Short-period Rayleigh Waves Research Articles

Automatic determination of focal depth with the optimal period of Rayleigh wave amplitude spectra at local distances

SUMMARY Focal depth of earthquakes is essential for studies of seismogenic processes and seismic hazards. In regions with dense seismic networks, focal depth can be resolved precisely based on the traveltime of P and S, which is less feasible in case of sparse networks. Instead, surface waves are usually the strongest seismic phases at local and regional distances, and its excitation is sensitive to source depth, thus theoretically important for estimating focal depth even with a limited number of seismic stations. In this study, short-period (0.5–20 s) Rayleigh waves are explored to constrain focal depths. We observe that the optimal period (the period corresponding to the maximum amplitude) of Rayleigh waves at local distances (≤200 km) shows an almost linear correlation with focal depth. Based on this finding, we propose an automated method for resolving the focal depth of local earthquakes using the linear regression relation between the optimal period of Rayleigh wave amplitude spectra and focal depth. Synthetic tests indicate the robustness of this method against source parameters (focal mechanism, source duration and non-double-couple component) and crustal velocity structure. Although the attenuation (Q factor) of shallow crust can introduce complexities in determining focal depth, it can be simultaneously estimated if a sufficient number of stations are available. The proposed method is applied to tens of small-to-moderate earthquakes (Mw 3.5–5.0) in diverse tectonic settings, including locations in the United States (Oklahoma, South Carolina, California, Utah, etc.) and China (Sichuan, Shandong). Results demonstrate that reliable focal depth, with uncertainty of 1–2 km, can be determined even with one or a few seismic stations. This highlights the applicability of the method in scenarios characterized by sparse network coverage or historical events.

Characteristics of azimuthal anisotropy in SE Tibetan plateau and its relationship with the background of block structure

The southeastern Tibetan Plateau, which includes the Tibetan Plateau, Yangtze Block, and Cathaysia Block, is geotectonically situated in the compound part of the Tethys-Himalayan tectonic domain and the Peninsular Pacific tectonic domain. It is one of the critical regions for studying the scientific problems of plateau material lateral escape, lithosphere deformation, geotectonic properties of blocks, and deep dynamics. In this study, we use ambient noise data recorded by 401 broadband stations to obtain high-resolution short-period (T = 4–32 s) Rayleigh wave azimuthally anisotropic phase velocity maps. These could provide fresh clues for an in-depth understanding of the crust-mantle velocity structure, deformation mechanism, and geotectonic evolution in the southeastern Tibetan Plateau. Within the Simao block, the strikes of the faults and the orientations of the principal compressive stress of the stress field both generally coincide with the fast-wave polarization direction (FPD). The FPD near the Lancangjiang fault zone in the west is in the NE-SW direction, near the Wuliangshan fault zone in the center is near the NS direction, and near the Red River fault zone in the east is the NW-SE direction. We estimate that the compressive stress in the southwest direction of the Tibetan Plateau material has a controlling effect on the crustal deformation of the Simao block, which is likewise blocked by the Lincang granite belt, resulting in strong tectonic deformation. The FPD of the crust in the middle Red River fault zone is NS direction, significantly different from the fault strike. Combining with the seismic activity and GPS results, the depth of 8 km below the surface of the middle Red River fault is completely locked, and we conclude that the anisotropy of the upper crust of the middle part of the Red River fault zone is related to the action of the regional tectonic stress field. Taking into account geochemical and thermochemical results, we speculate that the complex tectonic stress at the junction of the blocks leads to prominent regional characteristics of the FPDs of azimuthal anisotropy in the crust, suggesting that the Shizong-Mile fault zone may be the western boundary between the Yangtze block and the Cathaysia block.

