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Evidence of geologic activity still occurring in the Rio Grande rift (RGR) includes quaternary faulting, seismicity, and widening at a small rate. We map the crustal thickness and seismic velocity ratio to generate constrained crustal model cross sections that highlight the regional extension of the southern RGR (SRGR). Specifically, we computed receiver functions and receiver function stacks for 147 stations from EarthScope USArray Transportable Array and previously collected data in the southwestern United States, and interpolated the crustal and velocity results using a kriging interpolation scheme. We include a new optimized gravity inversion approach using the receiver function results as a constraint for inverted density profiles. We produce constrained crustal models that characterize the SRGR as showing a shallower Moho (30 km) in the region of the SRGR, which is thicker than previously suggested. The crust appears to be delaminated west of the RGR in the Basin and Range Province and thicker east of the RGR toward the Great Plains with a denser lower crustal layer. We also find that the Delaware basin shows a significant geophysical anomaly, likely due to thick saturated sediments. Given that there is no deep mantle anomaly determined from other studies in this region, we suggest that rifting could be the result of small‐scale convection of the uppermost mantle, and we conclude that the RGR is at a minimum the boundary between the Basin and Range Province and the Great Plains, perhaps reflecting the terminus of flat slab subduction.

An earthquake swarm took place from 27 August 2013 to December 2013 in the southern Basin and Range province of Chihuahua, Mexico. During this period, the Seismological Service of Mexico located approximately 60 earthquakes in the region. The largest earthquake took place on 21 September 2013 ( M w 5.3). A temporary seismic network was installed in the area for a period of two weeks. The hypocenters were relocated using the double‐difference algorithm. The results show a cluster of seismicity along the Vallecillo fault, where most of the seismicity took place, and on the Penasco fault. These two faults lie within the foothills of the sierras of the same name that bound the El Caballo desert basin. The seismicity suggests that both faults bounding the El Caballo basin slipped during the swarm. The main event of 27 August 2013 apparently took place on the Vallecillo fault. A third cluster of seismicity was located on Sierra La Mezcalera, about 20 km to the south of the Vallecillo fault, at the western end of the La Boquilla dam. Focal mechanisms of the larger events indicate right‐lateral slip with extensional deformation on the Vallecillo fault, with T axes oriented in a northeast–southwest direction. These source mechanisms reflect a regime of extensional tectonics in the southern Basin and Range province. The Vallecillo and Penasco fault system is exposed for approximately 12 km along strike. Scaling relations suggest that earthquakes as large as M w 6.4 may take place on them. This magnitude is similar to that of the 1928 Parral earthquake that occurred about 125 km to the south of the swarm. However, segmented faults may rupture sequentially and generate a larger earthquake than the one predicted on the basis of the length of individual mapped faults.

We used intensity information from five calibration earthquakes to estimate an intensity attenuation model for the Rio Grande rift region (encompassing west Texas, New Mexico, and southern Colorado). The attenuation model lies between similar models developed for the Basin and Range Province and central and eastern North America. Analysis of two test earthquakes occurring in 1931 and 2014 validated that the intensity attenuation relation adequately predicted the events’ magnitudes and epicenters. We then analyzed intensity information for nine historical (1906–1960) earthquakes to determine their magnitudes and epicenters. Our results indicate that the largest earthquakes of the 1906 Socorro, New Mexico, sequence had intensity magnitudes ( M I ) 5.7–5.9 and may represent north‐to‐south migration of seismicity associated with the Socorro magma body. Moderate earthquakes in 1935 and 1960 also are associated with the magma body. The 1938 Arizona–New Mexico border earthquake sequence occurred about 50 km northeast of the ongoing 2014 Arizona–New Mexico border sequence. The 17 September earthquake of the 1938 sequence had an M I (4.1) that was about 1.0 units less than that of the 29 June 2014 Arizona–New Mexico event ( M I 4.9).

