Differential Travel Times Research Articles

Shear generation in a confined, composite layer of cross-bedded porous rock

Abstract

Relative Time Corrections for Historical Analog Seismograms Using the Single-Day Ambient Noise Correlation Function

ABSTRACTThis study examines analog seismograms that were generated when most seismic stations had their own clock for timing, making precise comparison of time between different stations difficult. Availability of accurate relative timing facilitates differential travel-time analyses, such as seismic tomography and local earthquake relocations, to be performed using data originally recorded on paper or other physical media. These analyses allow for the investigation of longer-term processes like the earthquake cycle or climate change. We take advantage of the continuous nature of seismic noise to determine the relative time correction between two stations by leveraging the symmetry of the noise correlation function. This procedure is applied to two Global Positioning System-timed stations in the Hawaiian Volcano Observatory network demonstrating subsecond time accuracy. The technique is then applied to analog records from comparable stations between 7 and 10 August in 1988, and relative time corrections of up to about 6 s are obtained. These corrections are confirmed by the relative arrival times of teleseismic P waves of earthquake doublets.

Separation of diffracted waves via SVD filter

Diffracted seismic waves may be used to help identify and track geologically heterogeneous bodies or zones. However, the energy of diffracted waves is weaker than that of reflections. Therefore, the extraction of diffracted waves is the basis for the effective utilization of diffracted waves. Based on the difference in travel times between diffracted and reflected waves, we developed a method for separating the diffracted waves via singular value decomposition filters and presented an effective processing flowchart for diffracted wave separation and imaging. The research results show that the horizontally coherent difference between the reflected and diffracted waves can be further improved using normal move-out (NMO) correction. Then, a band-rank or high-rank approximation is used to suppress the reflected waves with better transverse coherence. Following, separation of reflected and diffracted waves is achieved after the filtered data are transformed into the original data domain by inverse NMO. Synthetic and field examples show that our proposed method has the advantages of fewer constraints, fast processing speed and complete extraction of diffracted waves. And the diffracted wave imaging results can effectively improve the identification accuracy of geological heterogeneous bodies or zones.

Can entry of high-speed rail increase air traffic? Price competition, travel time difference and catchment expansion

While many empirical studies find that high-speed rail (HSR) exercises a downward pressure on air traffic, several studies observe an increase in air traffic after HSR enters the overlapping markets, especially in long-haul markets (over 1000 km). The paper provides a possible theoretical and empirical explanation on the seemingly conflicted findings. With a model of differentiated price competition, we show that air-rail competition can induce more air traffic after the entry of HSR as long as the air travel time is sufficiently shorter than the HSR travel time. The mixed empirical results could be caused by the failure to incorporate both modes’ travel times. Thus, in the empirical part of this paper, we use the difference of HSR and air flight travel times to capture the relative competitiveness of these two competing modes of transport after controlling for the potential catchment expansion effect of HSR. Other route characteristics such as GDP per capita and population of the two endpoint cities, time-invariant route fixed effect and year fixed effect are controlled for in the model as well. Based on a sample of Chinese air routes, our regression analysis confirms the theoretical prediction. In particular, air traffic tends to increase after the entry of HSR if the HSR travel time is over 5 h longer than air travel time. Otherwise, the air traffic tends to reduce. This implies that a large share of sampled Chinese routes, including both medium-haul and long-haul routes, may experience an increase in air traffic.

Modeling spatial access to cervical cancer screening services in Ondo State, Nigeria

