Travel Time Residuals Research Articles

Bimodality of P-wave travel time residuals found in the CTBTO seismological data (REB-bulletins)

Bimodality of P-wave travel time residuals found in the CTBTO seismological data (REB-bulletins)

Using Extensive Core‐Reflected Phases to Constrain Sharp Inner Core Boundary Beneath East Asia

AbstractThe physical properties of the Earth's inner core boundary (ICB) are crucial for understanding inner core growth and geodynamo generation. In this study, we analyze the differential travel time residuals (DTTRs) and waveform similarities of the core‐reflected phases (PKiKP and PcP) to investigate fine‐scale seismic structures of the ICB. To study the ICB beneath East Asia, we collect a total of 4,272 PKiKP and PcP phase pairs from 37 earthquakes occurring from January 2009 to December 2018 recorded by permanent stations in the Chinese Digital Seismic Network (CDSN). This extensive PKiKP dataset allows for a geographically continuous depiction of the ICB beneath East Asia, revealing a diverse scope on its topography and sharpness. There are three main findings in our study. First, we collect a comprehensive PKiKP and PcP dataset with extensive ray coverage across East Asia. Numerous weak precritical PKiKP signals are detected, enabling robust constraints on the ICB across wide regions. Second, the PKiKP‐PcP DTTRs exhibit an average offset of −0.25 s (ranging from −1.5 to 1.0 s) relative to the PREM model, suggesting the outer core is approximately 1.3 km thinner than predicted by the PREM model, although it remains largely consistent with it. Third, the bin‐stacked PKiKP and PcP waveforms show high similarity across most sampled areas, indicating that the ICB thickness is no more than 2 km mostly beneath East Asia, which aligns with the hypothesis of a rapid transition from the solid inner core to the liquid outer core in our study region.

Patchwork structure of continental lithosphere captured in 3D body wave images of its anisotropic fabrics

This paper presents an overview of research conducted for more than five decades around Vladislav Babuška and collaborators on large-scale seismic anisotropy in tectonically different regions of continental lithosphere in Europe. A wide range of independent data sets and methods are covered. It also briefly touches laboratory measurements of velocity anisotropy on rock samples from the crust and the upper mantle, and emphasizes the importance of considering anisotropy in studies of the Earth structure. The anisotropy is responsible for even larger velocity variations than those due to composition of the most abundant upper mantle rocks (peridotites). The large-scale in-situ measurements of the upper mantle anisotropy capture fabrics of the mantle lithosphere, and enables mapping lateral changes in its structure. The joint inversion/interpretation of the teleseismic body-wave anisotropic parameters, such as variations of directional terms of relative travel time residuals of P waves, shear-wave splitting or the coupled anisotropic-isotropic teleseismic P-wave tomography, image the continental lithosphere as a mosaic of anisotropic domains. Each of the domains has its own thickness and fossil fabric characterized by tilted symmetry axes. We map boundaries of the domains in dependence on the fabric changes. The boundaries can be either narrow and steep or broader and inclined, with an offset relative to boundaries of the related crustal bocks, which can reach several tens of kilometres. This overview presents the European lithosphere-asthenosphere boundary (LAB) and shows examples of anisotropic fabrics of the mantle lithosphere domains and their boundaries in different parts of the European plate.

Induced seismicity at the Laúca reservoir, Angola Craton: Focal mechanisms and implications for the stress field in Western Central Africa

