Exploring uncertainties in millennial slip rates along the eastern Kunlun fault, NE Tibet Nathan Harkins PSU Geosciences SCEC Joint Fault System History

Exploring uncertainties in millennial slip rates along the eastern Kunlun fault, NE Tibet

Nathan Harkins Nathan Harkins PSU GeosciencesPSU Geosciences

SCEC Joint Fault System History / SCEC Joint Fault System History / SoSAFE WorkshopSoSAFE Workshop

1/31/20081/31/2008

Eric Kirby and Xuhua Shi

Motivation: Terrace risers as slip markers

Bounding terrace ages provide a range of allowable rates


Bounding terrace ages provide a range of allowable rates

Many workers assume riser age = age of lower bounding terrace


More than 50% of slip rate estimates in Tibet from offset terracesMore than 50% of slip rate estimates in Tibet from offset terraces


Many of these sites possess wide ranges of allowable ratesMany of these sites possess wide ranges of allowable rates

Data from Data from Cowgill et Cowgill et al. 2006; al. 2006; Meriaux et Meriaux et al. 2004; al. 2004; Van Der Van Der Woerd et al. Woerd et al. 20022002

Rates in m/ka


Many of these sites possess wide ranges of allowable ratesMany of these sites possess wide ranges of allowable ratesExplore uncertainties, reduce rage of allowable ratesExplore uncertainties, reduce rage of allowable rates

Data from Data from Cowgill et Cowgill et al. 2006; al. 2006; Meriaux et Meriaux et al. 2004; al. 2004; Van Der Van Der Woerd et al. Woerd et al. 20022002

Eastern Kunlun fault field site:

Flights of offset terrace Flights of offset terrace risers at 2 locationsrisers at 2 locations

~3 km from one another~3 km from one another

Eastern Site

Western Site Corona high altitude photography

100 m

N

-Well preserved terrace flight -Well preserved terrace flight on east side of channelon east side of channel

-Single fault trace -Single fault trace

The eastern slip-rate site (Ken Mu Da):

View direction

Corona high altitude photography

Radiocarbon age control at the eastern site:

T1 ~5,000 cal YBPT1 ~5,000 cal YBP

T3 ~12,500 cal YBPT3 ~12,500 cal YBP

T4 ~15,400 cal YBPT4 ~15,400 cal YBP

T4 surface Loess

GravelSample

offset MIN rate MAX rate

T1/T2 31 ± 5 m 6.0 ± 1.0 T2/T3 52 ± 3 m 4.2 ± 0.3 T3/T4 62 ± 3 m 4.0 ± 0.3 4.9 ± 0.2

Best estimate of slip rate at the eastern site…

100 m

N

-Broad terrace treads -Broad terrace treads (>400 m wide)(>400 m wide)

-Two fault strands-Two fault strands

The western slip-rate site (Deng Qin):View direction

200 m

N

Corona high altitude photography

Radiocarbon age control at the western site:

T2 T2 older thanolder than 3,370 YBP3,370 YBP

T4 ~16,400 cal YBPT4 ~16,400 cal YBP

T5T5 older thanolder than ~28,500 YBP ~28,500 YBP

T4

Loess

Gravel

Sample

offset MIN rate MAX rate

T2/T4 60 ± 5 m 3.7 ± 0.4 17.2 ± 2.0 T4/T5 115 ± 40 m 3.9 ± 1.2 7.2 ± 2.5

T2 T2 older than 3,370 YBPolder than 3,370 YBP T4 ~cal 16,400 YBPT4 ~cal 16,400 YBP T5T5 older thanolder than 28,500 28,500 YBP YBP Rates in m/ka

Best estimate of slip rate at the western site…

200 m

N

Summary of slip-rates..…

Ken Mu Da 3.9-5.1 m/ka

Deng Qin 2.7-19.2 m/ka

• • Sites < 3 km apart - spatial variation in rate unlikelySites < 3 km apart - spatial variation in rate unlikely• • Isochronous terraces - no evidence for temporal variationIsochronous terraces - no evidence for temporal variation

Best estimate of slip rate along here is 4-5 m/kaBest estimate of slip rate along here is 4-5 m/kaWhat can morphologic ages of risers reveal about uncertainties?What can morphologic ages of risers reveal about uncertainties?

Morphologic Ages of Scarps (risers)

Scarps in alluvium decay Scarps in alluvium decay over timeover time

Decay can be described as a Decay can be described as a diffusive flux of material diffusive flux of material down a topographic gradientdown a topographic gradient

= dZ

dTd2zdx2

Many workers have Many workers have exploited this description in exploited this description in age analyses of fault scarps, age analyses of fault scarps, paleo-shorelines, terrace paleo-shorelines, terrace risers, etc.risers, etc.

t = morphologic age

Photos from USGS.gov

= dZ

dTd2zdx2

-1-D,-1-D,FTCSFTCS diffusion model diffusion model

Approach: Determining Riser Morphologic Ages

= dZ

dTd2zdx2

-1-D,-1-D,FTCSFTCS diffusion model diffusion model -multiple forward models -multiple forward models

under a range of under a range of t (varyt (vary - - compute RMS error compute RMS error


= dZ

dTd2zdx2




= dZ

dTd2zdx2




= dZ

dTd2zdx2



best fit tfor riser


Riser Morphologic Age at Ken Mu Da; calibration of a local value

Local Local of 3.2-5.4 m of 3.2-5.4 m22/ka/ka

Diachronous Riser Ages across fault trace

Preferential riser ‘protection’ Preferential riser ‘protection’ or ‘exposure’ to erosion due or ‘exposure’ to erosion due to fault motionto fault motion

Riser Morphologic Age at Deng Qin

•T2/T4 riser fit by T2/T4 riser fit by tt ~49-70 m ~49-70 m22

•Age range of 9.1-21.9 ka Age range of 9.1-21.9 ka ((T4 T4 1414C age ~16.4 kaC age ~16.4 ka))

•Age must be 9.1-16.4 ka Age must be 9.1-16.4 ka Revised slip rate of 3.4-7.1 m/kaRevised slip rate of 3.4-7.1 m/ka

Significance

Site history at Site history at Deng Qin Deng Qin revealedrevealed

T4

T2

Age of Age of upper upper terrace terrace appropriate appropriate

Significance

Contrasting modes of terrace age vs. Contrasting modes of terrace age vs. riser offset records at adjacent sitesriser offset records at adjacent sites

Significance/ Conclusions

Deformed terraces Deformed terraces important slip-important slip-markers globallymarkers globally

Uncertainty in Uncertainty in appropriate model appropriate model of riser age vs. of riser age vs. fault offset can lead fault offset can lead to wide ranges in to wide ranges in allowable ratesallowable rates

Lower terrace age Lower terrace age not always not always appropriateappropriate

Consideration of riser morphologic age can help reduce Consideration of riser morphologic age can help reduce uncertaintiesuncertainties

Documents

Exploring uncertainties in millennial slip rates along the eastern Kunlun fault, NE Tibet Nathan Harkins PSU Geosciences SCEC Joint Fault System History