BackgroundWomen in low- and middle-income countries (LMIC) remain at high risk of developing cervical cancer and have limited access to screening programs. The limits include geographical barriers related to road network characteristics and travel behaviors but these have neither been well studied in LMIC nor have methods to overcome them been incorporated into cervical cancer screening delivery programs.MethodsTo identify and evaluate spatial barriers to cervical cancer prevention services in Ondo State, Nigeria, we applied a Multi-Mode Enhanced Two-Step Floating Catchment Area model to create a spatial access index for cervical cancer screening services in Ondo City and the surrounding region. The model used inputs that included the distance between service locations and population centers, local population density, quantity of healthcare infrastructures, modes of transportation, and the travel time budgets of clients. Two different travel modes, taxi and mini bus, represented common modes of transit. Geocoded client residential locations were compared to spatial access results to identify patterns of spatial access and estimate where gaps in access existed.ResultsOndo City was estimated to have the highest access in the region, while the largest city, Akure, was estimated to be in only the middle tier of access. While 73.5% of clients of the hospital in Ondo City resided in the two highest access zones, 21.5% of clients were from locations estimated to be in the lowest access catchment, and a further 2.25% resided outside these limits. Some areas that were relatively close to cervical cancer screening centers had lower access values due to poor road network coverage and fewer options for public transportation.ConclusionsVariations in spatial access were revealed based on client residential patterns, travel time differences, distance decay assumptions, and travel mode choices. Assessing access to cervical cancer screening better identifies potentially underserved locations in rural Nigeria that can inform plans for cervical cancer screening including new or improved infrastructure, effective resource allocation, introduction of service options for areas with lower access, and design of public transportation networks.

Fine Structure of the Chenghai Fault Zone, Yunnan, China, Constrained From Teleseismic Travel Time and Ambient Noise Tomography

AbstractTo derive high‐resolution fault zone (FZ) structure of the Chenghai fault in Yunnan, southwestern China, we deployed a linear dense array crossing the fault from January to February 2018. The array consisted of 158 short‐period (5 s) three‐component instruments and spanned an aperture of ~8 km with average station spacing of 40–50 m. During the 1‐month deployment, 20 teleseismic earthquakes with moment magnitudes larger than 5.5 and 41 local earthquakes were recorded. We first analyzed the travel times of P and S waves from teleseismic earthquakes to determine the boundaries of the FZ. After correcting the station‐event geometry effects, teleseismic travel time differences between the reference station, and all other stations clearly marked a zone of 3.4 km in width with distinct travel time anomaly, suggesting a low‐velocity zone (LVZ) surrounding the Chenghai fault. We then conducted ambient noise tomography and found that the S wave velocity of the LVZ was reduced by 60% and 40% compared to the northwest and southeast of the LVZ, respectively. Our ambient noise results suggested the LVZ extending to ~1.5 km in depth, consistent with the travel time anomalies of teleseismic earthquakes. Integrating ambient noise tomography with teleseismic travel times in a dense array with such an aperture is an effective approach for resolving FZ structure and depth extent.

Patients' perspectives on a new delivery model in primary care: A propensity score matched analysis of patient-reported outcomes in a Dutch cohort study.

Rationale, aims and objectivePrimary Care Plus (PC+) focuses on the substitution of hospital‐based medical care to the primary care setting without moving hospital facilities. The aim of this study was to examine whether population health and experience of care in PC+ could be maintained. Therefore, health‐related quality of life (HRQoL) and experienced quality of care from a patient perspective were compared between patients referred to PC+ and to hospital‐based outpatient care (HBOC).MethodsThis cohort study included patients from a Dutch region, visiting PC+ or HBOC between December 2014 and April 2018. With patient questionnaires (T0, T1 and T2), the HRQoL and experience of care were measured. One‐to‐two nearest neighbour calliper propensity score matching (PSM) was used to control for potential selection bias. Outcomes were compared using marginal linear models and Pearson chi‐square tests.ResultsOne thousand one hundred thirteen PC+ patients were matched to 606 HBOC patients with well‐balanced baseline characteristics (SMDs <0.1). Regarding HRQoL outcomes, no significant interaction terms between time and group were found (P > .05), indicating no difference in HRQoL development between the groups over time. Regarding experienced quality of care, no differences were found between PC+ and HBOC patients. Only travel time was significantly shorter in the HBOC group (P ≤ .001).ConclusionResults show equal effects on HRQoL outcomes over time between the groups. Regarding experienced quality of care, only differences in travel time were found. Taken as a whole, population health and quality of care were maintained with PC+ and future research should focus more on cost‐related outcomes.

