1. This work is supported by the National Science Foundation’s Transforming Undergraduate Education in STEM program within the
Directorate for Education and Human Resources (DUE-1245025).
BACKGROUND INFORMATION ON FIRST
MOTION MODELS FOR EARTHQUAKES
Bruce Douglas (Indiana University—Bloomington)
Gareth Funning (University of California—Riverside)
Version: Dec 7, 2015
2. We use a specific set of symbols
to identify faulting geometry on
maps. The symbols are called
earthquake focal mechanisms or
sometimes seismic “beach balls.”
A focal mechanism is a graphical
summary of the strike, dip, and
slip directions.
An earthquake focal mechanism
is a projection of the intersection
of the fault surface and an
imaginary lower hemisphere
surrounding the center of the
rupture.
EARTHQUAKE FOCAL
MECHANISMS
3. First motions
On a seismogram, the first motion
is the direction of ground motion
as the P wave arrives at the
seismometer.
Upward ground motion indicates
an expansion in the source
region; downward motion
indicates a contraction.
EARTHQUAKE FOCAL MECHANISMS
4. Seismic wave generation
• Rupture starts at focus and spreads
erratically and non-uniformly
• Irregularities on fault plane
(asperities) may act as barriers and
temporarily slow propagation in
certain direction
• Rupture stops when rocks not
sufficiently strained to allow it to
continue
• After rupturing ceases, adjacent
sides of fault rebound
• Seismic waves radiate from
numerous places on fault plane
• Rupture velocity variable
• sometimes ~ walking speed
(1960 Chile quake took an hour
for full rupture)
• Typically 2–3 km/sec
Fault
plane
Radiating
rupture
surface
F
SEISMIC WAVE
GENERATION
5. FIRST MOTIONS
• In the early 20th Century, Japanese seismologists began
investigating the sense of motion that accompanied the very first
seismic wave arrival at a seismometer.
• They found that these first motions are either upward or away from
the source, or arrive downward or toward the source.
• Though initially thought to be the result of different types of
earthquakes, it was soon discovered that a single earthquake could
produce both types of motion.
• This knowledge, combined with the developing theories of fault
rupture, led to the suggestion that these first wave motions were
indicators of slipping motion from the actual fault rupture
producing the seismic waves.
6. FIRST MOTIONS
• Imagine an east-west oriented, right-lateral strike-slip fault.
• Now imagine standing on the south side of that fault, facing east,
when an earthquake begins to rupture the stretch of fault in front
(east) of you.
• What would you feel?
7. FIRST MOTIONS
• The first motion you would experience would be a “push,” pushing you
away from the source of the earthquake, as your side of the fault
experiences compressional force from the motion of the fault rupture.
• The areas of the block diagram that turn red experience compression
when the fault moves.
• Those areas that fade to white experience the opposite kind of motion,
dilatation, as they are initially pulled toward the source with “pull”
motion.
8. FIRST MOTIONS
• Analysis of records made from the seismic instruments surrounding the
source of an earthquake allow seismologists to determine the sense of slip
of that earthquake, even if the rupture does not reach the surface.
• This is done by creating a model of the initial rupture motion, called a
focal mechanism.
10. FOCAL MECHANISMS — SEISMOLOGICAL “BEACH BALLS”
• A focal mechanism is a model of the exact orientation and sense of
slip of the fault rupture that generates an earthquake.
• The model can be described with a sphere, cut by two
perpendicular nodal planes that intersect in the center, dividing the
sphere into four equal quadrants.
11. FOCAL MECHANISMS — SEISMOLOGICAL “BEACH BALLS”
Each quadrant has either
push or pull
• Since focal mechanisms are constructed using first motions, the direction
of the first deflection recorded by a seismometer as that instrument
experiences the initial arrival of seismic waves.
• It takes a large number of seismometers in the area surrounding the
hypocenter to produce a reliable focal mechanism.
• Gaps in coverage will increase
the uncertainty in the model.
12. CREATING FOCAL MECHANISMS
• Now we need to plot the first motions
recorded from our earthquake.
