Relative Location Accuracy Research Articles

Location Accuracy Improvement of Long-range Lightning Detection Network In China by Compensating Ground Wave Propagation Delay

Very low frequency (VLF) electromagnetic waves distort along the long propagation path, and that causes the arrival time of the signals measured by the long-range lightning system to be delayed. In this paper, based on the propagation correction method by compensating the peak time delay of the ground wave, the location accuracy of the long-range lightning detection network in China is greatly improved. The improvement of the relative location accuracy and location offsets are evaluated by comparing with the Advanced Direction Time Lightning Detection System (ADTD) datasets. It shows that the mean relative accuracy is improved from 7.74 km to 4.32 km, and the median relative accuracy is improved from 7.28 km to 2.46 km. The mean westwards offset of the total lightning location data drops from 2.05 km to 0.93 km, and the mean southwards offset drops from 1.19 km to 0.63 km. In addition, it is found that the location accuracy will be greatly improved if the observation site affected by the terrain is removed. The mean relative location accuracy is further improved to 4.11 km and the median to 2.32 km.

Automatic Registration of INSAT-3D Daily Images Using Mutual Information and Stochastic Optimization Technique

An automatic image registration approach is presented here can be used to register daily images of Indian geostationary satellite system INSAT-3D acquired every 30 min’ interval without use of any ground control points (GCPs). There is always a pressing need to register meteorological images that are acquired over earth from geostationary platforms every 15–30 min, covering almost one-third of the earth. Weather forecast activities include derivation of atmospheric motion vectors, which demand immediate processing of such images to a reasonable accuracy in terms of its relative location accuracy. Generally followed approaches make use of image navigation models and GCPs drawn from known landmarks in land ocean boundaries and correlate image features before estimating a transform to warp the current acquisition to a known geometry. However, the hierarchical (coarse to fine) approach explained here makes use of intensity based Mutual Information as a similarity measure from a population of pixels selected randomly and uses stochastic gradient descent optimizer to estimate an affine transform between registering image pair, delivers satisfactory results.

Location of the mainshock and aftershock sequences of the <italic>M</italic>6.9 Mainling earthquake, Tibet

We measured the location of the mainshock of the November 18, 2017, earthquake ( M 6.9) that occurred in Mainling County, Nyingchi City, Tibet, by using arrival-time data recorded by temporary seismic stations deployed on the Namche Barwa syntaxis and surrounding regions. The seismic stations offered good coverage of the hypocenter region and provided a good opportunity to precisely locate the mainshock and aftershock sequences. The Mainling earthquake was located at 29.87° N, 95.05° E, and the focal depth is 12 km. We investigated the focal mechanism of the mainshock by applying the W phase source inversion algorithm from the broad-band records of global seismic networks accessed from the IRIS data center. First, we resampled the original waveform to 1 SPS and then removed the mean, trend, and instrument response of the waveform, filtering it with a bandwidth of 150–70 s. After inversion, we obtained a solution with M w = 6.45 and a best double couple given by strike/dip/rake=319.9°/61.7°/101.0°. This result suggested that the seismogenic fault of the Mainling earthquake was characterized by a high-angle and NE-dipping thrust. We determined the location of the aftershock sequences by applying Geiger and double difference (DD) algorithms. Due to the lack of the local-scale tomographic model and reliable Poisson’s ratio data in the hypocentral region, we used only the P-wave arrival times to locate the aftershock sequences. We determined absolute locations for 148 aftershocks with high quality P-wave arrivals recorded by 14 temporary seismic stations and estimated the locations using the 1-D velocity model, in which the P-wave velocity was 6 km/s in the upper crust, 6.6 km/s in the lower crust, and 8.1 km/s in the uppermost mantle. The location errors were 0.6 km in the horizontal direction with an average residual of 0.07 s. In addition, we used the DD location algorithm in an attempt to minimize uncertainties in the aftershock locations. It was confirmed that the DD location algorithm greatly improved the relative location accuracy of events that are close to one another compared to the traditional Geiger location method. After DD location, the location errors of the aftershock sequences are 0.23, 0.26, and 1.43 km in the north-south, east-west, and vertical directions, respectively. The results indicated that the aftershocks were distributed along both sides (NW and SE directions) of the mainshock, which spread approximately 40 km in length and 10 km in width. Results of the focal depth profiles suggested that the aftershocks were located at depths above 15 and 10 km in the NW and SE directions of the mainshock, respectively. The aftershocks were mainly located at two sub-parallel faults. In the first two hours, the M 6.9 Mainling earthquake and aftershocks were mainly distributed at fault F1, and after that the aftershocks were distributed at fault F2. This suggested that the Xixingla fault zone is composed of a series of high-angle and NE-dipping faults, and the M 6.9 Mainling earthquake occurred at the south most one. The M 6.9 Mainling earthquake triggered seismic activities of a sub-parallel fault located to the northeast. The occurrence of the M 6.9 Mainling earthquake indicates that the Namche Barwa syntaxis is still in a high tectonic compression state at present. The Namche Barwa terrane continues moving towards the NE direction, leading the shortening deformation in the interior of the terrane. It is possible that a large earthquake may occur at Motuo and Mainling faults in the future along both sides of the Namche Barwa syntaxis.

