The Beware of Movies! series is meant to point out some of the scientific inaccuracies of popular movies, specifically in points related to the geological sciences.
This post will point out the major inaccuracies portrayed in movies about earthquakes, and the mistakes that are made regarding how the important theory of Plate Tectonics works.
Let’s start with earthquakes. Earthquakes are shaking of the earth, typically due to motion along a fault. There are other things that can cause earthquakes, but we won’t worry about those here. Not yet, anyway.
So, what’s a fault?
Most of us have a general sense of what a fault is. It’s a big crack in the Earth’s crust, across which motion (or slip) can occur. Americans usually think of the San Andreas Fault, which cuts California from the northwest to the southeast.
There are tons of misconceptions about faults, some of which are carried into the movies and TV that we watch. Let’s first talk about how faults work, and then address these misconceptions.
TYPES OF FAULTS
Faults are divided into to main types: Strike-slip and dip-slip. Strike slip faults are those where the rocks on each side of the fault slide past each other in a horizontal fashion, to the right or to the left. Dip-slip faults occur when one side of the fault moves up or down relative to the other.
The San Andreas Fault is a strike-slip fault. The rock on the west side of the fault is moving northward with respect to the rock on the east side. If you stood on the Sierra Nevada mountains and looked to the West, across the fault, it would look like the west side was moving to the right. Hence, the San Andreas fault is a right-lateral strike-slip fault.
Beware of Movies: In the TV movie “10.5” (and in other movies like the original “Superman”), it was portrayed as if activation of the San Andreas fault would cause California to sink into the ocean. In fact, lots of people still seem to think this. The truth is that more likely, western California would slide up the western edge of North America and collide with Alaska. But don’t worry. That would take millions of years!
Many other faults in western North America are dip-slip faults. The fault surface or plane on dip-slip faults tends to be tilted, rather than vertical as in a strike-slip fault. If one were to open up such a fault and try to climb up it, on one side, a person could walk up and on the other a person would need ropes to hang off it. For this reason we call one side of a dip-slip fault the ‘footwall’ and the other side the ‘hanging wall.’
For a dip-slip fault, the motion of the hanging wall relative to the footwall is how we know what caused the fault to form. In faults where the hanging wall moved up with respect to the footwall, we know that compression caused the faulting. This is called a ‘reverse’ fault. If the hanging wall moves down with respect to the footwall, the faulting was caused by stretching, and the fault is called a ‘normal’ fault.
The Wasatch Fault, that runs through Salt Lake City, for example, is dip-slip. It is an example of a normal fault that formed as the continent of North America was stretched out on the west side. All of the mountains of the Basin and Range in the West are bounded by normal faults.
Reverse faults are common in big mountain belts like the Rocky Mountains and the Appalachians. These mountains formed by tremendous forces of compression. There is a special category of reverse faults called ‘thrust’ faults. Thrust faults are very low angle (close to horizontal) and can slip for hundreds of kilometers. Thrust faults can stack on top of each other (called duplexing) and take up tremendous amounts of shortening of the Earth’s crust.
These terms for faults are general. It is important to be aware that most faults don’t fall exactly into one of these categories. For example, there is a little bit of compression that occurs across the San Andreas Fault. The Wasatch Fault has a bit of horizontal motion. Faults are categorized by the type of faulting (strike-slip versus dip-slip) that dominates the motion. If a fault’s motion is between strike-slip and dip-slip (it has components of both kinds of slip), a fault might be called oblique-slip. One such fault might be described as “normal right-slip.”
When there is an earthquake along a fault, the whole fault doesn’t move at once. Parts of it move, while other parts remain stationary. A fault will remain stationary for a long time as stress builds up across it, then SNAP! It goes.
Earthquakes have epicenters, which most people understand to be where the quake originated. More specifically, the epicenter is the spot on the surface of the land directly above the part of the fault that actually moved. There’s a similar term, hypocenter, which refers to the actual spot, deep under the surface, where the fault moved.
To find the epicenter and hypocenter, a geoscientist looks at the seismic waves from the earthquake as recorded by at least three independent seismic stations. There are several types of waves generated by earthquakes, most importantly p- and s-waves. p-waves are “primary” waves, and arrive at seismic stations first. These are compressional waves. s-waves (“secondary waves”) arrive next. s-waves are shear waves, so they won’t pass through liquids. The separation in time between the s- and the p-waves tells the geoscientist how far away the earthquake happened, but not what direction. With several seismic stations, the actual point (the epicenter) of the earthquake can be found.
Beware of movies: The movie “10.5” had all sorts of gems about epicenters, hypocenters and seismic waves. One quote was “the s-waves are off the chart!” which is interesting because it’s not the p- or the s-waves that are the big sweeping squiggles on a seismogram. The big squiggles are from the surface waves, which come much later. The characters also became excited as they looked at seismograms shouting about side-by-side motion. Honestly, I don’t even know what that is. The characters were delighted that the hypocenter was deeper than they could measure (“sub-asthenosphere” even), which is bizarre. Read on about that.
