P-wave Travel-time Residuals Research Articles

P-wave travel-time residuals in the Newfoundland Appalachians

Teleseisms and quarry blasts were recorded at 24 temporary seismograph sites along a line across the northern Dunnage Zone of Newfoundland. Azimuthally dependent functions were also determined for teleseismic P-wave travel-time residuals at the permanent St. John's (STJ) and Corner Brook (CBK) seismographs. In order to minimize the effects of reading errors and structure far from the receivers, absolute residuals at the field sites were then obtained by correlating the records with those from STJ and CBK and using the residual functions for these two stations. The P-wave residuals in north-central Newfoundland tend to be up to half a second later than at STJ and CBK, with the more positive values towards the east of the region. The direction of fastest approach for rays varies from southeast at STJ to southwest towards the western side of the Dunnage Zone. The travel-time residuals are generally similar to variations observed elsewhere in the Appalachians.The residuals and gravity data indicate that the crust is about 40 km thick over most of the Dunnage Zone, possibly increasing by several kilometres under the Gander Zone and western Avalon Platform, which could be consistent with a former subduction zone in the region. The low crustal velocity of 6.2 km s−1 may support an allochthonous model for central Newfoundland, with average crustal thickening of about 10 km compared with adjacent platform areas.

Three-dimensional P-wave velocity image under the Carpathian arc

An inversion of P-wave travel time residuals from selected earthquakes in the distance range 30°–98° to two seismic station networks was used to model P-wave velocity anomalies down to 250 km depth. In the first inversion experiment a region between 43.5°–47.5°N and 21°–29°E was modelled, using 35 seismic stations, while in the second one a region between 44°–47°N and 25°–29°E was modelled, using 19 seismic stations. The 4-layer block model of the first inversion offers 19% reduction in residual variance, while the 5-layer block model of the second one offers 26% reduction, the rest being explained by noise and smaller scale heterogeneities. The obtained velocity anomalies correlate remarkably well with the gravity anomalies and with the tectonic model for the Vrancea region of Fuchs et al. (1979).

Elastic lenses in the Earth

Seismic waves in the earth's crust and mantle are known to be sensitive to density contrasts over large volumes of rock, which contrasts tend to cause focusing effects. The end results of such effects observed at seismograph stations are hard to detect unless sufficient arrivals are sampled. It is a common fault to confuse such effects with those of local structures and properties. In a study of teleseismic, short‐period (1 s) P‐wave travel‐time residuals and variations of amplitudes in western North America. R. Butler of the Hawaii Institute of Geophysics has found a high level of correlation to which he attributes qualitatively the focusing and defocusing of seismic waves (Nature, December 15, 1983). The correlation indicating that slow travel times relate to higher, and fast travel times to lower, amplitudes of seismic waves measured in western North America. Conversely, faster travel times and higher amplitudes are generally observed in eastern North America (defined as stations located east of the Rocky Mountain front). Although there may be less attenuation of seismic waves in the upper mantle beneath eastern North America, indications are that the degree of attenuation is highly variable. According to Butler, “On the large scale, the variations between western and eastern North America are probably rooted in lateral differences in temperature. Higher temperatures beneath the tectonically active west produce higher attentuation of P‐waves, lower velocities in the upper mantle, and high surface heat flow.” The focusing and defocusing effects of low and high velocity lenses, respectively, may be most effecitive if such lensese are located close to a seismic station. Butler noted that lenses, or anomalous regions, must have dimensions of one or more wavelengths of a P‐wave, which translates to a minimum dimension in the earth of 6–8 km (for 1‐s period waves). Positive correlations have been observed characteristically over large seismic arrays, suggeting the existence of lenses of several tens of square kilometers in cross section.

Travel time inversion for three-dimensional P-wave velocity anisotropy.

