Explain how the epicenter, focal mechanism and slip vector of an earthquake are
An Earthquake focal point is determined by drawing a map of the area, marking the
seismometers (at least three) and then drawing an arc of a circle of radius r1 about
seismometer 1, another radius r2 about seismometer 2 and another radius r3 about
seismometer 3. Then focus of the earthquake is
then at the intersection of the three arcs. The
epicenter is the point vertically above the focal
point which is determined by Latitude and
Longitude. The shortest distance on the surface of
a sphere between any two points on the sphere is
along the great circle which intersects the two
points. The epicenter distance is therefore the
length of this great-circle arc and is usually quoted
in degree except for local earthquake studies, in
which case it would be quoted in kilometers. Also,
calculated time between P and S wave arrival times recorded by the seismogram can be
used to get distance to epicenter
The direction of slip and orientation of an Earthquake is called the focal mechanism.
Information from seismograms are used to calculate the focal mechanism and is usually
showed on a map as a “beach ball” symbol. The beach ball represents that projection of a
horizontal plane of the lower half of an imaginary, spherical shell (focal sphere)
surrounds the earthquake source. A line is drawn where the fault plane intersects the
shell. The orientation of the stress-filed during the time of rupture determines the vector
of slip on the fault plane, which is shown on the beach ball drawing. In the drawing to
the right, the gray quadrants contain the tension axis (T),
which reflects the minimum compressive stress direction and
the white quadrants shows the pressure axis (P), which
reflected the maximum compressive stress direction.
The direction of P-first motion observed at a station is used
to determine focal mechanisms. However, depending on the
distribution and quality of first motion data more than one
focal mechanism solution may fit the data equally well.
Identification of fault plane on which slip occurred is harder
to identify for mechanisms calculated from first-motion
directions, due to the fact that slip occurred from the
orthogonal, mathematically equivalent, auxiliary plane. Example B in the schematic
above illustrates this fact.
The fault plane responsible for the earthquake will be parallel to one of the nodal planes
of an Earthquake mechanism plot (beach ball plot), the other being called the auxiliary
plane. Unfortunately it is not possible to determine which of the nodal planes the fault
plane is in fact. For this other geological or geophysical evidence is needed to remove the
ambiguity. The slip vector along the fault lies within the fault plane, 90 deg from the N-
2. How would you distinguish between Love waves and Rayleigh waves if you were
given an earthquake record from a WWSSN station?
Rayleigh, also known as “ground roll” waves occur close to the surface of a semi-infinite
medium. They are generated by the interaction of
P- and S- waves at the surface of the earth. Their
velocity is lower than the P-, S- and Love wave
velocities. Rayleigh waves travel about ten (10)
times the speed of sound in air. The particle motion
of these waves is confined to a vertical plane
containing the direction of propagation. They are
recorded both by the vertical component and by the
horizontal component of the WWSSN station seismometer. The amplitude of Rayleigh
waves decreases rapidly with depth of the earthquake. Earthquakes may generate
Rayleigh waves that travel several times around the world before dissipating. In contrast,
Love waves occur when there is a general increase of S-wave velocity with depth. They
propagate by multiple internal reflections of horizontally polarized S-waves (SH-waves)
in this near-surface medium and thus propagate in a
waveguide. The Love-wave particle motion is
transverse and horizontal, so they will be only
recorded by the WWSSN station horizontal
seismometer. Love waves take a log time to dissipate
due to their high energy content. This makes Love
waves most destructive in the immediate area of the
focus or epicenter of the earthquake.
3. a) Which types of seismic waves can propagate in an unbounded solid medium?
Body waves travel through unbounded solid medium.
b) Which types of seismic wave can be detected by a vertical-
A vertical seismograph will only record a horizontal S wave because the vertical
components of such waves vibrate vertically. The vertical seismograph will not record a
horizontal P wave approaching it as such a wave would be vibrating back and forth
horizontally in the direction of propagation and therefore, would not be recordable on a
vertical seismograph. This is because vertical seismographs only records vertical motion.
Most seismometers detect ground motion using a
mass which is suspended in some fashion by a
spring. When the ground moves (say in an up and
down or vertical motion), the seismometer's frame
also moves, but the mass tends to remain relatively
steady because of inertia. The relative motion of the
mass with respect to the frame (and thus the ground)
is then converted to an electrical signal (voltage
variations with time) using a capacitor plate, a
galvanometer or a magnet and coil assembly.
Seismometers are usually sensitive to one
component (direction) of ground motion (vertical or
horizontal) and thus three components are used to completely characterize the seismic
signals. A typical vertical component seismometer design is illustrated in Figure 1.