The dimensions of scatterers in the lower mantle using USArray recordings of S-wave to P-wave conversions

Seismic recordings from the USArray of the October 1, 2013 Sea of Okhotsk earthquake and the May 24, 2010 Western Brazil earthquake include high-amplitude SxP signals due to S-wave to P-wave scattering about 150 to 200 km below the 660-km discontinuity below these events. The scattering structures have been mapped previously and interpreted as fragments of folded crust that subducted into the uppermost lower mantle. The USArray recordings are unique because SxP is visible at all stations of the USArray over a distance of 2000 km. The arrival times of SxP with respect to the P arrival times vary by 2–3 s across the USArray.The 2–3 s arrival time variations of SxP suggest the finite dimensions of the scattering structures. We estimate the dimensions by beamforming waveforms for subsets of USArray stations and the uncertainty ranges by bootstrapping. In our analysis we assume that the high-frequency wave traveltimes can be modeled using ray theory and that wave-speed heterogeneity between the scattering point and the array does not affect SxP-P differential traveltimes. The results indicate that the scattering structures beneath the Sea of Okhotsk and western Brazil have dimensions of 20–160 km but the uncertainties are large. The variation of the SxP amplitude by a factor of two to three is likely due to the complex geometry of the folded crust. We attribute the down-and-up SxP waveform to the interference of S-wave to P-wave conversions at the top and bottom of a 10–20 km thick portion of the folded crust. Our results suggest that the subducted crust can retain thicknesses of 10–20 km even after being folded by subduction into the viscous lower mantle.

Evolution of Transportation Network Companies and Taxis through 2013–2018 in Chicago

The objective of this paper is to highlight important differences between taxis and transportation network companies (TNCs) in a large urban area. We analyze the publicly available dataset from Chicago which includes taxi and transportation network company (TNC) utilization and the level of service measures from five months in 2013–2014 and the same five months in 2018–2019. We compare and contrast the data from these two points in time to document utilization of taxis and TNCs and to measure differences in travel times, travel distances, fares, destinations served, and the spatial and temporal distribution of these trips. Travel to and from airports has been evaluated separately owing to the exceptionally high number of trips they generate. Striking differences between pooled and unpooled TNC trip volumes and other travel metrics have been assessed to highlight their operational diversity despite being considered as the same mode. The exploratory analysis has been carried out across the shared-ride, time, and mode dimensions. The study revealed both similarities and differences in taxi trip characteristics between the two evaluation periods and also outlined how the ridehailing market has grown over the years despite the near stagnation in population and employment in the city. We believe that assessing how taxis have fared through this time and highlighting the intrinsic differences between how the old and new mode of on-demand ride services coexist is important. This study aims to help understand how new-age mobility services are impacting transportation in one of the largest cities in the U.S.

Adverse Effects of an Edge Diffractor in Seismic Reflection Interferometry

To understand steeply dipping events in seismic reflection interferometry (SRI), we derived an expression that describes the difference in travel time (Δτ) from a diffractor to two receivers in two dimensions. For a fixed receiver interval, the expression shows that Δτ is zero when the diffractor is at the midpoint of the paired receivers, increases with an apparent velocity of half the medium velocity as the diffractor moves toward either receiver, and remains constant for a diffractor located on the same side of both receivers. The horizontal portion of Δτ is slightly skewed during the normal moveout correction, yielding a maximum peak of the horizontally stacked trace at a slightly smaller time than Δτ. Accordingly, the diffracted waves have an apparent velocity slightly higher than half of the medium velocity in a horizontally stacked image. This conformed to virtual data for an elastic two-layer model with a vertical boundary. We then generalized the expression to three dimensions, in which listric travel time curves were predicted for an oblique edge diffractor, a vertex diffractor offline from the receiver pair, or a buried diffractor. Based on both two- and three-dimensional analyses of the edge diffractor, we tentatively interpreted the linear and listric dipping events observed in the passive SRI image across the Korean Peninsula to have been caused by diffractors near the intersection of the profile and geologic boundaries.

A scalable non-myopic atomic game for a smart parking mechanism

We propose a novel non-myopic smart parking mechanism which aims to decrease the cruising time spent in searching for parking, with the assumption of elastic demand for both on-street parking lots and parking garages. A non-myopic atomic game is formulated to address competition for parking through assignment of vehicles to candidate parking facilities that takes into account the differences in travel times for the vehicles from their point of origin to the parking facilities and the differences in walking times for the drivers from the parking facilities to their final destination, as well as dynamic pricing, cruising times, and occupancies of the parking facilities. This study integrates a socially efficient price that accounts for the waiting times of drivers in their search for parking. We incorporate a game model into the social optimum problem by considering the competition of drivers for parking spaces where the drivers’ preferences are reflected in a collective decision such as social welfare. Using actual parking data for the city of San Francisco, we found that under our proposed dynamic parking system the average social welfare per vehicle improved by up to 54% compared to other parking strategies.

