We know the Earth shakes during earthquakes, and we know that waves on the Earth’s surface, produced by earthquakes, can cause great damage. But how do these waves move from the hypocenter, or focus—the earthquake’s point of origin at depth—to the surface, and what are the different types of waves?
Earthquakes, which start when pressure is released in the subsurface as slabs of rock slide past each other, create two categories of waves—body waves and surface waves. Body waves travel beneath the surface, and surface waves, unsurprisingly, travel on the surface.
You may have heard of the two types of body waves, p-waves and s-waves. The P in p-waves stands for primary, because these are the first waves to reach the surface; they’re faster than s-waves. The S in s-waves stands for secondary. Let’s take a closer look at each type of body wave and see why it’s helpful for us to know that they travel at different speeds.
P-waves are compressional waves. Their longitudinal natures means that the wave motion—the oscillation—moves in the same direction as the wave itself. Particles in the path of the wave alternate between compressing (squeezing together) and dilating (spreading apart). P-waves can travel through matter in any state—solid, liquid or gas. P-waves reach the surface first because they travel between 1 and 14 kilometers per second, depending on the substance through which they are propagating.
S-waves are shear waves. Their transverse nature means that the wave motion is perpendicular to the direction of wave propagation. In other words, the wave movement can be thought of as up-and-down or side-to-side or any direction perpendicular to the wave’s trajectory. S-waves cannot travel through fluids because liquids and gases cannot be sheared. Slower than p-waves, s-waves move at speeds between 1 and 8 kilometers per second.
Because the two types of waves travel at different speeds, seismologists working from at least three geographic points at the surface can figure out the epicenter of the waves’ origin (the epicenter is the point on the surface directly above the earthquake’s focus).
One of our experts demonstrates the motion of the waves and the processes for monitoring and assessing earthquakes in this video.
Transcript
Earthquake Fundamentals – Cliff Frohlich – The University of Texas at Austin
Earthquakes are a common natural occurrence in many parts of the world. Recently some areas that are typically quiet, like Texas, have experienced increased earthquake activity. Some researchers believe these are induced earthquakes that can be attributed to industrial operations. Let’s take a look at natural earthquakes as a primer to understanding both natural and induced seismic events.
To better understand earthquakes we need to consider several topics, the types of seismic waves generated by earthquakes, how to detect these waves, how to determine magnitude and how to find the earthquake’s focus. Focus is a location within the earth, where the earthquake originates. The epicenter is the point on the earth’s surface directly above the focus, or its location on a map. Earthquakes produce elastic waves that travel away from the focus. These waves may be felt by people and measured by scientific instruments.
Let’s look at three types of seismic waves and what we can determine about an earthquake from them. There are two main categories of waves, body waves and surface waves. Body waves travel through the interior of the earth, while surface waves travel along the earth’s surface, like ripples on a pond. Surface waves often cause the most damage for large earthquakes, but body waves are more useful for finding the focus and then the epicenter of an earthquake.
There are two types of body waves in a solid, compressional, or P waves and shear, or S waves. P waves are the fastest kind of waves. They can move through solid rock and through liquids. S waves are slower and don’t exist in liquids. All waves oscillate, or cause back and forth motion as they travel through a material, but the oscillations for P and S waves are different. The motion of P waves is longitudinal, that is as a P wave passes the back and forth motion is along the same direction the wave travels, by contrast an S wave causes motion side to side in a transverse direction, perpendicular to the direction the wave travels.
A simple way to demonstrate this is with a Slinky. If I squeeze some coils together like this and let them go we get a longitudinal, or P wave. The wave travels this direction and the motion in the Slinky coils is this direction. Let’s try that again, but if I pull a coil down and let it go we get an S wave, or a transverse wave. The wave travels in this direction, but the motion in the Slinky is perpendicular.
How do we detect seismic waves? Most people have seen early seismographs, where a rotating drum records vibrations with ink and paper. Today’s seismographs are digital and often connected to computer networks, so seismic data from different locations can be quickly analyzed. Besides charting the amplitude of seismic waves the seismometer records when each wave arrives. This is very important information, because it can help us figure out where the wave originated.
P waves move faster than S waves. Both types originate at the earthquake’s focus, at the same moment. This means we can use the gap in their arrival times at a seismograph station, to calculate the distance between the earthquake origin and the station. How? Think of two cars starting from the same unknown location, that would be the earthquake’s focus and traveling to the same known destination, that would be the seismic station. We know they left at the same time, but we don’t know where they started from. We know the blue car travels at 60 miles per hour and the red car travels at 30 miles per hour. The red car arrives 15 minutes after the blue car. Can you figure it out? Solving this finds that the distance each car traveled was 15 miles.
Back to our earthquake. Now, we know the epicenter is 15 miles from our seismometer, but 15 miles in which direction? One way to answer this is to draw a circle around the seismograph station with a radius of 15 miles. If we have measurements from at least three stations, in three different locations, we can overlay those circles. The place they meet reveals the epicenter of the earthquake. Today we use computers to solve for the location, but it is P and S arrival times that constrain the location just the same.
Now that we’ve located the epicenter we need to determine the strength, or magnitude of the earthquake. Although scientists determine magnitude using several different methods, or scales all of them depend on measuring the maximum amplitudes of waves recorded on seismographs. To find the magnitude they measure the amplitude, take its logarithm then adjust for differences in distances between the seismographs and the epicenter, because of this distance adjustment magnitude is a measure of the strength of the earthquake at its origin.
The Richter scale, developed in the 1930s, is the most well known scale for magnitude. It and all magnitude scales are logarithmic. This means that an earthquake with magnitude four is 10 times as strong as a magnitude three, or a hundred times as strong as a magnitude two. A two, or smaller magnitude earthquake is usually called a microearthquake. People generally don’t feel microearthquakes and their record is typically limited to local seismographs.
Understanding these qualities about all earthquakes informs the conversation about the potential for induced seismicity, in a given area. Collecting data from seismic events can pinpoint an earthquake’s timing, location and magnitude. Analyzing data from many events helps to establish a record and a base line for naturally occurring earthquakes, which in turn can be used to evaluate induced seismic events.
What waves cause most of the destruction associated with earthquakes? Although slower than body waves, surface waves are the typical culprit of earthquake damage because their particle motion is larger compared to body waves.
The interaction of P and S waves at the surface produces Rayleigh waves. These surface waves are similar to ocean waves in that they roll along the surface, and exhibit both compressional and transverse motion. Rayleigh waves produce most of the damage caused by earthquakes.
Love waves are faster than Rayleigh waves, move the ground horizontally during an earthquake and are only observed in special geological situations.
Images: “Waveform” by Bro Studio via Shutterstock