Earthquakes
Physical Geology 11/e, Chapter 16
Earthquakes and Plate Tectonics
• At divergent boundaries,
– tensional forces produce shallow-focus quakes on normal faults
• At transform boundaries,
– shear forces produce shallow-focus quakes along strike-slip faults
• At convergent boundaries,
– compression produces shallow- to deep-focus quakes along reverse faults
World Earthquake Distribution
• Nearly all intermediate- and
deep-focus earthquakes occur
in Benioff zones
– inclined seismic activity
associated with descending
oceanic plate at subduction
zones)
Most earthquakes occur at < 100 km depth
Earthquakes can occur 100-670 km below the surface
Causes of Earthquakes
• Elastic rebound theory earthquakes are a sudden
release of strain progressively
stored in rocks that bend until
they finally break and move
along a fault
Seismic Waves
• Seismic waves originate at the focus (or hypocenter) - the point
of initial breakage and movement along a fault
• Epicenter - point on Earth’s surface directly above the focus
(used for 2-D map)
Types of Seismic Waves
• Two types of seismic waves
are produced during
earthquakes
– Body waves - travel outward
from the focus in all directions
through Earth’s interior
– Surface waves - travel along
Earth’s surface away from the
epicenter
Body Waves
• P wave - compressional
(longitudinal or push-pull)
body wave in which rock
vibrates back and forth parallel
to the direction of wave
propagation
– Fast (4 to 7 kms/sec) wave that is
the first or primary wave to
arrive at recording station
following earthquake
– Pass through solids and fluids
– P-P-P
Body Waves
• S wave - shearing (transverse) body wave in which rock
vibrates back and forth perpendicular to the direction of
wave propagation
– Slower (2 to 5 km/sec) wave that is the secondary wave to arrive at
recording station following earthquake
– Pass through solids only
(can’t travel through the
outer core)
– S-S-S
Surface Waves
• Slowest type of seismic waves
produced by earthquakes
– Love waves - side-to-side motion
of the ground surface
• Can’t travel through fluids
– Rayleigh waves - ground to
moves in an elliptical path
opposite the direction of wave
motion
• Extremely destructive to buildings
Measuring Earthquakes
• Seismometers - used to measure seismic waves
– Seismographs - recording devices used to produce a
permanent record of the motion detected by
seismometers
Locating Earthquakes
• P- and S-waves leave earthquake focus at the same time
• P-wave gets farther and farther ahead of the S-wave with
distance and time from the earthquake
Locating Earthquakes
• Plotting distances from 3 stations on
a map, as circles with radii equaling
the distance from the quake, locates
earthquake epicenter
• Depth of focus beneath Earth’s
surface can also be determined
– Shallow focus
0 - 70 km deep
– Intermediate focus 70 - 350 km deep
– Deep focus
350 - 670 km deep
Measuring the “Size” of Earthquakes
• Size of earthquakes measured
in two ways - intensity and
magnitude
• Magnitude is a quantitative
measure of the amount of
energy released by an EQ
– Richter scale
• Logarithmic scale
• Amplitude increased 10 times
for every one step up scale
• How much ‘worse’ is a mag.8
compared to a 6 EQ?
Measuring the “Size” of Earthquakes
• Moment magnitude - more objective measure of energy
released by a major earthquake
– Uses rock strength, surface area of fault rupture, and amount of movement
– Smaller earthquakes are more common than larger ones
Measuring the “Size” of Earthquakes
• Intensity - a measure of the
effects an earthquake produces
(on both structures and people)
– Modified Mercalli scale
– Qualitative
– Multiple values for one EQ
• Amount of shaking controlled by
amount of energy released
(magnitude), distance from the focus,
‘soil’ type
– Useful for assessing historical EQs
Earthquake Risk
• Large seismic risks or hazards exist around New Madrid, Missouri
• Seismic risk determined based on the assumption that large future
earthquakes will occur where they have occurred in the past
Effects of Earthquakes
• Liquefaction occurs when water-saturated soil or sediment
sloshes like a liquid during a quake
Tsunami
• Tsunami (seismic sea
waves) - very large sea
waves caused by sudden
upward or downward
movement of the sea
floor during submarine
earthquakes
Earthquake Prediction and Seismic Risk
• Accurate and consistent shortterm earthquake prediction not
yet possible
• Three methods assist in
determining probability that an
earthquake will occur:
– Measurement of changes in rock
properties due to buildup of strain
• magnetism, electrical resistivity,
seismic velocity, porosity, etc.
