Correlating aftershock sequences properties to earthquake physics J. Woessner S.Wiemer, S.Toda.
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Transcript of Correlating aftershock sequences properties to earthquake physics J. Woessner S.Wiemer, S.Toda.
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?