Performance assessment of Beijing Lightning Network (BLNET) and comparison with other lightning location networks across Beijing

The performances of Beijing Lightning Network (BLNET) operated in Beijing-Tianjin-Hebei urban cluster area have been evaluated in terms of detection efficiency and relative location accuracy. A self-reference method has been used to show the detection efficiency of BLNET, for which fast antenna waveforms have been manually examined. Based on the fast antenna verification, the average detection efficiency of BLNET is 97.4% for intracloud (IC) flashes, 73.9% for cloud-to-ground (CG) flashes and 93.2% for the total flashes. Result suggests the CG detection of regional dense network is highly precise when the thunderstorm passes over the network; however it changes day to day when the thunderstorms are outside the network. Further, the CG stroke data from three different lightning location networks across Beijing are compared. The relative detection efficiency of World Wide Lightning Location Network (WWLLN) and Chinese Meteorology Administration - Lightning Detection Network (CMA-LDN, also known as ADTD) are approximately 12.4% (16.8%) and 36.5% (49.4%), respectively, comparing with fast antenna (BLNET). The location of BLNET is in middle, while WWLLN and CMA-LDN average locations are southeast and northwest, respectively. Finally, the IC pulses and CG return stroke pulses have been compared with the S-band Doppler radar. This type of study is useful to know the approximate situation in a region and improve the performance of lightning location networks in the absence of ground truth. Two lightning flashes occurred on tower in the coverage of BLNET show that the horizontal location error was 52.9m and 250m, respectively.

Automatic stitching method for Chang’E-2 CCD images of the Moon

The lunar map is a product of primary scientific objectives of lunar exploration. Aiming at the characteristics of the Chang’E-2 CCD data, an automatic stitching method used for 2C level CCD data from Chang’E-2 lunar mission is proposed. Combining with the image registration technique and the characteristics of Chang’E CCD images, the fast method proposed not only can overcome the contradiction of the high spatial resolution of the CCD images and the low positioning accuracy of the location coordinates, but also can speed up the processing and minimize the utilization of human resources to produce lunar mosaic map. Meanwhile, a new lunar map from 70oN to 70oS with spatial resolution of less than 10 m has been completed by the proposed method. Its average relative location accuracy of the adjacent orbits CCD image data is less than 3 pixels.

