Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 1

Geodetic and Geologic slip rate estimates for earthquake hazard

assessment in Southern California

Ken HudnutUSGS

March 30, 2001

University of Utah

Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 2

North American and Pacific plate relative motions

- is it 52 or 56 mm/yr?

San Andreas fault zone

- is it 22 mm/yr or 34 mm/yr along the Mojave segment?

Eastern CaliforniaShear Zone

– Estimated rates range between 6 and 12 mm/yr (geological & geodetic)

– Is this difference in rates real?– Does it indicate elevated strain rate during

a clustering of seismic (and aseismic) events?

– Is the ECSZ energetically ‘easier’ than the Big Bend?

Different path integrals yield differences

Measuring rates by geology or geodesy are both difficult and prone to pitfalls

Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 3

SCEC crustal motion map

• Combined

EDM, VLBI,

survey-mode

and continuous

GPS rigorously

• Released as a

SCEC product

• Set the bar very

high for the

SCIGN project

Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 4

SCEC Crustal Motion Map - Version 3

• 315 GPS vectors added for a total of 678 vectors (version 2 had 363 vectors)

• Early SCIGN site velocities now included

• Version 3 will include time series for all stations

Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 5

Generalized fault map of southern California with known slip rates circa 1990

from Tom Rockwell’s SCEC2 talk

Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 6

Effects of lower crust viscosity on geodetic velocity field

Weak lower crust => large variations in time

Strong lower crust

early

late

Borrowed from Brad Hager’s SCEC2 talk

‘Spectacular’AgreementIn CarrizoPlain

Rates may disagree the most wherethe lower crust is the weakest…

Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 7

Examples of differences in rate• Garlock fault

– Geologic rate 7 +/- 2 mm/yr– Geodetic rate 2 +/- 2 mm/yr– Geodesy < Geology => weak lower crust

• Eastern California Shear Zone– Geologic rate summed over all faults is ~6 mm/yr– Geodetic rate across ECSZ is ~10–12 mm/yr– Geodesy > Geology => clustering or new higher tectonic rate?

• Imperial Valley– Geologic rate of 20 mm/yr– Geodetic rate across valley of ~50 mm/yr => missing a major fault?

• Sierra Madre – Cucamonga fault zone– Geologic rate of 0.5 mm/yr– Geodetic rate of a

• Raymond fault– Geologic rate of 1.5-4 mm/yr – Geodetic rate of b a + b ~ 6-8 mm/yr

Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 8

Conclusions• In the past decade, our understanding of rates has changed

quite radically in So. Calif. – both in terms of geodetic and geologic estimates

• Where we now think there are differences, it is important to confirm the observations and then attempt to explain the differences

• Possible to explain these differences in a variety of ways– Bad data (e.g., missing a fault, off-fault deformation, etc.)– Bad modelling (e.g., dipping fault & layered structure, or variable

strength modelled as elastic half-space)– Weak lower crust or strong lower crust (earthquake cycle)

• Hypothesis testing approach needed where rates differ• Don’t assume geologic rate to be correct as a default position