We present the first case of reservoir induced seismicity (RIS) ever reported in the Angola Craton, Laúca reservoir. Impoundment of the reservoir began in late 2017 and seismicity started in March 2018 shortly after the water level reached 86 m. Earthquakes continued to be recorded in the following years, with 287 events detected between March/2018 and April/2024. The largest event had magnitude 3.0 ML. The dam is 156 m tall, and the total reservoir volume is 5,044.85 Hm3. The reservoir area is monitored by two stations, LAUC and ZERO. Seismic data is of high quality, with sharp P- and S-wave arrivals at both stations. We derive a half-space velocity model for the lake area using a composite Wadati diagram and minimization of travel time residuals and locate 90 events with arrivals at both stations. S-to-P converted phases at the surface were used to help constrain hypocentral depths and epicentral locations. We use waveform modeling to determine focal mechanisms for ten events using the program FMNEAR, and invert for the local stress field. We find the faulting regime in the area to be strike-slip and SHMax oriented roughly NE-SW. We compare our results to the broader stress field in western Central Africa and find them to be in agreement with nearby stress determinations from focal mechanisms in the Congo Basin. Our results make an important contribution to the state of knowledge of the stress field in western Central Africa, which can be taken into consideration in future geodynamic models of the Nubian plate as more data is gathered and help further our understanding of stress sources in this region.

3D Multiresolution Velocity Model Fusion with Probability Graphical Models

ABSTRACT The variability in spatial resolution of seismic velocity models obtained via tomographic methodologies is attributed to many factors, including inversion strategies, ray-path coverage, and data integrity. Integration of such models, with distinct resolutions, is crucial during the refinement of community models, thereby enhancing the precision of ground-motion simulations. Toward this goal, we introduce the probability graphical model (PGM), combining velocity models with heterogeneous resolutions and nonuniform data point distributions. The PGM integrates data relations across varying resolution subdomains, enhancing detail within low-resolution (LR) domains by utilizing information and prior knowledge from high-resolution (HR) subdomains through a maximum posterior problem. Assessment of efficacy, utilizing both 2D and 3D velocity models—consisting of synthetic checkerboard models and a fault-zone model from Ridgecrest, California—demonstrates noteworthy improvements in accuracy, compared to state-of-the-art fusion techniques. Specifically, we find reductions of 30% and 44% in computed travel-time residuals for 2D and 3D models, respectively, as compared to conventional smoothing techniques. Unlike conventional methods, the PGM’s adaptive weight selection facilitates preserving and learning details from complex, nonuniform HR models and applies the enhancements to the LR background domain.

Application and Research of Microseismic Monitoring System and Hydraulic Fracturing Technology in Coal Mines

In order to improve the effectiveness of coal mine gas control and enhance the level of coal mine safety production, the application of a microseismic monitoring system and hydraulic fracturing technology in coal mines was studied. Applying hydraulic fracturing technology to coal mine gas treatment, firstly, the geological structure and gas concentration in the mining area are detected using the radio tunnel perspective method and infrared differential absorption method. Then, the relevant parameters of hydraulic fracturing are determined, and finally, hydraulic fracturing technology is implemented. Microseismic monitoring technology is used to monitor the cracks formed during hydraulic fracturing construction and evaluate the fracturing effect. The instantaneous energy envelope is obtained from the microseismic data of each detection channel after stacking and Hilbert transform static correction. A microseismic in-phase inversion positioning objective function based on travel time residuals is constructed, and under the constraints of polarization analysis, the optimal solution is obtained through search iteration to complete microseismic in-phase inversion positioning. Experimental results have shown that after applying this method to coal mine gas control, the gas concentration decreases below the execution standard, achieving good control effects. Under microseismic monitoring in coal mines, the hydraulic fracturing effect can be effectively and reasonably evaluated, and the safety production level of coal mines can be improved.

Detailed 3D Structures of the Western Edge of the Pacific Large Low Velocity Province

AbstractLarge Low Velocity Provinces (LLVPs) are situated oppositely in the lowermost mantle beneath the Pacific Ocean and Africa. Deciphering the detailed seismic structures at the edge of LLVPs can provide key information on the composition and dynamics in the deep Earth. Here, we provide a detailed seismic image at the western edge of the Pacific LLVP by dense recordings. Differential travel time residuals and amplitude ratios between ScS and S outline the S‐wave western boundary of the Pacific LLVP, suggesting the complex structures including low/high‐velocity patches in the lowermost mantle in our study region. We determine the 3D low‐velocity structure by modeling the delayed ScS and high‐velocity D″ layer structure by modeling the anomalous Scd, with tight constraints from multiple events data. The drastically varied waveforms in azimuth suggests a sharp transitional boundary among the complex structures. After comparing the velocity structures in adjacent regions, we propose that the 3D structures of the western edge of the Pacific LLVP are strongly influenced by the vigorous mantle flow associated with the actively subducted slab.

