Ginger, thank you.
Coming up, more troubles for a…Earthquake!
We’re having an earthquake.
Have you ever felt an earthquake or witnessed the effects of one?
Seismic intensity, which is the shaking you experience, can be gentle or severe.
Short or long. Jerky or rolling.
Unlike Earthquake magnitude, which is a measure of the energy released and is the same for
all locations.
the seismic intensity you feel depends on where you are.
Intensity is controlled by three main factors: Magnitude: how big was the earthquake?
Distance from the hypocenter: Intensity varies from place to place, and
Local rock and soil conditions: Before we address each factor, let’s clarify
the difference between magnitude and intensity by comparing a lightbulb to an earthquake.
The lightbulb represents the location within the Earth called the hypocenter where the
earthquake begins.
The magnitude, or size, of an earthquake is like the wattage of a lightbulb.
Just as the wattage represents the amount of power of the lightbulb, the magnitude is
related to the total amount of energy released by the earthquake source.
The intensity, or shaking level, is like the amount of light from a lightbulb that is received
at any spot in a room.
A small lightbulb in one area of a room will make that area bright with high intensity
light, but it will leave the distant areas of the room dim with low intensity light.
So a given earthquake has only one magnitude but will produce different intensities of
ground shaking at different locations as shown on a ShakeMap.
But where did Shake Maps come from?After the 1906 magnitude 7.8 San Francisco earthquake,
an intensity map was created using reports by California residents.
Numbered colors represent the Modified Mercalli Intensity Scale, which reflects the effects
of an earthquake.In zone V moderate shaking was felt, houses rocked cracking plaster,
and some water tanks toppled over.
In zones VIII and IX, people, horses, and cattle were thrown off their feet, bridges
wrecked, and wood-frame buildings slid off their foundations.
The lower numbers generally dealt with how the earthquake was felt by people.
The higher numbers were based on observed structural damage.
Human observations still help scientists understand earthquake ground shaking but intensity is
now measured by seismometers and illustrated by USGS’ ShakeMaps like this modern reconstruction
for the 1906 earthquake.
Ginger, thank you. Coming up, more troubles for a…Earthquake!
We’re having an earthquake. Have you ever felt an earthquake or witnessed
the effects of one? Seismic intensity, which is the shaking you
experience, can be gentle or severe, short or long, jerky or rolling. Unlike Earthquake
magnitude, which is a measure of the energy released and is the same for all locations.
the seismic intensity you feel depends on where you are. Intensity is controlled by
three main factors: Magnitude: how big was the earthquake? Distance from the hypocenter:
Intensity varies from place to place, and Local rock and soil conditions:
Before we address each factor, let’s clarify the difference between magnitude and intensity
by comparing a lightbulb to an earthquake. The lightbulb represents the location within
the Earth called the hypocenter where the earthquake begins.
The magnitude, or size, of an earthquake is like the wattage of a lightbulb. Just as the
wattage represents the amount of power of the lightbulb, the magnitude is related to
the total amount of energy released by the earthquake source.
The intensity, or shaking level, is like the amount of light from a lightbulb that is received
at any spot in a room. A small lightbulb in one area of a room will make that area bright
with high intensity light, but it will leave the distant areas of the room dim with low
intensity light. So a given earthquake has only one magnitude
but will produce different intensities of ground shaking at different locations as shown
on a ShakeMap. But where did Shake Maps come from?After the 1906 magnitude 7.8 San Francisco
earthquake, an intensity map was created using reports by California residents. Numbered
colors represent the Modified Mercalli Intensity Scale, which reflects the effects of an earthquake.In
zone V moderate shaking was felt, houses rocked cracking plaster, and some water tanks toppled
over. In zones VIII and IX, people, horses, and cattle were thrown off their feet, bridges
wrecked, and wood-frame buildings slid off their foundations. The lower numbers generally
dealt with how the earthquake was felt by people. The higher numbers were based on observed
structural damage. Human observations still help scientists understand
earthquake ground shaking but intensity is now measured by seismometers and illustrated
by USGS’ ShakeMaps like this modern reconstruction for the 1906 earthquake. Colors represent peak ground acceleration.
