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. 

Friday, January 16, 2015

What kind of house base absorbs the most shock?

There are real, practical consequences from geology that affect every single one of us. Here's another, though you have to think like an engineer to understand all the issues involved.

Q: What kind of house base absorbs the most shock during an earthquake? - Maddie D-N

A: There are two different aspects of the same issue here: a "walking" building, vs a shock-absorbing building.

1. A building foundation that is anchored in bedrock will SHIFT the least. The foundations of my house in Washington State are built (excavated) down into bedrock. In addition, all parts of the foundation are tied together with reinforced concrete. This will keep my house from "doing the splits" when the next Cascadia earthquake hits. Being anchored in bedrock means I have a better chance that my house won't take a ride - walk - over to my neighbor's property, either.  During the Loma Prieta earthquake of 1989 in northern California, some houses that "walked" and some that "did the splits." They were build on landfill in North Beach, landfill made up largely of debris from the 1906 Earthquake dumped there nearly a century earlier. Their foundations failed - sagged, did the splits - because they were not tied together, nor were they anchored in bedrock. That landfill turned partially liquid with the shock waves passing through it. In geology-ese, this is "liquifaction."

2. There are some (generally rare) structures designed with shock-absorbing materials between the bedrock-grounded base and the structure itself. Some examples include the Trans-Alaska pipeline, and the underground facilities hosting NORAD (North American Air Defense Command) in Colorado Springs, CO. Designing a structure to ABSORB the most shock is a very expensive thing to do, however, and when NORAD was built during the early Cold War, cost was not an issue. 
   However, no matter what the structure is, engineers must decide on how BIG an event to design for.(i.e., how much displacement can they anticipate). There is not enough money to design everything in the country to survive a magnitude 8 or 9 earthquake, as we sadly learned with the Great Tohoku earthquake of 2011. The Fukushima Dai-Ichi nuclear plant had been built to survive a Magnitude 7+ event. It was unable to withstand the consequences of a magnitude 9 event (the initial shaking and the 15-meter tsunami that followed). To build it for this, the facility would have cost one or two orders of magnitude more than it did, and no one had ever experienced a M = 9 event in Japan before. 

People around the Pacific Rim will live for many years with the consequences of that structural inadequacy. 

Friday, January 2, 2015

Landscape Change – the Consequences

Landscapes change – sometimes gradually, but also sometimes in fits and spurts. There are real-life consequences to this change.

Q: When the area drifting from Africa eventually fully separates will any animals become extinct? How many new animals would be expected to evolve? How long would it take for the scenery to greatly differ from how it once was?
- Veronica V

A: Your question is ambiguous, so I will take it upon myself to infer that you mean the ~6,000-kilometer-long Great Rift Valley of Africa.

That name itself is somewhat ambiguous, since it combines features from a number of separate although related rift and fault systems stretching from Jordan to Mozambique. This continental split has been forming since at least the Miocene, 22–25 million years ago, and is currently pulling apart at a rate of about 7 mm per year. At that rate of extension, a complete rupture will occur within 10 million years, and the Somalian plate will break off, forming a new sea between it and Africa not unlike the Red Sea.

As to the animals, well, if history is any predictor of the future you can expect several things:

1. VAST numbers of animal species will go extinct this century. This is due to habitat elimination, over-hunting, and climate change. You are watching this “Sixth Extinction” happen right now, as poachers decimate African Rhinos and Elephants for their horns and tusks, and Tigers in Asia for their internal organs and bones, just to satiate a bottomless appetite in Yemen and China.

2. Those that survive on the “New Madagascar” that will be the eventual Somali Island will evolve to fit their altered ecosystem. Often this means they will grow smaller – evolve to better use the limited resources of a now-limited landscape.