4. Contrast the distribution of earthquake foci in and around the Atlantic and
Pacific Oceans and describe the mechanism of earthquakes on the mid-ocean
ridges. What light do these facts through on the plate?
Earthquake distribution in both Atlantic and Pacific Ocean follow active plate
boundaries. These earthquakes are as a result of plate movements. Most earthquakes in
the Pacific Ocean occur in the so called “Ring of Fire”. This is an area of 40,000 km, half
circle shaped associate with volcanic
arcs, oceanic tranches, and volcanic belts.
This place is the location of 75% of the
earth’s active and dormant volcanoes.
80% of the earth’s largest earthquakes
and 90% of all earth quakes occur at this
location. As illustrated by the figure the
right, the earthquakes in the pacific range
in depth from 70 to 800 km
(www.wikipedia). As illustrated by the
picture to the right, most deep
earthquakes in the pacific occur along the
Australian, Philippine, Cocos and Nacza plate boundaries. In contrast the Atlantic Ocean
earthquakes are mostly associated with the mid-oceanic ridge. These earthquakes tend to
be shallower in the range of 0 -70 km (see picture above). These earthquakes show that in
convergent plate boundaries such as the boundaries that surround the Pacific plate,
earthquakes tend to be deeper and larger in magnitude. In contract, at divergent plate
boundaries such as the Atlantic Mid-Oceanic ridge, earthquakes there tend to be
shallower and lower in magnitudes.
5. How much greater is a nuclear-explosion body wave amplitude likely to be than
an earthquake body-wave amplitude if both waves have the same Ms (Surface Wave
Magnitude) value and are recorded at the same distance?
Nuclear explosions are less efficient at generating surface waves than are earthquakes
with the same body-wave magnitude. In general, nuclear explosion body waves have Mb
values of 1.0 – 1.5 units greater than those for Earthquakes with the same Ms Values.
6. How do seismologists differentiate between earthquake and nuclear
In order to differentiate a nuclear exploration from an earthquake one must consider
parameters such as location, depth and shape of the waveforms. Nuclear explosions have
all been shallower than 2 km. Thus an event with focal depth of 200 km is most likely not
a nuclear explosion. The first P-wave form from an explosion to arrive at any seismic
station should be compressional, where as the first arrival from an earthquake will be
either dilatational or compressional, depending upon the azimuth.
In addition, nuclear explorations are compressional source and generate strong P-waves
in all directions with little shear energy (S-waves and surface waves). In contrast,
earthquake energy is produced by a slippage on a fault surface thus the first motion can
be both compressional and extensional.
As nations trying to conceal their nuclear testing activities use smaller yield tests, it
becomes more and more challenging for forensic seismologist to identify nuclear
explosion for the natural and man-made none nuclear events. To help with this difficult
situation, scientist have complied a huge data base of naturally occurring and other none
nuclear events to help computer programs model the nuclear events.
Key algorithms provide
discriminates between nuclear and
none nuclear explosions is the
characteristic features of a
waveform (peak-to-peak distance,
height, width, or some ratio). A
particularly useful discriminator,
for example, is the ratio of P-wave
amplitude to S-wave amplitude.
The P (or primary) wave is a
compression wave that is the first
to arrive at a station. The S wave or shear wave has a
slower propagation speed and arrives behind the P
As seen in Figure 1 the seismogram from a representative earthquake clearly differs from
that of the nuclear test. Discriminants that are refined based on P and S waves were
strongly indicative of an explosion, not an earthquake or other seismic source, at all
frequencies tested (0.5 to 8 hertz). Seismologists explain that the differences in seismic P-
and S-wave energy provide one method of discriminating explosions from earthquakes.
Seismic P waves are compressional waves, similar to sound waves in the air. Shear (S)
waves are transverse waves, like those that propagate along a rope when one end is
shaken. Because underground explosions are spherically symmetric disturbances, they
radiate seismic P waves efficiently. In contrast, earthquakes result from sliding or rupture
along a buried fault surface and strongly excite the transverse motions of S waves. Thus,
we expect that explosions will show strong P waves and weak S waves and that
earthquake will show weak P waves and strong S waves, as seen in Figure above.
According to Seismologists, one way to quantify this difference is by determining the
ratio of P-wave to S-wave energy measured from the seismograms. Explosions should
have higher P/S ratios than earthquakes, but the frequency at which the best separation
occurs varies by region and station. Figure 3 shows the P/S ratio for a nuclear test and for
earthquakes shown in Figure 1. The measurements in Figure 3 were made at four
different frequencies. The nuclear test has a higher P/S ratio than the earthquakes, as