VOCABULARY
Paleoseismology:
the
study of ancient
earthquakes.
Q
Transparencies made from Master 2.3f, Ditch Creek, 1830 to 1994
Q
Student copies of Master 2.3g, Ditch Creek Template
PART ONE
S
AG
P
OND
P ROC E DUR E
A. Introduction
Ask students: Have your parents or grandparents experienced an
earthquake in their lifetime? Explain: Geologists assume that the
earthquakes we observe today are similar to those that happened 50
years ago, 100 years ago, and even before human beings recorded
history. Their impact may be different because of differences in
human population patterns, but geologic processes and the natural
principles that govern them have operated in essentially the same way
throughout geological time. This assumption is expressed in the
principle of uniformitarianism. By studying the traces of recurring
earthquakes, those that have happened numerous times in the same
area, we can speculate about the history of very old earthquake events
and even make general predictions about the future. Introduce the
principle of superposition and the principle of cross-cutting
relationships. The activity that follows uses a generalized model of a
strike-slip fault to illustrate these principles.
B. Lesson Development
1. Project the first transparency made from Master 2.3d, Sag Pond,
1830 to 1993. Tell students that the diagrams they see show the effect
of faulting in successive earthquakes. Because faulting causes surface
dislocations, certain types of faulting will form hills and valleys in the
Earth’s surface. With strike-slip faulting, if movement occurs as
shown by the arrows, the following topography can be created:
Peat:
a deposit of semicarbonized
plant remains in a water-saturated
environment. Peat is an early stage
in the development of coal.
Principle of crosscutting
relationships:
the principle stating
that a rock is always younger than
any other rock across which it cuts.
Earthquake faulting illustrates this
principle: Faults are always younger
than the rocks they cut.
Principle of superposition:
the
principle upon which all geologic
chronology is based, stating that in
any sequence of sedimentary layers
that has not been overturned or
faulted, each layer is younger than
the one beneath, but older than the
one above it.
Principle of uniformitarianism:
the
fundamental principle stating that
geologic processes have operated in
essentially the same way throughout
geological time.
Recurrence interval:
the actual or
estimated length of time between
two earthquakes in the some
location.
Sag pond:
a small body of water
occupying an enclosed depression
formed by strike-slip fault movement.
Strike-slip faulting:
faulting in which
movement is horizontal.
TEACHING CLUES
AND CUES
If students have
questions at this point,
tell them that the activity
that follows may answer
them. Any questions that are not
answered by the activity can be dealt
with in the concluding discussion.
Right Lateral Strike-Slip Fault
X = area where ground
is being pulled apart
or extended, forming a sagpond
.
Left Lateral Strike-Slip Fault
Y = area where ground
is being squeezed or compressed, forming a
hill or ridge.
A G U
/
F E M A
70
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E I S M I C
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L E U T H S