Inversion method for estimating the three-dimensional isotropic seismic velocity structure developed by Aki and his colleagues is extended to include the effect of velocity anisotropy. Assuming weakly inhomogeneous anisotropic media with some symmetry in anisotropy, we reduce the number of unknown parameters. We present two sets of linearized equations of quasi P-wave travel time residuals suitable for inversion. One set is for a weakly anisotropic medium with a hexagonal symmetry about a horizontal axis, and the other for a medium with a spheroidal P-wave velocity surface. Using these equations, we invert P-wave travel time residual data to estimate the isotropic and anisotropic velocity perturbations in the subdivided three-dimensional blocks within the modeling space. For local earthquakes, the hypocentral parameter perturbations should also be included in the inversion. This three-dimensional anisotropic velocity inversion is a natural extension of the previous isotropic velocity inversion. The three-dimensional anisotropic inversion method is applied to seismic data in Southwest Japan to determine velocity perturbations in the blocks which were well-determined in the previous isotropic study. The isotropic velocity perturbations are well determined, but the anisotropic velocity perturbations are less resolved. In spite of the rather poor resolution, our preliminary analysis shows some interesting features. Within the descending Pacific and Philippine Sea plates, the fast directions of P-wave velocity seem to be perpendicular to the magnetic lineations in the sea-floors near the trenches, suggesting that the descending lithosphere maintains the anisotropic structures which were formed before subducting.

P-wave travel time and amplitude in western North America

A study of teleseismic, short-period (1 s) P-wave travel-time and amplitude variations in western North America reported here reveals a high level of correlation: early arrivals relate to low amplitudes and late arrivals to higher amplitudes. This trend is opposite to observations noted for the USA and southern Canada as a whole1. With some exceptions, seismic stations east of the Rocky Mountain front generally record faster travel times2–8 and higher amplitudes9–15 than those to the west. These differences have been attributed to higher attenuation and lower velocities in the upper mantle beneath the tectonically active western region11,16. The correlation in the west between P-wave travel-time residuals and relative amplitudes is consistent with elastic focusing and defocusing effects and is found for both World Wide Standardized Seismograph Network (WWSSN) and Long Range Seismic Measurement (LRSM) stations. These results are relevant to the estimation of yields of nuclear explosions because by reciprocity similar elastic focusing and defocusing effects can alter the amplitudes and travel times of P-waves from explosions.

Upper mantle heterogeneities beneath Eastern Europe

P-wave travel-time residuals for seismograph stations in eastern Europe as reported by ISC for the years 1964–1977 were used for constructing a seismic image of upper mantle heterogeneities in the network region. For the depth range 0–100 km, dominant tectonic features like the Pannonian Basin and the Aegean Sea and western Turkey correlate well with pronounced velocity lows which a ppear to extenddown to a 300 km depth. The velocity anomaly patterns in the depth intervals 300–500 km and 500–600 km are broadly similar but quite different from those of shallower depths. The observed seismic heterogeneities are briefly discussed in terms of large-scale tectonic and geophysical (heat-flow) characteristics of eastern Europe.

The effect of the descending lithosphere beneath the Tonga island arc on P-wave travel-time residuals at the Warramunga Seismic Array

P-wave travel-time residuals at the Warramunga Seismic Array (WRA) in the Northern Territory, Australia, have been studied from 49 earthquakes with epicenters south of 19°S in the Fiji-Tonga region. Focal depths are between 42 and 679 km as determined from pP-P. Using the Jeffreys-Bullen and the Herrin travel-time tables the epicentral parameters have been redetermined by considering only “normal” seismic stations in the location procedure. These are those stations where P-wave travel times are probably not affected by lateral heterogeneities caused by the lithosphere descending beneath the Tonga trench. Epicenters of deep earthquakes below 300 km have been relocated by using stations at Δ > 25° only. Epicenters from shallower-depth earthquakes have been recalculated without using stations between 35 < Δ < 75° epicentral distance. In both cases focal depths were determined from pP-P times. The resulting pattern of P-residuals at WRA does not show any significant change with depth below 350 km. The residuals become more negative for shallower earthquakes above about 250 km. P-waves to WRA are advanced by approximately 2 s compared with those from deep earthquakes. The results do not essentially differ for the two different travel-time tables used. The observations can be interpreted by P-wave velocities that are higher in the sinking slab down to 350–400 km by 5±2% than in both the Jeffreys-Bullen and Herrin models. Without considering possible elevations of phase boundaries this estimate yields a temperature contrast of 1000±450°C between slab and normal mantle material in this depth range.

