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)

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.

CURRENT GLOBAL CRITERIA

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

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 Keranen et al.

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?

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)

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

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

NETWORK QUALITY METRIC(BONDÁR ET AL. 2009)

Produces a metric 0 ≤ ΔU ≤ 1 0 = uniformly

distributed 1 = all stations

in the same azimuth

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.

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

Primary Gap Secondary Gap

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.

PROBLEMS WITH THESE MEASURES

Both metrics have a ΔU value of .45 Very different arrangement Differences in distance from event

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

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

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%!

RESULTS

Proposed criteria for GT595% location in the Main Ethiopian Rift

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

QUESTIONS?

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