Travel Time Delay Research Articles

Seismotectonics of the 20 August 1999 Red Rock Valley, Montana, Earthquake

On 20 August 1999, a magnitude 5.3 earthquake occurred in southwestern Montana, ending a 25-year hiatus for magnitude 5+ seismicity in Montana. This earthquake occurred in the central part of the Red Rock Valley, a northwest-trending graben bounded by late Pleistocene and Holocene faults. A focal depth of 12.4 km and a normal-faulting focal mechanism suggest that this earthquake resulted from continued graben development, although not along graben-bounding faults mapped at the surface. The 1999 Red Rock Valley earthquake occurred near the northern end and within the footwall block of the northeast-dipping Red Rock fault. We deployed a temporary network close to the mainshock epicenter and located 65 aftershocks over 3 days, including a magnitude 4.0 aftershock on 26 August that allowed determination of P -wave travel-time delays for regional seismograph stations. Using these station delays to improve relative hypocenter locations, we recomputed hypocenter locations for >1000 Red Rock Valley area earthquakes that occurred since 1989. Relocated hypocenters avoid the Holocene portion of the Red Rock fault but do surround it. The mainshock epicenter location is close to that of a magnitude 5.0 earthquake in 1965.

Upper mantle structure of the Kaapvaal craton from surface wave analysis—a second look

Surface waves from earthquakes recorded during the two‐year Southern Africa Seismic Experiment have been used to estimate the average velocity structure beneath the Kaapvaal craton. By cross‐correlating the data with synthetics derived using mode summation techniques, I measure frequency‐dependent phase delays for Love and Rayleigh waves, as well as higher modes, for 12 events ranging in epicentral distance between 12° and 91°. I simultaneously invert the traveltime delays for structure beneath the array and outside of the array. In this way, heterogeneity along the path lying outside of the array is not mapped erroneously into structure beneath the array. The differential dispersion between the Love and Rayleigh waves require some anisotropy in the upper mantle; however, models with a range of anisotropic depths (as shallow as 150 km and as deep as 220 km) can equally well fit the data. The sensitivity of the fundamental mode surface waves used in this study begins to decline significantly below ∼200 km depth limiting interpretation of deeper structure. Nonetheless, this range of anisotropic depths is significantly different than the 100 km depth proposed in a recent study by Freybourger et al. [2001] using a similar technique and different data from the same array. I attribute the difference in their dispersion measurements and modeling to the improper use of a phaseless (i.e. frequency‐dependent) filter on the data, which systematically biased their measurements at lower frequencies. In addition, I find no evidence of a low‐velocity layer as proposed by Qui et al. [1996] and Priestley [1996].

The terminal problem: Model-data comparisons of basin scale transmissions including effects of shallow bathymetry near source and receiver

Acoustic transmissions on basin scale ranges are being used to determine depth-dependent temperature variability. With travel time being the primary observable, stationary sources and nearly stationary receivers are experimental requirements. This has led to the use of bottom-mounted sources and receivers to reduce travel time variability. The NPAL (North Pacific Acoustics Laboratory) experiment has transmitted broadband acoustic pulses from two bottom-mounted sources near the sound channel axis. Recordings have been taken on the NPAL billboard array, a linear series of five vertical line arrays moored in 1800 m of water near Monterey, CA. Additional recordings have been taken from the SOSUS system throughout the Pacific basin. The effects of the near source and near receiver scattering are examined. In particular, near source scattering leads to excess high-angle energy entering deep water with a travel time delay of nearly 1 s due to the low group speeds of high-angle rays/modes in shallow water. We also compare the energetics of the arriving rays that have bounced once on the rising seafloor near the NPAL receivers. Comparisons of models and data for bottom interacting acoustics lead us to the perennial issue of geoacoustic parameters.

A regional study of shear wave splitting above the Cascadia Subduction Zone: Margin‐parallel crustal stress

Recordings of local earthquakes from 16 three‐component broadband seismic stations in southwestern British Columbia, Washington, and northern Oregon are used to study regional variations of shear wave anisotropy in the North American plate above the subducting Juan de Fuca plate. There is evidence for shear wave splitting at all sites, with good agreement of fast polarization directions and travel time delays at adjacent stations. Most stations exhibit fast directions parallel to the strike of the margin, with anisotropy of 1–2%. These fast polarization directions are consistent with earthquake focal mechanisms and borehole stress studies, indicating that the observed anisotropy is likely due to crustal stresses (i.e., extensive dilatancy anisotropy theory). The margin‐parallel stresses may be due to oblique subduction of the Juan de Fuca plate. However, at the station closest to the coast (OZB), the fast direction shows a more margin‐normal orientation that may be associated with the proximity of the locked portion of the underlying subduction thrust fault.