Smith Valley is bounded on its western edge by an active normal fault and the Pine Nut mountains. Displacement on the fault is responsible for development of one of the westernmost basins of the Basin and Range province of the western United States. Interpretation of an exposure afforded by excavation of a trench across the range-bounding fault places the most recent surface-rupture earthquake after 5176±130 cal yr B.P. and suggests it was close in time to 3530±82 cal yr B.P. Utilization of tephrochonologic and cosmogenic analyses and mapping of fault scarps across the Artesia Road fan are the basis for putting forth an initial estimate of the late Pleistocene vertical slip rate of the fault at between 0.125 mm/yr and 0.33 mm/yr.

We calculated the spectral decay parameter (κ) in the southern Basin and Range province, using records from earthquakes located near the Pitaycachi fault. We found that for a given distance and recording site the values of κ vary considerably, possibly as a result of the lateral heterogeneity of the crust. Individual values of κ do not show a clear trend with magnitude, nor with the average values of κ. The average values of κ tend to increase with distance up to about 70-80 km and then to decrease between 80 and 100 km, suggesting higher S-wave attenuation at shallow depths and lower attenuation for deeper paths. We analyzed the possible dependence of κ on earthquake size by determining empirical curves that describe the behavior of κ with distance and magnitude. Our results indicate that κ is independent of earthquake size within the magnitude range ( M< 3:5) of the events analyzed. The nonparametric curves also provided site-specific estimates of κ near the surface (κ0) for the stations and groups of stations analyzed. We found that the average κ0 value of 0.04 sec in northeastern Sonora is similar to values of κ0 reported in other regions. We conclude that κ0 depends not only on the rock type but also on the degree of fracturing and erosion of the rocks near the recording site.

During the great 3 May 1887 Sonoran earthquake (surface-rupture end-to-end length: 101.8 km; M W 7.5±0.3), an array of three north–south-striking Basin and Range Province faults (from north to south: Pitaycachi, Teras, and Otates) slipped along the western margin of the Sierra Madre Occidental Plateau of northern Mexico. This detailed field survey of the previously undocumented 1887 earthquake rupture zone along the Otates fault includes mapping the rupture scarp and measurements of surface deformation at 25 sites. The surface rupture coincides with the mapped trace of the Otates fault, dips 68° W, and has an end-point-to-end-point length of 18.2 km. The lateral rupture terminations are controlled by cross faults. Striation measurements and the style of rupture indicate extensional dip slip without significant lateral displacement. The average shortening ( P ) and extension ( T ) kinematic axes inferred from the striations are similar to published composite focal mechanisms of the microseismicity recorded in the epicentral region of the 1887 earthquake. Based on surface deformation, maximum slip along the Otates fault in the 1887 earthquake was 270 cm, and the average slip was 189 cm. A quadratic best fit to the along-rupture surface offsets indicates an asymmetric distribution with the maximum offset near the northern end of the rupture segment, which can be explained by the mechanical interaction of the Teras and Otates surface-rupture segments through their stress fields. The average recurrence interval of 1887-sized earthquakes on the Otates fault is 30–42 kyr based on the long-term geologic dip-slip rate of the fault (0.06 mm/yr) and its amount of slip in the 1887 earthquake.

Earthquakes in central Nevada (1932-1959) were used to develop a modified Mercalli intensity (MMI) attenuation model for estimating moment mag- nitude M for earthquakes in the Basin and Range province of interior western North America. M is 7.4-7.5 for the 26 March 1872 Owens Valley, California, earthquake, in agreement with Beanland and Clark's (1994) M 7.6 that was estimated from geologic field observations. M is 7.5 for the 3 May 1887 Sonora, Mexico, earthquake, in agreement with Natali and Sbar's (1982) M 7.4 and Suter's (2006) M 7.5, both estimated from geologic field observations. MMI at sites in California for earthquakes in the Nevada Basin and Range appar- ently are not much affected by the Sierra Nevada except at sites near the Sierra Nevada where MMI is reduced. This reduction in MMI is consistent with a shadow zone produced by the root of the Sierra Nevada. In contrast, MMI assignments for earthquakes located in the eastern Sierra Nevada near the west margin of the Basin and Range are greater than predicted at sites in California. These higher MMI values may result from critical reflections due to layering near the base of the Sierra Nevada.