Cross double-difference inversion for microseismic event location using data from a single monitoring well

The double-difference (DD) location method has long been applied for locating a cluster of earthquakes with data recorded at surface seismic stations. This method has also been used for locating microseismic events with multiple monitoring wells during hydraulic fracturing. We first extended the approach for locating a cluster of microseismic events with data recorded from a single well. To do this, we have reduced the 3D location problem to 2D by projecting all of the events onto a vertical ([Formula: see text]) plane, with a vertical [Formula: see text]-axis and a horizontal [Formula: see text]-axis representing the distance to the monitoring well, considering the symmetric character of the 1D velocity model and the vertical monitoring well. We then performed a 2D location inversion and projected the results back to 3D using the event azimuths, which were determined from a separate analysis of the initial P-wave polarizations. However, although the DD method could determine relative locations of events reasonably well, it yielded poor absolute locations. We have developed a cross DD (CDD) approach using the cross traveltime difference between the P-wave arrival of one event and the S-wave arrival of another event for inversion instead of the arrival-time differences of the same phases as in the DD method. The CDD method contains more information on absolute locations than the DD method, resulting in a much more stable absolute location determination. The synthetic and field data tests indicated that the CDD method could improve the accuracy of relative and absolute event locations in microseismic clusters.

PERFORMANCE ASSESSMENT AND GEOMETRIC CALIBRATION OF RESOURCESAT-2

Resourcesat-2 (RS-2) has successfully completed five years of operations in its orbit. This satellite has multi-resolution and multi-spectral capabilities in a single platform. A continuous and autonomous co-registration, geo-location and radiometric calibration of image data from different sensors with widely varying view angles and resolution was one of the challenges of RS-2 data processing. On-orbit geometric performance of RS-2 sensors has been widely assessed and calibrated during the initial phase operations. Since then, as an ongoing activity, various geometric performance data are being generated periodically. This is performed with sites of dense ground control points (GCPs). These parameters are correlated to the direct geo-location accuracy of the RS-2 sensors and are monitored and validated to maintain the performance. This paper brings out the geometric accuracy assessment, calibration and validation done for about 500 datasets of RS-2. The objectives of this study are to ensure the best absolute and relative location accuracy of different cameras, location performance with payload steering and co-registration of multiple bands. This is done using a viewing geometry model, given ephemeris and attitude data, precise camera geometry and datum transformation. In the model, the forward and reverse transformations between the coordinate systems associated with the focal plane, payload, body, orbit and ground are rigorously and explicitly defined. System level tests using comparisons to ground check points have validated the operational geo-location accuracy performance and the stability of the calibration parameters.

PERFORMANCE ASSESSMENT AND GEOMETRIC CALIBRATION OF RESOURCESAT-2

Abstract. Resourcesat-2 (RS-2) has successfully completed five years of operations in its orbit. This satellite has multi-resolution and multi-spectral capabilities in a single platform. A continuous and autonomous co-registration, geo-location and radiometric calibration of image data from different sensors with widely varying view angles and resolution was one of the challenges of RS-2 data processing. On-orbit geometric performance of RS-2 sensors has been widely assessed and calibrated during the initial phase operations. Since then, as an ongoing activity, various geometric performance data are being generated periodically. This is performed with sites of dense ground control points (GCPs). These parameters are correlated to the direct geo-location accuracy of the RS-2 sensors and are monitored and validated to maintain the performance. This paper brings out the geometric accuracy assessment, calibration and validation done for about 500 datasets of RS-2. The objectives of this study are to ensure the best absolute and relative location accuracy of different cameras, location performance with payload steering and co-registration of multiple bands. This is done using a viewing geometry model, given ephemeris and attitude data, precise camera geometry and datum transformation. In the model, the forward and reverse transformations between the coordinate systems associated with the focal plane, payload, body, orbit and ground are rigorously and explicitly defined. System level tests using comparisons to ground check points have validated the operational geo-location accuracy performance and the stability of the calibration parameters.