Low seismic-wave speeds and enhanced fluid pressure beneath the Southern Alps of New Zealand

A region of low seismic-wave speed is detected beneath the central Southern Alps of New Zealand on the basis of traveltime delays for both wide-angle reflections and P-waves from teleseismic events. Respective ray paths for these P-waves are mutually perpendicular, ruling out anisotropy as a cause of the delays. The low-speed region measures about 25 km by 40 km, has a speed reduction of 6%–10%, and is largely above the downward projection of the Alpine fault. The most likely cause of the low-speed zone is high fluid pressure due to excess water being released by prograde and strain-induced metamorphism into the lower crust. Because enhanced fluid pressure reduces the work required for deformation, the existence of the central Southern Alps low-speed zone implies that this part of the Australian-Pacific plate boundary is relatively weak.

Resolution of tomographic models of the mantle beneath Iceland

The locations of volcanic islands may be controlled by thin or extending parts of the lithosphere over a partially molten asthenosphere [Anderson and Bass, 1984; Favela and Anderson, 2000], by edge effects near the boundaries of thick cratonic lithosphere [Anderson, 1998], or by narrow jets of hot mantle rising from deep within the mantle [Campbell and Griffiths, 1992; Morgan, 1971; Wilson, 1986]. Many hotspots are found on or near ridges, at lithospheric discontinuities, or in extensional environments, so high resolution seismic images are required to determine whether it is lithospheric structure, stresses in the lithosphere, or the deep mantle that is the controlling factor for the location of these volcanoes. In this study, we perform a simple experiment in which we use basic geometrical arguments to better understand the resolution of tomographic images of the upper 400 km of the mantle under Iceland. Our results indicate that a narrow, deep seated mantle plume is not required in order to explain the observed travel time delays in this region. Results of tomographic inversions are often viewed as unique; however, recent seismic studies of the Icelandic Hotspot have illustrated the non‐unique nature of these models.

Wide-angle seismic constraints on the internal structure of Tenerife, Canary Islands

We have used wide-angle seismic data to constrain the internal structure of Tenerife, Canary Islands. The experiment was designed as a seismic fan profile to detect azimuthal variations in the seismic structure of the volcanic edifice and its flanks. Seismic energy was generated using a 75-l airgun-array on board the RRS Charles Darwin fired every 40s along a quasi-circular profile around the island of Tenerife, centered on Teide volcano. We present the results obtained from the data recorded by five portable land stations distributed on the island. The travel-times indicate that the averaged P-wave velocity within the volcanic edifice is greater than 6km/s. The observed travel-times were reduced to residual travel-times by removing the effects of variations in the bathymetry along the profile, variations in the shot–receiver distance, and from local heterogeneities. Negative residual travel-times up to 0.8s in amplitude indicate that the southwestern part of Tenerife is characterized by a high P-wave velocity zone, coincident with a gravity maximum that was previously modeled as a high-density body forming the core of an old, large mafic volcano. We estimate velocities greater than 7.3km/s within the anomalous body, suggesting that it represents an intrusive plutonic complex. This high-velocity, high-density body may have played an important role in the evolution of Tenerife, buttressing Las Cañadas edifice and preventing the occurrence of landslides in the southern and western areas of Tenerife. The bathymetric high between Tenerife and La Gomera is associated with travel-time delays up to 0.4s, suggesting that it may be composed of large deposits of lava flows and volcaniclastic materials, probably erupted from the shield massifs of Teno, Roque del Conde, and La Gomera. The post-shield volcanic zones of Santiago and Dorsal rifts also seem to be characterized by moderate high P-wave velocities.

