Interstation Paths Research Articles

S-wave velocity structure of northeastern China from joint inversion of Rayleigh wave phase and group velocities

SUMMARY We imaged the crust and uppermost mantle structure beneath northeastern (NE) China with fundamental mode Rayleigh waves recorded by 125 broad-band stations deployed in the region. Rayleigh wave phase and group velocities along more than 700 interstation paths were estimated using the wavelet transformation method and then these data were utilized to construct 2-D phase and group velocity maps in the period range of 15–60 s. Owing to the dense ray coverage and short ray path, our results provide better lateral resolution in the NE China region compared with previous phase and group velocity studies. The regularized Rayleigh wave phase and group velocity dispersions at each cell were jointly inverted to determine 1-D shear wave velocity structure using the linear inversion method and then assembled into a 3-D model. Our results show that obvious low velocities exist in the uppermost mantle beneath the Changbaishan volcanic region, which may be due to asthenospheric upwelling. The thin lithosphere with fast S-wave velocity in the lower crust of the Songliao Basin implies that the lithospheric mantle beneath NE China is partly removed.

Crustal and uppermost mantle velocity structure beneath northwestern China from seismic ambient noise tomography

SUMMARY In this paper, we conduct ambient noise seismic tomography of northwestern China and adjacent regions. The data include 9 months (2009 January to 2009 September) three-component continuous data recorded at 146 seismic stations of newly upgraded China Provincial Digital Seismic Networks and regional Kyrgyzstan and Kazakhstan networks. Empirical Rayleigh and Love wave Green's functions are obtained from interstation cross-correlations. Group velocity dispersion curves for both Rayleigh and Love waves between 7 and 50 s periods were measured for each interstation path by applying the multiple-filter analysis method with phase-matched processing. The group velocity maps show clear lateral variations which correlate well with major geological structures and tectonic units in the study region. Shear wave velocity structures are inverted from Rayleigh wave and love wave dispersion maps. The results show that the Tibetan Plateau has a very thick crust with a low-velocity zone in its mid-lower crust. Along the northern margin of the plateau where a steep topographic gradient is present, the low-velocity zone does not extend to the Tarim basin which may indicate that crustal materials beneath the Tarim basin are colder and stronger than beneath the plateau, therefore inhibit the extension of mid-lower crustal flow and deformation of the Tibetan Plateau, resulting in very sharp topography contrasts. In the northeastern margin with a gentle topographic gradient toward the Ordos platform, the low-velocity zone diminishes around the eastern KunLun fault. Meanwhile, our results reveal obvious lateral velocity changes in the crust beneath the Tarim basin. In the upper crust, the Manjaer depression in the eastern Tarim basin is featured with very low velocities and the Bachu uplift in the western Tarim basin with high velocities; in the mid-lower crust, the northern Tarim basin in general displays lower velocities than the southern part along latitude ∼40° N with an east–west striking, which is consistent with the high magnetic anomaly zone and may be related to the central suture belt connecting the south and north of Tarim basement blocks together in Pre-Sinian.

Lg Attenuation Characteristics across the Indian Shield

Abstract New measurements of Lg Q are obtained using a reliable two-station method to understand the attenuation characteristics across the Indian subcontinent. We use data from 514 events with magnitude≥3.5 and depth less than 40 km collected during 2006–2008 from 17 stations deployed across India. We measure 1-Hz Lg Q ( Q 0 ) values between many pairs of stations. Finally, 34 high-quality interstation paths were selected from 336 possible pairs that are (1) aligned approximately with the source and (2) separated enough to permit the use of the standard two-station method for Lg Q measurement. We found some spatial variation in Lg wave attenuation across India. Low- Q regions are present in western India. We attribute this to the recent tectonic activities in this region. The central Indian platform shows a moderate to high Q 0 value. The southernmost part of India exhibits higher values, which may indicate that the crust in this part is more stable. The obtained values correlate with the regional crustal heterogeneities and tectonic trends. As a whole, the estimated interstation Q 0 values compare well with the areas of moderate seismicity across the Earth.