– Studies of the slip rate along fault zones
– Paleoseismology studies
Earth’s Interior and
Geophysical Properties
Physical Geology 11/e, Chapter 17
Evidence from Seismic Waves
•
Seismic waves or vibrations from a large earthquake (or underground nuclear test) will pass through the
entire Earth
– Affected by the properties of Earth materials,
• Density and state (solid or liquid) may result in changes in seismic wave
velocity, reflection, refraction, or both reflection and refraction.
• Allow geologists to ‘see’ into the Earth
Evidence from Seismic Waves
•
Seismic reflection - the return of some waves to the
surface after bouncing off a rock layer boundary
– Sharp boundary between two
materials of different densities
will reflect waves
Evidence from Seismic Waves
•
Seismic refraction - bending of seismic waves as they pass from
one material to another with different seismic velocities
– V2 > V1 = ‘upward’ refraction
– V2 < V1 = ‘downward’ refraction
•
Seismic velocity (and density) of the mantle increases uniformly
with depth
– Curved ray paths result because the
wavefronts are continuously refracted
Earth’s Internal Structure
•
The crust, mantle and core, the three main layers within the Earth, have been determined based on seismic
evidence.
The Crust
•
•
•
The crust is the outer layer of rock that
forms a thin skin on Earth’s surface
Seismic wave studies indicate that the
crust is thinner and denser beneath the
oceans than on the continents
Seismic wave velocities are different in
oceanic (7 km/sec) vs. continental (~6
km/sec) crustal rocks
– Indicate different
compositions
•
Oceanic crust is mafic
– Primarily of basalt and
gabbro (basaltic and
gabbroic)
•
Continental crust is felsic
– Average composition is
similar to granite and
rhyolite (granitic and
rhyolitic)
Crust-Mantle Boundary
•
The crust-mantle boundary, called the Mohorovičić discontinuity or Moho, is marked by a jump in
seismic wave velocity.
The Mantle
•
•
•
•
•
•
The mantle is a thick shell of dense rock that separates
the crust above from the core below
Seismic wave studies indicate the mantle, like the
crust, is made of solid rock with only isolated pockets
of magma
Higher seismic wave velocity (8 km/sec) of mantle vs.
crustal rocks indicate denser, ultramafic composition
Crust + upper mantle = lithosphere, the brittle outer
shell of the Earth that makes up the tectonic plates
Lithosphere averages 70 km thick beneath oceans and
125-250 km thick beneath continents
Beneath the lithosphere, seismic wave speeds abruptly
decrease in a plastic low-velocity zone called the
asthenosphere
The Core-Mantle Boundary
•
Core-mantle boundary (D” layer) is marked by great changes in
seismic velocity, density and temperature
– P-wave velocitites dramatically decrease
• Ultralow-velocity zone (ULVZ)
– Due to hot core melting lowermost mantle
or reacting chemically to form iron silicate
‘sediments’
The Core
•
The core is the metallic central zone of the Earth
– Subdivided into a liquid outer core and solid inner core
•
Seismic wave studies have provided primary evidence for existence and nature of Earth’s core
The Outer Core
•
Seismic shadow zones
– Specific areas on the opposite side of
the Earth from large earthquakes do
not receive seismic waves
– S-wave shadow zone (≥103° from
epicenter) suggests outer core is liquid
• Liquids have no shear strength
– P-wave shadow zone (103°-142° from
epicenter) explained by refraction of
waves encountering core-mantle
boundary
• Decreased velocity causes wavefront
to be refracted ‘downward’
The Inner Core
•
•
Careful observations of P-wave refraction patterns indicate
that inner core is solid
Core composition inferred from its calculated density,
physical and electro-magnetic properties, and composition
of meteorites
– Iron metal (liquid in outer core and
solid in inner core) best fits observed
properties
– Iron is the only metal common in
meteorites
Isostasy
•
Isostatic adjustment - rising or sinking of crustal blocks
to achieve isostatic balance
– Crust will rise when large mass is
rapidly removed from the surface,
as at end of ice ages
– Rise of crust after ice sheet removal
is called crustal rebound
• Rebound still occurring in northern
Canada and northern Europe
Gravity Measurements
•
Gravity meters - detect tiny changes (anomalies) in gravity at
Earth’s surface related to total mass beneath any given point
– Gravity slightly higher (positive gravity
anomaly) over dense materials (metallic
ore bodies, mafic rocks)
– Gravity slightly lower (negative gravity
anomaly) over less dense materials (caves,
water, magma, sediments, felsic rocks)
Earth’s Magnetic Field
•
A magnetic field (region of magnetic force) surrounds the Earth
– Field has north and south magnetic poles
– A compass detects Earth’s magnetic field
– Recorded by magnetic minerals (e.g.,
magnetite) in igneous rocks as they cool
Earth’s Magnetic Field
•
Magnetic reversals - times when the poles of Earth’s magnetic field
switch
– After next reversal, a compass needle will point
toward the south magnetic pole
– Direction recorded in magnetic minerals
– Minerals also recorded the time that they formed
• Radioactive decay
• Reversals have occurred many times
• Timing appears chaotic, no discernable pattern
Magnetic Anomalies
•
•
Positive and negative magnetic anomalies represent larger and smaller
than average local magnetic field strengths, respectively
Can detect metallic ore deposits, igneous rocks (positive anomalies), and
thick layers of non-magnetic sediments (negative anomaly) beneath
Earth’s surface
Heat Flow Within Earth
•
Geothermal gradient - temperature increase with depth
– Tapers off sharply beneath lithosphere
– Due to steady pressure increase with depth, increased temperatures
produce little melt (mostly within asthenosphere) except in the outer core
The Sea Floor
Physical Geology, Chapter 18
form?