No regional anisotropic domains in the northeastern U.S. Appalachians

A fabric “frozen” within the continental lithosphere should lead to “regional anisotropic domains” with close correlation between tectonics, seismic anisotropy, and shear wave travel time delays. Alternatively, seismic anisotropy could be maintained by active deformation in the sub‐lithospheric mantle. This scenario would predict broad spatial consistency in shear wave splitting and weak correlation between the splitting, surface tectonics, and travel time delays. A coherent pattern of shear wave splitting is observed in the northeastern United States, covering large parts of New York and New England, spanning the Appalachian Orogen and areas underlain by the Grenvillian basement. The direction of fast shear wave propagation, as measured from core‐refracted SKS, SKKS, and PKS phases, varies systematically with event back azimuth at all sites examined, and is fit well by two‐layers of anisotropy with subhorizontal symmetry axes. Relative travel time delays of shear waves that sample the region vary over 100 km length scale, and correlate with surface geology. Tomographic images suggest lateral shear velocity variations ∼3%, including a slow anomaly beneath the Grenvillian basement exposed in the Adirondack Mountains. There is little correlation between this horizontally rough seismic velocity and the horizontally smooth anisotropic model consistent with shear wave splitting. We therefore conclude that in the northeastern US the concept of “regional anisotropic domains” (i.e. distinct regions within the lithosphere characterized by coherent anisotropic properties reflective of present or past deformation) does not apply.

Can a narrow, melt‐rich, low‐velocity zone of mantle upwelling be hidden beneath the East Pacific Rise? Limits from waveform modeling and the MELT Experiment

One of the goals of the Mantle Electromagnetic and Tomography (MELT) Experiment is to determine whether a narrow zone of enhanced melt concentration consistent with focused upwelling exists beneath the East Pacific Rise. Using SKKS, sScS, and S phases from two intermediate‐depth earthquakes in the Banda Sea and the Tonga‐Kermadec region, we demonstrate that there is no positive evidence for the existence of such a zone and that travel time delays for shear waves traveling through it must be <0.5 s. To test whether diffraction and wave front healing could obscure evidence for its existence, we employ a pseudospectral method to simulate finite frequency teleseismic waves propagating through narrow, vertical low‐velocity zones. A rich set of reflected, diffracted, and guided waves is generated when S waves encounter such a low‐velocity channel, particularly at high frequencies. Limiting the frequency content to the lower‐frequency bands with good signal‐to‐noise ratios in the observed phases obscures these waveform complexities. The travel time anomaly is broadened and reduced in amplitude but remains detectable unless the low‐velocity zone is very narrow or has only modest velocity contrast. The lower limit of detectability corresponds to a 5‐km‐wide channel of partial melt extending from 10 to 60 km below the seafloor at the ridge axis with a shear velocity contrast of 0.5 km/s. Although these limits are severe, 3 to 4% melt retention might cause a large enough viscosity reduction and anomalous buoyancy to dynamically focus upwelling into a 5‐km‐wide channel that falls within the limits. Strongly focused, dynamic upwelling beneath the ridge, however, is probably not compatible with the existence of a broad region of very low shear velocities in the surrounding mantle.

Vibroseis recording techniques and data reduction from the Jemez Tomography Experiment

Abstract The Jemez Tomography Experiment (JTEX) is a multidisciplinary study focused on the Valles Caldera and the Jemez Mountains, New Mexico. The objectives of the project are to create a high resolution crustal model of the subsurface structure of this silicic volcanic system and to develop an interpretation of its volcanic evolution. Use of Vibroseis sources in the acquisition of refraction/wide-angle reflection seismic data provided challenges beyond conventional explosive-source data. Processing of the JTEX Vibroseis data is an involved procedure consisting of sorting, cross-correlating, filtering, and stacking numerous individual seismograms in the production of final record sections. However, excellent results (high signal-to-noise seismograms at relatively small spacings) are obtainable with coherent arrivals at source-receiver distances of more than 60 km. The primary drawback in this approach lies in the massive volume of data that is necessary to produce record sections. One benefit of the Vibroseis source used during JTEX was a method to decrease effective seismogram spacing. This technique, dubbed a “source-offset” technique, provides smaller overall seismograph station spacing by moving the Vibroseis sources during acquisition and leaving deployed seismographs stationary. After station corrections, this method effectively decreases station spacings and increases detail in resulting record sections. Various shallow crustal heterogeneities create travel-time advances and delays that affect the source-offset data differently than single-source data. Synthetic modeling demonstrates small travel-time discrepancies associated with the source-offset technique. However, the addition of traces with smaller station intervals clarifies secondary arrivals within record sections and aids in interpretation of these arrivals with a minimum amount of field effort required.