Effect of earthquakes on ambient noise cross-correlation function

Surface wave tomography method based on analysis of ambient noise is widely used during the last decade. It is assumed that correlated component of noise is composed of surface waves generated by sources distributed over the Earth’s surface more or less uniformly. In such a case the cross-correlation function (CCF) at two stations may be considered as the Green’s function of surface wave. This function should be symmetric relatively to zero time. However analysis of CCF at the stations located at the East-European Platform shows that as a rule CCF is characterized with a strong asymmetry. Since “purered noise cannot be extracted from seismic records due to superposition of earthquake signals, the method for calculation of CCF includes amplitude normalization for suppression of earthquakes that reduces signals from earthquakes to a noise level. The parts of records containing waves from earthquakes are neglected because of their short duration. Present study shows that this contribution turns out to be dominant at periods larger than 20–40 s. In other words, what is assumed as a “noisered in reality is a superposition of signals from earthquakes. This fact results in distortion of the Green’s function and of surface wave dispersion curve used in surface wave tomography if in the time interval used for calculation of CCF many earthquakes occur within a small area apart of an extension of the interstation path (clustering). Numerical modeling shows how clusters of sources affect CCF and dispersion curve correspondingly. Means for reducing this effect are outlined.

Velocity structure of the upper mantle of the East European platform according to seismic noise data

Crosscorrelation functions of noise are construct ed on 119 interstation paths from seismic noise records at stations of Eastern Europe. Dispersion curves of the group velocity of Rayleigh waves obtained from the crosscorrelation functions are used for constructing the threedimensional distribution of the velocity of transverse waves on the East European platform and in adjacent regions by methods of surfacewave tomog� raphy. The mean velocity in the crust is minimum in the region of the Caspian depression and Black Sea basin ( 3.7 km/s). The upper mantle beneath the Baltic and Ukrainian shields is characterized by increased velocity and the absence of the asthenospheric layer. Reduced velocities are noted in the upper mantle of the Black Sea basin. A lowvelocity anomaly in the shape of a ver� tical column is revealed at depths of 200 −300 km in the central part of the Dnieper-Donets aulacogen, which confirms the existence of a paleorift in this region.

Rayleigh wave phase velocity maps of Tibet and the surrounding regions from ambient seismic noise tomography

Ambient noise tomography is applied to the significant data resources now available across Tibet and surrounding regions to produce Rayleigh wave phase speed maps at periods between 6 and 50 s. Data resources include the permanent Federation of Digital Seismographic Networks, five temporary U.S. Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) experiments in and around Tibet, and Chinese provincial networks surrounding Tibet from 2003 to 2009, totaling ∼600 stations and ∼150,000 interstation paths. With such a heterogeneous data set, data quality control is of utmost importance. We apply conservative data quality control criteria to accept between ∼5000 and ∼45,000 measurements as a function of period, which produce a lateral resolution between 100 and 200 km across most of the Tibetan Plateau and adjacent regions to the east. Misfits to the accepted measurements among PASSCAL stations and among Chinese stations are similar, with a standard deviation of ∼1.7 s, which indicates that the final dispersion measurements from Chinese and PASSCAL stations are of similar quality. Phase velocities across the Tibetan Plateau are lower, on average, than those in the surrounding nonbasin regions. Phase velocities in northern Tibet are lower than those in southern Tibet, perhaps implying different spatial and temporal variations in the way the high elevations of the plateau are created and maintained. At short periods (<20 s), very low phase velocities are imaged in the major basins, including the Tarim, Qaidam, Junggar, and Sichuan basins, and in the Ordos Block. At intermediate and long periods (>20 s), very high velocities are imaged in the Tarim Basin, the Ordos Block, and the Sichuan Basin. These phase velocity dispersion maps provide information needed to construct a 3‐D shear velocity model of the crust across the Tibetan Plateau and surrounding regions.

Seismic noise tomography in the Chile ridge subduction region

SUMMARY We used cross-correlation of ambient seismic noise recorded in the Chile Triple Junction (CTJ) region to estimate interstation surface wave time-domain Green’s functions, and then inverted traveltimes to obtain crustal surface wave velocity models. Interstation distances within the Chile Ridge Subduction Project (CRSP) temporary seismic network ranged from 40 to ∼100 km. We selected 365 d, and cross-correlated and stacked 24 hr of vertical component data at 38 stations pairs, resulting in nominally 703 traveltimes along assumed-straight interstation paths. Velocities in 2-D cells of 30 km × 30 km were calculated using a linear least-squares inversion of the Rayleigh wave group velocity traveltimes. Furthermore we performed a Rayleigh wave group velocity dispersion analysis to estimate the sensitivity of different period waves at depth and to calculate a 3-D shear velocity model of the Patagonian crust. The process was applied to cross correlation pairs determined in two period bands, 5–10 s, corresponding to shallow crustal velocities down to approximately 10 km depth, and 10–20 s, for velocities down to around 20 km. Our results show that cell velocities correlate well with known geological features. We find high-crustal velocities where the Patagonian Batholith outcrops or is likely present at depth, and low velocities correlate with the active volcanic arc of the Southern Volcanic Zone and the subducted Chile ridge in Taitao peninsula, where thermal activity of hot springs is present. High velocities in the mountainous portions of the southeastern study area appear to correlate with outcropping older metamorphic units. Low velocity in the east correlate with sequences of volcaniclastic deposits.

