Download HW #2 Earthquake Causes 1. Describe the process causing

Design for Wind & Seismic, Fall 08
HW #2
Earthquake Causes
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1. Describe the process causing earthquakes.
Rising of the earth’s molten mantle causes the earth’s crust to spread outward at mid-ocean ridges,
called rift valleys. Pieces of the earth’s crust, called plates, move relative to one another. Friction
forces develop at the boundaries between plates. Sudden release of these forces generate shock
waves which propagate outward in all directions.
2. What are the names of the two plates that meet along the west coast of the U.S.?
The North American and the Pacific Plates.
3. Name and describe the type of relative movement between the plate boundaries at:
3.1. San Francisco
The Pacific Plate moves lateral (northward) relative to the North American Plate forming a
transform boundary.
3.2.
Alaska
The Pacific Plate moves beneath (northward) the North American Plate at this subduction
zoner.
4. Which of the two types of boundary movement above generate larger earthquakes?
Subduction zones generate larger earthquakes.
5. A rupture of the earth’s crust is called a fault_ .
6. Describe the two main types of ruptures, name them, and draw a sketch for each.
Strike-Slip Fault: Blocks of the earth’s crust moves laterally. If the far block moves left relative to the
near block, it’s called “left lateral”
Normal Fault: Blocks move away from each other (normal) or toward each other (reverse or thrust).
HW #2
Earthquake Causes
Design for Wind & Seismic, Fall 08
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7. The equations for the velocities of compression waves (Vp) and shear waves (Vs) are shown below.
where K is the bulk modulus, G is the shear modulus, and  is the mass density, and the values
indicated are for typical seismic waves through the earth’s crust.
7.1 Assuming the s-wave arrives at a particular location 10 seconds after the p-wave, how far is the
earthquake’s epicenter from this location?
T P = travel time of p-wave, TS = travel time of s-wave
Tp
D
, Ts
Vp
TS
TP
10 sec
D(
D(
D
,
Vs
1
VS
1
)
VP
1
3.45 km / s
1
),
8.0 km / s
D 60.7 km
7.2 Develop an equation to easily calculate distance from the epicenter based on difference in p and
s-wave arrival times.
let T
then D
TS
T
TP
1
1
VS
1
VP
T
1
1
3.45 km / s
1
8.0 km / s
,
D
T 6.07 km / s
7.3 How could you locate an earthquake’s epicenter if you knew the locations and differences in p vs
s-wave arrival times for three seismographs? Draw a sketch as part of your answer.
On a map, draw a circle with radius = Di around the location of each seismograph, where Di is the
distance from the seismograph to the epicenter from the equation in 7.2. The intersection of the
circles represents the location of the epicenter. See example on next page.
Design for Wind & Seismic, Fall 08
HW #2
Earthquake Causes
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