Dipping Moho Research Articles

Moho depth and Poisson's ratio in the Western-Central Alps from receiver functions

Current knowledge about deep crustal structure of the Alpine orogen has mainly been derived from P-wave velocity models obtained from active and passive seismic experiments. A complementary S-wave model to provide lithological constraints necessary for unique structural interpretation has been missing to date. In this paper, we present important new information on S-wave velocity model in the Alps. We applied the receiver function method using 6 yr of high quality data from 61 permanent and temporary stations sampling the Western—Central Alps. We determined first-order crustal features Moho depth (H) and average Vp/Vs ratio (κ) with the H–κ stacking technique that uses timing of direct and multiple P-to-S converted phases from the Moho interface. Synthetic tests reveal a dipping Moho interface, expected beneath an orogen, causes a systematic bias of H and κ potentially leading to misinterpretation. We thus applied corrections determined from synthetic data to remove the bias, providing better fit of recovered Moho depths with active seismic estimates. For each site, we also obtained independent H and κ estimates based on the timing of the strong Ps-phase. Our results show a gently south—southeast dipping European Moho at a depth of ∼24–30 km beneath the Northern Alpine Foreland, steepening rapidly towards the Europe—Africa suture zone to reach a maximum depth of ∼55 km. South of the suture, the Moho of the Adriatic crust, promontory of the African plate, is at ∼35–45 km depth. In the previously ill-constrained Western Alps, we found the European Moho at ∼30 km depth beneath the more external units dipping east—northeast to reach ∼50–55 km in the inner core of the Alps. The Poisson's ratio clearly correlates with the tectonic units that comprise the Alps. Average crustal values in the European Alpine Foreland are close to 0.25. In the Alps, we observe low values (0.22) in the highly deformed nappes of the Mesozoic Helvetic and Southern Alps indicating a thickening of felsic upper-crustal material. In contrast, the Poisson's ratio is significantly higher (0.26) in the Penninic and Austroalpine units near the suture zone. This rapid and significant change marks a clear rupture between the Alpine forelands and the suture domain. We assign this high Poisson's ratio to doubling of mafic lower crust consistent with results from previous active seismic experiments. A continuation of the lower crustal wedge into the central part of the Western Alps, however, seems unlikely based on low observed Poisson's ratios.

Crustal structure across the Grand Banks-Newfoundland Basin Continental Margin - I. Results from a seismic refraction profile

SUMMARY A P-wave velocity model along a 565-km-long profile across the Grand Banks‐Newfoundland Basin rifted margin is presented. Continental crust ∼36 km thick beneath the Grand Banks is divided into upper (5.8‐6.25 km s −1 ), middle (6.3‐6.53 km s −1 ) and lower crust (6.77‐ 6.9 km s −1 ), consistent with velocity structure of Avalon zone Appalachian crust. Syn-rift sediment sequences 6‐7 km thick occur in two primary layers within the Jeanne d’Arc and the Carson basins (∼ 3k m s −1 in upper layer; ∼ 5k m s −1 in lower layer). Abrupt crustal thinning (Moho dip ∼35 ◦ ) beneath the Carson basin and more gradual thinning seaward forms a 170-km-wide zone of rifted continental crust. Within this zone, lower and middle continental crust thin preferentially seawards until they are completely removed, while very thin (<3 km) upper crust continues ∼60 km farther seawards. Adjacent to the continental crust, high-velocity gradients (0.5‐1.5 s −1 ) define an 80-km-wide zone of transitional basement that can be interpreted as exhumed, serpentinized mantle or anomalously thin oceanic crust, based on its velocity model alone. We prefer the exhumed-mantle interpretation after considering the non-reflective character of the basement and the low amplitude of associated magnetic anomalies, which are atypical of oceanic crust. Beneath both the transitional basement and thin (<6 km) continental crust, a 200-km-wide zone with reduced mantle velocities (7.6‐7.9 km s −1 ) is observed, which is interpreted as partially (<10 per cent) serpentinized mantle. Seawards of the transitional basement, 2- to 6-km-thick crust with layer 2 (4.5‐ 6.3 km s −1 ) and layer 3 (6.3‐7.2 km s −1 ) velocities is interpreted as oceanic crust. Comparison of our crustal model with profile IAM-9 across the Iberia Abyssal Plain on the conjugate Iberia margin suggests asymmetrical continental breakup in which a wider zone of extended continental crust has been left on the Newfoundland side.