3-D shear wave velocity structure in the shallow crust of the Tanlu fault zone in Lujiang, Anhui, and adjacent areas, and its tectonic implications

The Tan-Lu fault zone is a large NNE-trending fault zone in eastern China. Investigations of the structures of the fault zone and its surrounding areas have attracted much attention. In this study, we used dense-array ambient noise tomography to construct a three-dimensional shear wave velocity model of shallow crust in an area about 80km × 70km in Lujiang, Anhui Province, eastern China. For approximately one month we collected continuous ambient noise signals recorded by 90 short-period seismographs in the region, and obtained the short-period Rayleigh wave empirical Green's functions between stations by the cross-correlation method; we also extracted 0.5–8 s fundamental mode Rayleigh wave group velocity and phase velocity dispersion curves. Based on the direct surface wave tomography method, we jointly inverted the group velocity and phase velocity dispersion data of all paths and obtained the 3-D shear wave velocity structure in the depth range of 0–5 km. The results revealed important geological structural features of the study area. In the north region, the sedimentary center of the Hefei Basin — the southwestern part of the Chaohu Lake — shows a significant low-velocity anomaly to a depth of at least 5 km. The southwestern and southeastern regions of the array are the eastern margin of the Dabie orogenic belt and the intrusion area of Luzong volcanic rocks, respectively, and both show obvious high-speed anomalies; the sedimentary area within the Tan-Lu fault zone (about 10 km wide) shows low-velocity anomalies. However, the volcanic rock intrusion area in the fault zone is shown as high velocity. Our shallow crustal imaging results reflect the characteristics of different structures in the study area, especially the high-speed intrusive rocks in the Tan-Lu fault zone, which were probably partially derived from the magmatic activity of Luzong volcanic basin. From the Late Cretaceous to Early Tertiary, the Tan-Lu fault zone was in a period of extensional activity; the special stress environment and the fractured fault zone morphology provided conditions for magma in the Luzong volcanic basin to intrude into the Tan-Lu fault zone in the west. Our 3-D model can also provide important information for deep resource exploration and earthquake strong ground motion simulation.

Rayleigh waves from correlation of seismic noise in Great Island of Tierra del Fuego, Argentina: Constraints on upper crustal structure

In this study, the ambient seismic noise cross-correlation technique is applied to estimate the upper structure of the crust beneath Great Island of Tierra del Fuego (TdF), Argentina, by the analysis of short-period Rayleigh wave group velocities. The island, situated in the southernmost South America, is a key area of investigation among the interaction between the South American and Scotia plates and is considered as a very seismically active one. Through cross-correlating the vertical components of ambient seismic noise registered at four broadband stations in TdF, we were able to extract Rayleigh waves which were used to estimate group velocities in the period band of 2.5–16 s using a time-frequency analysis. Although ambient noise sources are distributed inhomogeneously, robust empirical Green's functions could be recovered from the cross-correlation of 12 months of ambient noise. The observed group velocities were inverted considering a non-linear iterative damped least-squares inversion procedure and several 1-D shear wave velocity models of the upper crust were obtained. According to the inversion results, the S-wave velocity ranges between 1.75 and 3.7 km/s in the first 10 km of crust, depending on the pair of stations considered. These results are in agreement to the major known surface and sub-surface geological and tectonic features known in the area. This study represents the first ambient seismic noise analysis in TdF in order to constraint the upper crust beneath this region. It can also be considered as a successful feasibility study for future analyses with a denser station deployment for a more detailed imaging of structure.

3-D crustal structure of the western United States: application of Rayleigh-wave ellipticity extracted from noise cross-correlations

We present a new 3-D seismic model of the western United States crust derived from a joint inversion of Rayleigh-wave phase velocity and ellipticity measurements using periods from 8 to 100 s. Improved constraints on upper-crustal structure result from use of short-period Rayleigh-wave ellipticity, or Rayleigh-wave H/V (horizontal to vertical) amplitude ratios, measurements determined using multicomponent ambient noise cross-correlations. To retain the amplitude ratio information between vertical and horizontal components, for each station, we perform daily noise pre-processing (temporal normalization and spectrum whitening) simultaneously for all three components. For each station pair, amplitude measurements between cross-correlations of different components (radial–radial, radial–vertical, vertical–radial and vertical–vertical) are then used to determine the Rayleigh-wave H/V ratios at the two station locations. We use all EarthScope/USArray Tranportable Array data available between 2007 January and 2011 June to determine the Rayleigh-wave H/V ratios and their uncertainties at all station locations and construct new Rayleigh-wave H/V ratio maps in the western United States between periods of 8 and 24 s. Combined with previous longer period earthquake Rayleigh-wave H/V ratio measurements and Rayleigh-wave phase velocity measurements from both ambient noise and earthquakes, we invert for a new 3-D crustal and upper-mantle model in the western United States. Correlation between the inverted model and known geological features at all depths suggests good resolution in five crustal layers. Use of short-period Rayleigh-wave H/V ratio measurements based on noise cross-correlation enables resolution of distinct near surface features such as the Columbia River Basalt flows, which overlie a thick sedimentary basin.

