Compilation to date
Much of the deformation at the Wrightwood site is
distributed across complicated small faults and folds that,
in the few places we have deeply exposed them, root into
low angle structures. To calculate the slip associated with
these structures we have used a cross sectional area
balancing approach often used to relate growth strata or
erosional unconformities to detachments at depth.
Event
W1857
W1812
W3
W4
W5
PC-T3
W6
W7
W8
W9
W10
W11
W12
W13
W14
Mean age
(1σ range)
1857 (Historic)
1812 (Historic)
1685 (1662-1700)
1536 (1518-1542)
1487 (1463-1502)
1360 (1343-1370)
1263 (1230-1286)
1116 (1071-1152)
1016 (981-1039)
850 (825-864)
781 (758-794)
722 (706-729)
697 (676-708)
634 (602-658)
534 (464-594)
Mean
Interval
44
130
150
49
127
97
148
101
165
70
60
31
65
110
See Sept. 2004 GSA Today for details
Offset (m),
(1σ range) 1
1.0 (0.5-2.0)
3.5 (1.0-7.0)2
3.5 (1.0-7.0) 2
7.0 (3.0-9.0)
0.7 (0.0-2.8)3
0.7 (0.0-2.8) 3
3.7 (1.9-5.6)
1.8 (1.1-3.4)
1.5 (0.7-3.1)
6.6 (3.0-9.9)
5.2 (2.5-7.5)
3.0 (1.2-6.3)
4.1 (1.4-8.2)
1.8 (1.0-5.1)
1.9 (1.0-3.8)
Summary
of the
Wrightwood
Upper Section’s
ages and offsets.
Note the huge
uncertainties in
slip. We did not
go here to
measure slip per
event. But we
could not ignore
the evidence we
were collecting,
so we did our
best to put it
together.
We know the probability of an earthquake (in M) rupturing the ground
surface.
Once the surface is ruptured we have relationships between M, L, u
(slip)
We know how slip varies along strike for a given average displacement.
So given an earthquake, we can say what the displacement (including its
uncertainty) will be at a point (trench site) on the fault.
Using data from trenches, we can determine the resolution (both
temporal and to the amount of displacement). Fundamentally based on
thickness and frequency of deposition of discrete sedimentary layers.
We also need a “rake” factor (angle of slip to bedding) and a “facies”
factor (how fast a layer changes character or thickness).
With a sedimentation rate of ~1 m/100 yrs, and individual clastic units
representing on average 5-10 years, we can distinguish events that are likely
separated by decades.
Here are two earthquakes separated by just 20-30 cms, and thus likely 20-30 years
(as the C-14 shows).
There are several transtensive stepovers within the closed basin at the
Frazier Mtn site.
We are working to construct 3d
surfaces of critical stratigraphic
horizons to quantify the folding
associated with individual paleoevents.
By comparing the folding formed
during prehistoric events to that
generated by the ~5 m of 1857 slip we
can estimate paleo-slip.
In a few places we have used 3d trenching to determine
the slip on minor faults within the deforming zone.
Note how the blue unit changes in thickness from NW
(top) to SE (bottom) and across the fault. The plot
above shows the resulting reconstruction of the lateral
component (right!) of the slip.
modern
Young
Section
12 meters
44 clastic layers
15 earthquakes
Composite Stratigraphic Column
BSSA: Biasi, 2002; Fumal, 2002; Scharer, 2007
1500 B.C.
Old
Section
4 meters
44 clastic layers
14 earthquakes
1500 A.D.
Middle
Section
3000 B.C.
Wrightwood Stratigraphy
• 6,000
years of deposition
• Marsh
capped periodically by debris
flow deposits
• No
fluvial deposits before 1850’s
Volume
Time
Sedimentation Event
Return Interval
37
yr
Cumulative Frequency
Young Section
12
yr
236
yr
Clastic Accumulation Rate
(cm/yr)
0.6 cm/yr
Cumulative Thickness (cm)
Earthquakes
2 cm/yr
Dated
0.4 cm/yr
Calendar Year (A.D.)
Interpolated
WW Young - Clastic Sedimentation Rates
Starting w/ W13
1200
avg
avg
1000
Age Range
800
600
# eqs
# DF layers
DF thickness
DF RI
634
890
5
13
43
20
890
1191
2
6
15
50
1191
1338
1
1
65
147
1338
1857
5
21
16
25
W6
400
200
C14 dated
interp
eqs
0
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
Variation around avg thickness (25 cm) - Modeled Dates
Age of debris flow
layer is age of
underlying peat.
100
clastic
80
60
Clastic (cm)
500
40
20
0
-20
-40
500
600
700
800
900
1000
1100
1200
1300
Calendar Year
1400
1500
1600
1700
1800
1900
2000
Pallett Creek
100%
90%
80%
Slip rate category
(mm/yr)
70%
<0.2
60%
0.2-1.0
50%
1-5
40%
>5
30%
20%
10%
0%
<1,600,000
<130,000
<15,000
Age Category (years)
<150
• Of the 111 UCERF-2 slip rates we can associate with
USGS Quaternary Faults Database slip rate categories,
96 UCERF rates fall in the consistent USGS category, 6
UCERF rates are too high, and 9 UCERF rates are too
low.
• We need to check the 15 that are not consistent and do
some work to associate the rest of the UCERF rates
with their USGS categories so we can check their
consistency.
• We can use the USGS Quaternary Faults Database to
assign slip rate categories to UCERF faults that we
don’t have slip rates for.