Crustal structure across the Grand Banks-Newfoundland Basin Continental Margin - II. Results from a seismic reflection profile

SUMMARY A P-wave velocity model along a 565-km-long profile across the Grand Banks‐Newfoundland Basin rifted margin is presented. Continental crust ∼36 km thick beneath the Grand Banks is divided into upper (5.8‐6.25 km s −1 ), middle (6.3‐6.53 km s −1 ) and lower crust (6.77‐ 6.9 km s −1 ), consistent with velocity structure of Avalon zone Appalachian crust. Syn-rift sediment sequences 6‐7 km thick occur in two primary layers within the Jeanne d’Arc and the Carson basins (∼ 3k m s −1 in upper layer; ∼ 5k m s −1 in lower layer). Abrupt crustal thinning (Moho dip ∼35 ◦ ) beneath the Carson basin and more gradual thinning seaward forms a 170-km-wide zone of rifted continental crust. Within this zone, lower and middle continental crust thin preferentially seawards until they are completely removed, while very thin (<3 km) upper crust continues ∼60 km farther seawards. Adjacent to the continental crust, high-velocity gradients (0.5‐1.5 s −1 ) define an 80-km-wide zone of transitional basement that can be interpreted as exhumed, serpentinized mantle or anomalously thin oceanic crust, based on its velocity model alone. We prefer the exhumed-mantle interpretation after considering the non-reflective character of the basement and the low amplitude of associated magnetic anomalies, which are atypical of oceanic crust. Beneath both the transitional basement and thin (<6 km) continental crust, a 200-km-wide zone with reduced mantle velocities (7.6‐7.9 km s −1 ) is observed, which is interpreted as partially (<10 per cent) serpentinized mantle. Seawards of the transitional basement, 2- to 6-km-thick crust with layer 2 (4.5‐ 6.3 km s −1 ) and layer 3 (6.3‐7.2 km s −1 ) velocities is interpreted as oceanic crust. Comparison of our crustal model with profile IAM-9 across the Iberia Abyssal Plain on the conjugate Iberia margin suggests asymmetrical continental breakup in which a wider zone of extended continental crust has been left on the Newfoundland side.

Moho geometry and upper mantle images of northeast India

Images of the crust and mantle beneath northeast India obtained by 2D migration of ∼1000 broadband P‐receiver functions clearly trace a northward dipping Moho from the Himalayan foredeep reaching depths up to 50 km further north beneath the Himalayan convergence zone. Also, these images reveal presence of largely coherent 410‐km and 660‐km discontinuities that conform to the IASP91 model. Marginal variations in the depth of the 410‐km interface are observed, that appear region specific. The thickness of the mantle transition zone does not deviate significantly from a global average of ∼250 km. Interestingly, our results reveal consistent presence of a signal from an interface around 300 km. Origin of such a boundary, known as X‐discontinuity and unrelated to the Lehmann discontinuity, is discussed. Possible presence of the X‐discontinuity from the Indian region is reported here for the first time.

Lateral variations of crustal seismic attenuation along the INDEPTH profiles in Tibet from Lg Q inversion

Fourier spectra of crustal guided Lg waves are collected to study seismic attenuation, or Q, along the INDEPTH profiles. The 1 Hz Lg Q (Q0) values are measured between many pairs of two stations and input to a back projection algorithm to obtain laterally varying Q0 models. Along the INDEPTH III profile in central Tibet, the Q0 model consists of low and nearly constant values of ∼90. These low values are consistent with abnormally high temperature as well as partial melts in the crust. Along the INDEPTH II profile in southern Tibet the Q0 values increase southward. They are extremely low (∼60) over the northernmost segment but increase by a factor of 2 over a distance of ∼100 km between the Indus‐Yalong Suture and the Kangmar Dome. Farther south and into the high Himalayas, Q0 values are higher than 300. Regional Rayleigh waves observed along the northern INDEPTH II profile are used to infer a low‐velocity, low‐Q layer at midcrustal depths (between ∼15 and 30 km) in southern Tibet. The aqueous fluid trapped in the upper crust and a midcrust partial melting zone, associated with the underthrusting of the Indian lithosphere, are the likely causes of the high Lg attenuation in southern Tibet. Low Q, rather then a systematic Lg conversion along a dipping Moho, is the main cause of the previously observed Lg blockage over paths crossing the southern Tibetan boundary.

