Geol 344: Earthquakes and Seismic Hazards
Spring 2011
Homework Set 4: Earthquake Locating and Focal Mechanism Solutions
1. On 27 June 1988, a magnitude 5.4 earthquake was recorded at three Californian seismograph
stations (BKS, JAS, and MIN), as shown on the attached map. The difference between P and S wave
arrival times are shown in the table below, along with the calculated distances to the epicenter
based on the P-S time interval.
California Seismograph Station Data
S-P
Distance
(sec)
(km)
BKS
21.0
190
JAS
20.4
188
MIN
12.9
105
Station
(a) Use this data to determine the location of the earthquake epicenter on the attached map. You
must use a compass to draw the circles around the seismograph stations. Note the scale on the
map for reference.
(6)
(b) It was not expected that an earthquake would occur along the faults in this area for a long time
in the future. Examine your determined epicenter location relative to nearby features and use this
to develop a hypothesis regarding why this earthquake occurred where it did and when it did.
You must explain your answer in the context of either the Coulomb failure criterion or what may
have happened on a Mohr circle failure diagram representing the region where the earthquake
occurred.
(4)
2. (i) Could a seismograph attached to a buoy just below the ocean surface detect the seismic waves
from an earthquake that occurred below the ocean floor? Explain your answer.
(3)
(ii) Would it be possible to calculate the distance to this earthquake from the seismogram? Again,
explain your reasoning.
(2)
Geol 344: Earthquakes and Seismic Hazards
Spring 2011
3. Considering that the Richter magnitude scale for earthquakes is a logarithmic scale, how much
more energy would be released by a M9.0 earthquake than by a M7.0 earthquake? Show your
calculations.
(2)
Geol 344: Earthquakes and Seismic Hazards
Map for Question 1:
Spring 2011
Geol 344: Earthquakes and Seismic Hazards
Spring 2011
4. The figure below shows focal mechanism solutions for three earthquakes. The shaded areas
represent the compressional regions in first arrivals at seismograph stations. The letters P and T
refer to the compressional and extensional strain axis orientations, respectively.
A.
B.
--------P
#---------------####----------------#######-----------------##########------------------############------------------T#############--------------####
###############----------########
################-----############
################-################
#############----################
##########--------###############
#####-------------#############
-------------------############
-------------------##########
------------------#######
-----------------####
----------------#
-------
C.
a) Draw the lines onto the fault plane
solutions to show the orientations of the two fault
planes in each double-couple solution. Don’t forget,
fault planes always intersect the edges of the circle
at two points that are exactly on opposite sides of
the circle, 180º apart.
(6)
b) If there is any strike-slip motion on any of
the faults, indicate this using arrows along the fault
lines (i.e., indicate whether left-lateral or right-lateral). (3)
Geol 344: Earthquakes and Seismic Hazards
c)
Spring 2011
Use a table to document the following information for EACH of the six faults. Be sure to use
the right-hand rule when determining the fault strike.
(18)
I.
fault strike (in degrees)
II.
fault dip (in degrees)
III.
dip direction (compass direction)
IV.
fault type (if a strike-slip component exists, mention whether left-lateral or right-lateral)
5. (a) Use the focal mechanism (aka moment tensor) data below to construct a focal mechanism
diagram for the MW 7.4 Izmit earthquake in Turkey in 1999. Use the circle provided on the
attached sheet to create your focal mechanism. Your strikes must be plotted with 100% accuracy
so use a protractor to find the right angle around the circle. Use the attached stereonet to
determine the curvature of the fault plane on the focal mechanism. Your dip must be accurate to
within 5º accuracy. Remember, dips increase away from the circle boundary (0º dip) towards the
circle center (90º dips). For both fault solutions, the strike direction is given below using the righthand rule. FP = fault plane.
Also, use the orientations of the principal strain axes to identify the shaded and unshaded
quadrants of the focal mechanism solution. Plot the locations of these strain axes as points on
your focal mechanism.
(12)
99/08/17 00:01:38.56 TURKEY
MOMENT TENSOR SOLUTION
Best Double Couple:
Principal strain axes:
FP1: Strike=092 Dip=75 S
T Azimuth= 049
FP2: Strike=183 Dip=88 W
P Azimuth= 317
(b) What are the two types of faults (i.e., the sense of slip) that potentially produced the earthquake?(4)
(c) The earthquake occurred along the North Anatolian fault. Given this information, which fault plane
solution is more likely to be the correct one, and why?
(2)
Geol 344: Earthquakes and Seismic Hazards
Spring 2011
Geol 344: Earthquakes and Seismic Hazards
Spring 2011
Focal mechanism for Question 5:
6. On the attached figure, seismograph locations are plotted on a lower hemisphere stereographic
projection with respect to an earthquake epicenter at the center of the projection. Stations with
compressive first arrival P-waves are shaded squares; dilatational first arrivals are open circles.
Geol 344: Earthquakes and Seismic Hazards
Spring 2011
(a) Use this information to construct a focal mechanism solution using the double-couple assumption.
Shade in the compressional areas. Remember, the two fault planes must divide up the
compressional first-arrival quadrants from the dilatational first-arrival quadrants. The easiest way to
do this is to figure out where the two lines should intersect the outside of the circle in order to
separate the shaded squares from the open circles. Then determine how curved each line needs to
be. Remember, any fault plane always intersects the edges of the circle at two points spaced 180º
apart. Also, the two fault planes must be mutually perpendicular in 3-D space (try to picture it in
your head to see if the two planes your drew obey this requirement).
(8)
(b) What are the strike, dip, dip direction, and slip sense of each fault that was potentially responsible
for this earthquake?
(6)
[76]
Geol 344: Earthquakes and Seismic Hazards
Question 6:
Spring 2011