Intensity of earthquakes is usually measured on the Richter scale, where greater numbers mean a bigger quake. The Richter scale is logarithmic, meaning that a magnitude 5 quake is 10 times as powerful as a magnitude 4 quake. It is measured in reference to how large the surface waves generated by the quake actually are. The first surface waves are usually the biggest, and then they taper off. The fault moves one time – suddenly – then stops. Aftershocks (renewed motion) might occur, but each of those come with their own seismic signature with p-waves, s-waves, and intensity.
Beware of movies: Here’s the thing: Magnitude of a quake is calculated after the quake is over. In 10.5, they’re measuring (somehow) the intensity of a quake as it is happening. What’s more, the intensity of the quake (in the movie) increases over time. That does not happen with real earthquakes!
WHY ARE THERE FAULTS AT ALL?
Obviously, something has to be driving all this compression and stretching and shearing that causes faults to exist at all. The theory of Plate Tectonics provides the best explanation for the existence of faults and the forces that drive their motion.
As mentioned in a prior Beware of Movies! post, the Earth’s surface (lithosphere – down to about 100 km depth) is broken into several plates, which move around. These plates can be divided into two categories depending upon their thickness and composition. Oceanic plates are under the oceans. They are much thinner but are made of very dense material. Continental plates are the continents, and are thicker but not all that dense (as rocks go). Some plates, like the North American plate, have parts that are continental (all of North America) and parts that are oceanic (the North American plate extends halfway across the Atlantic Ocean).
TYPES OF BOUNDARIES BETWEEN PLATES
There aren’t gaps between the plates, so something has to happen so that the plates can move. There are three general types of plate boundaries: convergent, divergent, and transform. Convergent boundaries exist where two plates are coming toward each other. Divergent boundaries occur where plates are moving apart. When plates slide past each other, we have transform boundaries.
Convergent boundaries involve compression, so it’s no surprise that faults associated with such boundaries are usually reverse faults. The nature of the boundary itself is dependent upon whether the convergence is between two continental plates, or if oceanic plates are involved. If two continental plates are converging, there will be a collision, just like when India hit Asia millions of years ago resulting in the Himalayas. The Appalachian Mountains of North America are remnants of an ancient collision between Africa and North America (which have since moved apart).
An oceanic plate can sink under another plate, resulting in subduction, where one plate overrides another. A subduction zone is like a colossal reverse fault, though we don’t generally call it as such. Subduction also results in mountain ranges, like the Rocky Mountains and the Andes. Subduction is also associated with volcanoes. The volcanoes of the Cascades and of the Andes are related to subduction.
When plates move apart, stretching and thinning of the plates occurs, along with lots of normal faults. The lithosphere gets so thin that magma comes up from the mantle (below the lithosphere) causing a line of volcanoes. When such stretching begins, especially in the middle of a continent, it is called ‘rifting.’ The East African Rift system is a prime example of this. Lake Victoria sits in the depression caused by the rifting.
At some point the rift becomes so deep that it is filled with ocean water. New oceanic crust is formed by the volcanic eruptions. This is happening in the Red Sea today. As this continues, a whole new ocean forms. The entirety of the Atlantic Ocean was once just a little rift between North and South America and Europe and Africa.
When plates slide past each other, we get transform faults. These are strike-slip faults. Sometimes there’s a bit of volcanism associated with these but usually the big activity there is earthquakes. The San Andreas Fault is a transform boundary between the Pacific plate and the North American plate.
Beware of Movies: The movie “Volcano” is based on the premise that the plate boundary between the Pacific Plate and the North American plate could spawn a new volcano, similar to those in the Cascades. The problem is that there is no subduction along the San Andreas Fault. There is subduction below the Cascades, but it’s not the Pacific Plate that’s being subducted. It’s the small Juan de Fuca Plate. The Juan de Fuca plate is a remnant of a once much larger plate (the Farallon) that has been completely subducted under North America.
WHERE DO FAULTS OCCUR?
Plate tectonics explains that most faults occur due to motions of the lithospheric plates, resulting in a limitation of where faults might be seen on the Earth’s surface. Faults also are limited to the lithosphere, or the upper 100 km or so of the Earth. The lithosphere – the crust especially – tends to deform in a brittle fashion. That is to say, if you put pressure on the rock, it will likely crack and snap. Below the lithosphere, heat and pressure are so high that rock (though it is still solid rock) deforms in a ductile or plastic fashion. It bends slowly or flows, due to individual motions of atoms. Big cracks and fissures do not exist below the lithosphere.
Beware of movies: In the made-for-TV movie “10.5,” the geologist claims that the massive earthquakes are being caused by faults existing 700 km down. Not only is that below the lithosphere, it’s in the lower mantle! Faulting cannot exist at such a depth.
The take-home message here is that earthquakes do not occur willy-nilly all over the surface of the Earth. They are most often associated with plate tectonic boundaries or mountains. There are a few that pop up in unexpected places. Some are even devastating, like the New Madrid earthquake that hit the mid-western United States in 1812. In that case, the earthquake resulted from the re-activation of an extremely ancient fault system that is no longer active, but had accumulated some stress over millions of years. I hasten to mention that the New Madrid fault is still in the crust!
Beware of movies: We don’t know where every fault is. We can’t predict earthquakes. Don’t believe it when characters in movies (like “Earthquake” or “10.5”) claim to be able to do so. It can’t be done. Not yet, anyway.