Seismic studies of the crust under the hydrothermal areas of the Taupo Volcanic Zone, New Zealand

Observations of relative P-wave travel-time residuals for local mantle earthquakes and P/S amplitude ratios for more distant events have been used to investigate the nature of the crust under the hydrothermal areas of the central Taupo Volcanic Zone. In contrast to some other regions of similar activity, such as the Yellowstone and Geysers geothermal areas, U.S.A., there is no evidence in the data for an extensive region of semi-molten or molten rock in the crust. Thus the large amounts of mafic intrusive rock and associated crustal melt inferred to be present on geological grounds must be now largely at temperatures below the appropriate solidus. The data do provide further evidence for previously suspected lateral inhomogeneity in the underlying mantle.

P-wave travel-time residuals and the crust and upper mantle lateral inhomogeneities in Africa

Teleseismic P-wave travel-time residuals of shallow earthquakes and nuclear explosions were analyzed for 27 African stations. The distribution of positive and negative residuals was determined for Northeast Africa, northern and southern parts of the East African Rift Valleys and South Africa. The existence of lateral inhomogeneities in the upper mantle was found in these regions. African rifts are characterized by lower velocity structure of the uppermost mantle than that of the surrounding areas.

On the relation between P-wave travel time residuals and the age of continental plates

A new set of P-station anomalies has been computed for 583 seismological observatories which sent their arrival times to the International Seismological Centre between 1964 and 1974. These P-residuals are used as a marker of the variations of the properties of the uppermost mantle. Important changes of P-residuals are observed within continents and particularly within Precambrian shields. It is shown that residuals tend to increase outwards from the centers of the North American and Scandinavian shields. As a first approximation, the P-residuals are a linear function of the root square of the age of the basement inside stable continental plates. For these stable plates, a P-residual is an indirect measurement of lithospheric thickness. The lithospheric thickness Z (in km) is related to its age t (in 10 9 years) by: z ≃ 10 + 140 t We then speculate on a possible use of P-residuals in the search of Archean nuclei inside Precambrian shields. Tiksi, in northern Siberia, is the “fastest” station on continents and could be the location of a very old continental nucleus.

Teleseismic delay times, Bouguer anomalies and inferred thickness of the African lithosphere

Teleseismic P-wave travel time residuals (delay times) have been determined for 38 African seismograph stations using a total of 104 presumed Russian underground nuclear explosions. The delay times ( T) for seismograph stations situated on Precambrian crust within the interior of Africa, where surface erosion is minimal, appear to be linearly related to both the station elevation ( E) and Bouguer anomaly ( B) such that: T=(1.12±0.32)E−(1.81±0.44)(1) T=−(0.013±0.005)B−(1.81±0.60)(2) where T is in seconds, E in kilometres and B in milligals. Equation 2 has been used to predict delay times more generally using the smoothed Bouguer anomaly map of Africa. The delay time map is tentatively used to derive a map of lithosphere thickness which takes into account lateral variations of velocity within both the lithosphere and asthenosphere. The lithospheric thickness model indicates a major zone of thin lithosphere (or thick asthenosphere) is associated with the East African Rift system and its continuation into southern Africa. Concomitant seismicity and volcanism further suggest that incipient separation of the continental plate is taking place along the axis of this zone.

Temporal stability of P-velocity anisotropy before earthquakes in central California

Abstract An examination of P-wave travel-time residuals from small earthquakes (source events) located near three larger earthquakes (4 ≦ M ≦ 5) that occurred on the San Andreas fault, near Bear Valley in central California, shows no temporal variations in the residuals extending over broad azimuth ranges (Δφ &amp;gt; ∼40°). Such variations could have resulted from changes in horizontal velocity anisotropy precursory to the larger events. The examination also shows (1) numerous azimuthal variations in the residuals within narrow azimuth bands (Δφ &amp;lt; ∼30°), apparently due to spatial heterogeneity of crustal velocity, (2) a dependence of residual on magnitude for a few stations but not for most stations, and (3) trends of residual versus source event focal depth for about one-third of the 75 source region-station pairs examined. Residuals in these cases typically change by 0.1 sec, and occasionally by 0.2 to 0.3 sec, over the 2- to 12-km focal depth range sampled. The trends vary from station to station in a complex manner. The assumption that traveltime changes are reflected in the residuals is tested by modifying arrival times from some source events according to assumed forms for the travel-time change, relocating those events, and comparing the resulting residuals with those from the unmodified data.