Using a matched-filter technique at the Mikhnevo small-aperture seismic array

The Mikhnevo small-aperture array (SAA) was designed as an instrument for various regional seismic studies, including the compilation of a detailed catalogue of industrial blasts in the East European craton. This array includes 12 observation points arranged in three circles and equipped with SM3-KV shortperiod seismometers. The Institute of Geospheres Dynamics launched the array in 2004, and since that time it has been detecting up to 1000 industrial blasts per year. The Mikhnevo SAA uses beam formation for array processing. The stacking of individual waveforms reduced to a reference point allows the suppression of microseismic noise and improving the signal-to-noise ratio (SNR) relative to a three-component station. An improved SNR for a given signal is equivalent to a reduced detection threshold: much weaker signals can be detected with the use of a beam-formation technique. In turn, much more signals from small industrial explosions are detected. Weak signals are difficult to identify because of the higher uncertainty in the estimates of such characteristics as azimuth, slowness, and amplitude. Having a ten-year catalog of industrial blasts and the archive of raw digital records for this period, we apply a waveform cross-correlation (matched filter) technique, which has an extremely high relative location accuracy and thus identification capability. We have created a set of master events with relevant waveform templates for automatic data processing and creation of an accurate catalogue of industrial blasts.

Three‐dimensional Vp and Vp/Vs models in the Coso geothermal area, California: Seismic characterization of the magmatic system

AbstractWe combine classic and state‐of‐the‐art techniques to characterize the seismic and volcanic features in the Coso area in southern California. Seismic tomography inversions are carried out to map the variations of Vp, Vs, and Vp/Vs beneath Coso. The velocities in the top layers of our model are correlated with the surface geological features. The Indian Wells Valley, with high silica content sediment strata, shows low‐velocity anomalies up to 3 km depth, whereas the major mountain ranges, such as the south Sierra Nevada and the Argus Range, show higher velocities. The resulting three‐dimensional velocity model is used to improve absolute locations for all local events between January 1981 and August 2011 in our study area. We then apply similar‐event cluster analysis, waveform cross correlation, and differential time relocation methods to improve relative event location accuracy. A dramatic sharpening of seismicity patterns is obtained after using these methods. We also estimate high‐resolution near‐source Vp/Vs ratio within each event cluster using the differential times from waveform cross correlation. The in situ Vp/Vs method confirms the trend of the velocity variations from the tomographic results. An anomalous low‐velocity body with low Vp, Vs, and Vp/Vs ratios, corresponding to the ductile behavior underlying the Coso geothermal field from 6 to 12 km depth, can be explained by the existence of frozen felsic magmatic materials with the inclusion of water. The material is not likely to include pervasive partial melt due to a lack of high Vp/Vs ratios.

Seismic velocity variations along the rupture zone of the 1989 Loma Prieta earthquake, California

We revisit the rupture zone of the 1989 Mw6.9 Loma Prieta earthquake, central California, by developing high‐resolution three‐dimensional (3‐D)Vp and Vp/Vs models. We apply the simul2000 inversion method and algorithm to a set of “composite” events, which have greater number of picks per event and reduced random picking errors compared with traditional master events. Our final P‐wave velocity model generally agrees with previous studies, showing a high velocity body of above 6.7 km/s in the southeast rupture zone of the main shock. The 3‐DVp/Vs model, however, has different features, with low Vp/Vs in the upper crust and high Vp/Vs anomalies in deeper layers of the rupture zone. We interpret the low Vp/Vs at shallow depths to be granitic rocks, whereas at greater depths the areas of higher Vp/Vs(around 1.725–1.75) presumably are mafic rocks. The resulting 3‐D velocity model was used to improve absolute locations for all local events between 1984 and 2010 in our study area. We then applied a similar event cluster analysis, waveform cross‐correlation, and differential time relocation methods to improve relative event location accuracy. Over 88% of the seismicity falls into similar event clusters. A dramatic sharpening of seismicity patterns is obtained after using these methods. The medians of the relative location uncertainties calculated by using the bootstrap approach are 5 m for horizontal and 8 m for vertical. Differential times from cross‐correlation are used to estimatein situnear‐sourceVp/Vsratio within each event cluster. The high‐resolutionVp/Vs method confirms the trend of the velocity variations from the tomographic results, although absolute values are slightly different.

