Improved Location Procedures at the International Seismological Centre

1. Improved Location Procedures at the International Seismological Centre István Bondár ESC 32nd General Assembly Montpellier, September 6-10, 2010

2. Outline • Motivation • Current ISC locator • Proposed ISC locator • Validation tests • Relocation of ~7,000 GT0-5 events • Comparison with EHB • Conclusions

3. Motivation • Correlated travel-time prediction errors along similar ray paths introduce location bias and result in underestimated error ellipses • Example: • The effect of USArray on event locations in South America • Without accounting for correlated errors, locations get worse!

4. Current ISC Locator • ak135, P[g|b|n], S[g|b|n] up to 100º • Jeffrey’s uniform reduction algorithm • Iterative reweighting, all phases are equal • Reidentifies phases in each iteration (S could become pP!) • Duplicates are explicitly down-weighted • Uncertainties are scaled to 95% confidence level • Attempts free-depth solution and cycles through all reported hypocentres until a convergent solution is found • If fails, fixes depth to that of the trial hypocentre • If fails, fixes depth to region-dependent default depth (10/35 km) • Depth-phase depth solution is obtained by inverting pP-P times • Network magnitude can be obtained from a single station magnitude; no magnitude uncertainties are calculated

5. New ISC Locator • Uses all ak135 predicted phases (including depth phases) in location • Ellipticity, elevation corrections • Bounce point and water depth corrections for depth phases • Attempts free-depth solution only if there is depth resolution • Accounts for correlated model error structure • Obtains initial guess via neighbourhood algorithm search • Performs iterative linearized inversion using a priori estimate of data covariance matrix • Scales uncertainties to 90% confidence level • Obtains depth-phase depth via depth-phase stacking • Provides robust network magnitude estimates with uncertainties

6. Data Covariance Matrix • When correlated errors are present, the data covariance matrix is no longer diagonal • The network covariance matrix accounts for correlated travel-time prediction errors due to similar ray paths • Estimated by an isotropic stationary variogram (Bondár and McLaughlin, 2009) derived from GT residuals • The a priori picking error variances add to the diagonal • CD = CN + CR Generic variogram model

7. Depth Resolution • Attempt free-depth solution if and only if there is depth resolution • We have depth resolution if • There is a local network • one or more defining stations within 0.2º • Or there are sufficient number of depth phases • 5 or more defining depth phases reported by at least two agencies • Or there are sufficient number of core reflections • 5 or more defining core reflections (PcP et al) • Or there are sufficient number of local/near-regional S • 5 or more defining S and P pairs within 5º • Otherwise fix the depth to regional default depth • Explosions are fixed to zero depth

8. Neighbourhood Algorithm Grid Search • The prime location may or may not be close to the global optimum in the search space • Reported hypocentres may exhibit a large scatter • Neighbourhood Algorithm (Sambridge and Kennett, 2001) to get an initial hypocentre • Grid search around the median of reported hypocentre parameters • NA explores the search space and rapidly closes in on the global optimum • Once close to the global minimum, we switch to the linearized inversion algorithm to obtain the final solution and formal uncertainties

9. Location Algorithm • Minimize by solving • G is the design matrix containing the travel-time derivatives for an event-station path, m is the model adjustment vector [Δx, Δy, Δz,ΔT]T and d is the vector of time residuals. • W is the projection matrix that orthogonalizes CDand projects redundant observations into the null space (Bondár and McLaughlin, 2009). • The SVD of CD is • We keep only the first p largest eigenvalues from the eigenvalue spectrum such that 95% of the total variance is explained. • This reduces the effective number of degrees of freedom of the data from N to p, with N-p dimensions of null space. • Let then • Solution via singular value decomposition • Aposteriori model covariance matrix

10. Depth-phase Depth 2001/01/28 17:15:27.363, 23.28, 70.04, 28.5 Depth-phase stack (Murphy and Barker, 2006) • Generate predicted moveout curves • TTdepth-phase – TTfirstP • Parametric on delta • Generate depth traces for each station • Boxcar at the corresponding depth for the observed moveout • The width of boxcar is the prior measurement error estimate • Stack depth traces • Calculate median and SMAD depdp=26.0 ± 8.9

11. Validation Tests • Relocation of some 7,200 GT0-5 events • Earthquakes and explosions • Some explosions are poorly recorded • Mimic automatic ISC location • ISC associations • Use only reported hypocentres • Ignore GT, EHB and ISC hypocentres • Cases • Baseline: current ISC locator (isc) • Independent errors, current ISC phases (ii) • Independent errors, all ak135 phases (ia) • Independent errors, all phases, grid search (iag) • Correlated errors, current ISC phases (ci) • Correlated errors, all ak135 phases (ca) • Correlated errors, all phases, grid search (cag)

12. Explosions • Abysmal coverage when correlated error structure is ignored

13. Test Sites French Polynesia NTS Semipalatinsk Lop Nor • Correlated errors • Significant improvements in French Polynesia • Deterioration in Novaya Zemlya • When the effect of conspiring stations is taken out, the poor ak135 regional TT predictions become apparent • Grid search • Does a good job in rectifying poor initial hypocentres Novaya Zemlya

14. GT5 Earthquakes (free-depth) Improved locations, depth and OT

15. GT5 Earthquakes (depth-phase depth) Significant improvements in location, depth and OT

16. Regional Default Depth • Get a reasonable default depth where there is seismicity • First attempt: • 1x1 degree grid of EHB free-depth solutions • Second attempt: • Improvements were shown for free-depth hypocentres of GT events with the new locator • Relocated the entire ISC bulletin with the new locator • 0.5x0.5 degree grid from relocated free-depth solutions + EHB free-depth and EHB reliable depth solutions (~800K events) • Otherwise use CRUST2.0

17. GT5 Earthquakes (fixed depth) • Default depth grid: version2 vs version1 • Significant improvements in depth • Improvements in location w.r.t. EHB grid • Most of the events now have a default depth based on seismicity

18. Effect of Correlated Errors All events • Area of error ellipse does not vanish with increasing number of stations • Actual coverage is maintained • Location and depth bias is reduced for large number of stations 90% percentile 50% percentile

19. Comparison with EHB • EHB earthquakes in IASPEI Reference Event List • Overall improvements in location • Depth estimation is consistent with EHB • Results are comparable or better than EHB

20. Conclusions • Accounting for correlated errors • Provides honest ~85-90% actual coverage • Reduces location bias due to correlated travel-time prediction errors • Improvements in location, depth and origin time for free-depth solutions are due to • Using all phases (including depth phases) in location • Testing for depth resolution • Default depth grid provides reasonable depth for fixed-depth earthquakes • Initial guess by neighbourhood algorithm grid search • Crucial for poorly recorded events with unreliable reported hypocentres • Location and depth accuracy is comparable to EHB