Improved Location Procedures at the International Seismological Centre István Bondár ESC 32 nd 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 C D = C N + C R 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 5 º 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 C D and projects redundant observations into the null space (Bondár and McLaughlin, 2009). The SVD of C D 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 A posteriori model covariance matrix

10 Depth-phase Depth Depth-phase stack (Murphy and Barker, 2006) Generate predicted moveout curves TT depth-phase – TT firstP 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 2001/01/28 17:15:27.363, 23.28, 70.04, 28.5 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 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 French Polynesia Lop Nor NTS Semipalatinsk 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 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 50% percentile 90% percentile All events

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