How the Earth works

Stephen Paul Phipps, associate professor of Earth and Environmental SciencePhoto credit: Candace diCarlo

Geology to most Philadelphians probably amounts to the study of the depths of a SEPTA tunnel, digging into the earth to reach busted water pipes or scaling mountain-like steps to the summit of the Comcast Center.

As a result, many East Coast geologists and geology majors at Penn come into the field relatively late, says Stephen Paul Phipps, an associate professor in the Department of Earth and Environmental Science.
“I’ve always said it’s because people around here don’t see a whole lot of geology unless they take a trip west or go up to the mountains in New Hampshire on vacation,” he says.

In the Midwest and West, where the oil business and the Rocky Mountains are prominent, geology is a daily presence. And although Phipps is a native Midwesterner, he admits he came to the field late, too. He was a physics major as an undergrad before he was “seduced by geology and decided to pursue it as a career.”

It was the geological field trips that lured him, “the idea that you could go outdoors and, by making some fairly simple observations, really understand some important things about how the Earth works,” he says. “It was very powerful as well as an awful lot of fun.”

A structural geologist, Phipps is interested in how mountain ranges form and how oceanic rocks are emplaced on land and incorporated into mountain ranges.
“People in my field study the way rocks are bent and broken at all scales, ranging from microscopic up to continental,” he says. “I work mostly in the intermediate scale, such as the scale of a single rock outcropping, or a region, like California coast ranges or Southeastern Pennsylvania Appalachians.”

Although not an earthquake expert, Phipps is interested in the natural hazards and works in a related field. He is also very knowledgeable about their causes and composition.

The Current recently sat down with Phipps to discuss the Haiti earthquake, earthquakes in general, and if, as many believe, we really are immune from earthquakes here on the East Coast.

Q. When you heard about the Haiti earthquake, were you surprised or was this something geologists have been expecting?
A.
That part of the Caribbean is a very active geological area. Anybody who knows very much about the area probably wasn’t surprised by this earthquake. The boundary between two of the major plates that make up the earth’s outer layers, the North American Plate and the Caribbean Plate, runs right through Hispaniola. A major branch of it runs right next to Port-au-Prince. And there hadn’t been an earthquake there in a long time, and that’s actually a danger sign. If you’re in an active area and you haven’t had an earthquake, it doesn’t mean you’re safe, it means the earth is storing up energy. The longer the wait, the bigger the earthquake. That’s why people are concerned about San Francisco and Los Angeles. Los Angeles hasn’t had a big earthquake in 150 years. That’s a long time for that fault. For San Francisco, it’s been 100 years, also a pretty long time. So in an active area, you’re safer if you’re having a whole lot of little earthquakes than if everything’s quiet.

Q. What geological forces cause an earthquake?
A.
It’s friction. The process is that the interior of the earth is hot and it’s trying to cool down, and the way it cools down is through convection, which means that large masses of warm material rise upward and reach the surface, which is colder, and spread out and give off their heat through a variety of processes, and then go back down into the earth. That’s what drives plate tectonics; the plates are kind of the skin that’s moving around on the hot interior of the earth. The convecting layer isn’t a liquid; it’s a very weak plastic solid. The outer layer is cold and brittle in comparison, so it gets carried around by these currents. As it gets carried around, pieces of it bump into each other, and it’s that bumping that causes a lot of mountains. When one plate gets pushed down under another, that’s the kind of motion that causes really big earthquakes, like the Sumatra-Andaman earthquake in 2004. That’s one kind of interaction. Another kind is where they split apart, like what’s happening at the Mid-Atlantic Ridge in the middle of the Atlantic Ocean. There are a lot of earthquakes in these areas, but most of them are quite small. The other type of boundary is like California and Haiti, where the plates are sliding past each other. Those are intermediate in danger. They don’t cause the biggest earthquakes, but they cause quite a few of them.

Q. Is there any way to predict when an earthquake will occur or are they just random events?
A.
They’re not random. One of the things that geologists are always struggling with is the tremendous difference between geologic time scales and human time scales. And not just human time scales, but the scale at which we have to make decisions, like buying a house, taking a job, which is just a year or two. That’s where we can’t do earthquake prediction. So the thing that just has eluded the science completely is the ability to make the kinds of forecasts like hurricane scientists can make about hurricanes.
For instance, the U.S. Geological Survey has made very good predictions about the probability of having a large earthquake in the San Francisco Bay area within 30 years or within the greater Los Angeles Basin within 30 years. Those are pretty good predictions, but the problem is that nobody can make a prediction that says, ‘It’s going to happen next month so let’s start evacuating now,’ let alone one that says, ‘It’s going to happen in two hours. Get out of unreinforced masonry structures.’ You pretty much have to make decisions in the way of, ‘I don’t want to live in California’ or ‘I don’t want to live within 10 miles of the San Andreas Fault.’ But there are lots of people who were born and grew old and had great-grandchildren and died who lived right on the San Andreas Fault and never experienced a great earthquake.

