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Lecture 11

# GEOL 106 Lecture Notes - Lecture 11: 1989 Loma Prieta Earthquake, Love Wave, Power Law

Department
Geological Sciences and Geological Engineering
Course Code
GEOL 106
Professor
David Alan Hanes
Lecture
11

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Geology 106: Week 4, Lecture 11
How frequent are earthquakes?
Graph of global number of earthquakes per year vs. magnitude =
straight line (magnitude of 10 – both are log scales)
oPower law relationship: on al log-log plot, you get a straight
line
oBig earthquakes happen rarely, oodles of small ones
Magnitude
o< 3  over 100,000/year
o> 3  over 30,000/year
o> 6  100/year
o>7  20/year
1. What exactly causes damage?
Determine this by:
a) Empirical observations
b) Laboratory experiments
c) Computer modelling
In general
1. The closer you are to the earthquake the greater the damage
2. The greater the magnitude the greater the damage
BUT this can be modi9ed by natural (nature of soils/rocks) and
anthropogenic (nature of the structures that we build) conditions
What causes damage?
1. Surface faulting
Structures on the fault wull be disrupted by the tearing motion
So don’t build on active faults  but people do (eg. San Francisco)
2. Ground Shaking
Generally the greatest threat to buildings and people  in a big
earthquake, the shaking can be severe even 100’s of kilometers
away from the epicenter
Most ground shaking is generally a direct consequence of surface
wave motion (Love waves and Rayleigh waves)
oAnd if you are close to the epicenter, the P and S waves
arrival
Material ampli9cation e@ect: when seismic waves slow down as
the go into another material  some of the energy is transferred
into greater shaking
oVelocity is highest in the rock (least shaking)  sand soil
(intermediate shaking)  and lower in the clay soil (most
shaking)
oMost severe = water saturated clay soil