Did a supermoon cause the March 11, 2011 earthquake in Japan?

We’re still getting so many questions about supermoons and earthquakes that I want to try to simplify it even more. There is no reason to think that a supermoon caused the March 11, 2011 8.9-magnitude earthquake in Japan because the March 11 moon was not a supermoon.

Here’s my earlier post about the supermoon, from two days ago, the day of the quake:
What’s true – and false – about the March 19 supermoon

The supermoon-earthquake connection idea depends on two things. First, an extra close moon. Second, a full moon – when the Earth, sun and moon make a line (more or less) in space. Here is why the March 11 moon could not have been an example of the supermoon-earthquake theory at work:

#1 The moon was not full on March 11. In fact, on March 11, the moon was only a day away from first quarter moon. So, on March 11, the moon was more or less at right angles to the Earth/sun line. It was nearly as far from being aligned with the Earth and sun as it can be.

#2 The moon was not closest to Earth on March 11.
Apogee – the moon’s farthest point from Earth for this month – was March 6. Perigee – the moon’s closest point to Earth for this month – will be March 19. On March 11, the moon was about halfway between its closest and farthest point to Earth.

So, on March 11, neither condition for a supermoon existed. The moon was not particularly close to Earth, and the Earth, sun and moon were not aligned. In fact – both in the sense of its distance from Earth, and also in the sense of an Earth/moon/sun alignment – the moon was far from the condition of “supermoon.” And yet there was this massive earthquake. What is the connection?

One more thing. My colleague at EarthSky – Bruce McClure, who writes most of our Tonight pages – passed along this link. You can see from the chart of perigee full moons on this page that these sorts of full moons – full moons coinciding with the moon’s closest point to Earth – happen about every 1 year 1 month and 18 days. You can also see from the chart that full moon distances do vary, but the variation in lunar distance is small in contrast to the moon’s overall distance.

For example, the last perigee full moon – on January 30, 2010 – was only 30 kilometers (20 miles) farther than the March 19, 2011 full moon will be. That’s in contrast to the moon’s mean distance of 384,400 kilometers (about 239,000 miles – or nearly a quarter million miles).

Supermoon conditions were not in effect during the March 11 8.9-magnitude earthquake in Japan. If a connection between supermoons and earthquakes exists, it should be easy to spot and track over time. Scientists will notice it and incorporate it into their efforts to predict earthquakes and save lives. Until then, the idea of a supermoon has not been supported by science.

Is it possible to predict earthquakes?

Understanding moon phases

Understanding full moon

Understanding first quarter moon

March 15, 2011

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Deborah Byrd

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