Crustal structure variations in northeast India from converted phases

Teleseismic receiver functions from a ten station network deployed in northeast India region sampling the Shillong plateau, Mikir Hills, Himalayan foredeep and the Himalayan convergence zone, are analyzed to obtain the crustal structure in this seismically active but less studied region. The Shillong plateau and Mikir hills, away from the convergent margins, reveal remarkably simple crust with thickness (∼35 km) and Poisson's ratio (∼0.25), akin to the Indian shield values. A surprisingly thin crust for the uplifted Shillong plateau may be explained invoking presence of an uncompensated crust that popped up in response to tectonic forces. In contrast, crustal signatures from Assam valley suggest a thicker crust and higher Poisson's ratio with evidences for a dipping Moho. Predictably, the crust is much thicker and complicated in the eastern Himalaya further north, with values in excess of 50 km.

Gravity anomalies, crustal structure and thermo-mechanical support of the Himalaya of Central Nepal

SUMMARY We use two gravity profiles that we measured across Central Nepal, in conjunction with existing data, to constrain the mechanical behaviour and the petrological structure of the lithosphere in the Himalayan collision zone. The data show (1) overcompensation of the foreland and undercompensation of the Higher Himalaya, as expected from the flexural support of the range; (2) a steep gravity gradient of the order of 1.3 mgal km x1 beneath the Higher Himalaya, suggesting a locally steeper Moho; and (3) a 10 km wide hinge in southern Tibet. We compare these data with a 2-D mechanical model in which the Indian lithosphere is flexed down by the advancing front of the range and sedimentation in the foreland. The model assumes brittle Coulomb failure and nonlinear ductile flow that depends on local temperature, which is computed from a steadystate thermal model. The computed Moho fits seismological constraints and is consistent with the main trends in the observed Bouguer anomaly. It predicts an equivalent elastic thickness of 40‐50 km in the foreland. The flexural rigidity decreases northwards due to thermal and flexural weakening, resulting in a steeper Moho dip beneath the high range. Residuals at short wavelengths (over distances of 20‐30 km) are interpreted in terms of (1) sediment compaction in the foreland (Dr=150 kg m x3 between the Lower and Middle Siwaliks); (2) the contact between the Tertiary molasse and the meta-sediments of the Lesser Himalaya at the MBT (Dr=220 kg m x3 ); and (3) the Palung granite intrusion in the Lesser Himalaya (Dr=80 kg m x3 ). Finally, if petrological transformations expected from the local (P, T) are assumed, a gravity signature of the order of 250 mgal is predicted north of the Lesser Himalaya, essentially due to eclogitization of the lower crust, which is inconsistent with the gravity data. We conclude that eclogitization of the Indian crust does not take place as expected from a steady-state local equilibrium assumption. We show, however, that eclogitization might actually occur beneath southern Tibet, where it could explain the hinge observed in the gravity data. We suspect that these eclogites are subducted with the Indian lithosphere.

Regional crustal thickness variations of the Peninsular Ranges, southern California

We used the teleseismic receiver function technique to obtain a profile of the crustal thickness of the northern Peninsular Ranges, California. Depth to the Moho varies from ∼37 km beneath the western Peninsular Ranges batholith to ∼27 km at the western edge of the Salton trough, an average apparent dip of ∼10° to the west over a lateral distance of 60 km. We previously obtained a similar result for a profile ∼100 km to the south (a Moho dip of ∼20° over 30 km lateral distance). In both cases, the Moho depth variations do not correlate with topography of the eastern batholith, but rather appear to parallel the trend of a boundary that separates compositionally distinct eastern and western terranes. These observations suggest that a steeply dipping Moho is a regional feature beneath the eastern Peninsular Ranges, and that compensation is through lateral variations in crustal or upper mantle density rather than through an Airy root.

