How to tell an earthquake from a nuclear explosion (2)

There were rumors and press releases vibrating through cyberspace yesterday, suggesting that another test occurred, that it might have been an earthquake, and that nobody had any idea what was going on.

Here is the report page for that event.

First, consider the location, shown on this map.
The event was East of Japan, and relatively distant from North Korea. In addition, the depth was approximately 30 km, or about 3 times deeper than the deepest man-made hole ever drilled.

To fully understand this event, however, we need to look at the historical context.


As is obvious from this figure, hundreds of similar quakes have occurred over the past 15 years. This earthquake is completely standard. The above link does show a great representation of the Japanese Benioff zone, however. As is shown by the color coding, the quakes in the downgoing slab increase in depth to the west. A rough schematic, using the same color coding as the USGS, is shown below.



Benioff zones are one of the classic indicators of plate tectonics, and thus are way more interesting than nuclear weapons.