Tone-Based Localization for Distinguishing Relative Locations in Wireless Sensor Networks

Bounding-box mechanism is a well-known low-cost localization approach for wireless sensor networks. However, the bounding-box location information cannot distinguish the relative locations of neighboring sensors, hence leading to a poor performance for some applications such as location-aware routing. This paper proposes a Differentiating Relative Locations (DRL) mechanism which uses a mobile anchor to broadcast tones and beacons aiming at distinguishing the relative locations of any two neighboring nodes. With the order of entering and leaving tone transmission range, each sensor is able to identify its relative location relation with each of its neighbors. In addition, two path planning mechanisms are proposed to guarantee that all sensors can identify their relative locations while the energy consumption of the mobile anchor can be effective. Theoretical analysis is developed to estimate the impacts of duty time of tone signals in terms of energy conservation and accuracy of relative locations. Experimental study reveals that the proposed mechanism effectively distinguishes relative locations of any two neighboring nodes and hence significantly improves the performance of location-aware routing in wireless sensor networks (WSNs).

Spatiotemporal Earthquake Clusters along the North Anatolian Fault Zone Offshore Istanbul

We investigate earthquakes with similar waveforms in order to characterize spatiotemporal microseismicity clusters within the North Anatolian fault zone (NAFZ) in northwest Turkey along the transition between the 1999 Izmit rupture zone and the Marmara Sea seismic gap. Earthquakes within distinct activity clusters are relocated with cross-correlation derived relative travel times using the double-difference method. The spatiotemporal distribution of microearthquakes within individual clusters is resolved with relative location accuracy comparable to or better than the source size. High-precision relative hypocenters define the geometry of individual fault patches, permitting a better understanding of fault kinematics and their role in local-scale seismotectonics along the region of interest. Temporal seismic sequences observed in the eastern Sea of Marmara region suggest progressive failure of mostly nonoverlapping areas on adjacent fault patches and systematic migration of microearthquakes within clusters during the progressive failure of neighboring fault patches. The temporal distributions of magnitudes as well as the number of events follow swarmlike behavior rather than a mainshock/aftershock pattern.

The Use of Geospatial Information and Spatial Cognition of Taxi Drivers in Tokyo

This study aims to clarify the process of taxi drivers’ acquisition and use of geospatial information, focusing on the effects of in-vehicle navigation systems (IVNS) on taxi drivers’ spatial cognition. To that end, we interviewed taxi companies and conducted a questionnaire survey with taxi drivers and university students in Tokyo, where more than 60% of the taxis are equipped with IVNS. The interview with taxi companies revealed that to be engaged in the taxi service, licensed taxi drivers in Tokyo must pass a stringent set of examinations on geography, and undergo training to prepare newcomers at each taxi company. A comparison on cognitive maps of taxi drivers with those of university students, based on the analysis of the questionnaire data, showed that taxi drivers’ knowledge of relative locations was more accurate than that of students; however, no significant difference was found in the knowledge of absolute locations. This indicates that the abundant route knowledge that taxi drivers acquire through experience influences the accuracy of relative locations on cognitive maps rather than of absolute locations. In addition, neither a taxi driver career nor the IVNS usage had any significant effect on the accuracy of cognitive maps, which suggests the possibility that IVNS enhances novice drivers’ spatial knowledge.

