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Alexandra Allen
Neotectonics Midterm Study Guide
ERTH 610: Neotectonics
San Francisco State University
1. Understand the concept of elastic rebound and the earthquake cycle.
Elastic rebound
Elastic rebound occurs when elastic strain drops coevally with stress due to rocks failing
resulting in permanent displacement. This rock failure and displacement is an earthquake event.
Following the earthquake, strain accumulates until the rock fails again and another earthquake
occurs. As rocks on opposite sides of a fault are subjected to stress and strain, they accumulate
energy and slowly deform until their internal strength is exceeded.
2. What’s the Moho and how is it defined?
The Mohorovichic Discontinuity is the crust-mantle boundary, entirely in the lithosphere and an
abrupt velocity boundary. P-waves’ and s-waves’ velocities increase at/below Moho. Mafic
rocks-->ultramafic rocks.
Average depth of 8km below oceanic crust, 32km below continental crust.
3. How do various factors affect the depth to the BPT and strength envelopes in general? At
about what depth is the BPT in CA and NV, and how do we know?
Qtz and feldspar rich rocks in continental crust at depth of 13–18 km.
CA & NV: western plate boundary, high geothermal gradient, basement of seismogenic zone
BDT is 10-15km.
BDT is the level in crust where downwards increasing brittle strength=upwards increasing
ductile strength. Strongest part of crust and the depth at which earthquakes occur. The level of
the transition zone depends on both strain rate and temperature gradient. Crustal composition
will also affect the depth at which this zone occurs.
-greater strain rate, greater differential stress, shallower BDT
-greater friction, greater stress, shallower BDT
-greater pore fluid, less differential stress, deeper BDT
-Slow deformation, high temperature, shallower BDT
-Fast deformation, low temperature, deeper BDT
Above BDT- rock strength is dependent on law of friction, independent of temperature
Below BDT- rock strength is independent on law of friction, dependent on temperature
4. Have a solid understanding of Byerlee’s Law and Andersonian theory of fault mechanics.
Byerlee’s Law of Friction: most rocks have similar ϻs value-0.6,
Increased normal stress, required shear stress increases (linearly), with the rate of increasing
actually decreasing, becoming independent of rock type. Used to determine when slip will take
place along a fault. Linear relationship between rock strength and depth independent of rock
type.
Strength increase with depth is different between strike-slip, extension, and confining scenarios
due to different orientations of fractures and normal stress acting on them related directly to
Andersonian Theory of Fault Mechanics
Andersonian Theory: style of fault is controlled by direction of least confined stress. Fault angle
not specific-coefficient of friction controls fault angle. Predict normal faults dipping at 60, thrust
faults at 30, and strike-slip faults at 30 to the regional stress field. Faults result from brittle
fracture so apply the Coulomb criterion to this problem. Faults sometimes form conjugate sets,
with planes inclined at acute angles on either side of the maximum principal stress and which
include the intermediate principal stress direction.
Normal: σ3=σh (acts perpendicular to fault strike)
σ2=σH (parallel to fault strike)
σ1=σv (lithostatic stress)
form at 60° dip
Thrust: the value of friction is the slope of failure (θ) relative to σ1
σ3= σv
σ2= σH
σ1= σh
form at 30° dip
Strike-slip: σ3=σh
σ2=σv
σ1=σH
form at 90° dip
5. What’s the difference between mean stress and differential stress, and how do variations in
their values affect rock/fault strength or prospects for rock/fault failure?
Differential Stress- the diameter of a mohr circle, the difference between highest and lowest
compressive stress on an object. Increasing σD, rock is stronger and BDT is shallower
Mean Stress- the average of the 3 Principe stresses. Increasing σm, rock is stronger and BDT is
shallower.
The higher the strain rate, the stronger the rock. The lower the temperature, the stronger the
rock. The greater the speed of deformation, the lower the strain rate and hence the rock is not
as strong. Lower strain rate means less differential stress is needed to go from elastic-plastic.
Strength and plasticity increase with greater confining pressure. Increasing friction, BDT
shallower.
Flow Stress EQTN:
Mean Stress EQTN:
in 3D & 2D
Differential Stress EQTN:
in 2D
6. Be able to distinguish the basic differences in the models for stick-slip fault behavior, which
include 1)periodic, 2) time-predictable, and 3) slip predictable behavior.
Periodic:
Same temperatures are reached each period, while stress drops equally each time. Same initial
shear stress each period. Slip and slip rate are constant as well as plate movement.
Time-predictable:
Temperatures constant throughout as well as fault strength. Slip rate is constant but no
constant recurrence period. Size of earthquake is a variable to recurrence intervals. No
background shear stress.
Slip-predictable:
Recurrence interval is noncalculable as there is no constant critical shear stress. Constant fault
friction and constant background shear stress though value of stress drop varies. Slip also varies
though the slip rate is constant. Magnitude is calculable from measured slip.
7. Why do some fault undergo aseismic creep? Explain the various factors that must be
considered.
Aseismic creep is fault slip occurring in the uppermost areas of the crust occurring between
intervals of large stress release along a fault or immediately after an earthquake and can last
days to years. This happens because faults are “locked” between intervals below the surface.
Closer to the surface, elastic shear strain accumulates to a critical stress threshold resulting in
elastic strain energy being released by seismic fault slip, essentially “creeping”.
8. Be able to show with sketches the radiation patterns for both p- and s-wave energy and
important aspects related to these patterns (such as nodal planes for each and first motion
patterns for P waves). How is the phenomenon directly related to radiation patterns of body
waves?
P-waves:
S-waves:
9. Understand the concepts of the seismic wave propagation, such as take-off angles, seismic
refraction, ray paths, etc…
P-waves: primary/first waves, push pull, ground motion is parallel to wave direction.