Crustal structure of the northwestern Vizcaino block and Gorda Escarpment, offshore northern California, and implications for postsubduction deformation of a paleoaccretionary margin

The Vizcaino block is an anomalously shallow region of the western U.S. continental margin located southwest of the Mendocino triple junction. It originated as part of the accretionary prism of the North America plate and was transferred to the Pacific plate in the Miocene as the Pacific‐North America plate boundary migrated ∼130 km eastward, forming the Gorda Escarpment at its northern boundary. We present hybrid crustal models for the northwestern part of the Vizcaino block derived from marine seismic and gravity data. The velocity and density structure of the northwestern Vizcaino block are compatible with paleoaccretionary complex material similar to San Simeon/Patton terrane overlying oceanic crust or a mafic layer. The most significant result of our modeling is an abrupt increase in Moho dip from ∼5° to ∼20–30° beneath the western edge of the Oconostota ridge along the northwestern margin of the Vizcaino block. This Moho dip is steeper than observed anywhere along the Cascadia subduction zone, indicating postsubduction deformation. We suggest that the paleotrench was deformed by compression, which reactivated preexisting thrust faults in the upper crust and thickened the crust within this apparent weak zone. At least part of the deformation predates late Pliocene Pacific‐North America plate convergence and may result mainly from north‐south compression between the Pacific‐Juan de Fuca plates across the Mendocino transform fault. North‐south compression continues today and may dynamically support the uplifted northern margin of the Vizcaino block, although the primary locus of deformation shifted to the relatively weak Gorda plate sometime prior to 3 Ma.

Moho dip and crustal anisotropy in northwestern Nevada from teleseismic receiver functions

AbstractReceiver functions are derived from teleseismic waves recorded during the 1988 to 1989 PASSCAL Basin and Range passive-source seismic experiment in northwestern Nevada. A velocity model involving both a planar dipping Moho and crustal anisotropy is needed to explain the radial and tangential motions of the observed PS conversions. An arrival-time difference often observed between radial and tangential Moho PS conversions suggests an anisotropic crust. The PS conversions are large and indicate a major discontinuity under the area. The particle motion directions for most of the tangential components change sign between South American events and events from the other two source areas (Japan and Tonga regions), providing good evidence for a Moho dipping approximately to the north. Also, the PS conversions from the Moho follow direct P arrivals by about 3.2 sec under the southern part of the array, 3.4 sec under the southern central part of the array, and 3.7 sec under the northern part of the array, indicating a Moho that varies in depth from about 26 to 31 km. A velocity model with the Moho dipping ∼9° in a nearly northerly direction and an anisotropic crust with a split time of ∼0.25 sec and a fast axis of ∼130° best explain these observations.

Transition from oceanic to continental crustal structure: seismic and gravity models at the Queen Charlotte transform margin

Seismic refraction data have been interpreted along a line crossing the Queen Charlotte transform, just north of the triple junction where the Explorer Ridge intersects the continental margin. These data, observed at three onshore sites, help to define the structure of the continental crust beneath the Queen Charlotte sedimentary basin. Sediment thicknesses of up to 4 km were determined from a coincident multichannel reflection line. Beneath the sediments, velocities increase from about 5.5 to 6.3 km·s−1 at 8 km depth, then increase from 6.5 to 6.7 km·s−1 at 18 km depth. Below this depth, the lower crust is partly constrained by Moho wide-angle reflections at the three receiving sites, which indicate a lower crust velocity of 6.8–6.9 km·s−1 and a Moho depth of 26–28 km. The crustal velocity structure is generally similar to that in southern Queen Charlotte Sound. It is in contrast to the velocity structure across Hecate Strait to the north, where a prominent mid-crust interface at ~15 km depth was observed. Seismic velocity models of the continental crust provide constraints that can be used in modelling gravity data to extend structures across the ocean–continent boundary. Along the profile just north of the Queen Charlotte triple junction, the gravity "edge effect" is very subdued, with maximum anomalies of &lt; mGal (1 mGal = 10−3 cm·s−2). To satisfy the gravity data along this profile, the modelled crustal thickness must decrease to oceanic values (5–6 km) over a horizontal distance of 75 (±10) km, which gives a Moho dip of about 14°. Farther north, refraction models across Hecate Strait provide similar constraints for gravity modelling; the gravity data indicate horizontal transition distances from thick to thin crust of 45 (±10) km, comparable with, but slightly smaller than, those nearer the triple junction, and Moho dips at an angle of 18–22°. The greater thinning near the triple junction is consistent with mass flux models in which ductile flow in the lithosphere is induced by the relative motion between oceanic and continental plates.

