Geography 458/558 -- Dr. C.M. Rodrigue
EARTHQUAKES
Physical dynamics
Defined as strong motion of the earth's surface
Causes
Tectonism
Faulting
Vulcanism
Subsidence
Wave attributes
Frequency
Amplitude
Attenuation with distance
Velocity
Acceleration
Stress
Strain – change in volume with stress
Modulus – resistance to strain
Materials will undergo strains:
Elastic deformation
Plastic deformation
Failure (quakes)
Magnitude is a direct function of the area of the fault plane
that ruptures -- more area to emit waves
Wave types
Body waves
Primary or comPressional
Secondary or Shear
Surface waves
Love
motion perpendicular to propagation and horizontal
along surface
develops in layered surfaces
Rayleigh
oscillatory motion, like a sea wave
develops in homogeneous surface materials
Stoneley -- interior crustal discontinuities
Channel -- low velocity crustal materials channels
Determinants of wave velocities
Direct with rigidity and uniformity
Inverse with density
Fault characteristics
Types
Normal (extensional)
Reverse (compressional)
Thrust (extreme compressional)
Strike-slip or transcurrent
Left lateral
Right lateral (most California faults, except Garlock)
Oblique
Fault measures
Strike (intersection with ground, plane parallel to ground)
Dip (angle to surface)
Seismogenesis
Greatest in destructive zones
Also high in conservative zones (San Andreas)
Less common and violent in constructive zones
Less and generally weakest in volcano-seismic zones (< 5M)
Origin points
Hypocenter or focus
Epicenter
Determination
P>S>L>R
No matter the material, relationships among wave types
is mostly constant
Vp/Vs = \/3/1 = 1.73
Vr = 0.92Vs
The difference in arrival times implies distance
One station only creates a radius
Two stations create intersecting radii = 2 points
Three or more stations allow triangulation
Epicenter is probabilistic, actually: +/- 10-20 km, if
hypocenter is shallow
This is why it takes a long time to nail it down
Equipment
Seismometer -> seismograph (expensive)
Accelerometer -> accelerograph (cheap but coarse)
Measurement of earthquakes
Magnitude = energy release
Charles Richter (1958)
Based on largest trace on a seismograph
Calculated for 100 km from epicenter and interpolated
Logarithmic -- 8 = 10,000 a 4
Energy release is 31.6 times greater for each unit
Ml = log (ampl/wave period) + assorted corrections
Based only on ground waves
Various other measures of magnitude for other wave types
Moment magnitude (M)
function of
surface area of fault place displaced
average length of movement
average rigidities of materials
Very similar to Richter, but can get above 9
Magnitude and frequency relationships: semi log-linear
very roughly:
log N = a + bM
where:
N = number of quakes of a given M
M = target magnitude
a and b constants to be calculated
the bigger they are, the rarer they are
those above 6.1 release about 90% of seismic energy
Intensity: Mercalli Modified
Based on the ensemble of observed effects (overhead)
Very roughly related to M
8-8.9 ~ XI-XII
7-7.9 ~ IX-X
6-6.9 ~ VII-VIII
5-5.9 ~ VI-VII
4-4.9 ~ IV-V
3-3.9 ~ II-III
Social dynamics
Earthquake impacts are generally transmitted to humans through
architecture failures of one kind or another.
Dams crack and fail under seismic stresses, with lethal consequences
downstream
Bridges and roads fail, posing great danger to those on them or under
them (bridges)
Lifelines of other types fail, such as water mains (with firefighting
consequences), electrical grids, and sewer lines, posing all sorts
of cascading secondary effects
Buildings fail, crushing their occupants or trapping them
There are a number of things that can be done to mitigate these
architectural failures and thereby increase the length of time the
structures can stand and the probabilities that their occupants or users
can be either gotten out of harm's way or survive the temblor in them
This is the task of engineering geology (which makes seismic
geological information available to the engineering and
architectural professions), seismic engineering, and planning
(which can limit construction by seismic hazard zones and can
implement and enforce building codes appropriate for the seismic
situation and the local culture).
The idea is to replace aseismic construction with earthquake resistant
design.