On the effect of topography on surface wave propagation in the ambient noise frequency range

Due to the increasing popularity of analyzing empirical Green’s functions obtained from ambient seismic noise, more and more regional tomographical studies based on short-period surface waves are published. Results could potentially be biased in mountainous regions where topography is not small compared to the wavelength and penetration depth of the considered waves. We investigate the effect of topography on the propagation of short-period Rayleigh waves empirically by means of synthetic data using a spectral element code and a 3-D model with real topography. We show that topography along a profile through the studied area can result in an underestimation of phase velocities of up to about 0.7% at the shortest investigated period (3 s). Contrary to the expectation that this bias results from the increased surface distance along topography, we find that this error can be estimated by local topographic contrasts in the vicinity of the receiver alone. We discuss and generalize our results by considering topographic profiles through other mountain ranges and find that southern Norway is a good proxy to assess the topography effect. Nevertheless, topographic bias on phase velocity measurements is in general not large enough to significantly affect recovered velocity variations in the ambient noise frequency range.

Basin Shear-Wave Velocities beneath Seattle, Washington, from Noise-Correlation Rayleigh Waves

Tomography with short-period Rayleigh waves, extracted using noise interferometry, can refine S -wave velocity ( V S ) models in urban areas with dense arrays of short-period and broadband instruments. We apply this technique to the Seattle area to develop a new shallow V S model for use in seismic-hazard assessment. Continuous data from the Seismic Hazards in Puget Sound (SHIPS) array and local broadband stations have interstation distances of 90 km or less. This spacing allows us to extract Rayleigh waves with periods between 2 and 10 s that are sensitive to shallow-basin structure. This new V S model for the Seattle basin is constructed using direct observations rather than using P -wave velocity ( V P ) observations and a V P / V S ratio as all previous 3D models at this scale have been constructed. Our results reveal greater detail in the upper 3.5 km than previous models. Earthquake simulations calculated using our new model better predict peak ground velocities (PGV) at periods between 1 and 2 s for two local earthquakes than the previous model used to calculate Seattle’s seismic-hazard map (Frankel et al. , 2007). We collected data from two local earthquakes and ran finite-difference simulations using our new velocity model as well as the previous velocity model used in development of the Seattle seismic-hazard maps to assess how well our model predicts ground motions relative to the previous model. With a recent deployment of Netquakes strong-motion stations by the Pacific Northwest Seismic Network (PNSN) and the U.S. Geological Survey, we are now able to make more comprehensive assessments of the predictions for recent events.

Crustal Structure of the Korean Peninsula Using Surface Wave Dispersion and Numerical Modeling

Surface wave dispersion is studied to obtain the 1-D average velocity structure of the crust in the Korean Peninsula by inverting group- and phase-velocities jointly. Group velocities of short-period Rayleigh and Love waves are obtained from cross-correlations of seismic noise. Multiple-filter analysis is used to extract the group velocities at periods between 0.5 and 20 s. Phase velocities of Rayleigh waves in 10- and 50-s periods are obtained by applying the two-station method to teleseismic data. Dispersion curves of all group and phase velocities are jointly inverted for the 1-D average model of the Korean Peninsula. The resultant model from surface wave analysis can be used as an initial model for numerical modeling of observations of North Korean events for a velocity model appropriated to the Korean Peninsula. The iterative process is focused especially on the surface sedimentary layer in the numerical modeling. The final model, modified by numerical modeling from the initial model, indicates that the crust shear wave velocity increases with depth from 2.16 km/s for a 2-km-thick surface sedimentary layer to 3.79 km/s at a Moho depth of 33 km, and the upper mantle has a velocity of 4.70 km/s.

