

I submitted:

when a rotating system contracts its kinetic energy increases (Angular momentum is *conserved*; rotational kinetic energy *increases*) So: during an Earth-rotation-rate-changing-earthquake the gravitational contraction of the Earth is doing work.

Here's my point: I argue that all of that work goes to the increase of Earth rotation rate, not to the quake!
I argue that *during the quake* the energy of the seismic waves comes entirely from the release of elastic potential energy that has been building up in the tectonic plates.

Elaborating on the above:
I think that the contraction of the Earth-as-a-whole is a consequence and not a cause. I think: when tectonic plates slip relative to each other their rearrangement allows a contraction of the Earth to occur, and following that there is an increase in Earth angular velocity.

My question: which one of the above described opposite points of view is likely to be the correct one?

share|cite|improve this question

The largest earthquakes are caused as a tectonic plate of cooler, and therefore denser, lithosphere slides down a subduction zone into the Earth’s mantle. Gravitational energy is released because the plate is sliding downhill. It’s just falling and some of the energy is released as earthquakes. No ‘contraction’ or ‘shrinking’ of the Earth is involved. Earth’s rate-of-rotation increases, and the length-of-day decreases slightly, because of the redistribution of mass closer to the axis of rotation.

Sorry, the thread on that blog was too confusing for me to comment, but I think this answers your question.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.