Differences in Travel Time to Cancer Surgery for Colon versus Rectal Cancer in a Rural State: A New Method for Analyzing Time-to-Place Data Using Survival Analysis.

Rectal cancer is rarer than colon cancer and is a technically more difficult tumor for surgeons to remove, thus rectal cancer patients may travel longer for specialized treatment compared to colon cancer patients. The purpose of this study was to evaluate whether travel time for surgery was different for colon versus rectal cancer patients. A secondary data analysis of colorectal cancer (CRC) incidence data from the Iowa Cancer Registry data was conducted. Travel times along a street network from all residential ZIP Codes to all cancer surgery facilities were calculated using a geographic information system. A new method for analyzing "time-to-place" data using the same type of survival analysis method commonly used to analyze "time-to-event" data is introduced. Cox proportional hazard model was used to analyze travel time differences for colon versus rectal cancer patients. A total of 5,844 CRC patients met inclusion criteria. Median travel time to the nearest surgical facility was 9 minutes, median travel time to the actual cancer surgery facilities was 22 minutes, and the median number of facilities bypassed was 3. Although travel times to the nearest surgery facilities were not significantly different for colon versus rectal cancer patients, rectal cancer patients on average traveled 15 minutes longer to their actual surgery facility and bypassed 2 more facilities to obtain surgery. In general, the survival analysis method used to analyze the time-to-place data as described here could be applied to a wide variety of health services and used to compare travel patterns among different groups.

Driving to Childhood Cancer Hub Hospitals: A Study on Hospital Accessibility in Japan.

Objective:In 2013, 15 childhood cancer hub hospitals in Japan were designated to provide quality medical treatment and care. The present study assessed hospital accessibility by investigating travel times and distances from patient residences. Methods:A total of 37,309 residence/hospital pairs were generated using the addresses of 15 hub hospitals that were designated in 2019 and local government offices in 2014. Using the Google Directions Application Programming Interface (API), travel times and distances were calculated on the assumption that each patient would arrive by driving to the hospitals by 10 am on Wednesday, November 6, 2019. Thus, after identifying the nearest hospital for each residence and deriving adjusted estimated travel times (AETT), the data were summarized according to the regional block using weighted population descriptive statistics for children under 15 years of age in 2015. The cumulative distribution functions of the weighted mean of AETT were also plotted. Results:Childhood cancer patients could access the nearest hub hospital by traveling approximately 1.78 hours (AETT, range: 0.1 to 41.8) and 91.86 km (range: 1.0 to 1438.0). Moreover, a total of 94.5% of patients had the nearest hub hospital within their own regional block. The cumulative distribution functions of AETT indicated that many children in three blocks with multiple hub hospitals have shorter travel times and better hospital accessibility than those in other blocks. Conclusions:Although feasibility is ultimately dependent on each patient’s condition and situation, child cancer patients on average can likely complete hospital visits from home and return within a single day. However, this is likely not the case for children who live at considerable distances from hub hospitals. We found regional differences in travel times and distances, depending on whether a given block contained multiple hub hospitals.

Pn uppermost mantle tomography of Central Tibet: Implication for mechanisms of N-S rifts and conjugate faults

The north-south convergence between the Indian and Eurasian continents since Cenozoic has produced the Tibetan plateau, and east-west extension structures have also developed since late Cenozoic. Whether these extensional structures are limited to the crust or extend to the deep lithosphere remains controversial. The uppermost mantle serves as a link between the crust and deep lithosphere, the velocity structure of which provides an important constraint on exploring the mechanism of extensional structures. Based on Pn traveltime data recorded by the SANDWICH network in central Tibet, we use the interstation traveltime difference method to obtain the uppermost mantle velocity structure. Our results show significant velocity differences for the uppermost mantle beneath the conjugated strike-slip faults and rifts, which indicates their different formation mechanisms. The high Pn velocity beneath the conjugated strike-slip faults support an eastward horizontal shear at the base of the upper crust rather than the lower crust has contributed to the development of the conjugated strike-slip faults. The low Pn velocity beneath the rifts in Lhasa may provide evidence for the rifts cutting through the entire crust and reflect the connection between the formation of the rifts and deep tectonic activities. The most prominent features in our results are the low Pn velocity zones beneath the Tangra Yum Co Rift (TYR), the Pumqu-Xianza Rift (PXR) and the zone between the two rifts, and we explain the low Pn velocity as the top of the mantle wedge upon the subducting Indian lithospheric slab.

