Ray Weldon - WGCEP

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22 Φεβ 2014 (πριν από 3 χρόνια και 8 μήνες)

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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
Mean age
(1
σ
range)
Mean
Interval
Offset (m),
(1
σ
range)
1
W1857
1857 (Historic)
44
1.0 (0.5-2.0)
W1812
1812 (Historic)
130
3.5 (1.0-7.0)
2
W3
1685 (1662-1700)
150
3.5 (1.0-7.0)
2
W4
1536 (1518-1542)
49
7.0 (3.0-9.0)
W5
1487 (1463-1502)
127
0.7 (0.0-2.8)
3
PC-T
3
1360 (1343-1370)
97
0.7 (0.0-2.8)
3
W6
1263 (1230-1286)
148
3.7 (1.9-5.6)
W7
1116 (1071-1152)
101
1.8 (1.1-3.4)
W8
1016 (981-1039)
165
1.5 (0.7-3.1)
W9
850 (825-864)
70
6.6 (3.0-9.9)
W10
781 (758-794)
60
5.2 (2.5-7.5)
W11
722 (706-729)
31
3.0 (1.2-6.3)
W12
697 (676-708)
65
4.1 (1.4-8.2)
W13
634 (602-658)
110
1.8 (1.0-5.1)
W14
534 (464-594)
1.9 (1.0-3.8)
See Sept. 2004
GSA Today

for details

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 step
-
overs 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 paleo
-

events.


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

1500 A.D.

1500 B.C.

3000 B.C.

Young

Section

Old

Section

BSSA: Biasi, 2002; Fumal, 2002; Scharer, 2007

Middle

Section

12 meters

44 clastic layers

15 earthquakes

4 meters

44 clastic layers

14 earthquakes

Composite Stratigraphic Column


6,000 years of deposition


Marsh capped periodically by debris
flow deposits


No fluvial deposits before 1850’s


Volume

Time

Wrightwood Stratigraphy

Dated

Interpolated

Calendar Year (A.D.)

Cumulative Thickness (cm)

Cumulative Frequency

Sedimentation Event

Return Interval

Clastic Accumulation Rate

(cm/yr)

37
yr

0.6 cm/yr

Young Section

Earthquakes

2 cm/yr

12
yr

236
yr

0.4 cm/yr

Variation around avg thickness (25 cm) - Modeled Dates
-40
-20
0
20
40
60
80
100
500
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
Calendar Year
Clastic (cm)
cl asti c
WW Young - Clastic Sedimentation Rates
0
200
400
600
800
1000
1200
500
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
C14 dated
i nterp
eqs
Age of debris flow
layer is age of
underlying peat.









avg

avg



Age Range



# 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

W6

1338

1857

5

21

16

25

















Starting w/ W13

Pallett

Creek

0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
<1,600,000
<130,000
<15,000
<150
Age Category (years)

<0.2
0.2-1.0
1-5
>5
Slip

rate category

(mm/yr)


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.