A geology lesson: Earth speaks to us through earthquakes
Pat Spencer, professor of geology, sits serenely in the Hall of Science atrium while the world seems to shake around him, thanks to a little photography software magic.
by Patrick Spencer, professor of geology
I grew up in California, in a home only a few miles from the main trace of the San Andreas fault. I remember in my youth sneaking in to the Crystal Springs Reservoir to fish. It was illegal, but we were young, and the fish were huge. At the time it did not occur to me why the reservoir was shaped the way it is; the folly of locating a domestic water supply in a valley that owes its existence to a major tear in Earth’s brittle crust was completely lost on me.
Earth speaks to us through earthquakes. They are Earth’s way of relieving the stress built up as crustal plates shift and drift. When I was young, earthquakes were a novelty and something to be wondered at, but I had little understanding of their causes and consequences. Earth speaks to me more clearly now.
What are earthquakes and why do they occur where they do? What does the Richter scale actually measure? Can we predict the next big one? What is the risk in our community for a destructive earthquake? These are all important questions. To answer them, or at least to try, a basic understanding of Earth processes is necessary.
Earth’s outer skin (usually called the crust) is solid and brittle. It is fractured in many places, and these fractures divide the brittle skin into many large and small crustal plates, which are in motion relative to each other. Their motion is slow, about as fast as your fingernails grow, but over geologically significant intervals of time, they can move long distances.
Where crustal plates interact at their edges, there is a constant bumping and grinding but at the same time, friction between the plates tends to keep them from moving. Stress builds up at plate edges. Rocks, like any other material, have a finite strength and when the stress applied to a rock exceeds its strength, it breaks. This is an earthquake. Earthquakes generate shock (seismic) waves, which cause the material through which they move to deform in a variety of ways. This is what we feel and what causes primary damage during an earthquake.
Earthquakes are rated by the amount of energy they release. Today, the Moment Magnitude scale is most commonly used; it measures energy as a function of length of surface rupture, depth of rupture, total amount of slip along rupture and rock strength. The advantages of this scale are that it considers specific measurable aspects of rock and rupture at the site of the earthquake; it has replaced the better-known Richter scale, which was developed to measure the energy of moderate earthquakes in California.
In general, an increase of one number translates to 33 times more energy released.
A 3 releases 33 times as much energy as a 2. An earthquake of magnitude 4 releases 33 times 33, or more than 1,000 times the energy of a 2, an important thing to remember if you live in earthquake country.
The “World Series” earthquake near Santa Cruz, Calif., in 1989 (M7) released a lot of the energy stored on that segment of the San Andreas fault. The 1906 San Francisco quake (estimated at M8) released 33 times as much energy. Large (M7-8) quakes have a frequency of about 100 years in the Bay Area. Since 1906, there have been half a dozen significant quakes, none of them above 7. The theory tells us that it takes 33 M7 quakes to release the energy of an M8. Even I can do that math. The Bay Area is overdue. So are western Washington and Oregon.
Another scale, less well-known but more relevant to the average citizen, is the Mercalli intensity scale, which measures destructiveness. A Mercalli intensity of I is negligible, while a XII is total destruction. The Mercalli scale is obviously meaningless in uninhabited areas. Also, depending on the type of buildings and construction, an earthquake with a low Moment Magnitude may have a high Mercalli intensity and vice versa. Haiti and Chile demonstrated this quite effectively: Haiti experienced near total destruction in its capital from the M7 quake. Chile experienced far less from the M8.
Humans need order and predictability in their lives; it is our nature. We like to know what the traffic will be like today; what the weather will be tomorrow. People who live in earthquake-prone areas want to know when and how big the next quake will be. The reality is that we cannot predict earthquakes. We are getting better at it, but we are nowhere close to actual prediction. There are too many variables beyond our control and understanding. How can we evaluate risk?
Seismic risk maps indicate the level of hazard across the country. Not surprisingly plate boundaries are high-risk areas. Surprisingly, Walla Walla is a high-risk area. It isn’t close to a plate boundary, so what gives? Seismic risk maps are based in part on the historical record; they are not predictive. They tell you that if there have been historical quakes, there must be a reason, and thus there might be more. In 1936, there was a magnitude 5.8 quake near Milton-Freewater, Ore., located about 10 miles south of Walla Walla. It remains one of the largest quakes in Oregon’s recorded history. The fault (or faults; there are two that intersect not far south of Milton-Freewater) that produced the earthquake may shift again and produce another.
We are better off recognizing the risk in our region and preparing for an earthquake by establishing strict building codes, zoning laws and evacuation plans. Masonry and unreinforced concrete are less flexible than wood frame construction. Unreinforced mud is less stable than bamboo and straw. While a brick house may protect you from the big bad wolf, a straw house would be better in an earthquake. Building foundations also are critical. Saturated mud (which moves a lot when it is shaken) makes a poor foundation. Bedrock, which transmits seismic waves with less movement, is preferable.
Earthquakes are a fact of life on our planet. Some places are at less risk than others, but no place is totally safe (just ask the residents of New Madrid, Mo., which is as far from a plate boundary as you can get in our country, and experienced a series of catastrophic quakes in 1811-12).
Since prediction is not presently an option, we are better off preparing. Understand and accept the risk. An earthquake may never strike our area again. But if it does, will we be prepared?
About the author: Patrick Spencer earned his bachelor’s degree in geology from the University of Washington, master’s degree from Western Washington University and doctorate from UW. He joined the Whitman faculty in 1984.