Anisotropic media hybrid grid tomographic inversion method based on angle domain imaging gathers

The grid tomography velocity inversion method prevails as the most efficacious technique for acquiring background velocity requisite for migration. Originating from imaging gathers, this method not only inverses velocity but also subsequently ascertains the anisotropic parameters field. Traditional approaches utilize a parameter field discretized in a rectangular grid for both ray tracing and parameter inversion. However, the rectangular grids may not provide optimal accuracy for structures with pronounced lateral parameter variations. In contrast, triangular grid discretization allows for grid division along steep slopes, offering sparsity and flexibility when juxtaposed with conventional rectangular grid methods. This paper introduces an optimized triangular grid travel time parameter tomography inversion method, formulated on the parameter sensitivity to travel time and employing angle gathers to derive travel time residuals. Analytical modeling and real data processing substantiate that the triangular grid ray tracing method ameliorates the ill-conditioned nature of the tomography equation and enhances the accuracy of the inversion parameter field effectively.

APLIKASI DOUBLE DIFFERENCE UNTUK IDENTIFIKASI ZONA PATAHAN MIKRO WILAYAH SULAWESI BARAT

West Sulawesi is an area in South Sulawesi province that is experiencing a sudden increase in seismic activity. There have been 264 earthquake events recorded during the 2021-2022 period. The increase in seismic activity occurred around the west Sulawesi segment and the Mamuju segment, which are local faults located in west Sulawesi. Sulawesi Island itself is the result of fragments due to larger complex reactions and has quite high seismic activity. Therefore, it is necessary to calculate the earthquake hypocenter relocation to obtain more accurate hypocenter parameters for fault zone identification and micro fault orientation. This study aims to relocate the earthquake hypocenter based on mathematical measurement data using the Double-Difference method for identification of fault zones and micro-fault orientation. The data used is BMKG earthquake catalog data for the western Sulawesi region with a range of 118.49 East - 119.59 East and 3.67 LS -2.45 LS in the 2021-2022 period. The Double-Difference method is a method based on the Geiger method using residual travel time data from each pair of hypocenters to the earthquake recording station. The principle is to compare two earthquake hypocenters that are close to the earthquake recording station with the assumption that the distance between the two hypocenters must be closer than the distance between the hypocenter and the earthquake recording station. This is done so that the raypath of the two hypocenters can be considered close to the same value. From the calculation of the earthquake hypocenter relocation, it was found that the hypocenter after relocation experienced changes in distribution that accumulated in the Mamasa fault segment area. From the residual histogram, it is found that the RMS residual is close to 0, indicating an improvement in data quality as evidenced by the observation travel time and the calculated travel time are almost close to the same value. In the cross-section results, it is obtained that the depth changes vary in the subsurface depth from the initial depth of 10 km before being relocated, indicating that the earthquake that occurred in West Sulawesi was included in the shallow crustal earthquake type.

