Students apply their knowledge of seismic waves to design seismograph prototypes that detect and record high- and low-amplitude seismic surface waves.
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How Seismic Waves Travel
Body waves from an earthquake have a higher frequency than surface waves, and they arrive at a seismograph before surface waves. Seismographs are devices with a sensor to detect the ground’s motion along with a recording system that graphs the seismic wave patterns. P-waves are the fastest kind of seismic wave, so they are first to reach a seismograph. They can travel through solids and liquids. They often travel upward through Earth’s layers, shaking the ground in a vertical motion.
S-waves are slower than P-waves, and they can only move through solids, not liquids. Because S-waves are transverse waves, they shake the ground back and forth perpendicular to the direction the wave is moving. Surface waves have a lower frequency than either kind of body wave, and they reach the seismograph last. They move the ground in all directions, and are responsible for most of the damage caused by earthquakes.
Discovering Earth’s Liquid Outer Core
By monitoring arrival times of seismic waves throughout the planet, scientists discovered that S-waves do not reach the opposite side of Earth after an earthquake. They concluded that there must be a liquid outer core.
This was an important discovery because the deepest well ever drilled was 12 kilometers. Earth’s radius is 6,370 km, so the only way to find out the details of Earth’s interior comes from monitoring seismic waves and drawing conclusions. Scientists also concluded that Earth’s crust is made of less dense matter than the mantle.
Measuring an Earthquake’s Energy
A seismograph includes a sensor that detects the ground’s motion and a recording system. Most seismographs have three separate elements so they can capture Earth’s movement in three directions: up-down, north-south, and east-west. A simple seismograph has a frame with a movable arm of some kind. The arm is attached to a pen or other recording device and moves whenever the ground moves. The pen, which is attached to the arm, records the relative motion between itself and the rest of the instrument, which is the motion of the ground.
Many modern seismographs use electronics to measure seismic waves. The scientists in Southern California used more than 5,000 highly sensitive seismographs. The surprisingly deep earthquakes they measured were microquakes, having magnitudes of no more than 2 on the Moment Magnitude Scale. This scale goes from 0-9+ depending on the amount of energy an earthquake releases.
The quakes were so deep that typical seismic sensors closer to the surface didn’t detect them at all. Scientists still don’t know the implications of these deeper earthquakes. One possibility is that the Newport-Inglewood fault could produce a larger earthquake than scientists have predicted.
Another possibility is that deep earthquakes remain small and isolated and so wouldn’t affect surface earthquakes. Scientists will continue to research these questions because many people would be affected by a massive earthquake. The Newport-Inglewood Fault runs underneath an area of Southern California where many people live. And scientists have predicted that the fault could produce an earthquake of a magnitude 7.4. An even larger earthquake would cause significantly more damage to the structures on Earth’s surface, including the homes, office buildings, roads, and bridges in the area.
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