Earthquakes

4 Pages 1043 Words

Earthquakes

I. Seismic Waves to Study Earth’s Interior
A. Travel Time, Reflection, Refraction of P & S Waves
1. Travel time of body waves gives us VP, VS, and  (density) as a function of depth.
2. Velocity of P & S waves increases with depth.
B. Earth’s Layered Composition
1. Crust
a. Quartz, feldspar, and basalt.
b. Continental Crust
• Mostly quartz and feldspar.
• Lower density than oceanic crust.
• Variable thickness (10-50 km).
c. Oceanic Crust
• Mostly basalt.
• Higher density than continental crust.
• Uniform thickness (10 km).
d. Isostacy
• Crust floats in equilibrium on the denser mantle beneath.
2. Mantle
a. Ultramafic Rocks
• Olivine.
• Pyroxene.
b. Low velocity zone: Convection in mantle distributes heat evenly.
c. Polymorphic phase transition
• 400 km and 660 km.
• Pressure changes the crystal structure of minerals causing a slow in seismic wave velocity.
3. Core
a. Core-Mantle boundary 2913 km.
b. Nickel and FE.
c. Inner core is 6271 km deep.
d. Outer core boundary is 2900 km deep.
e. Liquid outer core
• TM: Melting temperature of ultramafic rocks.
• S waves cannot propagate through the outer core (liquid).
• Iron, oxygen, and silicon.
C. Mantle is mostly olivine.
1. Seismic waves
a. VS, VP, and  measured in lab match seismic measurements.
2. Zenolith
a. Mantle rocks found in volcanoes with deep roots contain olivine.
b. Kimberlite Volcanoes
• Mantle material brought up very quickly contains olivine and diamonds. Do not erupt anymore.
3. Ophiolite Suite
a. Oceanic crest that’s been abducted onto continental crust contains olivine and the moho.
4. Primitive meteorites
a. Examining carboneous chondrites (primitive meteorites) with olivine.
D. Iron Catastrophy
1. This is how the earth became layered.
2. 4.3 billion years ago.

II. Interpreting Seismograms
A. Seismograms
1. Short period instruments: T 1 second (body waves)....