cosmogenic-36Cl, exposure-dating, paleoearthquakes, fault-scarps, Hebgen-Lake
Geological Society of America, 51st Annual Meeting, Rocky Mountain SectionApril 8-10, 1999, Pocatello, Idaho.
TIMING OF LATE QUATERNARY EARTHQUAKES ON THE HEBGEN LAKE FAULTBY COSMOGENIC CHLORINE-36 DATING OF BEDROCK FAULT SCARP
ZREDA, Marek, Department of Hydrology and Water Resources, University of Ari-zona, Tucson, AZ 85721, [email protected]; NOLLER, Jay S., Depart-ment of Geology, Vanderbilt University, Nashville, TN 37235.
Fault scarps along the Hebgen Lake fault, Montana, recorded multiple large pale-oearthquakes, including the most recent earthquake in 1959. We used cosmogenic36Cl in bedrock scarp faces exposed at the surface due to recurring faulting to deter-mine ages of paleoearthquakes at Hebgen Lake. The technique measures how long thedifferent, episodically offset parts of the scarp have been exposed to cosmic radiation.
Twenty-seven samples collected every 0.5 m from the bottom (0 m) to the top (12m) of the scarp yielded the following exposure ages: 0.4 (for the 1959 scarp), 1.7, 2.6,7.0, 20, 24 and 37 ky (maximum age).The data indicate two periods ofheightened earthquake activity duringwhich the displacement occurred:from 0 to 7 kyr ago and from 20 to 24kyr ago, and two periods of quies-cence: from 7 to 20 kyr and from 24 to37 kyr. This temporal pattern sug-gests that the Hebgen Lake fault maybe cyclic, with period of 15-20 kyr,presently in its active state. The aver-age displacement rate during the twoactive periods is about 1 m/kyr, twiceas high as that calculated over theentire geological history of the faultrecorded in the scarp.
Cosmogenic 36Cl age (kyr)
0 10 20 30
Dis
plac
emen
t (m
)
0
2
4
6
8
10
12
14
measured dataone average ratepiecewise average
Presentation outline
(1) Title [original graphics](2) Goal[original graphics]
Approach and methods
(3) Production and accumulation of36Cl [original graphics](4) Corrections[original graphics](5) Subsurface distribution of cosmic rays[original graphics](6) Episodic exposure of scarp[original graphics](7) Episodic exposure of fault scarp[original graphics]
Location and samples(8) Hebgen Lake map[original graphics](9) Hebgen scarp[original slide](10) Hebgen scarp[original slide](11) Hebgen scarp[original slide](12) Hebgen scarp[original slide]
Results and discussion
(13) Apparent36Cl ages[original graph]
(14) Corrected36Cl ages[table]
(15) Corrected36Cl ages[original graph](16) Clustering[original graph](17) Vertical slip rates[table]
Conclusions(18) Validity of dating approach[text](19) Advantages[text](20) Conclusions[text]
Fault scarps along the Hebgen Lake fault, Montana, recorded multiplelarge paleoearthquakes, including the most recent earthquake in1959. We used cosmogenic 36Cl in bedrock scarp faces exposed atthe surface due to recurring faulting to determine ages of pale-oearthquakes at Hebgen Lake. The technique measures how long thedifferent, episodically offset parts of the scarp have been exposed tocosmic radiation.Twenty-seven samples collected every 0.5 m from the bottom (0 m) tothe top (12 m) of the scarp yielded the following exposure ages: 0.4(for the 1959 scarp), 1.7, 2.6, 7.0, 20, 24 and 37 ky (maximum age).The data indicate two periods of heightened earthquake activity duringwhich the displacement occurred: from 0 to 7 kyr ago and from 20 to24 kyr ago, and two periods of quiescence: from 7 to 20 kyr and from24 to 37 kyr. This temporal pattern suggests that the Hebgen Lakefault may be cyclic, with period of 15-20 kyr, presently in its activestate. The average displacement rate during the two active periods isabout 1 m/kyr, twice as high as that calculated over the entire geologi-cal history of the fault recorded in the scarp.
