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.}
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