Geol 344: Earthquakes and Seismic Hazards
Spring 2008
Homework Set 5: 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
(2)
The term en echelon refers to an array of mutually parallel fracture segments that are all inclined
relative to the overall orientation of the array. The entire array is typically confined to a narrow zone
that generally reflects the orientation of an underlying fault plane.
(2)
(b) left-stepping
(2)
If fractures are left-stepping, then the geometric arrangement is such that one must steer to the left
to move orthogonally from the tip of one fracture segment to the location of the adjacent fracture
segment in an en echelon array or along a fault zone.
(2)
(c) right-stepping
(2)
If fractures are right-stepping, then the geometric arrangement is such that one must steer to the
right to move orthogonally from the tip of one fracture segment to the location of the adjacent
fracture segment in an en echelon array or along a fault zone.
(2)
Geol 344: Earthquakes and Seismic Hazards
Spring 2008
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)
See red lines on photo.
(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.
(5)
As the fault ruptured through the loose sediments at the earth’s surface, it produced a
complex pattern of small ruptures (“surface breaks”) that define a zone of deformation above
the underlying fault. The rupture zone is bounded by fairly continuous breaks that are parallel
to the underlying fault and confine most of the deformation between these bounding
ruptures. Nonetheless, some short (~2-4 m long) surface breaks occur outside of the main
rupture zone, indicating a broad zone of distributed surface rupturing. Within the main
rupture zone, there are numerous short (~2-10 m long) surface breaks, many of which are
oriented oblique to the overall orientation of the fault zone, defining a left-stepping en
echelon geometry in a few locations. Using the scale provided by the 2 m wide jeep trail, the
rupture zone is consistently about 6 m wide, although the distributed deformation extends
across a width of about 13 m in places.
(5)
Geol 344: Earthquakes and Seismic Hazards
Spring 2008
c) What was the 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?
(3)
Based on the manner in which the jeep trail has been broken and offset by the fault, the
motion is right-lateral.
(3)
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)
The left-stepping, en echelon pattern of surface ruptures is also an indication of right-lateral
motion.
(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, and why wasn’t the fault
visible to geologists prior to the 1999 earthquake?
(4)
The material at the surface here is unconsolidated sediment. The area is covered by loose
surface sediments that were likely deposited by non-perennial desert rivers and streams (such
as the one visible in the photo), or perhaps alluvial fans. These sediments have been
accumulating since the fault last ruptured more than 10,000 years ago, resulting in the last
surface rupture having been covered and obscured by sediments. Therefore, geologists were
unable to recognize that a fault existed beneath these sediments.
(4)
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.
(4)
The pattern of surface ruptures does not extend all the way to the base of the seismogenic
zone. It is a near-surface phenomenon where the rupture broke rapidly through the
unconsolidated sediments at the surface. The ruptures likely all coalesce into a single fault
plane at some depth below the surface, likely beneath the surface sediments.
(4)
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.
a) Use a 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)
See red lines drawn onto the map below.
(4)
Geol 344: Earthquakes and Seismic Hazards
Spring 2008
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?
(4)
The portions of the San Andreas fault that show the least amount of seismicity are those
portions that historically produced the largest earthquakes (i.e., the regions of the 1857 Fort
Tejon earthquake rupture and the 1906 San Francisco earthquake rupture). There is also a
portion along the southern part of the San Andreas fault (Coachella Valley segment) that has
very little background seismicity, but this region has not produced a large earthquake in
historic times. The implication is that regions of low seismicity do not reflect portions of the
fault that are incapable of producing large earthquakes. In fact, the opposite may be true.
Accordingly, one might be concerned that the Coachella Valley segment of the fault may
produce a large magnitude earthquake in the future (as will the 1857 and 1906 rupture
regions when earthquakes occur there in the future).
(4)
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.
(4)
The regions of the fault showing the most amount of background activity are those that are
creeping at plate motion rate of 32-34 mm/yr. This is particularly true of the Parkfield segment
of the San Andreas fault. The constant creeping motion actually involves numerous small
Geol 344: Earthquakes and Seismic Hazards
Spring 2008
magnitude earthquakes (hence resulting in the large amount of background seismicity), but
because there is no long-term build-up of elastic strain due to the creeping, there is no
likelihood of large magnitude earthquakes.
(4)
d) Numerically label the following important features of the Pacific/North American plate
boundary on the map:
(1) Eastern California Shear Zone
(2) Walker Lane
(3) Coachella segment of the San Andreas fault
(4) The “Big Bend”
(5) Cape Mendocino
(6) Hayward fault
(7) Calaveras fault
(8) San Jacinto fault
(9) Elsinore fault
(10) The epicenter of the M7.8 1906 earthquake
See numbered locations on the map above.
(10)
(10)
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