Scalar moment rates estimated from a 146-year seismicity catalog agree, within uncertainties, with the deformation rate of the Basin and Range province determined by using space geodesy. Seismic-moment rates have been estimated from a new catalog of earthquakes complete for M ≥ 5. The catalog was compiled from 15 preexisting catalogs, supplemented by the review of 44 journal articles. Throughout the catalog compilation, care was taken to obtain the moment magnitude or a reasonable, and not inflated, equivalent. Seventy-six percent of the moment release occurred during 10 earthquakes of magnitude M W ≥ 6.79. The spatial distribution of earthquakes and their moment release matches the geodetic pattern of deformation. All three are concentrated in a ∼200-km-wide zone along the western boundary of the Great Basin, with this zone widening to the north. Several techniques, ultimately traceable to Kostrov and Brune, are used to translate the geodetic strain rates into rates of seismic-moment release. The agreement between geodetic and seismic- moment rate suggests that, within uncertainties, the rate of historic earthquakes within the Basin and Range province, taken as a whole, provides a reasonable estimate for the future rate of seismicity. These results support the hypothesis that even a few years of detailed geodetic monitoring can provide a good constraint on earthquake occurrence rate estimates for large-enough regions.

The Santa Rosa Range fault zone (srrfz) is one of the most topographically prominent normal fault systems in the northern Basin and Range province of the western United States. It has been assigned high rates of vertical slip by others and has been identified as a possible site of the future extension of the central Nevada seismic belt (cnsb). We use detailed trench mapping and luminescence dating to estimate displacements and timing of the last several large-magnitude paleoearthquakes on the southern part of the srrfz at a trench site near Orovada, Nevada. Coseismic vertical displacements ranged from 1 to 2.8 m for each of the last four events. Luminescence ages provide time limits for the last three events of 125–155 ka, 90–108 ka, and 11–16 ka. These data yield recurrence intervals of 17–65 k.y. and 74–97 k.y. and an elapsed time of 11–16 k.y. since the youngest event. Slip-rate determinations at the Orovada site are complicated by multiple fault strands, but rates calculated from a variety of data are surprisingly low (0.01–0.16 mm/yr), given the topographic prominence of the Santa Rosa Range. A lack of compelling patterns in a comparison of paleoseismic parameters indicate that the srrfz is no more likely a location for a large-magnitude earthquake than previously identified seismic gaps or along faults that lie directly north of the cnsb.

Frequency-dependent crustal attenuation (1/ Q ) is determined for seven distinct physiographic/tectonic regions of the continental United States using high-quality Lg waveforms recorded on broadband stations in the frequency band 0.5 to 16 Hz. Lg attenuation is determined from time-domain amplitude measurements in one-octave frequency bands centered on the frequencies 0.75, 1.0, 3.0, 6.0, and 12.0 Hz. Modeling errors are determined using a delete-j jackknife resampling technique. The frequency-dependent quality factor is modeled in the form of Q = Q 0 f η. Regions were initially selected based on tectonic provinces but were eventually limited and adjusted to maximize ray path coverage in each area. Earthquake data was recorded on several different networks and constrained to events occurring within the crust ( m b 3.5 in size. A singular value decomposition inversion technique was applied to the data to simultaneously solve for source and receiver terms along with Q for each region at specific frequencies. The lowest crustal Q was observed in northern and southern California where Q is described by the functions Q = 152(±37) f 0.72(±0.16) and Q = 105(±26) f 0.67(±0.16) , respectively. The Basin and Range Province, Pacific Northwest, and Rocky Mountain states also display lower Q and a strong frequency dependence characterized by the functions Q = 200(±40) f 0.68(±0.12) , Q = 152(±49) f 0.76(±0.18) , and Q = 166(±37) f 0.61(±0.14) , respectively. In contrast, in the central and northeast United States Q functions are Q = 640(±225) f 0.344(±0.22) and Q = 650(±143) f 0.36(±0.14) , respectively, show a high crustal Q and a weaker frequency dependence. These results improve upon previous Lg modeling by subdividing the United States into smaller, distinct tectonic regions and using significantly more data that provide improved constraints on frequency-dependent attenuation and errors. A detailed attenuation map of the continental United States can provide significant input into hazard map mitigation. Both scattering and intrinsic attenuation mechanisms are likely to play a comparable role in the frequency range considered in the study.

