Saturday, February 7, 2015

Do Small Earthquakes Prevent Large Earthquakes?

Here's another question about earthquakes. It doesn't really address "slow-slip" earthquakes or induced seismicity, but clearly the questioner has been reading...

Q: Do small earthquakes prevent larger earthquakes from occurring? 
- Laurie F

A: There is an argument to that effect within the earthquake research community. Theoretically, a series of small events might accommodate (re-equilibrate, redistribute) at least some of the strain being built up by tectonic forces. In the practical world this works only imperfectly. For instance, we know: 

2....that waste water injection into oil wells north of Denver, CO, led to a significant cluster of micro-earthquakes. Apparently the fluid lubricated fault surfaces that were collecting strain. There wasn't a lot of energy released by this process, but it caused earthquake scientists to sit up and listen.

2....that there are "slow slip" earthquakes on subduction faults that cannot normally be felt, but are only "seen" by noting slow displacement changes in continually-recording GPS instruments. An example of this has been measured in the Olympic Peninsula of Washington State, and another example has been observed on the south coast of the Big Island of Hawai'i. 

This small-event re-equilibration process works for parts of a subduction fault surface - for instance the shallow and deep parts of the down-going Juan de Fuca oceanic crustal slab currently being subducted beneath the Pacific Northwest Cascades. However, there is a section of this (and other) subduction fault(s) that does NOT release strain in small increments like this. This part remains "locked."

When these locked sections "rip" (fail) there can be many meters of abrupt displacement. THIS process is the source of the greatest earthquakes ever recorded, including the last great Cascadia earthquake of January 1700 AD, which caused an "orphan tsunami" that devastated the Sendai coast of Japan many hours later without any warning.