The M5.0 Suining-Tongnan (China) earthquake of 31 January 2010: A destructive earthquake occurring in sedimentary cover

Usually, only crystalline basement is strong enough to store the massive strain energy that can be released in a damaging earthquake. By contrast, sedimentary cover is weak, because of its relatively high porosity and fluids. Therefore, it generally cannot accumulate enough energy for strong earthquakes. On January 31, 2010, a M5.0 earthquake occurred near the border of Suining and Tongnan in China. It excited strong short-period Rayleigh waves Rg, indicative of its shallow focal depth. The focal depth is constrained to less than 4 km, most probably in the range of 1–3 km, by modeling amplitude dependence on the frequency and waveforms of teleseismic depth phases (pP, sP). Because the local Mesozoic sedimentary cover is about 6 km thick, this earthquake should have occurred in the sedimentary cover. Though some shallow earthquakes with magnitudes up to M4 occur in Paleozoic sediments, this earthquake is the first M5 event studied in Mesozoic sedimentary cover. This event provides a rare opportunity to study seismogenic processes of damaging earthquakes in sedimentary basins.

Multiscale Ambient Noise Tomography of Short-Period Rayleigh Waves across Northern Taiwan

Abstract Using the ambient noise cross-correlation technique, we derived empirical Green’s functions (EGFs) of short-period Rayleigh waves from the continuous vertical component of data recorded by 34 seismic stations in northern Taiwan. We measured the group velocity of the EGFs in the frequency range of 0.2–0.5 Hz. The derived EGFs and group velocity measurements were carefully examined, and only quality results were used for the tomographic inversion. We implemented a wavelet-based multiscale inversion technique to construct the group velocity map for 0.35-Hz Rayleigh waves in northern Taiwan. The resulting model shows excellent correlation to the surface topography and geological units. To compare with models derived from traditional body-wave tomography, we calculated the group velocity maps and their fit to the noise-derived EGFs for two recent body-wave models. The comparison indicates that body-wave velocities, particularly V S , at shallow depth in the west coast plain are overestimated in two recent body-wave-derived models, and/or a much higher V P / V S ratio may exist for this area. Our results suggest that the noise-derived short-period EGFs may provide important constraints on shallow Earth structure, in complement to that provided by body waves.

Short-Period Rayleigh-Wave Group Velocity Tomography through Ambient Noise Cross-Correlation in Xinjiang, Northwest China

Abstract High-quality, short-period Rayleigh-wave group velocity tomography is conducted through ambient noise cross-correlation among 54 stations in the newly upgraded Xinjiang Provincial Digital Seismic Network in northwest China. The velocity maps obtained in this study give more details in most parts of Xinjiang than previous surface wave studies that mostly focused on large scale patterns in Eurasia, Asia, or the whole of China. The tomography results have a high correlation with the major tectonic and geological features in the study area. In general, low velocities are associated with the two basins, and high velocities are associated with the two mountain belts in the Xinjiang area. The southernmost part of the Tianshan mountains has the same velocity structure as that of the neighboring part of the Tarim Basin, implying that they may have the same crustal composition. The velocity in the Tarim Basin is spatially inhomogeneous, due to the fact that it comprises several sub-blocks that have different thickness of sediments and crustal layers. In the Dzungaria Basin, as the period becomes longer, the low-velocity zone shrinks toward the south, reflecting the fact that the sediments and the upper crust in the southern part of the Dzungaria Basin are thicker than in the northern part.

Shallow-depth shear wave velocity structure of the Southern Korean Peninsula obtained from two crustal-scale refraction profiles