The 1994 Double Spring Flat earthquake ( M W 5.8) occurred within a densely faulted step-over between the Genoa and Antelope Valley faults, two principal normal faults of the transition zone between the Basin and Range Province and the northern Sierra Nevada. The earthquake created zones of ground cracks from 0.1 to 2.8 km long along at least five northwest- to north-northwest-striking faults in the epicentral area. Individual cracks had extensional openings generally from 1 to 10 mm wide. No cracks displayed obvious vertical separation, and only one zone showed permissive evidence of right-lateral separation. Over the 8 days following the mainshock (the period over which the cracks were found), aftershocks formed a dominant northeast trend suggesting the earthquake occurred along a northeast-striking structure. However, no ground breakage was found along faults striking parallel to this northeast aftershock alignment, and subsequent aftershocks formed a conjugate northwest trend. Based on the location and character of the five zones, the observed cracks are attributed to secondary fault slip and shaking effects. The earthquake also created ground cracks along at least two faults 15-25 km from the epicenter. In both of these cases, the faults had documented histories of prior ground cracking, indicating that they are particularly susceptible to such triggered deformation. Manuscript received 21 August 2002.

Analysis of historical accounts of the ∼ M 6.3 1884 earthquake in northern Utah reveals that the earthquake had an epicenter near 42.3° N, 111.4° W, approximately 30 km northwest of the event's original location. We use detailed reports of damage to structures and the consequences of ground shaking to define a felt area of approximately 70,000 km2 and estimate the peak ground accelerations as 100-300 cm/sec2. Analysis of the geologic structure of the area indicates that the epicentral area is a half-graben bounded on the east by the listric Bear Lake fault and on the west by the steeply dipping West Bear Lake fault. The earthquake epicenter was on the west side of the basin, and we interpret the event to have been the result of slip on the West Bear Lake fault zone at a depth of 4-5 km. This zone consists of steeply dipping antithetic faults in the hanging wall of the East Bear Lake fault. These data suggest that moderate-magnitude earthquakes on antithetic or small-displacement faults pose a significant, if local, seismic hazard in the northeastern Basin and Range province. We also demonstrate the utility of combining geological and historigraphic analyses to examine pre-instrument-era earthquakes. Online material : Felt reports and town summaries. Manuscript received 16 July 2002.

North–south-striking and west-dipping Basin and Range province normal faults form the western edge of the Sierra Madre Occidental plateau in northeastern Sonora. These faults and associated half-grabens extend over a distance of more than 300 km between the San Bernardino basin in the north and the Sahuaripa basin in the south. An earthquake in 1887 ruptured three neighboring segments of this major fault zone. Our field mapping in this region indicates that the surface rupture of the 1887 earthquake extends farther to the south and is considerably longer (101.4 km end-to-end length) than previously reported. A compilation of the seismicity in the epicentral region of the 1887 earthquake shows the epicenters to be distributed in well-defined clusters at the northern end of the 1887 surface rupture, in the step-overs between the three rupture segments, on a neighboring fault in the west (Fronteras fault), and on fault segments farther south along the same fault zone (Granados region). The distribution of seismicity correlates well with calculated changes in Coulomb failure stress resulting from the 1887 earthquake.