Simultaneous determination of hypocenter location and crustal structure in the Virginia seismic region

Simultaneous inversions for one‐ (1‐D) and three‐dimensional (3‐D) seismic structure and hypocenters (SSH) for both the central and western Virginia seismic regions are performed, using direct and refracted phases from about 60 earthquakes. Optimal inversion models are computed by employing regularization techniques of Tikhonov and of Backus and Gilbert. Improvements for the residual sum squared, of up to 70% are obtained for the 3‐D models and somewhat less for the 1‐D models. For the central Virginia seismic region the SSH 1‐D velocity models show a strong negative bias in the deduced Pn velocity which could only be alleviated after a ray‐tracing travel time correction for the Pn phases due to the east‐west downdipping Moho has been applied. A 4° Moho dip turns out to be optimal for best explaining the observed travel time residuals and returning a realistic Pn velocity of 8.2–8.3 km/s. However, the deduced velocity in the lower crust is only ∼ 6.35 km/s, which is lower than that predicted from gravity modeling but can be explained in terms of a lower crustal petrology of anorthosite‐rich granulite. A more detailed analysis of the lower crust shows that the hypothesized high‐velocity layer at the bottom of the crust (Pratt et al., 1988) is not clearly identifiable by the present travel time data and so can neither be corroborated nor refuted. Many of the 1‐D crustal results are further substantiated by the 3‐D model, which also provides a reduced lower crustal velocity under the Goochland Terrane. Positive velocity anomalies, which are found in the upper and middle crust under the Blue Ridge/Piedmont boundary, correlate qualitatively with the high‐density, thrust‐faulted slab deduced by Pratt et al. (1988) from gravity modeling. The vertical movements of the relocated hypocenters in the central region are of O(2–5 km) and are such as to further widen the aseismic gap under the Goochland Terrane. In the eastern section of the seismic zone (at the Piedmont/Atlantic Coastal Plain boundary) the downward focal shifts at the lower seismic boundary place events well into the Grenville basement, indicating a lowering of the brittle‐ductile transition boundary. For the western Virginia seismic region the SSH 1‐D velocity models result in a gradual increase in velocity with depth. Significant shifts in the hypocentral relocations are obtained which indicate a further deepening of the brittle‐ductile transition. This and results of thermomechanical creep models favor a quartz‐poor, diorite rich, middle and lower crust in the western region. The 3‐D velocity model of the western region shows a variety of features which can be related to the geology and the tectonics of the Valley and Ridge province. In the surficial layer a velocity low in the Mississippian‐Permian molasse of the West‐Virginian Valley and Ridge province and a velocity high in the thicker Cambrian‐Ordovician carbonate thrust and fault sheets of the Giles county area of the Valley and Ridge are obtained. In the lower crust, higher velocities are detected in the western Valley and Ridge province, which correlates well with the thickening of the crust in the western section of the Virginian Appalachian.

The Queen Charlotte Islands refraction project. Part I. The Queen Charlotte Fault Zone

The active margin between the continental North American plate and oceanic Pacific plate west of the Queen Charlotte Islands was the site of an extensive onshore–offshore seismic refraction project in 1983. An airgun line shot over two ocean-bottom seismographs (OBS's) and a 32-charge explosion line recorded on the two OBS's and eight land-based seismographs (LBS's) deployed across northern Moresby Island were selected to study the structure of the predominantly transform Queen Charlotte Fault Zone and the associated offshore terrace. Two-dimensional ray tracing and synthetic seismogram modelling produced a pronounced laterally varying velocity structural model showing three major crustal components (oceanic, terrace, and continental) separated by an outer, crustally pervasive fault and active Queen Charlotte Fault, respectively. The 3 km thick block-faulted upper terrace unit, overlain by deformed sediments, is indistinguishable from adjacent oceanic sediments and upper crustal basalts located to the west. The upper part of the 10–17 km thick lower terrace unit has anomalously low velocities relative to the adjacent oceanic and continental crustal units. A high gradient increases terrace velocity rapidly with depth until the contrast becomes negligible at approximately 17 km depth. Changes in depth to Moho beneath the terrace suggest an increase in eastward Moho dip from 2–5 °observed west of the terrace to 19 °below it. Tectonic mechanisms proposed to explain the anomalous terrace structure involve sediment accretion during subduction of oceanic lithosphere, alternating or combined with compressive upthrusting of material along near-vertical fault planes during periods of active transform motion.