P-wave travel time residuals caused by a dipping plate in the Aegean Arc in Greece

Lateral inhomogeneities in the earth's crust and upper mantle give rise to residuals in P-wave travel times that are dependent on azimuth as well as distance from a seismic station. A special negative residual for P-waves at stations in Greenland is apparent for earthquakes near Rhodes in the eastern part of the Aegean Arc in the Mediterranean Sea. This residual of approximately 2 seconds is interpreted as caused by a lithospheric plate of high P velocity dipping below the Aegean Arc. The pattern in azimuth and distance of the P-wave travel times from earthquake hypocenters near Rhodes indicates that the seismic zone is close to the upper boundary of the dipping plate, while the lower part of the plate is free from earthquakes.

A large-scale three-dimensional seismic structure under the Japan Islands and the sea of Japan.

A large-scale three-dimensional seismic velocity structure down to 650 km under the Japan Islands and the Sea of Japan has been investigated to clarify the subduction of lithospheric plates by applying the inversion method of Aki and others. About 3500 data of P-wave travel time residuals observed at 330 Japanese and foreign stations from 20 intermediate and deep-focus earthquakes around Japan were used to determine the velocity anomalies in a number of blocks and source corrections simultaneously. This analysis has well revealed lateral heterogeneities in the upper mantle above 450km. A high velocity zone corresponding to the descending Pacific plate has been clearly identified, particularly in the north Honshu arc, indicating good correlations with the horizontal and vertical distributions of a number of earthquakes around there. It was found that intermediate and deep-focus earthquakes take place mainly in the upper portions of the descending plate with a thickness of about 100km. The highest velocity anomalies are +5-6%(50-250km), +3%(350km), +2%(450km) within the plate, and about -2% down to 350km over a region on the continental side of the descending plate.

A dynamic model of the Hellenic Arc deduced from geophysical data

Combined gravity and seismic data from Greece and the adjacent areas have been used to explain the high seismicity and tectonic activity of this area. Computed 2-D gravity models revealed that below the Aegean region a large “plume” of hot upper-mantle material is rising, causing strong attenuation of the crust. The hot “plume” extends to the base of the lithosphere and has very probably been mobilized through compressional processes that forced the lithosphere to sink into the asthenosphere. The above model is supported by: high heat flow in the Aegean region; low velocity of the compressional waves of 7.7 km/sec for the upper mantle; lower density than normal extending to the base of the lithosphere; teleseismic P-wave travel-time residuals of the order of +2 sec for seismic events recorded at the Greek seismic stations; volcanics in the Aegean area with a chemical composition which can be explained by assuming an assimilation of oceanic crust by the upper mantle; deep seismicity (200 km) which has been interpreted by various authors as a Benioff zone.

Temporal and spatial variations of travel-time residuals in central California for Novaya Zemlya events

abstract Eight large nuclear explosions in Novaya Zemlya, from October 1969, through November 1974, were used to monitor long-term variations in crustal seismic velocity near the San Andreas fault in central California. Relative P-wave travel-time residuals appear to be accurate to approximately ±0.1 sec. Of the over 100 stations used, none show clearly significant temporal variations in residual greater than this amount, corresponding to about a 4 per cent change in velocity in the upper crust. Average relative residuals at individual stations show a large spatial variation of about 1.5 sec. These variations reflect both a complex crustal geology and changes in crustal thickness and provide a potentially powerful tool for studying crustal structure.

Teleseismic p-wave traveltime residuals and deep structure of the Aegean region

Teleseismic P-wave traveltime residuals have been measured at the Greek seismic stations with respect to the Herrin 68 tables. In spite of the large scatter, some insight into crustal and upper-mantle structure of the Aegean region can be gained. The average absolute residuals (observed minus Herrin traveltimes) are of the order of + 2 s. The most plausible interpretation is an efficient low-velocity zone in the upper mantle. Simple estimates of densities and subsequently gravity with the aid of Birch's law suggest that the Aegean region is underlain by hot expanded upper mantle, perhaps involving partial melting. The relative P residuals (observed minus Herrin traveltime differences between a station and Athens) are generally positive and can be interpreted with lateral variations of the LVZ or of the crust. The latter interpretation is supported in some cases by seismic refraction data. The azimuthal variation of the relative residuals at stations on the non-volcanic arc bears a distinct relation with the arc orientation. At Archangelos (Rhodes) where we “see” through the Benioff zone, the residuals from N to W are between -1 and -2 s and indicate a high-velocity slab sinking below the Aegean sea. At Vamos (Crete), Valsamata (Kephallenia), and Joanina (Pindus Range) the largest (smallest) residuals are along directions parallel (perpendicular) to the arc. This can be interpreted by crustal thickening under the sedimentary arc and/or by velocity anisotropy with the maximum perpendicular to the arc. On the whole, our study supports the hypothesis that the Aegean region is a trench—island-arc—marginal-sea system.