Analyzing Seismicity Pattern of the Gorontalo Region and Its Surroundings in the Form of Hypocenter Relocation Using Double Difference Method

The level of seismic activity due to earthquake events in Gorontalo is strongly influenced by the tectonic system model of the Gorontalo region and the presence of a subduction zone in the Sulawesi Sea. These characteristics make the Gorontalo region have high seismic activity. To describe the pattern of seismicity in the Gorontalo region, it is necessary to conduct an analysis in the form of earthquake hypocenter relocation using the double difference (hypoDD) method. This method uses residual travel time data from each pair of hypocenters to the recording station. This study aims to analyze the results of hypocenter relocation and describe the subsurface model related to the seismicity pattern of Gorontalo and its surroundings. The data used are P and S waves for 7 years recorded at the Meteorology, Climatology and Geophysics Agency (BMKG) station. The earthquake hypocenters relocated in this study were 3,940 out of 4,598 earthquake events. The results of hypocenter relocation show a fairly good seismicity pattern obtained from good RMS values and distribution patterns following Gorontalo tectonic patterns. In general, the seismicity pattern of Gorontalo and its surroundings is strongly influenced by activity in the Sulawesi Sea subduction zone and the Gorontalo fault in the region. The seismicity caused by the subduction zone has a depth of 50 - 200 km, and due to the active faults in the Gorontalo region has a depth of 2 - 25 km.

Seismic imaging of the Northern Andean subduction zone from teleseismic tomography: a torn and fragmented Nazca slab

SUMMARY The Nazca-South America subduction zone in Ecuador is characterized by a complicated along-strike geometry as the slab transitions from flat slab subduction in the south, with the Peruvian flat slab, to what has been characterized as ‘normal’ dipping subduction beneath central Ecuador. Plate convergence additionally changes south to north as the trench takes on a convex shape. Highly heterogeneous bathymetry at the trench, including the aseismic oceanic Carnegie Ridge (CR), and sparse intermediate-depth seismicity has led many to speculate about the behaviour of the downgoing plate at depth. In this study, we present a finite-frequency teleseismic P-wave tomography model of the northern Andes beneath Ecuador and Colombia from 90 to 1200 km depth. Our model builds on prior tomography models in South America by adding relative traveltime residuals recorded at stations in Ecuador. The complete data set is comprised of 114 096 relative traveltime residuals from 1133 stations across South America, with the added data serving to refine the morphology of the Nazca slab in the mantle beneath the northern Andes. Our tomography model shows a Nazca slab with a fragmented along-strike geometry and the first teleseismic images of several proposed slab tears in this region. At the northern edge of the Peruvian flat slab in southern Ecuador, we image a shallow tear at 95–200 km depth that appears to connect mantle flow from beneath the flat slab to the Ecuadorian Arc. Beneath central Ecuador at the latitudes of the CR, the Nazca slab is continuous into the lower mantle. Beneath southern Colombia, the Malpelo Tear breaks the Nazca slab below ∼200 km depth.

Emulation of seismic-phase traveltimes with machine learning

SUMMARY We present a machine learning (ML) method for emulating seismic-phase traveltimes that are computed using a global-scale 3-D earth model and physics-based ray tracing. Accurate traveltime predictions based on 3-D earth models are known to reduce the bias of event location estimates, increase our ability to assign phase labels to seismic detections and associate detections to events. However, practical use of 3-D models is challenged by slow computational speed and the unwieldiness of pre-computed lookup tables that are often large and have prescribed computational grids. In this work, we train a ML emulator using pre-computed traveltimes, resulting in a compact and computationally fast way to approximate traveltimes that are based on a 3-D earth model. Our model is trained using approximately 850 million P-wave traveltimes that are based on the global LLNL-G3D-JPS model, which was developed for more accurate event location. The training-set consists of traveltimes between 10 393 global seismic stations and randomly sampled event locations that provide a prescribed, distance-dependent geographic sample density for each station. Prediction accuracy is dependent on event-station distance and whether the station was included in the training set. For stations included in the training set the mean absolute deviation (MAD) of the difference between traveltimes computed using ray tracing through the 3-D model and the ML emulator for local, regional, and teleseismic distances are 0.090, 0.125 and 0.121 s, respectively. For tested station locations not included in the training set, MAD values for the three distance ranges increase to 0.173, 0.219 and 0.210 s, respectively. Empirical traveltime residuals for a global reference data are indistinguishable when ML emulation or the 3-D model is used to compute traveltimes. This result holds regardless of whether the recording station is used in ML training or not.