Three‐dimensional P and S wave velocity structures of southern Peru and their tectonic implications

In 1981 an 18‐station regional array was operated in southern Peru above the area where the dip of the subducted Nazca plate changes from virtually horizontal in the northwest to about 30° in the southeast. Arrival times of compressional (P) and shear (S) waves from microearthquakes recorded by this array are used in this paper to investigate the three‐dimensional velocity structure of the crust and upper mantle in the region between the coast and the Cordillera Occidental. The results suggest a crustal thickness of about 40 km beneath the coast, increasing to about 70 km beneath the Cordillera Occidental. This change in thickness occurs abruptly in the northwest but is gradual in the southeast. The inverse correlation between the dip of the Moho and the dip of the slab suggests a broad‐scale, causal relation between the two. At the same time, free‐air gravity anomalies suggest that the steepening of the Moho in the northwest may occur only in the region under the array. Because the crust is thicker in the southeast than originally surmised, a suite of earthquakes previously thought to occur in the mantle now appears to be confined to the crust. The association of these earthquakes with a shallow dipping Moho is suggestive of ongoing crustal deformation to the west of the Andes. However, other seismological and geological observations make the role of these earthquakes in crustal deformation unclear. S wave velocities in the mantle between 70‐ and 130‐km depth above the 30° dipping slab are low, suggesting the presence of partially melted asthenosphere that may be responsible for the magmatic activity observed in southern Peru.

Pnobservation and interpretation in Washington

abstractThe travel-time characteristics of Pn waves in the state of Washington were investigated using regional earthquakes as seismic sources and stations of the state-wide University of Washington telemetered network as receivers. There is a significant difference in upper mantle velocity between eastern Washington (8.20 ± 0.02 km/sec) and western Washington (7.79 ± 0.02 km/sec). Modifications of the time-term method were used to find the dependence of P velocity on distance and azimuth to the source. Pn velocity in eastern Washington depends on the distance to the source, implying a velocity gradient with depth in the upper mantle. No significant distance dependence was found for western Washington. Azimuthal anisotropy was found for Pn with a northwest direction of maximum velocity in eastern Washington and a north-northwest direction in western Washington. Assuming no lateral crustal velocity gradients, the Pn refracting interface dips to the west approximately 0.8° in eastern Washington and to the southeast approximately 3.2° in western Washington. Contour maps of the station time terms indicate significant variations in crustal structure across the state. The combination of low Pn velocity and small Moho dip beneath western Washington suggests that the subducting lithospheric slab underlying the continental margin lies below the level of transition to upper mantle velocity over much of this region.

Moho orientation beneath central California from regional earthquake travel times

Table Al is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B84‐011; $2.50. Payment must accompany order.Analysis of relative Pn travel times recorded by 238 stations of the U.S. Geological Survey central California seismic network (CALNET) from 77 regional earthquakes and explosions shows that arrivals are progressively delayed to the northeast in both the Coast Ranges and Sierra Nevada foothills. By dividing the CALNET into 10 subarrays and assuming laterally uniform velocities for a crust over an upper mantle half space, the orientation of a planar Moho beneath each subarray is calculated by inversion of relative Pn arrival times. The results show that the Moho dips to the northeast between 3° and 6° beneath the Coast Ranges and 8°–17° beneath the Great Valley and is horizontal beneath the foothills. Pn residuals west of the San Andreas fault are relatively early by 0.35 s but are interpreted as resulting from a lateral velocity contrast across the fault rather than variations in crustal thickness. Unexplained Pn travel time variance improves by approximately 60% after correction for the travel time due to dip and mean Pn residual at each station. The Pn data corrected for the travel time due to a dipping Moho indicate an isotropic upper mantle velocity of 7.97±0.05 km/s beneath the Coast Ranges, Great Valley, and foothills. Evidence of upper mantle velocity anisotropy in the azimuthal variation in Pn apparent velocity corrected for dip is only marginally significant at the 5% level. However, large azimuthal variations in corrected apparent velocity do exist and may be attributed to velocity heterogeneities in the crust, unmodeled higher‐order mantle structure, and gradients in upper mantle velocity. Arrival times of Cape Mendocino earthquakes recorded in the Coast Ranges south of San Francisco Bay indicate lateral variations in upper mantle velocity. The 7.4 km/s apparent velocity estimate from travel times of the Oroville earthquake recorded in the Coast Ranges out to distances greater than 400 km is anomalous in that Pn is normally observed at distances greater than 200 km. These arrivals may originate from updip propagation along a 6.9 km/s layer in the lower crust of the Coast Ranges subparallel to the Moho.

