1. Correlating aftershock sequences properties to earthquake physics J. Woessner S.Wiemer, S.Toda

2. S1 S2 S3 S1 S2 S3Motivation Landers, 1992 Big Bear, 1992 Hector Mine, 1999 Mc = 2.1±0.18 b = 1.28±0.16 Mc = 1.5±0.11 b = 0.73 ± 0.04 Stress tensor heterogeneity b-value diversity Wiemer et al. (2002)

3. Motivation S1S1 S2S2 S3S3 M W =7.3, Landers, 1992 Misfit angle ß [deg] Longitude [deg] Latitude [deg] S1S1 Background orientation : S 1 N7°E Emerson-Camp Rock Homestead valley Johnson valley Homogeneo us Heterogeneo us ß > 45° Rotation of maximum principal stress axis (S1)

4. Aftershock activity – Local stress field 1.Relate b-values to stress field heterogeneity ß: H 1 : Stress tensor heterogeneity correlates positively with b-values 2.Investigate spatio-temporal behavior of the local stress field using aftershock focal mechanisms: H 2 : Stress tensor heterogeneity decreases with time

5. Aftershock activity – Local stress field 3.Evaluate Heterogeneous Post Seismic Stress Field Hypothesis (Michael et al., 1990) : H 3 : The rotation of the stress tensor axis correlates with regions of high slip and readjusts with time to the regional stress field 4.Introduce conceptual model

6. Study Regions and Data Sets  1983 Coalinga  1989 Loma Prieta  1992 Landers  1994 Northridge  1999 Hector Mine  SCSN: - Parametric earthquake catalogs from SCEDC - Fault Plane Solution catalog (FPS)  NCSN: - Parametric earthquake catalogs from NCEDC - Fault Plane Solution catalog (FPS) Study RegionsData Sets

7. Method  Spatial and temporal analysis of the magnitude of completeness Mc (Woessner & Wiemer, 2005)  Spatial and temporal mapping of b-values LogN = a – bM (M ≥ Mc)  Spatial and temporal mapping of stress tensor (Michael, 1984; Gephart, 1984): determination the stress tensor axis orientation and heterogeneity

8. Measures of Heterogeneity Stress tensor variance: Describes the fit of the observed focal mechanism to a homogeneous stress tensor. High variance => high heterogeneity of the stress field! ß Angular misfit ß: Angular misfit between the slip direction from the aftershock and the uniform stress tensor perfectly fitting the main shock FM (Michael, 1984, 1987, 1990) S1 S2 S3 Summary: Measures are equivalent and scale linearly heterogeneou s

9. Stress tensor heterogeneity – b-value H 1 : Stress tensor heterogeneity correlates positively with b-values Is the heterogeneity related to the b- value distribution?

10. Landers: Magnitude of completeness Emerson – Camp Rock fault Homestead valley fault Johnson valley fault Northern part: Catalog complete for larger magnitudes Southern part: Catalog complete to small magnitudes

11. Stress tensor heterogeneity – b-value S1S1 S2S2 S3S3 b = 1.13± 0.08 b = 0.73± 0.03 M W =7.3, Landers, 1992

12. Stress tensor heterogeneity – b-value Downdip distance [km] Along strike distance [km] Slip [cm] b-value Misfit angle ß N S Emerson – Camp Rock fault Homestead valley fault Johnson valley fault Wald & Heaton (1994)

13. Stress tensor heterogeneity – b-value From Map From Cross-section Positive correlation: high b-values – high misfit angle! But only seen for Landers

14. Correlation for Other Events Coalinga 1983 Loma Prieta 1989 Hector Mine 1999 Coalinga: positive correlation Loma Prieta: ambiguous Hector Mine: ?

15. Temporal dependence of ß H 2 : Stress tensor heterogeneity decreases with time What is the temporal dependence of the stress tensor heterogeneity ßand the rotation of S1)? What is the temporal dependence of the stress tensor heterogeneity ß (and the rotation of S1)? How does it compare to the aftershock sequence duration?

16. Temporal dependence: Landers Prior to Landers Post Landers 1975-1992 2 months Homogeneous! Heterogeneous! Emerson / Camp rock fault Pre-mainshockTime series Post- mainshock ß=30° ß=78°

17. Temporal dependence: Coalinga / Loma Prieta Coalinga, 1983 Loma Prieta, 1989

18. Aftershock sequence duration: T a Landers: Emerson-Camp RockCoalinga T a  25.2–63.2y T a  17.1– 27.2y Loma Prieta T a  8.8– 11.7y

19. Summary 1.H 1 : Positive correlation of b and ß only for the Landers event a.Probably not the fundamental relation Differential stress (Schorlemmer et al., 2005) 2.H 2 : Stress tensor heterogeneity decreases with time a.Seen for all events b.Time-scale is smaller than aftershock sequence duration c.Results support the HPSSF-hypothesis

20. Main shock slip – Stress field : The rotation of the stress tensor axis correlates with regions of high slip and readjusts with time to the regional stress field H 3 : The rotation of the stress tensor axis correlates with regions of high slip and readjusts with time to the regional stress field How are the stress field heterogeneity and the rotations of the maximum stress axis S1 related to main shock slip?

21. Main shock slip - Stress tensor heterogeneity Morgan Hill (1984) Parkfield (2004) Morgan Hill (1984) Parkfield (2004) No heterogeneity observed! Heaton & Hartzell (1988)C. Ji (2005)

22. Main shock slip - Stress tensor heterogeneity Loma Prieta (1989) Beroza (1991)

23. Main shock slip - Stress tensor heterogeneity N S Downdip distance [km] Along strike distance [km] Slip [cm] Misfit angle ß Emerson – Camp Rock fault Homestead valley fault Johnson valley fault Landers (1992)

24. Conceptual model Longitude [deg] Latitude [deg] S 1 = without coseismic stress change (N7°E) S 1 = with coseismic stress change S 1 [East of North] Stress tensor inversionCoulomb stress transfer model

25. Conclusion & 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 field Homogeneous with slight variations  Explain temporal rotation depending on loading process: viscoelastic relaxation?