Jennifer Kokoska KB Boomer (Advisor) Richard Brazier (Advisor)

1. Empirically Based Ground Truth Criteria for Seismic Events Recorded at Local Distances on Regional Networks with Application to the Main Ethiopian Rift Jennifer Kokoska KB Boomer (Advisor) Richard Brazier (Advisor)

2. Accurate Locations of Seismic Events • A seismic event occurs, waves travel to recording station • Wave travel is affected by geological structures of the region • Velocity models are used to estimate epicentral location, depth, and origin time • Implications for detecting nuclear testing sites • The U.S. National Nuclear Security Administration (NNSA) maintains a knowledge base of well-located events. • In order to be added to the knowledge base, event must be GT5₉₅% • Ground Truth (GT)- the absolute location and depth error in the estimate. Goal: • Explore and identify the criteria necessary to obtain GT5 for events located in the Main Ethiopian Rift • Add more events to the knowledge base, provide means for others to do the same • Increase our understanding of the effects of network organization and regional geology on event location.

3. Current Global Criteria • Overly restrictive • Sparse networks cannot meet criteria • As a result, many GT events go uncataloged.

4. The Main Ethiopian Rift • Formed by the separation of two tectonic plates • Very geologically complex • Heterogeneity in structure both across and along the rift. • EAGLE data set includes 25 large shots were well recorded • Arrival times from these shots were provided by Keranenet al.

5. ProcessDevelopment of local Empirically Based Ground Truth (EBGT5₉₅%) criteria: • Single explosion is recorded on 670 stations. • What stations should we consider? • What network characteristics result in a GT5₉₅% estimation for the event? • How can we adjust our velocity models for more accurate estimates?

6. ProcessResampling • Models of data contain a residual error term • Typically assumed normal, average zero • Existing seismic literature claims errors are NOT normal • Resampling methods allow us to create sample distribution • Allows us to create a 95% confidence interval (NNSA requirement)

7. ProcessResampling: The Bootstrap • Radomly select k<n arrival times with replacement • Estimate overall epicentral change • Repeat (i.e. 10,000 iterations) to create sampling distribution • Sampling with replacement implies independence of observations • Large number of stations which capture the event reduces repeated stations which results in a more accurate estimate

8. Assessing Network Quality Using Azimuthal Coverage • Interest in the effects of azimuthal clustering on estimation • Need a measure of the distribution of stations around a seismic event • More useful than simply primary and secondary gap • Three metrics • Network Quality Metric (Bondar et al. 2009) • Kuiper’s Test • Circular Range Test

9. Network Quality Metric(Bondáret al. 2009) • Produces a metric 0 ≤ ΔU ≤ 1 • 0 = uniformly distributed • 1 = all stations in the same azimuth

10. Kuiper’s Test(N.H. Kuiper 1960) • Goodness-of-fit test • Examines the sum of maximum positive and negative deviations from the uniform distribution • A Kolmogorov-Smirnov type test • Null hypothesis of uniform azimuthal coverage.

11. Circular Range Test(N.I. Fisher 1993) • Goodness-of-fit test based on variable w, the smallest arc which can be drawn through all points on the circle (stations around the event). • 360˚ - Primary gap measure = w Secondary Gap Primary Gap

12. Problems with these measures • Lack of azimuthal coverage on the plateaus • A 0.05 level of significance for Kuiper's test and Circular Range test appear too liberal.

13. Problems with these measures • Both metrics have a ΔU value of .45 • Very different arrangement • Differences in distance from event

14. Pg/Pn Crossover • Event-to-station distance problems • Crossover distance- farthest point from the station where a direct arrival from the P-wave will still be maintained. • Eliminates travel below the crust • Maximum distance of 215 km radius from each station is preselected. • Region specific Pg/Pn crossover was also used in the design of the velocity model. • Stations in the rift – 140 km • Stations on plateaus – 215 km

15. Results • Due to geologic complexity, each event displayed independent challenges for constraining epicentral location, depth, and time. • Farther regions of the plateaus are not well covered azimuthally • Difficultly finding criteria for entire region without being overly restrictive • Central rift region is well covered azimuthally • Simple geology causes bias in analysis

16. Results • CDF shows 95% percentile for all samples • Network Quality Metric at or below .37 • Scatterplots show less than 5% of the data is above 5 km • GT595%!

17. Results Proposed criteria for GT595% location in the Main Ethiopian Rift

18. Future Work • Examine other local regions • Strategic geological structures • Apply current criteria or develop local criteria for the region • Tibet- similar plateau structures to Ethiopia • Attempt to apply region-specific criteria globally • Based on local geology • Allows more GT events to be accepted/added to knowledge base • Explore other statistical methods of modeling network arrangement and station deviation from Normality

19. Questions? Acknowledgement: The work is supported in part by DOD/AFRL grant FA9453-10-C-0211{PSU}, FA9453-10-C-0211{Bucknell Univ.}