The Genoa fault, a principal normal fault of the transition zone between the Basin and Range Province and the northern Sierra Nevada, displays a large and conspicuous prehistoric scarp. Three trenches excavated across this scarp exposed two large-displacement, late Holocene events. Two of the trenches contained multiple layers of stratified charcoal, yielding radiocarbon ages suggesting the most recent and penultimate events on the main part of the fault occurred 500-600 cal B.P., and 2000-2200 cal B.P., respectively. Normal-slip offsets of 3-5.5 m per event along much of the rupture length are comparable to the largest historical Basin and Range Province earthquakes, suggesting these paleoearthquakes were on the order of magnitude 7.2-7.5. The apparent late Holocene slip rate (2-3 mm/yr) is one of the highest in the Basin and Range Province. Based on structural and behavioral differences, the Genoa fault is here divided into four principal sections (the Sierra, Diamond Valley, Carson Valley, and Jacks Valley sections) and is distinguished from three northeast-striking faults in the Carson City area (the Kings Canyon, Carson City, and Indian Hill faults). The conspicuous scarp extends for nearly 25 km, the combined length of the Carson Valley and Jacks Valley sections. The Diamond Valley section lacks the conspicuous scarp, and older alluvial fans and bedrock outcrops on the downthrown side of the fault indicate a lower activity rate. Activity further decreases to the south along the Sierra section, which consists of numerous distributed faults. All three northeast-striking faults in the Carson City area ruptured within the past few thousand years, and one or more may have ruptured during recent events on the Genoa fault.

A new method based on the joint inversion of receiver functions and surface-wave phase velocities results in well-determined shear-velocity structures that are consistent with the compressional-wave structure, gravity, heat flow, and elevation data in the northern Basin and Range. This new inversion method takes advantage of average-velocity information present in the surface-wave method and differential velocity information contained in the receiver function method, thus minimizing the nonuniqueness problem that results from the velocity-depth trade-off. An unusually thick (38 km) and relatively faster crust and upper mantle are found in central and eastern Nevada compared to the thin (28 to 34 km) and slower crust and upper mantle of the western Basin and Range. We interpret the regions of thicker and faster crust and upper mantle as zones that have undergone less Cenozoic extension relative to the surrounding regions to the west and north. The thick crust and consequently greater depth to the dense mantle material is consistent with the gravity low pattern in central and eastern Nevada. Simple gravity modeling shows both local and regional isostatic compensation occur within 40 km of the surface, indicating a near-classical Airy type of compensation in the province. We analyze in detail the shear-wave (S-wave) velocity model derived from the receiver functions at station BMN and compressional-wave (P-wave) velocity models derived from the 1986 PASSCAL experiment in northwestern Nevada. The most interesting feature of these models is the presence of negative-velocity gradients in the S-wave model with no corresponding velocity decrease in the P-wave models between depths of 10 and 24 km. This combined velocity model may be explained by high pore fluid pressures at these depths. This model favors a layered fluid porosity model proposed in the literature to explain extensive middle- to lower-crust continental seismic reflections and high electrical conductivity. An upper-mantle, gradational low-velocity zone is present between 32 and 38 km in the S-wave model. This upper-mantle, shear-wave, low-velocity zone is consistent with partial melt, which may be the source material for magmatic underplating in this region.

Seismic waves from nuclear explosions and from well-located local earthquakes have been recorded along several profiles through western Nevada, southeast Oregon, and eastern California. Pn delay times from these data and from additional travel times of NTS explosions recorded across the western Nevada seismic network have been interpreted in terms of varying crustal thickness of the western Great Basin. Our interpretation of these data implies that the crust thins from greater than 32 km thick in the vicinity of Mono Lake to 20 to 22 km thick over a broad region in northwest Nevada. The Pn headwave propagates to a distance of about 550 km with a velocity of 7.8 km/sec. At distances greater than 600 km a low velocity (7.44 km/sec), larger amplitude phase is observed. This phase may result from long-period energy diffracting into a shadow zone. A prominent, high-velocity second arrival following Pn by 1 to 2 sec in the distance range 340 to 475 km, is interpreted as a reflection from the base of the lithosphere at 60 to 70-km depth.