Studying inner core and lower mantle structure with a combination of PKP and converted SKP and PKS waves

SUMMARYStructure of the inner core is often measured through traveltime differences between waves that enter the inner core (PKPdf) and waves that travel through the outer core only (PKPab and PKPbc). Here we extend the method to converted waves PKSdf and SKPdf and compare results to PKP wave measurements. PKSdf and SKPdf have a very similar path to PKPdf and if velocity variations are present in the inner core, all three wave types should experience them equally. Since traveltime deviations can be due to velocity changes (either isotropic or anisotropy) as well as wave path deviations born from heterogeneity, we simultaneously investigate wave path directions and traveltimes of PKP, SKP and PKS waves for several source-array combinations. One of the path geometries is the anomalous polar corridor from South Sandwich to Alaska, which has strong traveltimes anomalies for PKPdf relative to more normal equatorial path geometries. Here we use array methods and determine slowness, traveltime and backazimuth deviations and compare them to synthetic data. We find that path deviations from theoretical values are present in all wave types and paths, but also that large scatter exists. Although some of the path deviations can be explained by mislocation vectors and crustal variations, our measurements argue that mantle structure has to be considered when investigating inner core anisotropy. Our polar path data show similar traveltime residuals as previously published, but we also find slowness residuals for this path. Interestingly, SKPdf and PKSdf for the South Sandwich to Alaska path show traveltime residuals that are partly opposite to those for PKPdf, possibly due to an interaction with a localized ultra-low velocity zone where waves enter the core.

Wide‐spectrum reconstruction of a velocity model based on the wave equation reflection inversion method and its application

AbstractMost seismic data used for conventional exploration lack far‐offset signals and effective low‐frequency components. This makes it difficult for conventional full waveform inversion methods to recover the low‐wavenumber components of mid‐ and deep‐layer models. To resolve the bottleneck of ray‐theoretical reflection traveltime tomography, wave equations, such as reflection inversion methods, are receiving increasing attention. We reconstructed a wide‐spectrum velocity model based on the multiscale wave‐equation reflection inversion method for model decomposition. First, using the dynamic image warping method to obtain reflection traveltime residuals, the wave‐equation reflection traveltime inversion was used to recover the low‐wavenumber components of the background model, which also solved the cycle‐skipping problem. Second, the medium‐wavenumber component of the model is then supplemented by the reflection waveform inversion, while the reflectivity model is obtained relying on the least‐squares reverse time migration method. Based on this method, the background and perturbation models are updated alternately and iteratively. At the same time, the stratum structural tensor information was extracted using the perturbed model image to construct a preconditioned operator for the stratum structural constraint, suppress unreasonably high‐wavenumber components in the generalized gradient and improve the geological consistency of the inversion results. Testing of the model using the Sigsbee2b model and seismic field data from the East China Sea showed that, compared with the conventional reflection traveltime tomography method based on prestack depth migration, the wave‐equation reflection inversion strategy with well‐matched information from traveltimes to waveforms significantly improved the accuracy of the middle and deep velocity modelling and enhanced the imaging quality of the whole body.

Seismic Velocity Structure of Upper Mantle Beneath the Oldest Pacific Seafloor: Insights From Finite‐Frequency Tomography