Crustal structure of southwestern Montana and east‐central Idaho: Results of a reversed seismic refraction line

On October 20, 1982, we collected seismic refraction data on a 250 km line segment between Butte, Montana and Challis, Idaho. These data are from two open pit mine blasts; one at Anaconda Minerals Companys' southeast Berkely pit and the other at Cyprus Mines Corporations' Thompson creek mine. Eight of our ten recorders wrote clear records of both blasts with measured travel times accurate to +/−0.2 seconds. We supplemented these data with data from permanent stations in the Butte area. The Pg (crustal) velocity is 5.8 km/s to the southwest and 6.0 km/s to the northeast. The Pn (mantle) velocity is 7.9 km/s to the northeast and 8.1 km/s to the southwest; total travel‐times are equal. The S‐wave velocity in the crust is about 3.5 km/s. The symmetry of the travel‐time results, with their intersection within one kilometer of the midpoint between the pit blasts, strongly suggests the refraction line overlies a horizontal Moho or parallels the strike line of a dipping Moho surface. These data indicate the crust in southwestern Montana and east‐central Idaho is approximately 33 km thick. Contrasting tectonic processes, compression and thickening during the Laramide orogeny with extension and thinning since the Paleocene, resulted in a crust with slightly less than normal thickness.

A dipping Moho and crustal low-velocity zone fromPnarrivals at The Geysers-Clear Lake, California

abstractRelative Pn arrival times across an array of stations at The Geysers-Clear Lake region in northern California indicates that the upper mantle velocity is 8.0 km/sec, and that the Moho dips 5.3° at 57° to the northeast in this area, reflecting thickening of the crust toward the continent. These results agree with regional trends in the Bouguer gravity field. The travel-time residuals with respect to the dipping model suggest a crustal low-velocity zone beneath Mt. Hannah, consistent with reported teleseismic delays, and the low-velocity zone is interpreted as representing partial melt. No delays are observed in The Geysers, precluding the existence of an extensive magma body beneath the steam production area. Azimuthal variations in apparent velocity may reflect upper mantle azimuthal velocity anisotropy, but such an interpretation is uncertain due to the limited azimuthal distribution of earthquakes.

Determination of Moho dip using PS reflections

Summary. A clear PS reflection was recorded in the northern section of the LISPB line (shot-point N1, profile ALPHA). Travel times of PS and PP phases reflected at the same part of the Moho were used to model the lower crustal structure. The observed high apparent velocity of the PS phase (8.4 km/s) is interpreted as due to a localized Moho dip of 7 per cent (4°) towards the north. This is supported by observations of Pn refractions from other shots. It is shown that PS reflections could be very useful for determination of Moho dips.

The effect of planar dipping structure on source and receiver responses for constant ray parameter

A geometrical ray method is developed for wave calculations involving three-dimensional planar dipping interfaces. Justification for the method is based on analogy with first-motion approximations derived from generalized ray theory where frequency dependence in the reflection-transmission coefficients is related to changes in the complex ray parameter. The method is applied to finding the teleseismic response of an arbitrarily oriented dislocation source in dipping layered media and for receiver calculations which assume an impinging P or S wave beneath a stack of dipping layers. Source results indicate that wave forms from fast azimuthally varying sources, such as strike-slip faults, are significantly distorted from the plane layered case for simple structures. A simple dipping Moho for dips up to 10° does not significantly distort vertical and radial P waves for the receiver response. However, due to azimuth anomalies introduced by interface dip a significant tangential P component is produced. In addition, the S-wave response becomes a function of source mechanism due to the need for specifying the incident polarization angle. Polarization studies are suggested for finding dipping structure.