This paper presents a detailed analysis of S-wave data for two areas in western Nevada. Some of these data were used by Gupta (1973b, c) as the basis for his claim that large premonitory changes in the extent of stress-induced S-wave splitting are observed for moderate-sized earthquakes in Nevada. Analysis of particle motion for an earthquake sequence near Slate Mountain indicates that changes in S-wave splitting did not occur during that sequence. For the Mina area, comparison of S-wave signatures for 158 events occurring over a 3-year period resulted in the identification of numerous events that would be considered anomalous by Gupta's criteria, but these were not followed by larger earthquakes. The present state of knowledge on crustal structure and seismic source parameters in the western Basin and Range province is not sufficient to discriminate between stress-induced velocity anisotropy and the many other factors that contribute to the complexity of S-wave signatures.

Approximate Lg attenuation coefficient values are determined for paths in eastern North America at periods between 2 and 4 sec where no data were previously available. These new data, together with reported values at a period of 1 sec, are consistent with values predicted by a frequency-dependentent Qβ of the crust in which Qβ varies as ω0.2. The new data are inconsistent with values which would be predicted by models having prominent maxima or minima in Qβ−1 within the period range 1 to 5 sec.Lg attenuation coefficients were computed for the frequency-independent crustal Qβ models of Cheng and Mitchell (1981) for the Basin and Range Province and Colorado Plateau of the Western United States. The predicted values at 1 sec for the Colorado Plateau and Basin and Range Province are about twice as large, and three times as large, respectively, as those predicted for the Eastern United States. Recently reported values of Q for 1-sec Lg in those regions are consistent with the attenuation coefficients and Q values of Lg predicted by the Colorado Plateau and Basin and Range Province models. A model with a small degree of frequency dependence of Qβ would be consistent, but is not required by the data.

Two magnitude 3.8 earthquakes occurred south of the Wallowa Mountains in northeastern Oregon in August and September 1984. Locations of 15 aftershocks recorded on portable seismographs cluster around the Oregon-Idaho border near the mouth of the Powder River. A composite fault-plane solution indicates normal faulting on planes striking north or north-northwest, consistent with stresses observed in the Basin and Range Province to the east. The record of historical earthquakes in the region suggests periodicity with intervals of 11 to 21 yr between cycles of events consisting of several main shocks within a 3-yr time span. A magnitude 3.6 earthquake on 29 September 1981 may have begun the present cycle.

Slip vectors, derived from striations on variably oriented faults along the west and south sides of a bedrock spur north of Salt Lake City, Utah, indicate a consistent relative motion between the spur and the Salt Lake Valley during Quaternary time. The possibility of motion of coherent crustal blocks during basin and range type faulting suggests: (1) segmentation of major fault zones, such as the Wasatch fault zone, into independent crustal blocks, complicates the evaluation of earthquake hazards because of the unknown relationships between individual faults; and (2) if this pattern of crustal deformation is characteristic of the Basin and Range Province, then fault-plane solutions for this area should be carefully evaluated because they may reflect local displacements rather than the effects of the regional stress field.

A nine-station telemetered array was installed around the Boulder Basin portion of Lake Mead, Nevada-Arizona. During 112 years of monitoring, approximately 1,360 events were detected in the magnitude range −1.0 ≦ ML ≦ 2.9, and half this number were locatable. Many of the events, which ranged in depth from 0 to 13 km, can be associated with mapped faults. In particular, epicenter lineations clearly indicate activity on portions of steeply dipping faults on the east side of Boulder Basin, which generally confirms Carder's earlier work (Carder, 1945, 1948, 1970). The strike of these faults is approximately north. Focal mechanisms are in agreement with this strike and show right-lateral motion on near-vertical faults. The tension axis for this solution is oriented northwest-southeast in agreement with the stress pattern for other parts of the Basin and Range Province. Although lake load increased 20 per cent during the monitoring period, neither number of events nor energy release shows a correlation with this change. Carder (1970) found similar results for periods after 1949. A higher b value (1.45) was obtained than in the past, and lower monthly energy release was observed than in the late 1930's and 1940's when seismic activity at Lake Mead was first recorded. A 2-yr seismicity map of southern Nevada, including the monitoring period, shows that Lake Mead activity is now no greater than that of the surrounding area. The pre-Lake Mead seismic history is not well known because the founding of Boulder City occurred just a few years before Lake Mead was impounded. The existence of earthquakes in the region before the reservoir was filled has been questioned in the liteature. However, a search of the Las Vegas Review Journal for felt reports pre- and postimpoundment and compilation of a catalog of the largest events from the literature confirm that there was a significant increase in the number of felt events at Las Vegas after the reservoir was filled. The results indicate that the filling of Lake Mead has triggered release of tectonic stresses having the same orientation as the regional stress field. These stresses are probably being released in a way that is more dependent on tectonic stress buildup than on small changes in pore pressure (∼ 1 bar) due to fluctuating lake level. Given that the shear strength of rock decreases with increasing pore pressure, one might explain the decreasing energy release since the 1940's and the high b value as due to a decrease in elastic-energy density of the rock. A test of this hypothesis based on the order of magnitude of energy released seems to support it.

