Correlating aftershock Correlating aftershock sequences properties to sequences properties to

earthquake physicsearthquake physics

J. WoessnerJ. Woessner

S.Wiemer, S.TodaS.Wiemer, S.Toda

S1

S2

S3

S1

S2S3

MotivationMotivation

Landers, 1992

Big Bear, 1992

Hector Mine, 1999Mc = 2.1±0.18b = 1.28±0.16

Mc = 1.5±0.11b = 0.73 ± 0.04

Stress tensor Stress tensor heterogeneityheterogeneity

b-value diversityb-value diversity

Wiemer et al. (2002)Wiemer et al. (2002)

MotivationMotivation

S1

S2

S3

MW=7.3, Landers, 1992

Mis

fit a

ng

le ß

[deg

]

Longitude [deg]

Lati

tud

e [

deg

]

S1

Background orientation : S1 N7°E

Emerson-Camp Rock

Homestead valley

Johnson valley

Homogeneous

Heterogeneous

ß >

45°

Rotation of maximum principal stress axis Rotation of maximum principal stress axis (S1)(S1)

Aftershock activity – Local stress fieldAftershock activity – Local stress field

1.1. RelateRelate b-values b-values to to stress field heterogeneity ß:stress field heterogeneity ß:

HH11: Stress tensor heterogeneity correlates : Stress tensor heterogeneity correlates positively with b-valuespositively with b-values

2.2. Investigate Investigate spatio-temporal behavior of the local spatio-temporal behavior of the local stress fieldstress field using aftershock focal mechanisms: using aftershock focal mechanisms:

HH22: Stress tensor heterogeneity decreases with : Stress tensor heterogeneity decreases with timetime

Aftershock activity – Local stress fieldAftershock activity – Local stress field

3.3. Evaluate Evaluate HHeterogeneous eterogeneous PPost ost SSeismic eismic SStress tress FField Hypothesis (Michael ield Hypothesis (Michael et alet al., ., 1990) :1990) :

HH33: The rotation of the stress tensor axis : The rotation of the stress tensor axis correlates with regions of high slip and correlates with regions of high slip and readjusts with time to the regional stress readjusts with time to the regional stress fieldfield

4.4. Introduce conceptual model Introduce conceptual model

Study Regions and Data SetsStudy Regions and Data Sets

1983 Coalinga1983 Coalinga1989 Loma Prieta1989 Loma Prieta1992 Landers1992 Landers1994 Northridge1994 Northridge1999 Hector Mine1999 Hector Mine

SCSN:SCSN:- Parametric earthquake - Parametric earthquake catalogs from SCEDCcatalogs from SCEDC- Fault Plane Solution - Fault Plane Solution catalog (FPS)catalog (FPS)

NCSN:NCSN:- Parametric earthquake - Parametric earthquake catalogs from NCEDCcatalogs from NCEDC- Fault Plane Solution - Fault Plane Solution catalog (FPS)catalog (FPS)

Study RegionsStudy Regions Data SetsData Sets

MethodMethod

1.1. Spatial and temporal analysis of the Spatial and temporal analysis of the magnitude of completeness magnitude of completeness McMc (Woessner & (Woessner & Wiemer, 2005)Wiemer, 2005)

2.2. Spatial and temporal mapping ofSpatial and temporal mapping of b-valuesb-valuesLogN = a – bM (M LogN = a – bM (M ≥≥ Mc) Mc)

3.3. Spatial and temporal mapping of stress Spatial and temporal mapping of stress tensor (tensor (Michael, 1984Michael, 1984; Gephart, 1984):; Gephart, 1984):determination the stress tensor axis determination the stress tensor axis orientation and heterogeneityorientation and heterogeneity

Measures of Heterogeneity Measures of Heterogeneity

Stress tensor varianceStress tensor variance: : Describes the fit of the observed Describes the fit of the observed focal mechanismfocal mechanism to a to a homogeneous stress tensor. High homogeneous stress tensor. High variance => high heterogeneity variance => high heterogeneity of the stress field!of the stress field!

ß

Angular misfit ßAngular misfit ß:: Angular misfit between the slip Angular misfit between the slip direction from the aftershock and direction from the aftershock and the uniform stress tensor perfectly the uniform stress tensor perfectly fitting the main shock FM fitting the main shock FM (Michael, 1984, 1987, 1990) (Michael, 1984, 1987, 1990)

S1

S2

S3

Summary:Summary: Measures are Measures are equivalent and scale linearlyequivalent and scale linearly

heterogeneous

Stress tensor heterogeneity – b-valueStress tensor heterogeneity – b-value

HH11: Stress tensor heterogeneity : Stress tensor heterogeneity correlates positively with b-valuescorrelates positively with b-values

Is the heterogeneity related to the Is the heterogeneity related to the b-value distribution?b-value distribution?