Analysis of Component Errors in the Highway Capacity Manual Travel Time Reliability Estimations for Urban Streets

The Highway Capacity Manual 6th edition (HCM6) includes a new methodology to estimate and predict the distribution of average travel times (TTD) for urban streets. The TTD can then be used to estimate travel time reliability (TTR) metrics. Previous research on a 0.5-mi testbed showed statistically significant differences between the HCM6 estimated TTD and the corresponding empirical TTD. The difference in average travel time was 4 s that, while statistically significant, is not important from a practical perspective. More importantly, the TTD variance was underestimated by 70%. In other words, the HCM6 results reflected a more reliable testbed than field measurement. This paper expands the analysis on a longer testbed. It identifies the sources and magnitude of travel time variability that contribute to the HCM6 error. Understanding the potential sources of error, and their quantitative values, are the first steps in improving the HCM6 model to better reflect actual conditions. Empirical Bluetooth travel times were collected on a 1.16-mi testbed in Lincoln, Nebraska. The HCM6 methodology was used to model the testbed, and the estimated TTD by source of travel time variability was compared statistically to the corresponding empirical TTD. It was found that the HCM6 underestimated the TTD variability on the longer testbed by 67%. The demand component, missing variable(s), or both, which were not explicitly considered in the HCM6, were found to be the main source of the error in the HCM6 TTD. A focus on the demand estimators as the first step in improving the HCM6 TTR model was recommended.

Differential volumetric analysis combined with monitoring of differential ultra-sound travel time, an approach for tracing fine-structural compression behaviour of solids: a study on MgO / periclase and polyethylene / PE up to 1.5 GPa at room temperature

The study addresses the topic of fine-structural compression behaviour of solids. MgO / periclase and polyethylene / PE are studied as exemplary materials by volumetric and ultra-sound techniques at room temperature up to 1.5 GPa. The experiments conducted with a high-pressure device of the piston cylinder type rely on: (a) measurements of the piston displacement performed under continuously changing uniaxial load in combination with (b) simultaneous monitoring of the travel time of ultra sound. Both methods base on differential technique implying that all data are of relative nature. The volume-density relation (Δρ = Mconst / ΔV) is the fundamental relation which provides information on compressional as well as on acoustic behaviour of the sample. It serves, additionally, as mutual control of the observations. The data evaluation bases on the method of difference calculation: the data obtained from a run with sample [sample + (vessel + piston assembly)] are subtracted from those of a calibration run [(vessel + piston assembly)]. In most cases the differences retrieved are small and may be presented best in form of derivatives with respect to pressure. The compression behaviour of MgO under hydrostatic conditions is reported to be quasi-linear up to about 2 GPa. Under the uniaxial load exerted by the piston cylinder apparatus MgO reveals a compression behaviour characterized by two volumetric anomalies at 400 and at 1000 MPa. The ultrasonic measurements suggest that shear plays an important role. Comparative measurements on the visco-elastic material polyethylene corroborate that interpretation.

Seismic Wave‐Based Constraints on Geodynamical Processes: An Application to Partial Melting Beneath the Réunion Island

AbstractThe inversion of seismic observations leads to maps of the interior of the Earth that can be interpreted. Regions of low seismic velocity have historically been interpreted to be due to factors related to high‐temperature and high‐melt retention. Subsequently, geodynamic models can be used to test such interpretations. However, the inversions are nonunique, and arguably, it would be best to test geodynamic scenarios against observations rather than interpretations. Here we make a first attempt at this. At depths greater than 80 km below Réunion, a low shear‐wave velocity zone is imaged. Rather than interpret this inverted model, we test a forward model of melt generation and retention against seismic observations. Geodynamic model solutions are converted with a mineral parameter database to P wave and S wave velocity profiles from various initial temperatures T, upwelling velocities , and permeabilities k0. By embedding these velocity profiles, synthetic seismograms are generated. For a range of k0, T, and , we generate synthetic traces for 21 teleseismic events registered at a receiver on Réunion island. We measure the traveltime difference between observed and synthetic waveforms and the interphase differential travel times for 210 scenarios for several phase arrivals of three components, filtered between 0.01 and 0.2 Hz. The results indicate that upper mantle temperatures beneath Réunion lie within 1400–1450 °C, with permeability coefficients of 10−5–10−6 m2. These conditions are associated with porosities of &lt;0.28% and high‐melt extraction rates of 8.37–18.35 m·year−1. This study demonstrates the potential for fully comparing geodynamic scenarios with seismic observations.