That is, how quickly the ground shook and how swiftly it changed direction. The
red zones reached 1G acceleration, or 100% of gravity, a violence of shaking illustrated
in this shake-table experiment. In the green zone, moderate shaking occurred with accelerations
up to 10% of gravity. Let’s examine how intensity of ground shaking
is controlled by earthquake magnitude. We understand that a one hundred-watt lightbulb
produces more light everywhere than a 25-watt lightbulb. Similarly increasing earthquake
magnitude also means greater intensity of shaking everywhere. To see this principle
at work, let’s examine two earthquakes that occurred at the same depth beneath Kumamoto,
Japan but had different magnitudes. On April 14, 2016, a magnitude 6.2 earthquake generated
strong shaking up to forty km from the epicenter. The next day, a magnitude 7.0 earthquake released
twenty times as much energy in the same location. Strong ground shaking extended to one hundred
km from the epicenter. The larger magnitude earthquake “lit up” the surrounding area
with stronger ground shaking just like a larger wattage lightbulb casts brighter light everywhere
in a room Let’s now examine how intensity of ground
shaking is controlled by distance from the earthquake. This ShakeMap for a magnitude
7.0 earthquake in Mozambique nicely illustrates a bullseye map pattern with intensity decreasing
with distance along the surface away from the epicenter just like the light from a lamp
is less bright the farther away we are from the lamp. By analogy, we expect shaking to
decrease as distance increases away from the earthquake.
Remembering that earthquakes occur in the Earth, not on Earth’s surface, let’s consider
the effect of earthquake depth on ground shaking intensity. To see this principle applied to
earthquakes, let’s examine two earthquakes of the same magnitude with nearby epicenters
but with hypocenters at different depths. In 2015, the magnitude 7.5 Hindu Kush earthquake
occurred 212 km below Afghanistan. Although strong shaking was distributed over a broad
area only 399 lives were lost. The hypocenter of the 2005 magnitude 7.5 Kashmir earthquake
was only 23 km deep. Severe ground shaking was concentrated near the epicenter causing
extensive damage to buildings with over 87,000 fatalities.
In 1989, the magnitude 6.9 Loma Prieta earthquake occurred on the San Andreas Fault system near
Santa Cruz, California. Ground shaking intensity and resulting damage was strongly influenced
by local rock and soil conditions. In fact, the most severe damage occurred in
Oakland and San Francisco’s Marina District, 100 kilometers from the epicenter, where
violent intensity IX shaking was greater than that experienced by many areas closer to
the epicenter. Both are built on mud substrate. A dramatic and tragic example of shaking amplification
is the double-decker Nimitz Freeway, constructed on Bay Mud. Four seconds after drivers felt the earthquake,
the Nimitz Freeway shook and swayed, and sections collapsed taking 42 lives. Seismograms from nearby stations
document that seismic waves are amplified by a factor of two as they pass from bedrock
to sand and gravel deposits and the amplitude of ground shaking of soft and water-saturdated
Bay Mud is five-times larger than on bedrock. The section of the Nimitz Freeway that collapsed
was built on Bay Mud. The Marina District was constructed on loosely
compacted Bay Mud. In addition, debris from the 1906 earthquake had been dumped into the
Bay to make more land for city expansion. This weak ground experienced both amplification
of ground shaking and liquefaction of water-saturated sediment that caused differential settling
of buildings, and ruptured gas lines resulted in major fires.
So the soil and rock that your house is built on has a major influence on earthquake intensity
and potential damage due to ground shaking. Following the earthquake and during aftershocks
over 1,000 landslides were reported mostly near the epicenter in the steep terrain of
the Santa Cruz Mountains and along rocky bluffs of the Pacific Ocean.
In closing, If you feel the bump of an earthquake, it is important to know that the first waves
may not be the strongest. Taking immediate actions to Drop, Cover, and Hold On, before
strong shaking arrives. can save lives and reduce the risk of injury.