Landers: Magnitude of completenessLanders: Magnitude of completeness

Emerson – Camp Rock fault

Homesteadvalley fault

Johnson valley fault

Northern part: Northern part:

CatalogCatalog complete complete for larger for larger magnitudesmagnitudes

Southern part:Southern part:

Catalog Catalog complete complete to small to small magnitudesmagnitudes

Stress tensor heterogeneity – b-valueStress tensor heterogeneity – b-value

S1

S2

S3

b = 1.13± 0.08

b = 0.73± 0.03MMWW=7.3, Landers, =7.3, Landers,

19921992

Stress tensor heterogeneity – b-valueStress tensor heterogeneity – b-value

Dow

nd

ip d

ista

nce [

km

]

Along strike distance [km]

Slip

[cm

] b-v

alu

e M

isfi

t an

gle

ß

N S Emerson – Camp Rock fault

Homestead valley fault Johnson valley

fault

Wald & Heaton (1994)

Stress tensor heterogeneity – b-valueStress tensor heterogeneity – b-valueFrom MapFrom Map From Cross-section From Cross-section

Positive correlationPositive correlation: : high b-values – high misfit angle!high b-values – high misfit angle!

But only seen for LandersBut only seen for Landers

Correlation for Other EventsCorrelation for Other Events

Coalinga Coalinga 19831983

Loma Prieta 1989

Hector Mine 1999 Coalinga: Coalinga: positivepositive

correlationcorrelation

Loma Prieta: ambiguousLoma Prieta: ambiguous

Hector Mine: ?Hector Mine: ?

Temporal dependence of ßTemporal dependence of ß

HH22: : Stress tensor heterogeneity Stress tensor heterogeneity decreases with timedecreases with time

What is the temporal dependence of What is the temporal dependence of the stress tensor heterogeneity ßthe stress tensor heterogeneity ß (and and the rotation of S1)?the rotation of S1)?

How does it compare to the aftershock How does it compare to the aftershock sequence duration?sequence duration?

Temporal dependence: LandersTemporal dependence: Landers

Prior to Landers Post Landers1975-1992 2 months

Homogeneous!Homogeneous! Heterogeneous!Heterogeneous!

Emerson / Camp rock faultEmerson / Camp rock fault

Pre-mainshockPre-mainshock Time series Time series Post-mainshockPost-mainshock

ß=30° ß=78°

Temporal dependence: Coalinga / Loma Temporal dependence: Coalinga / Loma PrietaPrieta

Coalinga, 1983

Loma Prieta, 1989

Aftershock sequence duration: TAftershock sequence duration: Taa

Landers: Emerson-Camp Rock Coalinga

Ta 25.2–63.2y Ta 17.1–27.2y

Loma Prieta Ta 8.8–11.7y

SummarySummary

1.1. HH11: Positive correlation of : Positive correlation of bb and and ßß only for the only for the Landers eventLanders event

a.a. Probably not the fundamental relationProbably not the fundamental relationDifferential stress (Schorlemmer Differential stress (Schorlemmer et alet al., ., 2005) 2005)

2.2. HH22: Stress tensor heterogeneity decreases with : Stress tensor heterogeneity decreases with timetime

a.a. Seen for Seen for all eventsall events

b.b. Time-scale is smallerTime-scale is smaller than aftershock than aftershock sequence durationsequence duration

c.c. Results Results supportsupport the the HPSSFHPSSF-hypothesis-hypothesis

Main shock slip – Stress fieldMain shock slip – Stress field

H3: : The rotation of the stress tensor The rotation of the stress tensor axis correlates with regions of high slip axis correlates with regions of high slip and readjusts with time to the regional and readjusts with time to the regional stress fieldstress field

How are the stress field heterogeneity How are the stress field heterogeneity and the rotations of the maximum and the rotations of the maximum stress axis S1 related to main shock stress axis S1 related to main shock slip?slip?

Main shock slip - Stress tensor Main shock slip - Stress tensor heterogeneityheterogeneity

Morgan Hill (1984) Morgan Hill (1984) Parkfield Parkfield (2004)(2004)

No heterogeneity observed!No heterogeneity observed!

Heaton & Hartzell (1988) C. Ji (2005)

Main shock slip - Stress tensor Main shock slip - Stress tensor heterogeneityheterogeneityLoma Prieta (1989)Loma Prieta (1989)

Beroza (1991)

Main shock slip - Stress tensor Main shock slip - Stress tensor heterogeneityheterogeneity

N S

Dow

nd

ip d

ista

nce [

km

]

Along strike distance [km]

Slip

[cm

] Mis

fit

an

gle

ß

Emerson – Camp Rock fault

Homestead valley fault Johnson valley

fault

Landers (1992)Landers (1992)

Conceptual modelConceptual model

Longitude [deg]

Lati

tud

e [

deg

]

S1 = without coseismic stress change (N7°E)

S1 = with coseismic stress change

S1

[East o

f North

]

Stress tensor inversion Coulomb stress transfer model

Conclusion & OutlookConclusion & Outlook

Results support the HPSSF-hypothesis

Case studies show difference between small and large ruptures

More quantification needed!

Possibilities of conceptual model:

Constrain absolute size of regional background stress fieldHomogeneous with slight variations

Explain temporal rotation depending on loading process: viscoelastic relaxation?