A Search for Temporal Variations in Station Terms in Southern California from 1984 to 2002

We use relative arrival times and locations for similar earthquake pairs that are found using a cross-correlation method to analyze the time dependence of P and S station terms in southern California from 1984 to 2002. We examine 494 similar event clusters recorded by Southern California Seismic Network (SCSN) stations and compute absolute arrival-time variations from the differential arrival-time resi- duals obtained following event relocation. We compute station terms from the robust means of the absolute arrival-time residuals from all events recorded by each station at 3-month intervals. We observe nine stations with abrupt offsets in timing of 20- 70 msec, which are likely caused by equipment changes during our study period. Taking these changes into account could improve the relative location accuracy for some of the event clusters. For other stations, we generally do not see systematic temporal variations greater than about 10 msec. Analysis of residuals along individual ray paths does not reveal any clear localized regions of apparent velocity changes at depth. These results limit large-scale, long-lasting temporal variations in P and S ve- locities across southern California during this time period to less than about � 0:2%. However, there is an increased fraction of individual travel-time residuals exceeding 20 msec immediately following major earthquakes from source regions near the main- shock rupture.

In‐flight geometric calibration of different cameras of IRS‐P6 using a physical sensor model

AbstractThe IRS‐P6 satellite has multi‐resolution and multispectral capabilities on a single platform. A continuous and autonomous co‐registration and geolocation of image data from different sensors with widely varying view angles and resolution is one of the unique challenges of IRS‐P6 data processing. This requires in‐flight geometric calibration of the cameras. In‐flight calibration includes alignment calibration of individual sensors and calibration between the sensors. A method for in‐flight geometric calibration and quality assessment of IRS‐P6 images is presented in this paper. The objectives of this study are to ensure the best absolute and relative location accuracy of different cameras, and the same location performance with payload steering and co‐registration of multiple bands. This is done using a viewing geometry model, given ephemeris and attitude data, precise camera geometry and datum transformation. In the model, the forward and reverse transformations between the coordinate systems associated with the focal plane, payload, body, orbit and ground are rigorously and explicitly defined. System‐level tests using comparisons to ground check points have validated the operational geolocation accuracy performance and the stability of the calibration parameters.

The COMPLOC Earthquake Location Package

This article describes the programs included in the COMPLOC computer program package that are designed to apply the source-specific station term (SSST) method to solve for local earthquake locations using P- and S- wave phase data. These programs can greatly improve the relative location accuracy of nearby events by applying empirical corrections for the biasing effects of three-dimensional velocity structure. They have been tested on data from both the Southern California Seismic Network (SCSN) and Northern California Seismic Network (NCSN) The SSST method (Richards-Dinger and Shearer 2000; Lin and Shearer 2005) works by assigning each station a travel-time correction that varies as a function of source position. This approach provides relative location accuracy comparable to master event or hypocentroidal decomposition (Jordan and Sverdrup 1981) methods within compact event clusters, but is applicable to distributed seismicity. It has some similarities to the double-difference algorithm (Waldhauser and Ellsworth 2000, 2002) and can be shown to give comparable results in tests on synthetic data (Lin and Shearer 2005). However, the SSST approach has computational advantages for big data sets because the location and station term parts of the computation are separate, so that large matrix inversions are not necessary. In addition, our implementation of SSST provides the option to use L1-norm misfit measures, which are more robust than least squares in the case of occasional timing errors or bad phase picks. We implement the SSST approach by selecting nearby events located within a sphere of specified radius r max around the target event. The station term for the target event is then computed as the median (or mean) residual of these events. Different results will be obtained depending upon the size of the cutoff distance r max. If r max is set to a large enough distance, then the SSST …