Microseismicity, composite focal-mechanism solutions, and previously-published focal parameter data are used to determine the current tectonic activity of the prominent zone of seismicity in western Nevada and eastern California, termed the Nevada Seismic Zone. The microseismicity substantially agrees with the historic seismicity and delineates a narrow, major zone of activity that extends from Owens Valley, California, north past Dixie Valley, Nevada. Focal parameters indicate that a regional pattern of NW-SE tension exists for the western Basin and Range and is now producing crustal extension within the Nevada Seismic Zone. An eastward shift of the seismic zone along the Excelsior Mountains and left-lateral strike-slip faulting determined from a composite focal mechanism indicate transform-type faulting between Mono Lake and Pilot Mountain. Based on these results and other data, it is suggested that the Nevada Seismic Zone is caused by the interaction of a westward flow of mantle material beneath the Basin and Range Province with the boundary of the Sierra Nevada batholith.

A program has been written for the University of Nevada's IBM 1620 computer which locates epicenters of seismic events that occur within the Nevada region. The program utilizes readings of P and S waves at up to eight stations in the Basin and Range province, rejects readings for events which are outside the region of interest or events recorded at less than three Nevada stations, establishes trial epicenters for locatable events by means of a table-lookup routine which utilized P-time differences between pairs of stations, and adjusts the trial epicenter by a least-squares procedure based on a sloping-layer crustal model derived from recent U. S. Geological Survey seismic refraction profiles. Epicenters calculated by the program are found to be affected by near-surface structural irregularities in the vicinity of receiving stations, but are not affected either by near-surface features in the source region or direction of the propagation paths with respect to regional structural trends. Traveltime delays calculated for Nevada seismographic stations appear to correlate with Bouguer gravity anomalies at the station sites.

The variation of shear velocity with depth in the upper mantle for the Basin and Range province of western North America has been studied with direct measurements of dT/dΔ for S waves in the distance range 14° < Δ < 40°. Three orthogonal components of digital data were used and onset times were determined using the product of the horizontal radial and vertical components of motion and particle motion diagrams. A linear LRSM array in Arizona was used for the measurement of dT/dΔ. An S-wave velocity distribution is derived, compatible with P-wave velocity models for the same region. The derived model consists of a thin lid zone of shear velocity 4.5 km/sec overlying a low-velocity zone and a change in velocity gradient at a depth of 160 km. Two regions of high-velocity gradient are located at depths beginning at 360 km and 620 km.

Travel times of P waves from 25 earthquakes to stations in North America, in the distance range 32° to 100°, have been analyzed by a least squares technique similar to the “time term” method of refraction seismology. In this way, deviations from the Jeffreys-Bullen tables have been separated from the effects of station and source. Differences from J-B times were calculated at 2° intervals, and a smooth curve was fitted to the results with an average scatter of 0.07 sec. The station residuals show a marked regional trend, with arrivals up to a second early in the central United States, and up to a second late in the Basin and Range Province. On the assumption that the derived travel time curve is universal in character, station residuals were calculated for stations not in the United States. Although this part of the analysis is less exact, consistent regional trends are apparent. P arrivals are early in shield areas, late in areas of recent tectonic uplift, and early along parts of the Pacific margin where deep-focus earthquakes are common. Station residuals for stations in southern California agree well with those obtained in a study by Press and Biehler. Carder9s data from nuclear explosions in the Pacific show broad agreement with the present findings, but discrepancies exist which require further investigation.

