1. Earthquakes Physical Geology 11/e, Chapter 16

2. 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

4. 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 km below the surface

5. 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

6. 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)

7. 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

8. 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

9. 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

10. 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

11. Measuring Earthquakes
Seismometers - used to measure seismic waves
Seismographs - recording devices used to produce a permanent record of the motion detected by seismometers

12. 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

13. 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 km deep
Intermediate focus km deep
Deep focus km deep

14. 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?

15. 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

16. 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

17. 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

18. Effects of Earthquakes
Liquefaction occurs when water-saturated soil or sediment sloshes like a liquid during a quake

19. Tsunami
Tsunami (seismic sea waves) - very large sea waves caused by sudden upward or downward movement of the sea floor during submarine earthquakes

20. Earthquake Prediction and Seismic Risk
Accurate and consistent short-term 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

21. Earth’s Interior and Geophysical Properties Physical Geology 11/e, Chapter 17

22. 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

23. 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

24. 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

25. Earth’s Internal Structure
The crust, mantle and core, the three main layers within the Earth, have been determined based on seismic evidence.

26. The Crust Indicate different compositions
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)

27. Crust-Mantle Boundary
The crust-mantle boundary, called the Mohorovičić discontinuity or Moho, is marked by a jump in seismic wave velocity.

28. 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 km thick beneath continents
Beneath the lithosphere, seismic wave speeds abruptly decrease in a plastic low-velocity zone called the asthenosphere

29. 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’

30. The Core Subdivided into a liquid outer core and solid inner 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

31. 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’

32. 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

33. 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

34. 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)

35. 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

36. 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

37. 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

38. Heat Flow Within Earth Tapers off sharply beneath lithosphere
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

39. The Sea Floor Physical Geology, Chapter 18
form?

40. Continental Shelves & Continental Slopes
Continental shelf: shallow, submarine platform, 0.1º seaward dip
Continental slope: 4-5º steep slope from a depth of m at edge of continental shelf

41. 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

42. Passive Continental Margins
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.

43. Passive Continental Margins Continental Rise: Types of deposition
Turbidity currents – flowing down slope
Contour currents – flowing along slope

44. Passive Continental Margins 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.

45. Active Continental Margins
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.

46. Active Continental Margins Oceanic Trenches
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

47. Mid-Oceanic Ridges Mid-oceanic ridge: Rift Valley:
Undersea mountain range
Basalt
80,000 km long
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

48. 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

49. Seamounts, Guyots, & Aseismic Ridges
Guyots: Flattopped seamounts
Aseismic ridges: Submarine ridges not associated with earthquakes.

50. 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

51. 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.

52. 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