Ground shaking is made up of mixtures of waves with different periods and
frequencies, which vary from periods of seconds to periods so very
short that they set up an audible hum or roar. Similarly, amplitude
varies, as does specific direction of motion.
As the quake ground motion persists, buildings accumulate damage from
compression, tension, and shear stresses, which progressively
weakens them, sometimes to the point of failure. Duration is critical -great quakes may not necessarily generate higher amplitude motion -earth materials saturate in terms of the amount of motion they can
transmit -- the energy is released in duration of shaking -- and a
building that can "take" 20 seconds of intense shaking may not be able
to withstand 2 minutes of it.
Both structures and soil and rock formations have a fundamental period,
consisting of a wavelength at which they tend to oscillate more
strongly (like a bell oscillates at a constant tone when hit by a
clapper). You get the potential for severe damage when a quake goes
on long enough for the building to begin resonating at its fundamental
period, which then amplifies the ground motion by the addition of this
resonant motion to the ground motion itself. Another situation is when
there is a conflict in fundamental periods between a building and the
rock or soil formation on which it rests or between two adjacent
buildings or different parts of a complex structure. You get wave
interference, which increases amplitude at one point in the structure
and cancels out amplitudes elsewhere in the structure, creating terrific
shear and torsion stresses.
Tall buildings have longer fundamental periods and so do worse on soft
ground that also has a long fundamental period.
Low buildings with shorter fundamental periods do worse on firmer
ground that also has a short fundamental period.
Critical points in buildings are the joints between horizontal and
vertical elements of the load-bearing structure.
Performance of building types:
Adobe and mortared stone buildings are very common in much of the
more arid parts of the Third World due to their cheapness and
ease of construction. They have almost zero resistance to
horizontal motion and fail as an unsorted heap of rubble,
crushing their occupants. Coming up with cheap, easy folk
architecture is one of the great and so far unsuccessful missions
of development and disaster planning in the world today.
Wood-frame buildings can do very well in quakes due to their
flexibility, IF it is well tied together and anchored to the
foundation and not top-heavy (as in Kobe).
Steel-frame construction (beam and joist) can sway too much in tall
buildings, but at least the flexibility of steel beams allows them
to be overstressed and severely deformed without losing all
strength and failing completely (extreme plastic deformation).
Reinforced concrete structures can do well, too, if the rebar in columns
is very well tied in (otherwise the rebar breaks out of columns
and these may then collapse. As the rectangular structure
becomes a parallelogram, the fill material will sometimes crack
and pop out or just crack in this X pattern. X-shaped bracing
can improve support.
Building interiors are critical to survivability, too, not just the structural
elements. Loose materials become projectiles in quakes, flying
from one room into another (which is why standing in a doorway
is not necessarily a good idea in a quake unless you're in an
otherwise unreinforced structure). There are many nonstructural mitigations to improve survivability inside a building,
such as strapping water heaters, installing quake valves on gas
lines, removing heavy pictures and tchotchkes from the head of
the bed, installing baffled picture hooks to keep pictures
attached to the wall, and placing the bed along the interior walls,
keeping water by the bed in case you are trapped for a while,
trying to roll out of bed and lying on the floor beside it for the
small crawl space there (in apartments -- in houses, you may be
better off just lying there).
The idea in earthquake resistant construction is not necessarily to prevent
failure, for that is impossible in the largest quakes, but to allow the
building to deform or fail in a manner that protects occupants and puts
them in predictable crawl spaces for rescuers to look for.
There are tremendous inequities in the allocation of people among these
different structures, inequities that arise both from political-economic
forces and from cultural quirks.
In the US, economic forces tend to allocate poorer people to older buildings
and increase their population density in those structures.
Older buildings may not have been built to any code whatsoever or,
more commonly, to codes now superseded by subsequent
failures and learning in the engineering profession
Even in newer buildings, poorer people are in cheaper buildings, and
there is a greater chance that corners were cut in construction
and inspection
Also, poverty correlates with other demographic attributes, notably
ethnicity/race, age, and disability
In a quake, media bias can exacerbate the already uneven vulnerability
Recovery will be delayed in poorer and minority communities, worsening
vulnerability
Media?
Resources
Know-how in dealing with a bureaucracy
Lawsuits?
Immigration status concerns