Dispersion of Rayleigh and Love waves recorded at Seoul, Hong Kong, Shillong, New Delhi and Quetta by similar long-period seismographs has been studied for the Arctic region earthquake of 25th August 1964. The entire paths from the epicenter to the recording stations are continental. Waves recorded at Shillong, New Delhi, and Quetta pass through the high mountain regions of the Himalayas and Tibet Plateau while those recorded at Seoul and Hong Kong do not pass through these high mountain regions. Observed dispersion has been compared with theoretical dispersion curves computed for a three-layer earth model. This gives an average crustal thickness of about 45 km between the epicenter and Shillong, New Delhi and Quetta, 35-37 km between the epicenter and Seoul and Hong Kong and 65-70 km in the Himalayan and Tibet Plateau regions. INTRODUCTION Crustal structure beneath various mountain ranges and plateaus has been in- vestigated by many workers. Roots of mountains were confirmed for the first time by Gutenberg (1933) from the study of travel times of longitudinal waves across the Alps in Europe. Byerly (1938) reported a delay of a few seconds across the Sierra Nevada for longitudinal waves. The roots were confirmed by Caloi (1958) for the Alps and by Gutenberg (1943) and Byerly (1956) for the Sierra Nevada. Later work leaves little doubt about the existence of roots for the Sierra Nevada. Studies have been carried out by Savarensky and Regimov (1958), Ewing and Ewing (1959), Press (1960), Cisternas (1961), Thompson and Talwani (1964) and others for different regions. Kosminskaya and Riznichenko (1964) have concluded that in Eurasia, in general, considerable thickening of the consolidated crust is observed in mountain regions. By and large, seismological work seems to substantiate the Airy hypothesis of isostasy of roots under mountains. However, in certain areas seismological data suggest that the crustal thickness is the same or even less under certain mountain systems compared to the coastal plains. Pakiser and Steinhart (1964) in their report on "Explosion Seismology in the Western Hemisphere" conclude that high moun- tains have roots at many places while at others they do not. The crust in the Sierra Nevada thickens to more than 50 km compared with about 30 km in the Basin and Range Province to the east, whereas the crust beneath the northern Rocky moun- tains is thinner than the crust in the Great Plains farther east. Our knowledge of the thickness of the crust under the Himalayas--the highest mountains in the world--and the Tibet Plateau is very limited. Stoneley (1955), Tandon and Chaudhury (1963) and Saha (1965) have attributed the higher average crustal thickness found from surface-wave dispersion studies for paths crossing the Himalayas and Tibet Plateau to greater crustal thickness beneath them, but no

The unified magnitude, the ratio of the amiplitudes of S to P waves, and travel-time residuals were compiled from published data for the five Seismological Observatories, TFO, UBO, BMO, WMO and CBO. Using one of the stations as a reference, a relative measure of the above quantities was calculated for each of the other stations for each of a number of earthquakes. The stations in the Basin and Range Province are consistent with a markedly higher attentuation of P waves and a high attenuation of S relative to P when compared to the other stations. This latter observation indicates a high Poisson's ratio in the mantle under the Basin and Range. The delay times to these stations are also consistent with the high Poisson's ratio and with a low-velocity upper mantle. The ratio of the amplitudes of long-period S waves to short-period P waves varies by a factor of 4 among these stations.BMO, in eastern Oregon, has a high S/P amplitude ratio compared to other stations and a travel-time residual that is comparable to the observatories in the mid-continent. This may be another example of a seismic “window” into the upper mantle that is generated by underthrusting of the oceanic lithosphere.