• To do this, we check the waveforms recorded
by various seismic stations in the area and
mark the first arrivals as “up”
(compressional) or “down” (dilatational).
• Each station’s location relative to the
hypocenter is then projected onto our
circular diagram with a symbol representing
the type of motion—up or down—first
recorded there.
• Stations that fall within the “missing” upper
hemisphere (above the horizontal) are
translated appropriately onto our lower-
hemisphere projection.
13. CREATING FOCAL MECHANISMS
• Once the first motions are correctly plotted, it is
time to solve for the two nodal planes.
• The sphere is divided into quadrants using two
perpendicular planes that best fit the set of first
motions.
• Again, since the two-dimensional plot shows only
the projection of the inner surface of the lower
hemisphere, those planes will look like two
intersecting lines within a circle.
• Though the two nodal planes must intersect at
the hypocenter, the center of the sphere, the
intersection of these lines—since it occurs on the
sphere’s surface—need not be in the center of
our circular projection.
14. CREATING FOCAL MECHANISMS
• After the nodal planes have been identified, the
symbol is complete, but of somewhat limited
use, because either of the two nodal planes
could be the fault plane.
• Other types of data can be useful in determining
which plane is the fault plane, and consequently,
what type of slip occurred in the earthquake.
17. First Motions for a Reverse Fault:
The Dilation, Nodal Plane and Compression responses are shown along with the
deflection recorded by a seismometer responding to the initial arrival of seismic waves
18. READING FAULT PLANE SOLUTIONS
• Interpreting fault plane solutions can be a little
tricky.
• There are several things to keep in mind when
converting a symbol (like the one at lower left) to
a sense of slip and fault plane orientation.
• First, the fault plane solution is generally given as
a two-dimensional projection of the lower
hemisphere of a focal mechanism sphere, not
just an overhead view of the outside of that
sphere.
• Also keep in mind that the lines crossing the circle
represent the intersection of two perpendicular
planes with a sphere.
19. +
+
-
Normal dip-slip
fault
READING FAULT PLANE SOLUTIONS
• If you can determine which nodal plane on a
fault plane solution corresponds to the
orientation of the geologic fault plane, you
know that the other plane must be the
auxiliary plane.
• Because this plane is oriented perpendicular to
the direction of slip, its point of intersection
with the fault plane (and the lower surface of
the sphere) provides information about the
relative proportions of dip slip and strike slip
involved in the fault rupture.
• If the line of the auxiliary plane bisects the
fault plane’s line, this represents pure strike
slip.
20. FOCAL MECHANISMS — SEISMOLOGICAL “BEACH BALLS”
• Focal mechanisms really only describe the motion involved
at the start of a rupture—the hypocenter (also called the
focus, hence their name)—because they are calculated
using the very first wave arrivals from an earthquake.
• In an earthquake large enough to involve several kilometers
of fault rupture, slip will sometimes “evolve,” changing in
sense and/or orientation as the rupture propagates.
• This can happen in response to changes in fault geometry
or rupture boundary conditions.
• In such a case, the entire fault rupture may not exactly
match the model supplied by the focal mechanism.
• The focal mechanism will still provide insight into the initial
rupture behavior at the hypocenter.
21. READING FAULT PLANE SOLUTIONS
• Knowing the geology of the area will help you
identify the most likely fault geometry.
• Aftershocks can be of great assistance;
aftershock sequences can provide valuable hints
about fault plane orientation at depth.
• Though not always feasible, the use of these
insights can lead to a final determination of the
fault plane solution.
22. Fault plane solutions from first motions
Type of fault can be determined
remotely from first motions
on a seismogram
P-wave: detected as either a
push or a pull
P-wave first motions will depend
where seismograph is located
relative to fault
Those located at points where
fault at the focus is moving
away record pulls (dilatations)
Seismographs located such that
as fault is moving toward
them will record pushes or
compressions
+
+
-
+
-
-
Normal dip-slip
fault
Reverse dip-slip
fault