ÿþýüûúùýüøþ÷úöõõõ
Dating of paleoearthquakes
Marek Zreda&
Jay Noller
Pocatello, April 1999
BY COSMOGENICCHLORINE-36IN FAULTSCARPS
Fault scarps along the Hebgen Lake fault, Montana, recorded multiplelarge paleoearthquakes, including the most recent earthquake in1959. We used cosmogenic 36Cl in bedrock scarp faces exposed atthe surface due to recurring faulting to determine ages of pale-oearthquakes at Hebgen Lake. The technique measures how long thedifferent, episodically offset parts of the scarp have been exposed tocosmic radiation.Twenty-seven samples collected every 0.5 m from the bottom (0 m) tothe top (12 m) of the scarp yielded the following exposure ages: 0.4(for the 1959 scarp), 1.7, 2.6, 7.0, 20, 24 and 37 ky (maximum age).The data indicate two periods of heightened earthquake activity duringwhich the displacement occurred: from 0 to 7 kyr ago and from 20 to24 kyr ago, and two periods of quiescence: from 7 to 20 kyr and from24 to 37 kyr. This temporal pattern suggests that the Hebgen Lakefault may be cyclic, with period of 15-20 kyr, presently in its activestate. The average displacement rate during the two active periods isabout 1 m/kyr, twice as high as that calculated over the entire geologi-cal history of the fault recorded in the scarp.
ÿþýüûúùýüøþ÷úöõõõ
Goal
Dating ofbedrock fault scarps
Measuringdisplacement rates
To determine theapplicability ofcosmogenic36Cl to:
ÿþýüûúùýüøþ÷úöõõõ
Production and accumulation of36Cl
neutron activation:35Cl (n,γ) 36Cl
spallation:39K (n, 2n2p) 36Cl40Ca (n, 2n3p)36Cl
negative muon capture:40Ca (µ-, α) 36Cl
Time
36C
l/Cl
Pλλλλ--- 1 e λλλλt––( )
ÿþýüûúùýüøþ÷úöõõõ
Corrections
Global:
Latitude
Elevation
Local:
Topographic shielding
Subsurface production
Corrected 36Cl ages
Apparent 36Cl ages
ÿþýüûúùýüøþ÷úöõõõ
Subsurface distribution of cosmic rays
e0
e-4
e-3
e-2
e-1
0 200 400 600
Depth, g cm-2
Rel
ativ
e pr
oduc
tion
rate
slow muons
thermal neutronsfast neutrons
ÿþýüûúùýüøþ÷úöõõõ
Episodic exposure of scarp
E A F
B
C
D
E
A F
B
C
D
E
A F
B
C
D
E
A F
B
C
D
E
A F
B
C
D
t = 0
t = t1
t = t2
t = t3
t = tnow
First faulting episodeFace AB exposedCosmogenic buildup in ABSlower buildup in BC and CD
Second faulting episodeFace BC exposedBuildup continues in ABBuildup starts in BCSlower buildup in CD
Third faulting episodeFace CD exposedBuildup continues in AB, BCBuildup starts in CD
AB exposed from t1 till nowBC exposed from t2 till nowCD exposed from t3 till now
Prior to faultingSlow buildup of cosmogenic 36Cl below the surface
ÿþýüûúùýüøþ÷úöõõõ
Episodic exposure of fault scarp
0
2
4
6
8m
0 10 20
-6
-4
-2
Age (103 years)������������������������
yyyyyyyyyyyyyyyyyyyyyyyy
15
2now
8
bedrock
air
15
8
2
ÿþýüûúùýüøþ÷úöõõõ
Hebgen Lake area
P
PP
P
P
PzP
P
M
M
Q
Q
Q
Q
Q
pC
pC
pC pC
Hebgen Lake
Earthquake Lake
SCARP
Red Canyon FaultHE
BG
EN
L A K EF A
UL T
Montana
Explanation:Q QuaternaryM MezozoicP PaleozoicpC Precambrian
1959 surface rupturesurface rupture
111˚22.5' 111˚15' 111˚7.5'
44˚52.5'
44˚45'0 1 2 3 km
N