Short-period Rayleigh waves from the two crustal-scale seismic refraction profiles, KCRT2002 and KCRT2004, were analyzed to determine the shear wave velocity and attenuation structure of the uppermost crust in different tectonic regions of the Korean peninsula and to examine if this can be related to the surface geology and tectonics of the study area. The refraction profiles were obtained using large explosive sources along a 294-km WNW–ESE line in 2002 and a 335-km NNW–SSE line in 2004. The two refraction profiles, recorded on 2-Hz portable seismometers, contained Rayleigh waves in the period range of 0.2 to 1.2 s, and the distance range up to 30–60 km from the sources. The profiles, which traverse four tectonic regions (Gyeonggi massif, Okcheon Fold belt, Yeongnam massif and Gyeongsang basin), were divided into twelve subsections based on the tectonic boundary and lithology. We obtained shear wave velocity models in the upper 1.5 km of the crust. The average shear wave velocity of the twelve subsections increases from ~ 2.4 km/s at the surface to ~ 3.2 km/s at 1.5 km depth. Overall, the shear wave velocities for the Okcheon fold belt and Gyeongsang basin are lower than those for the Gyeonggi and Yeongnam massifs by ~ 0.3 km/s. Shear wave velocity differences also exist among the subsections within the same tectonic region when there are distinct differences in lithology. We obtained the values of Q β − 1 in the upper 0.6 km. Q β − 1 for the Okcheon fold belt (~ 0.025) is approximately twice larger than Q β − 1 for the massif areas (~ 0.012). The low shear wave velocity and high attenuation for the Okcheon fold belt seem to be related to the rock types of the area, which are mostly conglomerates and phyllites.

Effects of Seawater of Tokyo Bay on Short-Period Strong Ground Motion

The effects of the seawater of Tokyo Bay on strong ground motions are investigated in a short-period range of less than about 2.0 sec for engineering importance. A recent study based on observed accelerograms revealed that considerable short-period surface waves were included in strong ground motions in the reclaimed zone of Tokyo, and that those surface waves were mainly Love waves. However, it was not clarified whether short-period Rayleigh waves were present. The present study is aimed at assessing the excitation of short-period Rayleigh waves on land and at exploring the effects of the shallow (approximately 25 m) water layer of Tokyo Bay on Rayleigh waves in the reclaimed zone of Tokyo. We use a theoretical approach in which a 2D P-SV -wave field is calculated for a multilayered structural model. The model is composed of a sea zone and a land zone, and includes a water layer and soft-surface layers. To examine the effects of seawater, we use models without and with a water layer. As a result, it is concluded that in the reclaimed zone of Tokyo, stripping the water layer off of a typical model for shallow Tokyo Bay has little influence on the short-period ground motions around the theoretical predominant period of the ground of about 1.0 sec. It is not essential to incorporate the water layer in modeling shallow Tokyo Bay for short-period, ground-motion simulations.

Imaging the Upper Crust of the Korean Peninsula by Surface-Wave Tomography

Cross correlation of seismic-background motions (Campillo and Paul, 2003; Shapiro et al. , 2005) is applied to observations from the Korean Meteorological Administration seismic network to estimate the short-period Rayleigh and Love wave dispersion characteristics of the region. Standard processing procedures are applied to the cross correlation, except that signal whitening is used in place of one-bit sampling to equalize power in signals from different times. Multiple-filter analysis is used to extract the group velocities from the estimated Green’s functions, which are then used to image the spatially varying dispersion at periods between 0.5 and 20 sec. The tomographic inversion technique used inverts all periods simultaneously to provide a smooth dispersion curve as a function of period in addition to the usual smooth spatial image for a given period. The Gyeongsang Basin in the southeastern part of the peninsula is clearly resolved with lower group velocities.

Shear wave velocity and attenuation structure for the shallow crust of the southern Korean peninsula from short period Rayleigh waves