Tests of relative earthquake location techniques using synthetic data

We compare three relative earthquake location techniques using tests on synthetic data that simulate many of the statistical properties of real travel time data. The methods are (1) the hypocentroidal decomposition method of Jordan and Sverdrup (1981), (2) the source‐specific station term method (SSST) of Richards‐Dinger and Shearer (2000), and (3) the modified double‐difference method (DD) of Waldhauser and Ellsworth (2000). We generate a set of synthetic earthquakes, stations, and arrival time picks in half‐space velocity models. We simulate the effect of travel time variations caused by random picking errors, station terms, and general three‐dimensional velocity structure. We implement the algorithms with a common linearized approach and solve the systems using a conjugate gradient method. We constrain the mean location shift to be zero for the hypocentroidal decomposition and double‐difference locations. For a single compact cluster of events, these three methods yield very similar improvements in relative location accuracy. For distributed seismicity, the DD and SSST algorithms both provide improved relative locations of comparable accuracy. We also present a new location technique, termed the shrinking box SSST method, which provides some improvement in absolute location accuracy compared to the SSST method. In our implementation of these algorithms, the SSST method runs significantly faster than the DD method.

Analysis of similar event clusters in aftershocks of the 1994 Northridge, California, earthquake

We perform waveform cross‐correlation on over 14,000 aftershocks of the 1994 Northridge MW 6.7 earthquake in southern California as recorded by short‐period stations of the Southern California Seismic Network (SCSN). Approximately 10–30% of the events belong to similar event clusters, depending upon the similarity criteria that are applied. We relocate events within 218 of these clusters to a relative location accuracy of about 30 m using the differential times obtained from the cross‐correlation. These relocated event clusters often show planar features suggestive of faults at depth, and we apply principal parameter analysis to characterize the shape of each cluster and to compute best fitting planes. In several cases, these planes are parallel to the main shock fault plane; however, more generally, the seismicity planes exhibit a wide range of orientations, suggesting complexity in the aftershock faulting. Composite focal mechanisms can be obtained for each cluster by combining the P polarity data from individual events. A comparison of polarity measurement differences within similar event clusters provides constraints on the error rate in the individual focal mechanisms. For some clusters, we are able to resolve the primary versus auxiliary fault plane ambiguity by comparing the computed focal mechanisms with the best fitting seismicity planes. Individual event focal mechanisms are in general agreement with the composite focal mechanisms for the similar event clusters. Events occurring along the main shock rupture plane are mainly thrust, whereas events in the hanging wall are predominately strike‐slip.

Nuclear explosion locations at the Balapan, Kazakhstan, nuclear test site: the effects of high-precision arrival times and three-dimensional structure

We have investigated the potential contributions of improved arrival times (using waveform cross-correlation) and the use of three-dimensional (3-D) velocity models for seismic event location capability. Our analyses are applied to a dataset of nuclear explosions at Balapan, Kazakhstan, for which ground-truth locations and some absolute origin times are available. This ground-truth information allows us to determine excellent origin time estimates for the remaining explosions. The combination of excellent ground-truth location information and high-quality origin time estimates permits us to (1) carry out a detailed examination of the quality of ISC picks, (2) identify probable timing errors in the digital data, (3) evaluate relative and absolute location capability using data from a sparse network, (4) assess the influence of event signal-to-noise ratio (SNR) on relative location accuracy, (5) utilize the Balapan events as a source array for 3-D tomography beneath the test site, and (6) test the influence of 3-D structure (local and global) on relative location accuracy and precision in a “controlled” situation. Our principal finding is that improved arrival times are the primary contributor to improved locations. Joint and individual relocations of Balapan events using the full digital dataset result in average mislocations of less than 1 km and 95% confidence regions of a compatible size. To mimic a CTBT scenario more realistically, we also carry out relocations using very few stations (4–10 observations). Location accuracy degrades somewhat, but the high-quality picks generally result in mislocations less than 10 km, even for events with very large azimuthal gaps. Uncertainty is generally underestimated in these cases. Tests with artificially degraded SNR show that mislocation increases slowly as SNR decreases. 3-D velocity structure makes a smaller contribution to relative location accuracy than accurate time picks. Travel time variations due to global 3-D structure vary little across the source region, so that location scatter is not reduced when travel time corrections for global 3-D structure are applied. Travel time variations due to the local 3-D structure (estimated using source-region tomography) are also modest. Applying travel time corrections that account for the local structure does yield slight location improvement.