Fault scarps along the Hebgen Lake fault, Montana, recorded multiplelarge paleoearthquakes, including the most recent earthquake in1959. We used cosmogenic 36Cl in bedrock scarp faces exposed atthe surface due to recurring faulting to determine ages of pale-oearthquakes at Hebgen Lake. The technique measures how long thedifferent, episodically offset parts of the scarp have been exposed tocosmic radiation.Twenty-seven samples collected every 0.5 m from the bottom (0 m) tothe top (12 m) of the scarp yielded the following exposure ages: 0.4(for the 1959 scarp), 1.7, 2.6, 7.0, 20, 24 and 37 ky (maximum age).The data indicate two periods of heightened earthquake activity duringwhich the displacement occurred: from 0 to 7 kyr ago and from 20 to24 kyr ago, and two periods of quiescence: from 7 to 20 kyr and from24 to 37 kyr. This temporal pattern suggests that the Hebgen Lakefault may be cyclic, with period of 15-20 kyr, presently in its activestate. The average displacement rate during the two active periods isabout 1 m/kyr, twice as high as that calculated over the entire geologi-cal history of the fault recorded in the scarp.
ÿþýüûúùýüøþ÷úöõõõ
Hebgen Lake fault scarp
Fault scarps along the Hebgen Lake fault, Montana, recorded multiplelarge paleoearthquakes, including the most recent earthquake in1959. We used cosmogenic 36Cl in bedrock scarp faces exposed atthe surface due to recurring faulting to determine ages of pale-oearthquakes at Hebgen Lake. The technique measures how long thedifferent, episodically offset parts of the scarp have been exposed tocosmic radiation.Twenty-seven samples collected every 0.5 m from the bottom (0 m) tothe top (12 m) of the scarp yielded the following exposure ages: 0.4(for the 1959 scarp), 1.7, 2.6, 7.0, 20, 24 and 37 ky (maximum age).The data indicate two periods of heightened earthquake activity duringwhich the displacement occurred: from 0 to 7 kyr ago and from 20 to24 kyr ago, and two periods of quiescence: from 7 to 20 kyr and from24 to 37 kyr. This temporal pattern suggests that the Hebgen Lakefault may be cyclic, with period of 15-20 kyr, presently in its activestate. The average displacement rate during the two active periods isabout 1 m/kyr, twice as high as that calculated over the entire geologi-cal history of the fault recorded in the scarp.
ÿþýüûúùýüøþ÷úöõõõ
Scarp - close up
Fault scarps along the Hebgen Lake fault, Montana, recorded multiplelarge paleoearthquakes, including the most recent earthquake in1959. We used cosmogenic 36Cl in bedrock scarp faces exposed atthe surface due to recurring faulting to determine ages of pale-oearthquakes at Hebgen Lake. The technique measures how long thedifferent, episodically offset parts of the scarp have been exposed tocosmic radiation.Twenty-seven samples collected every 0.5 m from the bottom (0 m) tothe top (12 m) of the scarp yielded the following exposure ages: 0.4(for the 1959 scarp), 1.7, 2.6, 7.0, 20, 24 and 37 ky (maximum age).The data indicate two periods of heightened earthquake activity duringwhich the displacement occurred: from 0 to 7 kyr ago and from 20 to24 kyr ago, and two periods of quiescence: from 7 to 20 kyr and from24 to 37 kyr. This temporal pattern suggests that the Hebgen Lakefault may be cyclic, with period of 15-20 kyr, presently in its activestate. The average displacement rate during the two active periods isabout 1 m/kyr, twice as high as that calculated over the entire geologi-cal history of the fault recorded in the scarp.