We analyzed the short period Rayleigh waves from the first crustal-scale seismic refraction experiment in the Korean peninsula, KCRUST2002, to determine the shear wave velocity and attenuation structure of the uppermost 1 km of the crust in different tectonic zones of the Korean peninsula and to examine if this can be related to the surface geology of the study area. The experiment was conducted with two large explosive sources along a 300-km long profile in 2002. The seismic traces, recorded on 170 vertical-component, 2-Hz portable seismometers, show distinct Rayleigh waves in the period range between 0.2 s and 1.2 s, which are easily recognizable up to 30–60 km from the sources. The seismic profiles, which traverse three tectonic regions (Gyeonggi massif, Okcheon fold belt and Yeongnam massif), were divided into five subsections based on tectonic boundaries as well as lithology. Group and phase velocities for the five subsections obtained by a continuous wavelet transform method and a slant stack method, respectively, were inverted for the shear wave models. We obtained shear wave velocity models up to a depth of 1.0 km. Overall, the shear wave velocity of the Okcheon fold belt is lower than that of the Gyeonggi and Yeongnam massifs by ∼ 0.4 km/s in the shallowmost 0.2 km and by 0.2 km/s at depths below 0.2 km. Attenuation coefficients, determined from the decay of the fundamental mode Rayleigh waves, were used to obtain the shear wave attenuation structures for three subsections (one for each of the three different tectonic regions). We obtained an average value of Q β − 1 in the upper 0.5 km for each subsection. Q β − 1 for the Okcheon fold belt (∼ 0.026) is approximately three times larger than Q β − 1 for the massif areas (∼ 0.008). The low shear wave velocity in the Okcheon fold belt is consistent with the high attenuation in this region.

An Automated Short-Period Surface-Wave Detection Algorithm

We describe an automated short-period Rayleigh wave ( Rg ) detector designed to work on local (<2.5° epicentral distance) events recorded at three- component stations. The detector was modeled after an automatic 17- to 22-sec Rayleigh-wave detection method; however, we have modified the algorithms for local distance and short-period applications. We have tested the detector on a well-located cluster of mining events from central India and on a set of ground-truth events in the area. The Rg detector was also integrated into a semiautomatic event detection and location algorithm and applied on continuous data. Fourier and wavelet-based methods are evaluated for prefiltering. We observe that sample standard deviations of backazimuth estimates using the Rg detector, after wavelet prefiltering, are comparable to fk 3C P backazimuth estimates from event clusters. Our results indicate that using the Rg -phase backazimuths for event location is a promising alternative to using small signal-to-noise ratio first-arrival backazimuths. We recommend wavelet prefiltering versus Fourier prefiltering because it is more consistent for the detection of low signal-to-noise ratio events at local distances.

Analysis of Short-Period Rayleigh Waves Recorded in the Bohemian Massif Area During Celebration 2000 Experiment

The aim of this paper is to show the application of short-period surface waves recorded during deep seismic sounding experiment for constraining shallow velocity structure of the crust. Phase velocity of fundamental mode Rayleigh waves, observed along the CELEBRATION 2000 experiment profile CEL09, were obtained by a p-ω method and has been subsequently inverted for one-dimensional shear velocity models for the top 2 km. Multiple filter technique applied to one shot gather was used to carry out a joint inversion of phase and group velocity data and to provide γR data to be used for Qβ inversion. Validity of obtained VS and Qβ models was confirmed by the reflectivity method. Noticeably, no clear dispersive wawes were observed in the Tepla-Barrandian Unit. Quasi-2D model based on the individual 1D VS models is well correlated with the surface geology. Lower VS are observed in the Saxothuringian Zone in comparison to the Moldanubian Zone. In the vicinity of the Central Bohemian and Moldanubian Plutons, the near-surface VS values are relatively low, but below 1 km depth, they are higher than in surrounding areas. We interpret it as the result of the weathering and cracks within the granitoid rocks.

Analytical partial derivatives of the phase- and group velocities for Rayleigh waves propagating in a layer on a half-space

The dispersion relation for Rayleigh waves in a layer on a half-space is modified by introducing the quadratic wave number instead of the phase velocity. The implicit function theorem is then used to derive analytical formulae for the group velocity and for the phase- and group velocity partial derivatives with respect to the parameters of the medium. As two examples, the method is applied to the interpretation of dispersion curves for short-period Rayleigh waves observed in a sedimentary basin of the West Carpathians, and in the Moravo-Silesian region, Czech Republic.

