Spatio-temporal slip distribution around the Japanese Islands deduced from Geodetic Data

1. Spatio-temporal slip distribution around the Japanese Islands deduced from Geodetic Data Takeshi Sagiya Geographical Survey Institute ACES Workshop

2. Earthquake Simulations and Data Assimilation • Earthquake phenomena occur in the real world. • Incorporation of observational information is indispensable in order to make a forecast using simulation models. • What data can be assimilated? • How can we assimilate those data? ACES Workshop

3. Observation Data in Earthquake Studies • Seismicity • Related to stress • Seismicity rate <=> stressing rate • LURR • Assimilation model needs global applicability • More studies needed. • Crustal deformation • Well established model • Maruyama, Okada, etc. • Groundwater, SEMs, … • Needs for establishing quantitative model ACES Workshop

4. Assimilation of Deformation Data • Deformation data can be modeled in terms of stress/strain change in the earth. • Dislocation theory, FEM, FDM, etc. • GPS and InSAR provide deformation data with good accuracy and high (spatially and temporally, but not enough) resolution. • Conventional geodetic data are available for more than a century, which is comparable to a typical recurrence interval of subduction earthquakes. ACES Workshop

5. Assimilation of Geodetic Data • Limitations • Observations only at the Earth’s surface • Estimate on internal stress/strain/slip has limited resolutions. • Assumptions are necessary • Deformation caused by slip (or slip deficit) on the fault (or plate boundary). • Fault configuration needs to be assumed. • Isotropic elastic half-space is assumed in many cases. • Underdetermined problem • Introduce a priori constraints ACES Workshop

6. GPS Data 2000 Wesrwen Tottori Earthquake (Mw=6.6) • Daily coordinates • Accuracy: 2-3mm Horizontal, 10-20mm vertical • Near real-time: 2-14 days time lag • Average site interval in Japan: 20-25km • Network solution • systematic errors Length EW NS Velocity UD GPS site ACES Workshop

7. Vertical data by GPS • High temporal resolution • Rate estimates is getting reliable • Problems • Seasonal signal • Large scattering • Local effects, artificial subsidence • Comparison with InSAR will be a good solution. ACES Workshop

8. Leveling Data • Relative height change between neighboring benchmarks • High (~2km) spatial resolution along leveling routes. • Poor (1-10years) temporal resolution • Almost uniform quality for 100 years • Systematic errors • Still provides the best accuracy for vertical ACES Workshop

9. Horizontal Movements of Japan revealed by GPS Exaggeration: X500,000 Earthquakes(M>5) ACES Workshop

10. Data Assimilation with deformation data • Representation theorem • Observational equation • Response functions G includes time-dependent effects (viscoelasticity). • Linear equation is to be solved. • A priori constraints on smoothness • ABIC minimum criterion for optimum solution ACES Workshop

11. 20?? The Nankai Trough and the Philippine Sea Plate Subduction • Deformation can be attributed to slip (or slip deficit) on plate boundary interface. 2001 M6.7 1923 M7.9 1996 M6? 1944 M8.1 1946 M8.3 1997 M6.6 ACES Workshop

12. 1996 Boso Silent Earthquake • Detected by continuous GPS monitoring • Up to 20mm displacements within a week ACES Workshop

13. Inversion of A Slow Earthquake • Inversion of daily coordinates at 30 GPS stations • Elastic response functions • Constraints on spatio-temporal smoothness • Attribute to plate interface • Slip migration ACES Workshop

14. 2001 Tokai Slow Event Estimated source region of the Tokai Earthquake Hamamatsu(93054) Please check our updates at http://cais.gsi.go.jp/tokai/ (Captions in Japanese only, sorry) ACES Workshop

15. 2001 Tokai Slow Event • Moment release comparable to a M6.7 earthquake • Slip may slow down and slightly migrate to the east recently. Results by Ozawa et al. (2002) ACES Workshop

16. GPS: 950306 w.r.t. 950305 Length 950305 -2mm/yr -4.5mm/yr -5mm/yr 950306 East North Up ±0mm/y -10mm/y Tokai Slow Events Recorded in Legacy Data After Kimata and Yamauchi(1998) ACES Workshop

17. Earthquake Deformation Cycle • Coseismic, postseismic, and interseismic deformations documented by leveling in Shikoku Island, southwest Japan ACES Workshop

18. 30 20 10 Viscoelastic Inversion of Earthquake Deformation Cycle • Data analyzed • Vertical displacement in Shikoku and surrounding areas • Parameters estimated • Spatio-temporal distribution of slip rate on the plate interface • All the parameters for 100 years are estimated at once. • Response functions • Viscoelastic • Analytically calculated from a layed structure ACES Workshop

19. Viscoelastic Inversion of Earthquake Deformation Cycle • Preliminary results • Spatio-temporal distribution of slip deficit (blue color corresponds to plate locking) • Optimum solution is not yet obtained ACES Workshop

20. Obstacles in Data Assimilation: Problems to be solved • BEM-type problem • Need to solve a dense matrix • Parallelization • Assimilation in a spatio-temporal space • Large number of parameters/data • Kalman filtering • Viscorlastic (time-dependent) response functions applicable? • Faults/PBs may not be the only sources of deformation. • Only gives us kinematic conditions • Interpretation is not straightforward ACES Workshop

21. Asperities and Slip Deficits on Plate Boundary • Interpretation of kinematic Inversion results is not straightforward. It may not be a direct indication of mechanical coupling. ACES Workshop