ÿþýüûúùýüøþ÷úöõõõ
Scarp - side view
Fault scarps along the Hebgen Lake fault, Montana, recorded multiplelarge paleoearthquakes, including the most recent earthquake in1959. We used cosmogenic 36Cl in bedrock scarp faces exposed atthe surface due to recurring faulting to determine ages of pale-oearthquakes at Hebgen Lake. The technique measures how long thedifferent, episodically offset parts of the scarp have been exposed tocosmic radiation.Twenty-seven samples collected every 0.5 m from the bottom (0 m) tothe top (12 m) of the scarp yielded the following exposure ages: 0.4(for the 1959 scarp), 1.7, 2.6, 7.0, 20, 24 and 37 ky (maximum age).The data indicate two periods of heightened earthquake activity duringwhich the displacement occurred: from 0 to 7 kyr ago and from 20 to24 kyr ago, and two periods of quiescence: from 7 to 20 kyr and from24 to 37 kyr. This temporal pattern suggests that the Hebgen Lakefault may be cyclic, with period of 15-20 kyr, presently in its activestate. The average displacement rate during the two active periods isabout 1 m/kyr, twice as high as that calculated over the entire geologi-cal history of the fault recorded in the scarp.
ÿþýüûúùýüøþ÷úöõõõ
Scarp - sampling
ÿþýüûúùýüøþ÷úöõõõ
Apparent 36Cl ages
Scarp height (m)
App
aren
t 36 C
l age
(10
3 ye
ars)
0 2 4 6 8 10 12
0
10
20
30
40
0 1 2 3 4
1
2
3
4
12
3
4
5
61
2
3
ÿþýüûúùýüøþ÷úöõõõ
Surface exposure ages
Height [m]
9.1 - 11.4
7.2 - 9.1
5.1 - 7.2
3.8 - 5.1
2.6 - 3.8
0.5 - 2.6
Age [ky]
23.8 ± 1.1
20.3 ± 1.0
7.0 ± 1.5
2.6 ± 0.3
1.7 ± 0.2
0.4 ± 0.5
ÿþýüûúùýüøþ÷úöõõõ
Corrected 36Cl ages
Scarp height (m)
Mod
el a
ge (
103
year
s)
0 2 4 6 8 10 120
10
20
30
40
0
1
2
3
0 1 2 3 4 5
1 2 3
4
5
6
12
3
ÿþýüûúùýüøþ÷úöõõõ
Temporal clustering of earthquakes
Cosmogenic 36Cl age (kyr)
0 10 20 30
Dis
plac
emen
t (m
)
0
4
8
12
measured data
one average rate
piecewise average
ÿþýüûúùýüøþ÷úöõõõ
Vertical slip rates
Time [ky]
0 - 2.6
0 - 7.0
0 - 20
0 - 24
0 - 37
7.0 - 20
20 - 24
Slip rate [m/ky]
2.0
1.0
0.45
0.5
0.33
0
1.25
ÿþýüûúùýüøþ÷úöõõõ
Validity of 36Cl approach
��������Good chronology36Cl ages follow predicted pattern
they are compatible with scarp degradation data
��������Reasonable displacement rates
comparable to recent measurements
��������Clearly-defined clusters
ÿþýüûúùýüøþ÷úöõõõ
Advantages of36Cl approach
��������Can date previously undatable bedrockfault scarps
��������Can obtain complete record of multipleearthquakes at single site
��������Dating range: 10 3 - 105.5 years
��������Dating precision and accuracy: 10-20%
ÿþýüûúùýüøþ÷úöõõõ
Conclusions
Cosmogenic36Cl dating of bedrock fault scarps isfeasible
At Hebgen Lake, earthquakes are clustered in twotime intervals: 0-7 ky and 20-24 ky
During active periods vertical slip rate is 1-2 m/ky
Long-term vertical slip rate is 0.5 m/ky