Geology Reference
In-Depth Information
One EQ: regional uplift with
coeval local fault offset
One EQ: regional uplift only
fault
fault
1
paleo-sea
c
liff
paleo-sea
cliff
1
1
faulting events
offset raised
platform
offset raised
platform
raised platform
modern sea cliff
modern sea cliff
modern platform
modern platform
modern platform
A
B
Two EQs: local offset first; coeval
regional & local offset second
fault
Two EQs: both local offset only
fault
1,2
1,2
2
2
modern
sea cliff
paleo-sea cliff
higher raised p
latform
paleo-sea cliff
higher
raised platform
paleo-sea cliff
offset lower
raised platform
off
set lower raised pla
tform
lower raised platform
modern
sea cliff
modern platform
modern
platform
modern platform
C
D
modern platform
Fig. 6.2
Local surface uplift due to slip on surface-rupturing faults versus buried faults.
Displaced marine abrasion platforms (Fig. 2.1) provide a means to discern both local and regional uplift due to
surface-rupturing faults and buried faults, respectively. A vertical, dip-slip, surface-rupturing fault (up on the right-
hand side) occurs in the center of each scenario. The presence of a geomorphic marker (an abrasion platform) tied
to sea level permits recognition of displacement due to buried faults. Inset on upper right of each figure depicts
the activity and sequence of each fault: solid line for a ruptured fault plane; dashed line for an unruptured plane.
A. A single earthquake on the buried fault causes regional uplift that yields a raised platform, a paleo-sea cliff, and
modern sea cliff in a new position. No differential slip occurs on the fault at the surface. B. With coeval slip on both
fault surfaces, the right-hand side is differentially uplifted, creating a higher modern sea cliff and a higher raised
platform on that side. C. Two events that slip only on the splay create a second paleo-sea cliff and a second raised
platform on the right-hand side only. D. Two events, the first on the splay only and then on both the splay and buried
faults, create a complex pattern of features. Note that the alignment of the modern sea cliff all the way across the
modern platform indicates that the buried fault slipped in the second event. Otherwise, the sea cliff on the left would
align with the paleo-sea cliff on the right. Modified after Kelsey
et al
. (2008).
extensive chronology of past events. Two
different strategies are typically used for ori-
enting trenches with respect to faults. In most
situations, a single trench is excavated per-
pendicular to the trend of the fault. The
stratigraphy and structures revealed in the walls
of the trench are meticulously surveyed and
mapped, material for dating of various strati-
graphic horizons is collected, and a paleo-
earthquake history is interpreted based on these
data. Alternatively, along strike-slip faults, two
trenches parallel to the fault trace are sometimes
excavated (Fig. 6.3A). The stratigraphy on the
faces nearest to the fault trace is mapped,
surveyed, and dated in each fault. Special
attention is paid to linear features, such as
channels, planar crossbeds, shoreline features,
or unusual bedding configurations, that trend
approximately perpendicular to the fault. After
mapping the two trench walls parallel to the
length of the fault, the intervening strata that are
cut by the fault are “salami sliced” perpendicular
to the fault trace (Fig. 6.3B), which is to say that
they are incrementally cut back along vertical
planes. Each stratigraphic feature that could act
as a piercing point is traced to the fault and the
magnitude of offset
vis-à-vis
the correlative
feature on the opposite side of the fault is