S-waves: secondary/shear waves, ground motion perpendicular to wave direction. Usually have
greater amplitude than p-waves.
Love-wave: Sh motion- transverse component of s-wave
Rayleigh-wave: circular wave motion-vertical and radial component of s-waves as they are
refracted in the crust and bounce off earth’s surface continuously.
Take off angles are directly downward, facing towards the center of earth,
Xcritical is the distance from the source of earthquake where critical wave refraction occurs.
Refraction – Snell’s Law
Seismic wave hits boundary and can be refracted and converted into a different
wave or will continue moving through earth until reflection angle is reached.
10. What is attenuation? Compare and contrast the two types we discussed in class.
Attenuation is a form of energy loss in seismic waves, a decrease in intensity with distance as waves
spread geometrically. body waves attenuate faster than surface waves.
Scattering attenuation- When a seismic wave encounters a different kind of material in its travel, it will
redistribute its energy into different wave phases that are scattered. Some energy is lost each time a
wave is converted based on the kind of material it is bouncing off. Brittle Rock*
Intrinsic attenuation- Frictional heating between particles in elastic material absorbs energy from
seismic waves, slowing them exponentially. Known as, elastic wave propagation, intrinsic attenuation is
the permanent (non reversible) exchange between PE and KE through displacement and velocity. Energy
loss due to shear heating at grain boundaries. Plastic Rock*
11. What is a first-motion focal mechanism? How are they made? Know how to identify the
various parameters such as fault orientation (strike and Dip), slip vectors, P- and T-axes, and
sense of fault slip represented by each of the two nodal planes for a given focal mechanism.
Note that sense of slip fault is limited to only the following: right-lateral, left-lateral, normal,
reverse, right-normal-oblique, left-normal-oblique, right-reverse-oblique, left-reverse-oblique
slip.
Focal mechanisms are the deformation in the source region that generates seismic waves after
an earthquake. In the case of a fault-related event it refers to the orientation of the fault plane
that slipped and the slip vector. It can be derived from observing the pattern of "first motions”,
whether the first arriving P-waves are dilatational or compressional.
Sense of slip is confined to:
12. What is meant by double-couple mechanism? How does this relate to the fact that focal
mechanisms produce two possible fault plane solutions that could each satisfy the observed
first motion patterns?
A focal mechanism with a second order tensor (stress and strain) known as the moment tensor.
Elastic deformation compensates for torque in the moment. Elastic wave radiation from an
earthquake can be modeled in 2 (2 solutions) equivalent ways to represent shear faulting. There
must be an equal and opposite restoring force in an earthquake along the auxiliary plane.
13. What kinds of additional information can often be used by geologists and seismologists (and
geodesists) to determine which of the two nodal planes represent the actual fault that slipped
during the earthquake?
To accurately identify the nodal plane representing the fault, scientists should look for physical
evidence on the earth’s surface (fault trace, right/left offset from the fault, channel offset).
Seismic evidence is also important and these scientists should analyze information on the p and
t-axis in order to orient the regional stress field. Because earthquake waves travel in all
directions for long distances, they are recorded around the world so aftershock distributions as
well as seismogenic data is readily available in most cases and can be useful in determining the
nodal plane of the fault along which an earthquake occurred. Focal mechanisms are drafted to
calculate and show the orientation of a fault using the 2 different nodal planes.
14. Although velocity structure of the earth is known fairly well on a global scale, in what ways
can improvements to local velocity structure models help improve the quality of earthquake
locations and the focal mechanisms produced within the area a local seismic network such as
the greater SF Bay Region? What differences in the tectonic styles do Zoback et al. (1999) find
from their examination of newly derived focal mechanisms in the northern vs southern SF
Peninsula area.
Zoback’s idea of inverting data by refining velocity structure in an existing model using
earthquakes in a known area enabled her to back track ray paths to see if their location lined up
with the known data using quarry blasts.
She believed to improve the quality and precision in a focal mechanism by using multiple
earthquakes in a location, found a best-fitting 1-D velocity model, and inverted the information.
Regional earthquakes were included to better resolve the refractor velocities. She determined
individual station delays and computed them separately.
She found that station correlations also correlated well with surficial geology in the region.
Northern SF Peninsula- extensional tectonism
Southern SF Peninsula- compressional tectonism
15. Know the basic concepts of magnitude scales. How are they determined? What are some of
the pitfalls to various magnitudes scales? Understand why certain scales are said to saturate.
Richter Magnitude (local magnitude)(ML)
Scale based on amplitude (mm) of the largest s-waves recorded on a Wood-Anderson
seismograph (free period 0.8s) normalized to a standard epicentral distance of 100 km. reflects
the relative amount of seismic energy released by an earthquake up to a ML 6.0 (max) but
cannot accurately measure above that.
Body-Wave Magnitude (mb)
Short period body wave magnitude measured using peak body-wave amplitude (p-wave
amplitude) recorded at distances up to 1000km on instruments with a passband 1-10Hz (free
period 0.1-1s). First magnitudes recorded for an earthquake. Measures more distant
earthquakes than the Richter magnitude scale. Can accurately measure up to 6.5.
Surface-Wave Magnitude (Ms)
Peak wave amplitude is measured for surface (Rayleigh) waves with periods of 20s from long
period seismographs at teleseismic distances. Does not require a seismograph record within
100km. Can measure up to 7.3 accurately. Magnitude used in empirical comparisons of
magnitude vs earthquake rupture length/displacement.
Moment Magnitude (Mw)
The most recently developed scale. Does not rely on measured seismogram peaks but is
calculated using the seismic moment equation:
Represents the amount of energy released at the source of an earthquake rather than relying on
the effets of that energy on one or more seismographs at some distance from the source.
Moment magnitude is measured from the seismic moment: