Earthquake: A vibration caused by the
sudden breaking or frictional sliding
of rock in the Earth.
Fault: A fracture on which one body of
rock slides past another.
Focus: The location where a fault slips
during an earthquake (also called the
hypocenter).
Epicenter: The point on the surface of
the Earth directly above the focus of
an earthquake.
Fault trace: The intersection between
a fault and the ground surface.
Footwall: The rock mass that lies
below the fault plane.
Hanging wall: The rock or sediment
above an inclined fault plane.
(b) and (c) are showing 2 epicenters
and foci.
View of fault trace after 1906 SF
earthquake. Note that the
fracture is discontinuous.
Normal fault:
fault: hanging-wall block moves down the slope of the fault. Due to extension.
Reverse fault:
fault: A steeply dipping fault on which the hanging-wall block slides up.
Thrust fault:
fault : A gently dipping reverse fault; the hanging-wall block moves up the slope of
the fault.
Strike-slip or transform fault:
fault : one block slides horizontally past another, so there is no
relative vertical motion. Can be left lateral or right lateral.
10_07.jpg
Blind fault: fault that does not come to the surface.
Normal fault in South-central Texas
Reverse fault
Thrust fault north of Knoxville, TN
Strike-sip faults can have motion that is left lateral, or right lateral. This
means that if you were standing on one side of a fault, and the other side
moves to the left, than the fault is left lateral. Reversely, if the other side
of the fault moves to the right, then the fault is right lateral. It does not
matter which side of the fault you stand on. The motion will still be the
same.
Offset of this fence was caused
by the 1906 SF earthquake.
Note that the fence suggest
that the fault was right lateral in
movement.
Road is offset by 20 feet during the 1906 SF earthquake. Again note that
it is a right lateral sense of movement.
This shows the location of earthquakes on the surface of the earth. Note
that there is a difference between deep and shallow earthquakes. This is
due to the tectonic setting of the earthquakes.
Note that, with some
exceptions, the
locations of
earthquakes coincide
with the location of
plate boundaries.
Deep focus
earthquakes occur
along Wadati-Benioff
zone at convergent
plate boundaries (A
boundary at which two
plates move toward
each other so that one
plate sinks (subducts)
beneath the other; only
oceanic lithosphere can
subduct).
Wadati-Benioff zone: A
sloping band of
seismicity defined by
intermediate- and deepfocus earthquakes that
occur in the downgoing
slab of a convergent
plate boundary.
These types of
earthquakes occur in
the Pacific Northwest.
Oceanic transforms
and mid-ocean
ridges are common
locations for shallow
focus earthquakes.
mid-ocean ridges: A
submarine mountain
belt that forms along
a divergent oceanic
plate boundary.
transform plate
boundary: A
boundary at which
one lithosphere
plate slips laterally
past another.
divergent plate
boundary: A
boundary at which
two lithosphere
plates move apart
from each other.
And as we
discussed,
volcanic activity,
here shown as a
result of one plate
subducting under
another, has
shallow focus
earthquakes
associated with it.
Earthquakes that occur within plates occur along transform plate boundaries
(such as the San Andreas fault), in areas of active extension (such as the
western US basin and range an rift zones such as the east African rift) and in
collisional zones (such as the Himalayan mountains).
Why do earthquakes occur?
•They occur as stress built up on
opposite sides of a fault become
greater than the friction between the
two fault surfaces.
•The stress, in most cases, is due to
the relative motion of tectonic plates
past one another.
•We tend to think that constant motion
occurs across plate boundaries;
however, this is not the case. Usually
faults are inactive, causing the stress
to builds up due to the lack of motion.
•Faults that move all of the time are
said to creep. Earthquakes that occur
along these creeping faults are small,
and cause little damage.
We tend to think of faults as sharp, well defined lines. But in reality
they tend to be broad zones or systems of faults, that splay off of a
central fault. The San Andreas is a good example of this idea.
P-waves: primary waves. Compressional waves.
S-waves: Secondary waves. Shear waves.
R-waves (Rayleigh waves) or surface waves: cause ground to ripple
up and down.
L-waves (Love waves): cause ground to ‘snake’ from side to side.
R- and L-waves die out at
depth. This, however, is
not the case for P- and Swaves.
P- and S-waves will travel
throughout the entire
earth. There is a P-wave
shadow zone in the earth.
This is caused by the
refraction of P-waves
through the earth’s core.
The S-waves shadow zone
differs from the P-wave
zone because the physical
properties of S-waves do
not let them travel through
liquids. Therefore, they
cannot penetrate the liquid
outer core of the earth.
The inner core is solid.
The different physical properties of waves produced by earthquakes,
cause them to travel at different rates. P = fastest, S = moderate, surface
= slowest.
The different speed, or arrival times of earthquake waves, allows for the
creation of seismographs (an instrument that can record the ground motion
from an earthquake). Seismographs record the duration and intensity of
earthquakes.
Seismographs are usually fixed to
bed rock. They can record both
vertical (a) and horizontal (b) ground
motion.
10_16e.jpg
After an earth quake
occurs, seismographs
can be used to tell how
far away the epicenter is
located (use the velocity
of the waves, and the
duration of the quake,
and this gives a distance).
However, it is impossible
to tell what direction the
earthquake came from.
Epicenters are found
using the intersection of
three (or more) distances
measured from
seismographs
Richter scale, named after Charles Richter, is commonly used to
determined earthquake magnitude. This scale gives the largest ground
motion 100 km away from the epicenter of an earthquake.
Richter magnitude is
determined by first calculating
the interval between the S- and
P-waves, and then determine
the amplitude of the largest
wave produced by the
earthquake. A line is drawn on
special graph paper between
these two points, and where
this line intersects the
magnitude scale, can be read
off. Thus giving us the
magnitude of an earthquake.
The insert graph displays that as
the magnitude of the graph
increases, the number of quakes
decrease. This means that there
are more magnitude 1
earthquakes, than magnitude 8.
the larger graph shows that the
energy released by earthquakes
increases greatly with increasing
magnitude.
A second scale for earthquakes is the Mercalli intensity scale. This scale
measures the amount of damage that an earthquake causes. I causes little
damage, while XII =‘s total destruction. This scale is easier to use when
dealing with historical earthquakes.
Location of some historically significant earthquakes within the lower 48
states.
Ground shaking can cause sand to become mobilized, and rise up in a row
of sand volcanoes. It the sand layers are between mud layers, can become
disrupted and broken.
Sand volcano, 1979, El Centro, California
The sand becomes mobilized
due to water in the pore space.
Just as we discussed before, the
pore space opens up during
shaking caused by the quake,
and the sand grains no longer
touch each other.
Liquefaction: the process that
occurs when the motion of an
earthquake causes clay-rich
sediment to become a slurry of
clay and water. The sediment is
clay-rich, meaning it contains
clay, but is not only made up of
clay.
Liquefaction caused the collapse of Union St after 1906 quake in SF
Loma Prieta, California, Earthquake
October 17, 1989. Watsonville area.
Ground shaking triggered
liquefaction in a subsurface layer of
sand.
Secondary cracks on filled land
on Bluxom Street after the 1906
SF earthquake. Note the
devastation in the background.
This area had been developed
on fill that was placed in the
bay. When the quake occurred,
the sediment lost all of its
cohesion.
Landslide was triggered by 1906 earthquake.
Loma Prieta, California, Earthquake October 17, 1989. San Francisco
and San Mateo County Coast. Large slide at Daly City. This is the
largest slide encountered in San Mateo County. The base is about 152
meters (500 feet) across at its widest point, and it displaced
approximately 36,700 cubic meters (48,000 cubic yards) of material.
Here are some geological signs that indicate earthquake activity. 1) disrupted
bedding, 2) off set soil horizons, 3) sand volcanoes, 4) tilted trees, with
asymmetric tree rings, also buried and more recent faults.
Truncated ridges, with triangular escarpments are good indicators of normal
faults.