Earthquakes are the result of uncontrollable geologic forces. Their effects can range from
gentle, even unnoticed motions, to devastating and violent ground shaking. Few of us have
experienced such shaking, but we have all heard news reports of the damage that results
from ground motion or secondary phenomena such as liquefaction or triggered tsunamis.
The science of seismology has begun to answer some fundamental questions about these events.
- Why do we have earthquakes?
- How do earthquakes and related phenomena happen?
- What causes the ground to shake? And:
- How do we measure earthquakes?
To address these questions, we'll take a look at the roles of Plate Tectonics, Mechanics,
Seismic Waves, and Magnitude and Intensity.
To understand why we have earthquakes we need to step back for a big picture view of the
entire Earth as a system. To understand the role of Plate Tectonics in this system, we
can divide the Earth into four main compositional layers: the crust, mantle, outer core, and
inner core.
Convection currents are generated as part of the Earth's cooling process, with heat
rising and escaping to the surface, and cold material sinking.
The earth's solid inner core spins slowly, cools, and radiates heat.
The liquid outer core convects, generating the earth's magnetic field, and transferring
heat to the mantle.
Convection of the mantle is driven by cooling forces from above, and heating from below.
Heat is produced from within by the decay of radioactive elements.
A rigid cooled outer layer of the earth, consisting of the crust on top of a thicker layer of
stiff cooler mantle, forms what we call the Lithosphere.
The convection currents exert push and pull forces that break the Lithosphere into mobile
sections we refer to as tectonic plates.
Earthquakes rupture the active boundaries of these plates as they grind past, under,
and away from each other.
This simulation shows the last 200 million years of global plate tectonic motions. Notice
how the plates break apart and move about, eventually arranging themselves into the continents
as we recognize them today.
At Transform boundaries, plates slide past one another and form strike-slip faults.
At Convergent boundaries, continents collide and form mountains, or one plate slides beneath
another and forms a subduction zone.
At Divergent plate boundaries, plates move apart and form rifts within continents or
mid-ocean ridge spreading centers in the oceans.
To understand how earthquakes happen, we must examine the mechanics of the forces and motions
affecting plates and their interactions with one another.
You are viewing two oceanic plates moving away from a spreading center. In cross-section,
we can better see the convergence of plates at subduction zones, and the divergence of
plates at a mid-ocean spreading ridge or a rift valley.
Although the plate motions are generally smooth, the Mechanics of earthquakes arises from the
friction at the plate boundaries. Faults, held by this friction, remain stuck for long
periods of time and then suddenly break in major earthquakes.
There are three main types of faults that are associated with the three types of plate
boundaries.
Normal faults are generally found in divergent plate boundary zones, where the plates are
pulling apart from one another due to extensional forces. They release less energy or cause
the weakest shaking of all the fault types.
Thrust or reverse faults are generally found in convergent plate boundary zones, where
plates collide with one another and experience compressional forces. These faults cause the
largest earthquakes.
Strike-slip faults are generally found in transform zones, where the tectonic plates
slide horizontally past each other creating parallel shear forces. The San Andreas fault
is an example of a strike-slip fault.
If a subduction zone earthquake occurs along an underwater thrust fault, the associated
uplift can displace a huge volume of water. The resulting wave is known as a "tsunami."
Earthquakes on any type of fault follow a similar pattern. When stress reaches the breaking
point on the fault, a rupture begins at the hypocenter, and continues to break along the
fault surface. The epicenter is the geographic location at the surface of the earth directly
above the hypocenter.
To grasp what causes the ground to shake we need to understand the propagation of Seismic
Waves. Just as throwing a rock into a pond causes radiating ripples, rupturing of a fault
in an earthquake causes a transmission of energy in the form of seismic waves traveling
away from the hypocenter and epicenter.
Earthquakes produce many types of seismic waves. Body waves travel through the earth,
and surface waves travel along the Earth's surface. It is the arrival of seismic waves
that people feel when they experience shaking in an earthquake.
Body waves can be broadly classified in two categories.
The P wave, or primary wave, is a compressional body wave that alternately compresses and
expands the particles that it moves through within the Earth. P waves can pass through
any medium, and so travel through every layer of the earth, and even the air like sound
waves.
The S wave, or secondary wave, is a body wave that travels through the interior of the earth
with a shearing motion. S waves cannot travel through liquids, including the Earth's liquid
outer core.
There are likewise two kinds of surface waves.
Rayleigh waves have energy that causes the ground to roll up and down, like water waves
on the ocean.
Love waves move the ground side to side and are similar to shear waves, but can have larger
amplitudes.
Seismic waves carry the energy from the earthquake source to the surface where people can feel
the shaking.
Methods for measuring earthquakes quantify earthquakes and their effects. The magnitude
scale measures the size of different earthquakes, independent of where the seismic waves were
recorded. The larger the magnitude, the larger the offset and the area of fault that moved
in the earthquake. Also, for large earthquakes the shaking is stronger and lasts for a longer
time than for small ones.
Each whole number step in earthquake magnitude represents an increase in amplitude of ground
motion by a factor of ten. In a magnitude 6.0 earthquake, for instance, the ground shakes
10 times as much as in a magnitude 5.0, and in a magnitude 8.0, the ground shakes 1000
times stronger than in a magnitude 5.0.
In terms of energy, the increase is even greater, with each step in magnitude having approximately
32 times the energy of the previous. For instance, a magnitude 6.0 releases about 32 times the
energy of a magnitude 5.0, and a magnitude 8.0 releases around 32,000 times the energy
of a magnitude 5.0.
There is no upper limit on magnitude, however the largest recorded earthquake in history
was a magnitude 9.5.
Earthquake magnitude does not vary from place to place but is a characteristic of the total
energy released by a particular earthquake.
It is also helpful to use an intensity scale to quantify ground shaking due to the earthquake.
The shaking intensity can vary depending upon distance from the hypocenter and local geology,
and is a better measure for assessing damage risk.
This is the ShakeMap for the magnitude 6.7 Northridge earthquake, which occurred on January
17th, 1994. The areas that experienced strongest shaking are in warm colors like red, versus
areas of mild shaking in cooler colors like blue and green. Ground shaking recorded by
seismic sensors is used to make the ShakeMap.
We can also make subjective Intensity maps based on reports from local people from each
area through online resources like the "Did you feel it?" USGS webpage.
We have earthquakes because tectonic plates grind past, under, and away from each other
and generate earthquakes at their active boundaries.
Through seismological research we can better understand and measure the seismic waves traveling
from earthquake sources that cause ground shaking and potentially preventable damage.
As much as we know about earthquakes, there is still much more to learn through continued
earthquake research.