Continental Shelves &
Continental Slopes
• Continental shelf:
shallow, submarine
platform, 0.1º seaward dip
• Continental slope: 4-5º
steep slope from a depth of
100-200 m at edge of
continental shelf
Submarine Canyons
Submarine canyons: V-shaped valleys that run across
continental shelves & slopes
Abyssal fans: fan-shaped deposits of sediment at base of
submarine canyons
Turbidity currents: masses of sediment-laden water pulled
downhill by gravity
Passive Continental Margins
• Passive continental margin:
– Continental shelf, continental slope, and continental rise.
– Extends to abyssal plain at 5 km depth.
• Continental rise: at base of continental slope, wedge of
sediment from continental slope to deep-sea floor, slopes
0.5º.
Passive Continental Margins
Continental Rise: Types of deposition
• Turbidity currents –
flowing down slope
• Contour currents –
flowing along slope
Passive Continental Margins
Abyssal Plains
• Abyssal plains:
– Very flat regions at base of continental rise.
– Composed of horizontal sediment layers
probably deposited by turbidity currents.
– Flattest features on the Earth.
Active Continental Margins
• Active continental margin:
– Earthquakes, young mountain
belt, and volcanoes.
– Consists of continental shelf &
slope, and oceanic trench.
– Lacks continental rise and
abyssal plain.
– Associated with convergent
plate boundaries.
Active Continental Margins
Oceanic Trenches
• Oceanic trench:
– Narrow, deep trough
parallel to edge of
continent or island arc
– Continental slope steepens
to 10-15º
– Benioff seismic zone
– Volcanoes landward
– Low heat flow
– Negative gravity anomaly
Mid-Oceanic Ridges
• Mid-oceanic ridge:
–
–
–
–
–
Undersea mountain range
Basalt
80,000 km long
1500-2500 km wide
2-3 km above ocean floor
• Rift Valley:
–
–
–
–
–
Crust extension
Along ridge crest
1-2 km deep
Several km wide
Present in Atlantic & Indian
Ocean, absent in Pacific Ocean
North America
• Fracture zones:
– Major lines of
weakness of the
Earth’s crust
– Cross MOR at right
angles
– Rift valley is offset
– May extend onto
continents
Seamounts, Guyots, & Aseismic
Ridges
• Guyots: Flattopped
seamounts
• Aseismic ridges:
Submarine ridges not
associated with
earthquakes.
Reefs
• Reefs:
– Wave resistant ridges of coral, algae, & other calcareous
organisms
– Warm, shallow, sunlit, clean water
– Reef types
• Fringing reefs: Flat table-like, attached directly to shore
• Barrier reefs: parallel to shore, detached by lagoons
• Atolls: Circular reefs rimming lagoons, surrounded by deep water
Sediments of the Sea Floor
• Basaltic oceanic crust
• Terrigenous sediment:
– Land-derived sediment.
– Turbidity & contour currents
• Pelagic sediment:
– Fine-grained clay & skeletons
of microscopic organisms.
– Absent on ridge crests.
Oceanic Crust & Ophiolites
• Oceanic crust is thinner (7 km) and
denser (3.3 g/cm3) than continental
crust.
• Layer 1: Marine sediment (variable
thickness & composition).
• Layer 2: 1.5 km, pillow basalts
overlaying basalt dikes (closely
spaced, parallel, vertical).
• Layer 3: 5 km sill-like gabbros.
• Ophiolite: Slivers of oceanic crust
emplaced on land represented by
distinctive rock sequences