Quantitative analysis of the tremor wavefield at Etna Volcano, Italy

The properties of volcanic tremor wavefield at Mt. Etna Volcano, Italy, are investigated using data from two dense, small-aperture arrays of short-period seismometers deployed on the North and South flank of the volcano. Spectral analysis shows that most of the seismic energy is associated to several, narrow spectral peaks spanning the 1–5 Hz frequency band. Analysis of simultaneous recordings evidences that most of these peaks are common to different sites, thus suggesting a source effect as the origin of this energy. Frequency-slowness analyses evidence a complex wavefield, where body- and surface-waves alternatively dominate depending on the frequency band and component of motion taken into account. Surface waves are found to dominate at frequencies below 1 Hz and above 3 Hz. Conversely, the 0.8–2.3 Hz vertical- and radial-component wavefields at both arrays exhibit a nondispersive nature, with apparent velocities spanning the 1–2 s/km range. Particle motion analysis suggests these arrivals are associated to both P- and SV-waves inciding at shallow angles. At the northern array, back-azimuths of these waves encompass the whole summit crater area. At the southern array, back-azimuths are instead clustered around a direction pointing about 500 m east of the SE crater. At frequency around 4 Hz, the dominant direction of wave propagation at the southern site shifts about 30°W, pointing to the Bocca-Nuova/Voragine craters, and concordance of location is found with the source imaged by the northern array. The 0.8–2.3 Hz transverse-component of motion depicts velocities of about 0.5 km/s, a value which is about three times lower than those associated to the vertical and radial components. Results from polarization analyses at the two array sites depict the dominance of horizontal, linear particle motion oriented transversally with respect to the source direction. Polarization ellipsoids at the stations of the sparse network all depict a quasi-horizontal setting. With two exceptions, the direction of particle motion is always oriented tangentially to the summit volcanic edifice. The origin of the large transverse motion observed at the two array sites is thus attributed to SH waves generated by free-surface interaction of waves impinging the concave topography. The correlation method is used to derive the dispersion properties of short-period (0.5–5 Hz) Rayleigh waves, from which the shallow shear-wave velocity structures are derived for beneath the two semicircular arrays. Using a probabilistic approach, we invert slowness data measured at the two dense arrays for retrieving source location and extent. The joint inversion of slowness data from the two arrays point to different sources. This observation is interpreted in terms of ray bending associated to lateral heterogeneity and/or strong topographic effects on wave propagation. Once the propagation effects are taken into account, the most probable source locations are associated to a shallow region encompassing the summit craters and the eruptive fissures active at the time of the experiment (September 1999).

Evaluation of Short-Period, Near-Regional Ms Scales for the Nevada Test Site

Surface wave magnitude (M_s) estimation for small events recorded at near-regional distances will often require a magnitude scale designed for Rayleigh waves with periods less than 10 sec. We have examined the performance of applying two previously published M_s scales on 7-sec Rayleigh waves recorded at distances less than 500 km. First, we modified the Marshall and Basham (1972) M_s scale, originally defined for periods greater than 10 sec, to estimate surface wave magnitudes for short-period Rayleigh waves from earthquakes and explosions on or near the Nevada Test Site. We refer to this modification as ^(M+B) M_s(7), and we have used short-period, high-quality dispersion curves to determine empirical path corrections for the 7-sec Rayleigh waves. We have also examined the performance of the Rezapour and Pearce (1998) formula, developed using theoretical distance corrections and surface wave observations with periods greater than 10 sec, for 7-sec Rayleigh waves ^(R+P) (M_S(7)) as recorded from the same dataset. The results demonstrate that both formulas can be used to estimate M_s for nuclear explosions and earthquakes over a wider magnitude distribution than is possible using conventional techniques developed for 20-sec Rayleigh waves. These M_s(7) values scale consistently with other Ms studies at regional and teleseismic distances with the variance described by a constant offset; however, the offset for the ^(M+B) M_s(7) estimates is over one magnitude unit nearer the teleseismic values than the ^(R+P) M_s(7) estimates. Using our technique, it is possible to employ a near-regional single-station or sparse network to estimate surface wave magnitudes, thus allowing quantification of the size of both small earthquakes and explosions. Finally, we used a jackknife technique to determine the false-alarm rates for the ^(M+B) M_s(7)-m_b discriminant for this region and found that the probability of misclassifying an earthquake as an explosion is 10%, while the probability of classifying an explosion as an earthquake was determined to be 1.2%. The misclassification probabilities are slightly higher for the ^(R+P) M_s(7) estimates. Our future research will be aimed at examining the transportability of these methods.