Download Plate Boundaries and Earthquake Science

GEOL 0820
Ramsey
Natural Disasters
Spring, 2017
LECTURE #7: Plate Tectonics: Boundaries & EQ Science
Date: 26 January 2017
(lecturer: Victoria Buford)
I. Reminder: Exam #1: Feb 2nd
• one week from today
 be sure to be on time (no one will be allowed to take the exam if they show
up more than 15 minutes late!)
 bring a pencil, eraser, a photo ID, and your PeopleSoft number!
•
we will go over some example questions at the start of the lecture today
o material covered: first 8 lectures (including the video)
o up to next Tuesday's class
 Prof. Ramsey will return for that class and answer any exam-related
questions you may have
II. Review from Last Class
• brief history of Plate Tectonics
• two hypotheses that combined to become the Theory of Plate Tectonics
• 5 steps of the Scientific Method
III. Plate Tectonics
• three types of plate boundaries:
o divergent
 plates spreading apart
 small earthquakes
 example: ocean floors
o convergent
 plates come together
 subduction zones
 increased volcanic activity
 large earthquakes
 example: South America
o transform
 plates slide past one another
 moderate earthquakes
 example: San Andreas fault
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•
how do we know the structure of the Earth?
o seismology
 study of seismic waves as they move through the crust
 formed by earthquakes (EQ’s)
 can be VERY destructive
 but can also be used to “image” the structure of the Earth
•
generalized structure of the Earth
o how do we know what is below our feet?
 we have only drilled down several km’s
 never through the crust
 5km thick (ocean) and 35km thick (continents)
 in contrast, the Earth has a radius of 6,370km
o examining the seismic waves as they pass through the Earth
 waves change speeds when they pass through different rock compositions
and density
 seismologists can model these changes and estimate what the rock types
are, how thick they are, and what state they are in (e.g., solid or liquid)
IV. Seismology
• the branch of geology concerned with the study and analysis of EQ's, seismic
waves and their propagation through the Earth
•
seismic waves:
o energy in the form of waves (causing particle motion) radiating out from a fault
rupture (break)
o two main types: body waves and surface waves
o type 1: Body Waves:
 pass through the volume of the Earth
 two sub-types: P-waves, S-waves

P (primary) waves:
 compressional particle motion
 fastest average speeds in the crust (~ 6km/s)
 behave similar to sound waves in air
 can pass through solid, liquid and gas

S (secondary/shear) waves:
 second fastest wave (~ 3.5 km/s)
 shearing particle motion (side to side mov't with respect to the wave
propagation direction)
 only passes through solids
 more damaging to structures because of the shearing ground motion
o type 2: Surface Waves:
 confined to the very upper surface of the crust
 are slower than both P & S waves
 most damaging to structures
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

two main sub-types: Love-waves, Rayleigh-waves
Love waves:
 similar to the motion of S-waves

Rayleigh waves
 rotating particle motion
Seismic record of an EQ
o major compositional zones of the Earth
 crust (two types: oceanic and continental)
1. continental crust
 thicker (30-50 km)
 less dense (granitic rocks)
 older (some regions up to ~3 billion years old)
2. oceanic crust
 thinner (~ 5km)
 more dense (gabbroic rocks)
 much younger (only up to ~200 million years old)

mantle
 extends from the asthenosphere/lithosphere boundary (Moho) down to
the outer core
 2900 km
 some indirect sampling when material (called kimberlites) is brought to
the surface from very deep rooted, explosive volcanoes
 most information comes from seismic profiles





relatively uniform composition of rock
pressure/temperature increases with depth
low-velocity zone (upper 80-300 km) in upper mantle
only a few percent molten material
critical: this zone allows Plate Tectonics to happen
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
core (two zones that are VERY different)
 enriched in iron and nickel compared to the crust/mantle
 much more dense
 mostly iron (~ 5% nickel and 1-2% sulfur)
1. inner core
 solid (caused by the very high pressures)
 known because of seismic wave behavior
2. outer core
 liquid (caused by the very high temperatures)
 known because of seismic wave behavior
 rotation of this metallic liquid creates the Earth’s magnetic field
yellow arrow: “low-velocity” zone (small amount of liquid) – both P and S waves slow
red ellipse: outer core (all liquid) – P wave slows & S wave stops
o major mechanical zones of the Earth
 upper layer (lithosphere)
 defines a tectonic plate
 brittle behavior – does not flow, hence fractures when stress is applied
 all of the crust and part of the upper mantle (compositional units) define
the lithosphere

asthenosphere below
 plastic behavior – flows when stress is applied
 contains a “low-velocity” zone – some amount of partial melting
 all mantle material
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Earthquakes
V. Earthquakes
• what are earthquakes (EQ’s)?
o ground movement caused by the release of seismic energy
o seismic energy is usually caused by the brittle failure of rocks under stress
•
important hazards to understand:
o they kill the most people per year (> 1 million in total over the past century)
o commonly strikes without precursors or warning
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o 1000's of large EQ's every year
 only ~ 20 are > M7.0 (known as “major” – M7.0 to M7.9 or “great” – larger
than M8.0)
 account for 90% of the energy released
 and 80% of all the fatalities
o recall: most of the fastest growing cities lie on the Pacific Rim and are
threatened by large EQ's
VI. Earthquakes & Faults
• where and how do they occur?
o sudden release of stored energy as the result of rapid movement between two
blocks (plates)
o energy stored because of plate movement and friction
 release point is the focus or hypocenter
 energy radiates out in all directions
 point on the surface directly above the focus is the epicenter
 fault trace: line where the fault/fracture intersects the surface
Earthquakes in CA/NV during one week: http://earthquakes.usgs.gov
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