Realistic Models of Anisotropic Laminated Lower Crust

—The genesis of the laminated lower crust has been attributed to extensional processes leading to structural and textural ordering. This implies that the lower crust might be anisotropic. Laboratory measurements of lower crustal rock samples and xenolithes show evidence of anisotropy in these rocks due to oriented structure.¶In this paper we investigate the seismic shear-wave response of realistic anisotropic lower crustal models using the anisotropic reflectivity method. Our models are based on representative petrophysical data obtained from exposed lower crustal sections in Calabria (South Italy), Val Strona and Val Sesia (Ivrea Zone, Northern Italy). The models consist of stacks of anisotropic layers characterized by quantified elastic tensors derived from representative rock samples which provide alternating high and low velocity layers.¶The seismic signature of the data is comparable to seismic observations of in situ lower crust. For the models based on the Calabria and Val Strona sequences shear-wave splitting occurs for the Moho reflection at offsets beyond 70 km with travel-time delays up to 300 and 500 ms, respectively. The leading shear wave is predominantly horizontally polarized and followed by a predominantly vertically polarized shear wave. Contrastingly, the Val Sesia model shows no clear evidence of birefringence. Isotropic versus anisotropic modelling demonstrates that the shear-wave splitting is clearly related to the intrinsic anisotropy of the lower crustal rocks for the Val Strona sequence. No evidence of birefringence caused by thin layering is found.

The thin hot plume beneath Iceland

SUMMARY We present the results of a seismological investigation of the frequency-dependent amplitude variations across Iceland using data from the HOTSPOT array currently deployed there. The array is composed of 30 broad-band PASSCAL instruments. We use the parameter t*, de¢ned in the usual manner from spectral ratios (Halderman & Davis1991), to compare observed S-wave amplitude variations with those predicted due toboth anelastic attenuation and diiraction eiects.Four teleseismic events at a range of azimuths are used to measure t*. A 2-D vertical cylindrical plume model with a Gaussian-shaped velocity anomaly is used to model the variations. That part of t* caused by attenuation was estimated by tracing a ray through IASP91, then superimposing our plume model velocity anomaly and calculating the path integral of 1/oQ. That part of t* caused by diiraction was estimated using a 2-D ¢nite diierence code to generate synthetic seismograms.The same spectral ratio technique used for the datawas then used to extract a predictedt*.Thet* variations caused by anelastic attenuation are unable to account for the variations we observe, but those caused by diiraction do.We calculate the t* variations caused by diiraction for diierent plume models and obtain our best-¢t plume, which exhibits good agreement between the observed and measured t*. The best-¢t plume model has a maximum S-velocity anomaly of {12 per cent and falls to 1/e of its maximum at 100 km from the plume centre. This is narrower than previous estimates from seismic tomography, which are broadened and damped by the methods of tomography. This velocity model would suggest greater ray theoretical traveltime delays than observed. However, we ¢nd that for such a plume, wave-front healing eiects at frequencies of 0.03^0.175 Hz (the frequency range used to pick S-wave arrivals) causes a 40 per cent reduction in traveltime delay, reducing the ray theoretical delay to that observed.

Traffic Congestion at a Two-Tandem Bottleneck

Equilibrium queuing patterns are analyzed in the context of a two-tandem bottleneck with two origins and one destination. The individual in the model chooses his/her departure time, considering the trade-off between travel time and schedule delay. At equilibrium, no one can alter his/her departure time in the sense of the Wardrop principle. Four different configurations are analyzed associated with the capacities of the upstream and the downstream bottlenecks and queuing start times at both bottlenecks. Queuing patterns are determined in the model. It is found that, in some cases, a queue does not occur at the upstream since the departure rate is always equal to its capacity level at equilibrium. Therefore, in order to avoid traffic congestion at a two-tandem bottleneck, the downstream should be enlarged prior to the upstream.

