Tag Archives: Japan earthquake

How do we measure earthquake strain energy?

I was asked a very interesting question on one of my recent LinkedIn posts that I thought deserved a slightly detailed answer. Here’s the question:

QuestionAnd here’s my answer:

Hi David. Firstly, this is not a basic question at all. In fact, it’s one that is of considerable importance. Your question has two parts and I’ll address them individually.

How do we measure strain energy released in earthquakes?

The main way we do this is by measuring how much the ground moves in an earthquake, this can be done with high precision GPS instruments on the ground or from space based satellite measurements.

Here’s an example from the giant Tohoku earthquake, that struck Japan in 2011, of what we can do with GPS instruments. Each little arrow is a GPS station and it records how much the ground moved during the earthquake.

We can use these kind of measurements to calculate how much the ground has moved all along the fault and at depth during an earthquake. Once we have the full displacement of the fault we can relate that directly to the stress drop and strain release.

How do we measure strain energy being stored on faults?

A similar method is used to determine strain stored on a fault. The ground very slowly warps itself around a locked fault in the decades to centuries before an earthquake.

Here’s an animated model of what I mean (might need a screen refresh to play it). Imagine looking down onto the ground from above with the top half of the earth moving to the right and the bottom moving to the left.

The warping of the ground before an earthquake. Source: Philip England

If we can measure the degree of warping before the earthquake, again by using GPS and satellites, we can relate that to the strain energy getting stored on the fault. Here represented by the different colours, with red being the area of highest strain storage.

I hope that helps to answer your question!


Earthquake liquefaction causes ground to open and close

The 2011 magnitude 9 earthquake in Japan was one of the largest earthquakes ever measured. Strong shaking and the consequent tsunami caused the death of nearly 18,000 people.

Strong vibrations in earthquakes can cause soil particles in the ground to jiggle about and lose contact with each other. If there is a lot of water within the ground the shaking causes soil to behave more like a liquid than a solid. This process is called liquefaction.

Footage taken by an American tourist shows dynamic movements of the ground during the Japan earthquake. The changing pressures caused by the soil movement can force water out of the ground and form moving cracks on the surface.

Note that this is not ground movements caused by the actual shaking in the earthquake, but by secondary soil movements.