What will the shaking feel like?
Magnitude is a measurement of the energy produced by the earthquake but is not enough to predict what you feel during an event. What you feel is very complex -- hard or gentle, long or short, jerky or rolling -- and not describable with one number. Aspects of the ground motion are described by the peak velocity (how fast the ground is moving), peak acceleration (how quickly the speed of the ground is changing), frequency content (energy is released in waves and these waves vibrate at different frequencies just like sound waves), and duration (how long the strong shaking lasts).
Three factors are the most important to determine what you feel in an earthquake. These are:
- magnitude
- distance from the fault
- local soil conditions
Magnitude
At a given distance from a fault, you will generally feel more intense shaking from a big earthquake than from a small one. Big events also release their energy over a larger area and for a longer period of time.
An earthquake begins at a hypocenter, and from there the rupture front travels along the fault, producing waves all the time it is moving. That rupture front cannot travel faster than the speed of sound in rock -- about four miles (~6 km) per second because this is the first possible earthquake trigger. Every point crossed by the rupture front gives off shaking, so longer faults can produce bigger earthquakes that have longer durations. You can therefore use the duration to guess the magnitude of an earthquake you feel.
The actual durations (how long the Earth gave off energy) for 15 Nevada and California earthquakes are shown on page 30. For a magnitude 5 event, the actual process of rupturing the fault is over in a few seconds, although you might continue to feel shaking longer because some waves reach you after they bounce and echo within the earth.
The magnitude 7.8 earthquake on the San Andreas Fault in California (labeled Fort Tejon) in 1857 ruptured almost 230 miles (370 km) of the fault. At 2 miles (~3 km) per second, it took two minutes for that length of fault to rupture, so you would have felt the shaking for several minutes. If you are close to the fault, only 10 to 20 seconds of shaking originates from the part of the fault nearest you and will be very strong. Many of the rest of the waves you feel will be traveling from as far as 200 miles (~320 km) away.
Each cycle of shaking stresses buildings and can add to the damage. Because this is cumulative, the most damage happens at the very end of an earthquake.
Distance
Earthquake waves die off as they travel through the Earth, so earthquake shaking becomes less intense farther from the fault.
Low-frequency waves die off less rapidly with distance than do high-frequency waves (just as you can hear low-pitched noises from farther away than you can hear high-pitched noises). If you are near the earthquake, you will experience all the frequencies produced by the earthquake and feel "jolted". Farther away, the high frequencies will have died away and you will feel a rolling motion.
The amount of damage to a building does not depend solely on how hard it is shaken. Different structures respond differently to the various frequencies. In general, smaller buildings, such as houses, respond more to higher frequencies, so closeness to the fault is a very important factor. Larger structures, such as bridges and high-rise buildings, are more responsive to lower frequencies and will be more noticeably affected by the largest earthquakes. The shaking dies off with distance more quickly in Nevada than in the older, more rigid crust of the eastern United States (we are lucky in that respect), but Nevada is about the same as most regions that have large numbers of earthquakes.
Soil Conditions
Certain soils greatly amplify the shaking in an earthquake. Just as sound carries differently in water than in air, seismic waves travel at different speeds in different types of rock and soil. Passing from rock to soil, the waves slow down but get bigger. A soft, loose soil will shake more intensely than hard rock at the same distance from the same earthquake. The looser the soil is, the greater the amplification will be. An extreme example of this type of amplification was in Oakland and San Francisco during the 1989 Loma Prieta earthquake. That earthquake was 60 miles (~100 km) from San Francisco, and most of the Bay Area escaped serious damage. However, some sites in the Bay Area on soft soils experienced severe shaking, exemplified by the collapse of the elevated Nimitz freeway in Oakland and many homes and apartments in the Marina District in San Francisco. Ground motion at those sites was more than 10 times stronger than at neighboring sites on rock.
Other Factors Affecting Shaking
Several other factors can affect the shaking. Earthquake waves do not travel evenly in all directions from the rupture surface; the orientation of the fault and the direction of slip will change characteristics of the waves in different directions. This is called a radiation pattern. When the earthquake rupture moves along the fault, it focuses energy in the direction it is moving so that a site in that direction will receive more shaking than a site at the same distance from the fault but in the opposite direction. This is called rupture, or ground motion, directivity.
The valleys or basins that many of our communities are in also can modify earthquake waves in many ways, including collecting and amplifying waves from distant large earthquakes, sometimes to damaging levels. These are called basin effects.
The Quake Lab
Take a plastic bowl of Jell-O and a block of wood. Hit each one gently with a rubber hammer. The block of wood sits there but the bowl of Jell-O slops back and forth. The softer material amplified the shaking.
Back to Top | Previous | Next