Evaluating Networkwide Effects of Variable Message Signs in the Netherlands

The use of variable message signs (VMSs) is generally considered to be a powerful tool to influence route choice so as to increase safety and comfort during driving and improve network performance and to make optimum use of available capacity. Assessing whether such effects actually occur when VMSs are installed is a difficult task. This paper summarizes an extensive evaluation study carried out on the installation of 14 new VMSs on the Amsterdam orbital motorway. The study involves a number of issues. First, a networkwide analysis of aggregate performance indicators, such as traffic performance, severity of congestion, and travel time delay, is included. Second, the relation between the messages displayed on the VMS and the travel times experienced by drivers is analyzed. Finally, a stimulus-response analysis to find out how drivers actually respond to the messages is described. The study showed that the use of VMSs has a positive impact on network performance in the Amsterdam freeway system. Total congestion has decreased, and traffic performance has increased. Variation in congestion has decreased as well as variation in average travel speed. This finding implies that travel time has become more reliable and that the traffic flow is more homogeneous. The technical assessment showed that the displayed queue lengths, if interpreted correctly by drivers, are a good measure for expected delay. User response analysis showed that information has a significant effect on route choice. In general, the VMSs were found to improve the efficiency and reliability of the freeway system.

Statistical models for ground vibration and airblast

Abstract It is becoming increasingly important to be able to predict and reduce ground vibration and airblast so that mine operations may achieve their environmental requirements. The observed vibration and airblast at any monitoring location is influenced by variables such as the weight and type of explosive used per delay, the delay time sequence, scatter in that sequence, the spatial pattern of blastholes and properties of the transmitting medium. The present work shows how the separate influence of these variables may be analysed using a Monte Carlo model that has a predictive power superior to that of the traditional charge weight scaling laws. Charge weight scaling laws have sometimes been observed to give acceptable predictions at a fixed monitoring location for the consistent use of a particular blast design. However, unlike the Monte Carlo model, such laws cannot be used to predict the likely changes in vibration due to any planned changes in blast design. Monte Carlo predicted contours of airblast and vibration show that the blasthole locations and the direction of initiation have a considerable effect at any monitoring location. This is due to influences such as the finite travel time of the disturbance as well as the screening effect. The travel time delay between blastholes is more significant for airblast since the velocity (334 m/s) of sound in air is typically 10 times lower than the speed of vibrational waves through the ground. It is also well known that previously fired blastholes may provide a screen for both airblast and vibration produced by a current hole as it initiates. In the case of airblast, screening may occur due to the airborne particulate matter produced by previous holes; in the case of vibration, screening may occur due to ground fractured by these previous holes.

A new direct method for measuring the Hubble constant from reverberating accretion discs in active galaxies

We show how wavelength-dependent time delays between continuum flux variations of active galactic nuclei (AGNs) can be used to test the standard black hole—accretion disc paradigm, by measuring the temperature structure T(R) of the gaseous material surrounding the purported black hole. Reprocessing of high-energy radiation in a steady-state blackbody accretion disc with T ∝ R−3/4 incurs a wavelength-dependent light travel time delay τ∝λ4/3. The International AGN Watch multiwavelength monitoring campaign on NGC 7469 showed optical continuum variations lagging behind those in the UV by about 1 d at 4800 Å and about 2 d at 7500 Å. These UV/optical continuum lags imply a radial temperature structure T ∝ R−3/4, consistent with the classical accretion disc model, and hence strongly support the existence of a disc in this system. We assume that the observed time delays are indeed caused by a classical accretion disc structure, and derive a redshift-independent luminosity distance to NGC 7469. The luminosity distance allows us to estimate a Hubble constant of H0 (cos i/0.7)1/2 = 42 ± 9 km s−1 Mpc−1. The interpretation of the observed time delays and spectral energy distribution in the context of an accretion disc structure requires further validation. At the same time, efforts to minimize the systematic uncertainties in our method to derive a more accurate measurement of H0, e.g. by obtaining an independent accurate determination of the disc inclination i or statistical average of a moderate sample of active galaxies, are required. However, this remains a promising new method of determining redshift-independent distances to AGNs.

