Friday, January 30, 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. 


  1. Jeff- Came across your blog and hope you don't mind a question off the topic of your post. I noticed you live on Prune Hill, and my family is moving to a house on the NE side. Was curious about the geology and soil (we are at about 500ft). Is it fairly solid volcanic rock below? Would the geology by protective (or opposite) for earthquakes? I know much of the greater area has elevated radon risks due to Missoula flood, but wondering if the soil on the hill was more or less prone? Any advice on accessing landslide risk- what to look for? Information I could find was either very vague or too 'technical' for me to diigest.

    Thanks in advance and great blog

  2. Sorry I didn't see this comment until now!

    The answers are:
    1. Prune Hill is a volcanic remnant like Mt Tabor. From memory (don't have John O'Connor's map with me here) I think it's about 55,000 years old.
    2. My house has fragments of the Troutdale Formation in the crawl-space; this is an artifact of the 1st Missoula Flood about 12,000 years ago. My house, at least, has at least 15 vents in the crawl-space to mitigate against any radon accumulation.
    3. I live on the NW side, and the (inferred - it's never been seen in outcrop) Prune Hill Fault at the base of the hill farther to the northwest is the reason why I have such a great territorial view. There is another fault that is the reason for Lacamas Lake - it runs down the middle of the rather narrow lake and extends into the Columbia River, where about 6-8 years ago there was a M = 2.5 earthquake.
    4. There is no evidence of landslides on Prune Hill. There is one engineering study that says there WAS a landslide, but this claim was based on a *single augur hole* and the conclusion was written by someone who clearly never took Geology 101. Along the Lacamas Lake Heritage Trail you WILL see evidence of slow landslides: trees that are bent at the base. There is none of this on the rest of Prune Hill.
    4. The houses in my area have amazingly big, tightly-interconnected foundations that appear to have been excavated into bedrock. Safest place to be.
    I hope this answers your questions adequately.

  3. Thanks Jeff! Appreciate the detail and look forward to learning more about Geology now that I'm on top of such an interesting formation.