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Geol 344: Earthquakes and Seismic Hazards
Spring 2006
Homework Set 6:
Reverse Faults and Subduction Zones
1. Like other fault types, thrust faults tend to be segmented. This
poses a problem when trying to determine the along-strike length of
blind thrust fault segments. Why is it important to determine the
lengths of these segments and what evidence might we look for (or
techniques we might use) to identify potential segment boundaries at
depth?
(5)
2. Why are there thrust faults in the Los Angeles basin? Provide a
geologic context for this region and indicate why the thrust faults
tend to be approximately east-west oriented.
(5)
3. Why do blind thrust faults in the Los Angeles basin potentially
provide a higher seismic hazard to this metropolitan region than the
nearby San Andreas fault, despite the fact that the SAF can produce
M~8 earthquakes whereas the thrust faults have historically produced
earthquakes with M<7?
(3)
4. Seismic hazards maps of the Los Angeles basin have traditionally
been based on expected ground shaking predicted for mapped faults
and as well as the seismological history of the region (e.g.,
earthquake records and instrumental seismicity measurements). In
light of the Northridge earthquake in 1994, what are the
implications of this earthquake for assessing seismic hazard maps
that were produced prior to that event?
(5)
5. Examine the attached map showing the rupture extent of the December
2004 Sumatra-Andaman earthquake (M9.0). Then answer the following
questions:
i) What is the tectonic setting of this region that resulted in
such a massive earthquake?
(3)
ii) Search through the earthquake information website that we have
used in this class to find the focal mechanism for this event.
Include it with your answer and comment on the solution,
indicating how it compares to what you know about the fault the
produced the earthquake.
(5)
iii) What is the nature of the plate boundary further south along
the coast of Sumatra, beyond the southern extent of the rupture
zone, and what may have prevented the rupture from continuing
further south than it did?
(4)
Geol 344: Earthquakes and Seismic Hazards
Spring 2006
iv) On 28th March 2005, a M8.7 earthquake struck near the island
of Nias. Where is this relative to the December earthquake, and
is there likely to be a relationship between these two events?
Explain your reasoning in terms of the phenomenon called
“triggering.”
(5)
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Geol 344: Earthquakes and Seismic Hazards
Spring 2006
Figure 1: Rupture extent of the M9.0 December 2004 Sumatra-Andaman
earthquake