Mantle seismic structure beneath the MELT region of the east pacific rise from P and S wave tomography

Relative travel time delays of teleseismic P and S waves, recorded during the Mantle Electromagnetic and Tomography (MELT) Experiment, have been inverted tomographically for upper-mantle structure beneath the southern East Pacific Rise. A broad zone of low seismic velocities extends beneath the rise to depths of about 200 kilometers and is centered to the west of the spreading center. The magnitudes of the P and S wave anomalies require the presence of retained mantle melt; the melt fraction near the rise exceeds the fraction 300 kilometers off axis by as little as 1%. Seismic anisotropy, induced by mantle flow, is evident in the P wave delays at near-vertical incidence and is consistent with a half-width of mantle upwelling of about 100 km.

Fuzzy Ramp Metering: Design Overview and Simulation Results

A fuzzy logic ramp-metering algorithm was designed to overcome the limitations of conventional ramp-metering strategies. The fuzzy controller demonstrated improved robustness, prevented heavy congestion, intelligently balanced conflicting needs, and tuned easily. The objective was to maximize total distance traveled and minimize total travel time and vehicle delay, while maintaining acceptable ramp queues. A multiple-ramp study site from the Seattle I-5 corridor was modeled and tested using the freeway simulation software, FRESIM. For five of the six testing sets, encompassing a variety of traffic conditions, the fuzzy controller outperformed the three other controllers tested.

Fault location and rupture kinematics of the magnitude 6.8, 1992 Erzincan earthquake, Turkey, from strong ground motion and regional records

Abstract The hypocenter location of the 13 March 1992, M = 6.9 Erzincan earthquake (Turkey) and its rupture kinematics have been studied using local and regional seismic records to understand its relationship with the tectonic activity of the Erzincan basin and of the North Anatolian fault (NAF). Regional records of the mainshock and the largest aftershocks were used to locate the respective epicenters. The mainshock S-P travel time delay and the P-wave polarization of the near-source accelerogram record in Erzincan (ERZ), calibrated by the aftershock records, constrain the mainshock hypocenter to be located about 10 km to the east-southeast of Erzincan, 39°42.3 N, 39°35.2 E, and at a best-fit depth of about 9 km. The activated fault is a major branch of the NAF along the eastern edge of the basin. Hypocentral depths greater than about 16 km are not supported by the data. The ground velocity record at ERZ has been synthesized for constraining the northwestward rupture, assuming a kinematic model with constant slip and rupture velocity, and a strike 125°, for a large number of models. The latter were constructed by varying the rupture parameters in a discrete domain. The best-fit fault models are located within 1 km of the epicenter above and have all their northwestern end within 5 km of ERZ. The up-dip limit of the rupture in the vicinity of ERZ ranges from 5 to 2 km in depth, in agreement with the absence of surface breakage. The estimated rupture velocity is 3 km/sec; the rise time is 0.35 sec; and the mean slip of about 1 m is poorly resolved, with a standard uncertainty of 0.5 m, being strongly sensitive to the rupture velocity and to the fault location. Combining these results with the seismic moment and the source duration provided by other studies leads to a total rupture length ranging between 20 and 35 km. The rupture thus propagated bilaterally on the NAF and was confined within the limits of the basin, stopping to the southeast near the intersection of the NAF with the Ovaçik fault.

Anisotropic scattering and travel time delay analysis in Kilauea volcano, Hawaii, earthquake coda waves

We study the S wave coda of microearthquake multiplets (earthquakes showing similar seismograms) recorded on the Kilauea volcano, and we analyze the direction of emission for wavelets composing the coda. Our data set consists of 71 small magnitude, well‐relocated, clustered events that occurred in an area of about 1 km2 in the southern flank of Kilauea, recorded at hypocentral distances of 10 to 20 km. For each station, we apply a cross‐spectral moving‐window technique to each pair of multiplet records to estimate the travel time delays due to source location offsets. These delays are computed for consecutive windows, starting at the P wave arrival time and ending at 4 times the S wave travel time. They are next used to estimate the slowness vector for each window along the seismograms. Our analysis shows (1) that scattered waves are emitted anisotropically along some preferred directions as late as 4 times the S wave travel time, and (2) that for some stations, the azimuthal plane rotates during the seismogram with a clear travel time delay signature. Our analysis shows that travel time delay variation in the coda can be purely due to geometrical propagation effects. This conclusion is of special interest when trying to use travel time delays to infer temporal changes in the crust. Our results do not support a coda diffracted isotropically in the crust, in the magnitude range and at the hypocentral distances we investigated.