1. Effect of Velocity Models on the Accuracy of Earthquake Hypocenters Sudipta Sarkar, Youshun Sun, M. Nafi Toksöz Earth Resources Laboratory Massachusetts Institute of Technology USA

2. Big Picture Earthquake hypocenters could be in error by tens of kilometers Several factors contribute to errors –Poor station coverage –Phase reading errors –Poor knowledge of velocity –Lateral variations of velocity

3. Outline Earthquake location errors due to errors in velocity models: Examples - I. Local scale II. Regional scale Improving location accuracy by using a 3D velocity model China case A practical approach to improving location accuracy Source Specific Station correction Applicability to the Gulf region Conclusions

4. Velocity Model and Depth of Events

7. Depth Error in Local Events Due to Incorrect Velocity Model Vel #1Vel #2Vel #3

8. 1D(km)AK135(km) 11 Events (Explosions)1020 Earthquake1220 Location Errors Due to Lateral Variation of Velocity Our experience in CHINA

9. 3D(km)1D(km)AK135(km) 11 Events (Explosions)11020 Earthquake3.51220 Location Errors Reduced by Introducing a 3D Model Our experience in CHINA

10. Event Relocation Using 3D Travel Times: A Regional Study in China Method Determine 3-D, regional velocity structure in crust and upper mantle (tomography) Use 3-D velocity / travel time grids to locate events

11. A Regional Study in China

14. 3D(km)1D(km)AK135(km) 11 Events (Explosions)11020 Earthquake3.51220 Location Errors Reduced by Introducing a 3D Model Our experience in CHINA

15. Station Distribution and Events in the Gulf

16. Prospects of 3D Tomography and Event Location in the Gulf

17. A Practical Approach Doing true 3D locations is computationally expensive and most often not possible! A practical solution to this problem – use Source Specific Station Corrections.

18. Procedure Use 3-D velocity models to calculate travel times from all potential source regions to all stations Determine source specific station corrections relative to reference model used for locations Determine an initial location Apply the station correction to times at each station specific for the source region Relocate the event

19. Conclusions Local and regional seismic networks are improving rapidly Accurate event locations with these networks require accurate 3D velocity models Such velocity models can be derived through regional seismic tomography 3D travel times can be implemented through source specific travel time corrections New networks in the Gulf region provides an opportunity for 3D velocity models and more accurate locations

20. Event Mislocation Low VelHigh Vel Mislocations