Information Content of Ship Noise on a Drifting Volumetric Array for Passive Environmental Sensing

This study investigates the information content of ship noise received on a drifting volumetric array of hydrophones in shallow water marine environments for the purposes of conducting acoustic thermometry or other environmental inversions. Passive inversions for physical oceanographic parameters are conducted using travel time differences, determined by cross-correlating ship noise received on hydrophones suspended beneath drifting buoys. Ships are tracked using the automatic identification system. Information content gained from the inversion is assessed using traditional a posteriori error analysis. Numerical simulations using a standard normal mode propagation model are used to test limitations of the proposed approach with respect to frequency band, drifting receiver configuration, precision, and accuracy of the inversion results, along with sensitivity to environmental and position mismatch. Performance predictions using this model are compared with results from a field experiment using at-sea data collected off the coast of New London, CT, USA, in Long Island Sound. Information gathered using passive acoustic inversion methods on drifting arrays can be used to constrain general circulation models, in coastal environments, where ship noise is ubiquitous, environmental data is sparse, and the oceanography is dynamic and important for understanding large-scale ocean processes.

Antipodal observations of global differential times of diffracted P and PKPAB within the D″ layer above Earth's core–mantle boundary

SUMMARYAntipodal diffracted, compressional wave (Pdiff) data analysed diametrically opposite three large earthquakes have uniformly sampled 99% of the laterally heterogeneous zone above Earth's core–mantle boundary (D″ in seismic nomenclature). These antipodal data offer a fundamental gross Earth datum—a robust, global constraint on the average compressional velocity at the base of the mantle. We use for the first time the seismic phase PKPAB as a reference, which travels an identical mantle path as Pdiff, thereby cancelling common mantle heterogeneity. Differential traveltimes between Pdiff, PKPAB and PKIKP are measured, appropriately making allowance for the phase shifts acquired in propagation. We have independently confirmed the $\pi /4$ polar phase shift of Pdiff at the antipode. The global mean PKPAB − Pdiff time is 136.5 ± 0.6 s. The global mean apparent velocity (13.05 km s−1) and ray parameter (4.65 ± 0.01 s deg−1) are within the margin of error of prior Pdiff studies—which were dominated by Northern Hemisphere paths—indicating that complementary, southern hemisphere paths have a comparable, mean Pdiff apparent velocity. The seismic velocity constraints afforded by antipodal Pdiff and PKPAB suggest that the heterogeneous processes already observed in D″ may be broadly ascribed where D″ coverage has been lacking or poorly resolved.

Finite‐Frequency Sensitivity Kernels for Seismic Noise Interferometry Based on Differential Time Measurements

AbstractFull waveform adjoint tomography has achieved great success in applications from global Earth structure using earthquakes to exploration seismology using active sources. When combined with ambient seismic noise data, however, the ambient noise cross correlations are subject to strong variability and bias due to nonstationary and unevenly distributed noise sources. As a result, the shape of the structure sensitivity kernel can deviate significantly from the classic banana‐doughnut kernel of a point source, which has been used in previous studies as an inference from empirical Green's function and contains bias. In this study I calculate the sensitivity kernel for ambient noise cross correlation by introducing an additional station to the classic two‐station setting. I compute sensitivity kernels for differential traveltime measurements. These differential sensitivity kernels show promise for canceling the overlapping part of the original source and structure kernels for pairs of stations in interferometry, thus significantly reducing the effect of nonisotropically distributed and nonstationary noise sources. I apply the ambient noise differential sensitivity kernel to synthetic data examples based on 2‐D membrane waves, though the approach will apply for 3‐D as well. Our results for multiple station pairs show promise for velocity tomography based on seismic noise interferometry when perfect information on noise source distribution is not available.