Elevation of the olivine-spinel transition in subducted lithosphere: Seismic evidence

The top of the olivine-spinel phase change in subducted oceanic lithosphere can be located by the travel times of seismic waves which have propagated through the slab. P-wave travel-time residuals from deep earthquakes in the Tonga island are observed at Australian seismic stations are grouped according to the depth of the earthquake. The change in mean residual with a change in earthquake depth is related to the velocity contrast between slab and normal mantle at that depth. The curve mean residual versus earthquake depth displays a region of markedly increased slope between earthquake depths of about 250 and 350 km. The most probable explanation of this observation is an elevation by 100 km of the olivine-spinel phase change within the relatively cooler slab. No evidence was found for vertical displacements within the slab of any deeper phase changes. A temperature contrast between slab and normal mantle of about 1,000°C at 250 km depth is implied. This finding confirms current thermal models for subducted lithosphere but is inconsistent with the global intraplate stress field unless only a few percent of the negative buoyancy force at subduction zones is transmitted to the surface plates.

P-Wave Residual Differences and Inferences on an Upper Mantle Source for the Silent Canyon Volcanic Centre, Southern Great Basin, Nevada

Summary Teleseismic P-wave travel-time residuals from six large nuclear detonations in the caldera of the Silent Canyon volcanic centre of the Nevada Test Site have been reduced by corresponding P-wave residuals of a nearby reference event (Dumont) in an attempt to map the deep structure of this volcanic centre. Each set of residual difference data is plotted on a projection of the lower focal hemisphere of Dumont. After corrections were applied for known low velocity rocks in the Silent Canyon caldera, data clusters of at least -1.0 s were found to dominate observations in the distance range 60–90 degrees. These large negative delays indicate the existence of a major high velocity anomaly that is directly beneath the Silent Canyon volcanic centre. Teleseismic P-wave residuals of two additional nuclear explosions, outside both the Silent Canyon caldera and the region of the reference event, were also reduced by Dumont residuals. The resulting data are distinct from the Silent Canyon residual difference fields and serve as a control on the primary experiment. The observed differential residuals, independent data on trace and major element chemistry of Silent Canyon lavas, and direct observations of effective Q argue that the high velocity anomaly is in the upper mantle. A model that satisfies the observed residual differences and which is consistent with additional seismic data possesses an anomalous zone which begins beneath the Mohorovicic discontinuity, at a depth of 40–50 km, extends to a depth of 170–190 km, and has a volume of 350000–600000 km3. This zone is interpreted as representing upper mantle material that has been largely depleted of its partial melt component by various differentiation processes and by associated eruption of the partial melt material through the Silent Canyon volcanic centre.

Prospecting for dead slabs

At subduction zones that have only recently ceased to be active, the lithospheric slab may retain a seismic velocity greater than that of the surrounding mantle even after the slab becomes seismically dead. To seek the subduction zone thought to have been recently active along the western margin of North America, we examined the variation with propagation direction of P-wave travel time residuals from sources at various distances and azimuths to seismograph stations in Washington and California. The uncertainty in source location and origin time was removed by referring the travel-time delay to a nearby station overlying presumably more uniform mantle. An eastward-dipping band of anomalously early arrivals at several stations on the western flank of the Sierra Nevada and California Cascades may imply that a dead slab is present beneath northern California, though a definitive conclusion is premature at present because of a paucity of seismic sources in eastern North America. The position of the dead slab speculatively suggested by the travel-time data is roughly consistent with that predicted by others on the basis of heat flow and geochemistry in the Sierra Nevada, and the southward decrease in the magnitude of the travel-time advance associated with such a slab is in agreement with the history of subduction of the Farallon plate as reconstructed from ocean floor magnetic anomalies and continental tectonic activity.