Geol 344: Earthquakes and Seismic Hazards
Fall 2013
Homework Set 4: Strike-Slip Faults
1. Explain what is meant by the following terms. Include a sketch to accompany each explanation. Show the
sense of slip along the underlying fault for parts (b) and (c).
(a) en echelon
(b) left-stepping
(c) right-stepping
(2)
(2)
(2)
2. Consider the photo below taken after the Mw 7.1 Hector Mine earthquake on the Lavic Lake strike-slip
fault in the Mojave Desert region of southern California on 16 October, 1999. The jeep trail on the right
side of the photo can be used to determine the scale of observation.
a) Use a colored pen or pencil to trace out the detailed pattern of the surface ruptures on the
photo. Show every single rupture trace. Use the electronic version of the photo to see the
details of the rupture cracks, as they may be hard to see on a printed version of the image.
(4)
b) Now provide a detailed description of the geometric pattern of the surface rupture features
produced as a result of slip along the fault. Include an estimate of the width of the rupture zone
(assume that the jeep trail is 2 m wide), and comment on why it is this wide (as opposed to
being a single break at the Earth’s surface).
(4)
c) What was the amount and sense of slip along this fault (i.e., left-lateral or right-lateral) and
how can you tell based on features that were shifted laterally across the fault?
(2)
Geol 344: Earthquakes and Seismic Hazards
Fall 2013
d) If there hadn’t been visible offset features visible here, how could the pattern of surface rupture
cracks indicate the sense of slip?
(2)
e) Prior to this earthquake, geologists did not know that the Lavic Lake fault even existed!
Certainly, it had not ruptured to produce an earthquake in the last 10,000 years. Based on the
photograph, what type of material exists at the Earth’s surface here that would have made the
fault invisible to geologists prior to the 1999 earthquake?
(2)
f)
Does the exact pattern of surface rupture seen in this photograph extend all the way to the
base of the seismogenic zone (i.e., lots of small, meter-scale cracks), or is the fault different
below the Earth’s surface? Explain your answer.
(3)
3. The map below shows the seismicity of California and Nevada from 1970-2003. Felt earthquakes are
shown by blue squares; instrumental or background seismicity is shown by purple dots.
Geol 344: Earthquakes and Seismic Hazards
Fall 2013
a) Use a sharpie or colored pen to highlight the regions of the San Andreas fault that produced the
1857 Fort Tejon earthquake (M7.9) and the 1906 San Francisco earthquake (M7.8).
(4)
b) What portions of the San Andreas fault produce the least amount of background seismicity,
and what does this tell us about the earthquake behavior of those parts of the fault?
(3)
c) Some parts of the San Andreas fault show a large amount of earthquake activity. Does this
mean that these portions of the fault are most prone to producing damaging earthquakes?
Explain your reasoning.
(3)
d) Numerically label the following important features of the Pacific/North American plate boundary
on the map:
(1) Eastern California Shear Zone
(2) Imperial fault
(3) Coachella segment of the San Andreas fault
(4) Calaveras fault
(5) Hayward fault
(6) Elsinore fault
(7) San Jacinto fault
(8) The epicenter of the M7.8 1906 Great San Francisco earthquake
(9) The epicenter of the M7.2 2010 El Mayor-Cucapah earthquake
(10) The location of the M7.6 Owens Valley earthquake in 1872
(10)
[43]