Q. Was this one of the biggest earthquakes of your lifetime?
A.
This earthquake wasn’t that large; it was only a 7.0. There are seven or eight of those a year, sometimes more, around the planet. What made this one bad was that it was shallow. It was very close to the surface and it was close to Port-au-Prince. If the earthquake had been farther out, it would have been much less damaging, but the epicenter was 10 miles from the city. The other thing was that the construction in the city was very bad.

Q. There has been much talk about how the poor building construction in Haiti and the absence of building codes played a large role in the destruction and fatalities. Is it really possible to construct a building that can withstand a 7.0 earthquake?
A.
Yes. If unlimited resources were available, you could build, or rebuild, a city the size of Port-au-Prince that would have very few fatalities in an earthquake like this. The problem is that even in a rich country, it’s very expensive to do. In a city like San Francisco, many of the structures are 100, even 150 years old, and were built before we understood what the earthquake hazard was or what caused it. We didn’t know anything about plate tectonics and, most importantly, we didn’t know how to build buildings that were earthquake resistant. So if there is a great earthquake in San Francisco, if there’s an 8.0 as there was in 1906, there are probably going to be thousands of fatalities. Ironically enough, it’s the skyscrapers, which tend to be newer, that are relatively durable. They’ll have things like windows fall out and there will be hazards of falling glass, but the building itself is probably not going to collapse. The problem with Haiti is not only are a lot of the houses made of concrete blocks or bricks mortared together with no wood, which flexes and bends, and no steel, which flexes and bends, but also a lot of the bigger buildings are made in the way we’ve made a lot of the buildings on the East Coast, which is just putting up reinforced concrete piers and then putting concrete floors between them with no strong, flexible link. What can happen in a lot of these bigger structures is you’ll have shaking, a floor will collapse and it will come down and hit the floor below it, which will knock that floor down, and that floor will knock the next floor down and the whole building collapses. It’s called pancaking.

Q. There is an urban legend that a major earthquake on the San Andreas Fault could cause California to break off from the continental United States or fall into the ocean. Is there any truth to this?
A.
No. What is actually happening is the west side of California is moving steadily northward. What’ll eventually happen is that Los Angeles will end up next to San Francisco, which has all the San Francisco people horrified.

Q. Philadelphia is not known as an earthquake danger zone, but a recent ‘Good Morning America’ segment discussed the nearby Ramapo Fault. Are we in any danger?
A.
In this immediate area, the area around Lancaster and Reading tends to get a few of them. They’re typically not very strong; they’re maximum Richter 4 or 5, in that range. Typically what they’ll do is rattle windows. People who have little knickknacks on little hutches, those will fall off and break sometimes. Occasionally there will be broken windows. I don’t know of an injury from an earthquake in the area since I’ve lived here, which is 25 years now. But they do happen. I’ve never felt one but a lot of people have felt one or more. We think these earthquakes are happening on old faults that exist in this area, faults from 150 million years ago and older, when this area was as geologically active as California is today. This area was once a lot more like the West Coast is now. There are old faults in the earth’s crust and our best guess is that those faults occasionally move just a little bit because the entire planet is active and is under stress. In particular, we know that the plate that we’re living on, the North American Plate, is moving slowly and steadily westward off the Mid-Atlantic Ridge, which is the mountain range running down the middle of the Atlantic Ocean.
As the North American Plate moves off the Mid-Atlantic Ridge and moves westward, it may be running into some resistance in the Pacific area, and some of that stress from that sliding and pushing gets transmitted into the interior and causes these quakes. There have been very destructive earthquakes on the East Coast. In 1886, there was an earthquake in Charleston, South Carolina, that did a lot of damage. That’s one of the things that seismologists who study this area worry about because there’s no earthquake consciousness in this area and not much preparedness. ... It is theoretically possible to have a very destructive earthquake that would affect major East Coast cities. It’s just not very likely. It’s not something I worry about, let’s put it that way.

Originally published on February 4, 2010