AbstractThe oldest oceanic basin (160–180 Ma) in the western Pacific is the birthplace of the Pacific Plate and is thus essential for understanding the formation and evolution of the oceanic plate. However, the upper mantle structure beneath the region has not been thoroughly investigated because of the remoteness and difficulties of long‐term in situ seismic measurements at the ocean bottom. From 2018 to 2019, the Oldest‐1 experiment on the oldest seafloor was conducted as part of the international Pacific Array initiative. We present the first three‐dimensional P‐wave velocity structure down to a depth of 350 km based on the relative travel time residuals of teleseismic earthquakes recorded by 11 broadband ocean‐bottom seismometers operated during the Oldest‐1 experiment. Our result shows a fast P‐wave velocity anomaly (VP perturbation of 2%–4% faster than average) at a depth of 95–185 km beneath the northeast of the study area. This structure is interpreted as evidence of dry, viscous, and rigid materials at depths below the lithosphere. Two slow anomalies (VP perturbation of 2%–4% slower than average) are seen beneath the southwestern and eastern (the oldest seafloor >170 Ma) parts of the array site. The low‐velocity zones are found at depths of 95–305 km. The observed velocity structures can be indicative of plume activities that affected the upper mantle as the Pacific Plate migrated over hotspots from the southeast. Alternatively, the observed velocity features may provide seismic evidence for small‐scale sublithospheric convection.

Global empirical models for infrasonic celerity and backazimuth

SUMMARY Global empirical models for infrasound signal celerity (where celerity is defined as the epicentral distance divided by the total traveltime) and backazimuth deviation (the difference between the measured and predicted backazimuth assuming great circle propagation), can be used for the association of infrasound automatic detections, for event location and for the validation of acoustic propagation simulations. Using software developed to consistently analyse a global ground truth database, we observe 296 detections in the 0.32–1.28 Hz passband; predominantly stratospheric arrivals, with a smaller number of tropospheric and thermospheric arrivals, in agreement with previous studies. We develop an updated global celerity-range model and introduce a global backazimuth deviation model. These new models suggest that the variation in backazimuth deviation is not range-dependent; 93 per cent of the 296 detections studied have a deviation magnitude ≤5°. The maximum deviation observed is 8.9°. The variation in celerity, however, is range-dependent. An exponential range-dependent celerity model with bounds calculated using a quantile regression fit to the traveltime residuals is determined. This traveltime residual model, with uncertainties that increase with range, produces bounds on the celerity consistent with both the observations and current understanding of infrasound propagation. The traveltime residual model derived celerity bounds are not symmetric around the celerity model, which further reflects the physical processes. At long ranges (>3000 km), maximum peak-to-trough amplitude arrivals are not observed with celerities <280 ms−1, but even at long ranges, we occasionally observe celerities (>320 −1) more usually consistent with tropospheric arrivals.

Study of Seismicity Based on the Results of Hypocenter Relocation Using Double Difference (HypoDD) Method in West Sumatera and Its Surrounding

The presence of the Indo-Australian plate, the Eurasian plate and the active Sumatra fault zone makes the West Sumatra region and its surroundings have very high seismic activity. To describe the seismicity pattern, it is necessary to analyze the earthquake hypocenter relocation, one of which is using the double difference (hypoDD) method. This method basically uses the data residual travel time from each hypocenter pair to the earthquake seismic station. The purpose of this research is to analyze the relocation of the hypocenter and describe the seismicity pattern in the West Sumatra and surrounding areas. The data used in this study are the time arrivals of P and S waves during the period of 2022 obtained from the earthquake catalog at the Padang Panjang Geophysical Station. The results showed that the earthquake hypocenter was relocated 888 out of 934 earthquake events and the distribution of hypocenter relocation produces a good seismicity pattern. From the results of the earthquake hypocenter relocation, the seismicity pattern in the West Sumatra region and its surroundings is generally influenced by subduction zones and some due to active fault zones. Seismicity in the subduction zone and active faults of Sumatra has a depth of about 50 - 250 km and 5 - 20 km. This indicates that during the period of 2022, subduction zones with medium to deep depths are very active as well as in Sumatra's active fault zones, especially in the Sianok segment.

Upper Mantle Velocity Structure Beneath the Yarlung–Tsangpo Suture Revealed by Teleseismic P-Wave Tomography

We applied teleseismic tomography to investigate the 3D P-wave velocity (Vp) structure of the crust and upper mantle at depths of 50–400 km beneath the Yarlung–Tsangpo suture (YTS), by using 6164 P-wave relative travel-time residuals collected from 495 teleseismic events recorded at 20 three-component broadband seismograms. A modified multi-channel cross-correlation method was adopted to automatically calculate the relative arrival-time residuals of all teleseismic events, which significantly improved the efficiency and precision of the arrival-time data collection. Our results show that alternating low- and high-Vp anomalies are visible beneath the Himalayan and Lhasa blocks across the YTS, indicating that strong lateral heterogeneities exist beneath the study region. A significant high-Vp zone is visible beneath the southern edge of the Lhasa block at 50–100 km depths close to the YTS, which might indicate the rigid Tibetan lithosphere basement. There exists a prominent low-Vp zone beneath the Himalayan block to the south of the YTS extending to ~150 km depth, which might be associated with the fragmentation of the underthrusting Indian continental lithosphere (ICL) and induce localized upwelling of asthenospheric materials from the upper mantle. In addition, significant low-Vp anomalies were observed beneath the Yadong–Gulu rift and the Cona–Sangri rift extending to ~300 km depth, indicating that the tearing of the subducted ICL might provide pathways for the localized asthenospheric materials upwelling, which contributes to the widespread distribution of north–south trending rifts and geothermal activities in southern Tibet.

Characteristic reflector-based wave equation reflection traveltime inversion

Reconstructing the deep background velocity model is crucial for imaging subsurface structures. The reflection traveltime inversion (RTI) is capable of recovering the kinematic information in the velocity model under a limited seismic observation aperture. The estimation of the reflection traveltime residuals between the synthetic and observed seismic waves is the key to the RTI. However, it is difficult to accurately obtain the rapidly changing reflection traveltime shifts in the prestack data domain. Estimating the traveltime difference is especially difficult where the discrepancy between the noisy observed data and demigrated synthetic data includes a waveform distortion, inconsistent reflected events, and so on. To overcome this problem, we have developed a new reflection traveltime residual estimation method. In this method, after identifying a characteristic reflector in the prestack depth migration section with an initial velocity, the demigrated and observed seismic reflections in the data domain corresponding to the characteristic reflector share the same traveltime at the zero offset. Thus, their traveltime residual can be automatically tracked by their crosscorrelation with a modified dynamic programming algorithm based on a zero-offset control point, which can avoid traveltime picking or windowing of relevant reflection arrivals in the prestack seismic data domain. Then, the estimated reflection traveltime residual is applied to the RTI. To improve the inversion accuracy, we have further developed a layer-stripping velocity model-building workflow. Synthetic and field data tests demonstrate that the proposed method can obtain a robust reflection traveltime residual and further achieve a correct low-to-intermediate wavenumber velocity model update.

Evaluation of Station Timing Errors from Demeaned Arrival Time Delays

ABSTRACT We present an analytical solution for the evaluation of timing errors at seismological stations. The method makes use of differential P- and S-wave arrival time measurements demeaned over a network that recorded a set of densely located seismic events. In this configuration, one can assume coincident P and S ray paths between sources and receivers, and cancel out dependencies associated with absolute event origin times, event locations, and P- or S-wave velocities in the problem. Relative timing errors can be obtained by linear inversion, using only a limited amount of input data: differential P- and S-wave arrival times, and a local VP/VS ratio. By setting at least one reference station in the network, supposed to be devoid of any timing error, one can retrieve reliable timing errors for other stations. We validate the approach against synthetic and real data. We also analyze the sensitivity of results on errors in the input data. Although picking uncertainties do affect the variability of estimates, we also identified a significant bias when an incorrect VP/VS ratio is used. However, this bias can be reduced if one uses the optimal VP/VS value that minimizes the root mean square of travel-time residuals. Application of the method to a collection of manually picked arrival times for the 2002–2003 Tricastin, France, earthquake, swarm allowed us to identify nonstationary timing errors from tenth to tens of seconds during the monitoring campaign.