Approximations in seismic interferometry and their effects on surface waves

SUMMARY We investigate common approximations and assumptions in seismic interferometry. The interferometric equation, valid for the full elastic wavefield, gives the Green's function between two arbitrary points by cross-correlating signals recorded at each point. The relation is exact, even for complicated lossless media, provided the signals are generated on a closed surface surrounding the two points and are from both unidirectional point-forces and deformation-rate-tensor sources. A necessary approximation to the exact interferometric equation is the use of signals from point-force sources only. Even in simple layered media, the Green's function retrieval can then be imperfect, especially for waves other than fundamental mode surface waves. We show that this is due to cross terms between different modes that occur even if a full source boundary is present. When sources are located at the free surface only, a realistic scenario for ambient noise, the cross terms can overwhelm the higher mode surface waves. Sources then need to be very far away, or organized in a band rather than a surrounding surface to overcome this cross-term problem. If sources are correlated, convergence of higher modes is very hard to achieve. In our examples of simultaneously acting sources, the phase of the higher modes only converges correctly towards the true solution if sources are acting in the stationary phase regions. This offers an explanation for some recent body wave observations, where only interstation paths in-line with the prevailing source direction were considered. The phase error resulting from incomplete distributions around the stationary phase region generally leads to an error smaller than 1 per cent for realistic applications.

Lg Attenuation near the North Korean Border with China, Part I: Model Development from Regional Earthquake Sources

We have collected broadband data from three seismic networks—two portable and one permanent—in a remote part of northeast China bordering North Korea. The combined network coverage allows first-hand investigations of seismic- wave propagation in a region of import for nuclear test ban monitoring research. Application of the reversed two-station method to the newly acquired data has pro- duced stable measurements of the quality factor Q ( Q0f η ) for Lg waves. The re- sults, a weighted regional average over 23 interstation paths involving 51 events and 21 stations, point to 1 Hz Q (or Q0) of 345 and η of 0.38 in a jarring incongruity with what appears to be a popular perception of the region as being of very low attenuation for Lg waves. The results also show that Lg Q varies from one interstation path to another. The highest Lg-wave attenuation (110 <Q 0 < 140) is found along intersta- tion paths that crossed two recently active volcanoes (Huangyishan and Chang- baishan). The subregion exhibiting the next highest Lg attenuation is Bohai, an extensional basin known to feature above-average heat flow.

Stratified seismic anisotropy reveals past and present deformation beneath the East-central United States

Evolution of continental lithosphere during orogenies and the following periods of relative stability is poorly understood, largely because of the lack of relevant observational constraints. Measurements of seismic anisotropy provide such constraints, but due to limitations in the resolving power of available data sets and, more generally, of various data types, detailed mapping of lithospheric anisotropy has remained elusive. Here we apply surface-wave array analysis to data from the East-central U.S. and determine the layering of azimuthal anisotropy beneath the Grenville–Appalachian orogen in the entire lithosphere–asthenosphere depth range. Combined measurements of Rayleigh-wave phase velocities along 60 interstation paths constrain phase-velocity maps with statistically significant anisotropy. Distinct anisotropy patterns in three different period ranges point to the existence of three distinct layers beneath the orogen, with different anisotropic fabric within each. We invert phase-velocity maps and, alternatively, pairs of selected measured dispersion curves for anisotropic shear-velocity structure. The results confirm that three anisotropic layers with different fabric within each are present, two in the lithosphere (30–70 km; 70–150 km depths) and another in the asthenosphere beneath (> 150 km). Directions of fast wave propagation in the upper lithosphere are parallel to the Grenville and Appalachian fronts, suggesting that the region-scale anisotropy pattern reflects the pervasive deformation of the lower crust and uppermost mantle during the continental collisions. The fast-propagation azimuth within the lower lithosphere is different, parallel to the NNW direction of North America's motion after the orogeny (~ 160–125 Ma). This suggests that the lithosphere, 70-km thick by the end of the Appalachian orogeny, gradually thickened to the present 150-km while inheriting the fabric from the sheared asthenosphere below, as the plate moved NNW. Below 150 km, the fast-propagation direction is parallel to the present plate motion, indicating fabric due to recent asthenospheric flow. Anisotropy in narrower depth ranges beneath the region has been sampled previously. Published results (from observations of P n and SKS and waveform tomography) can be accounted for and reconciled by the three-layered model of anisotropy for the lithosphere–asthenosphere depth range constrained in this study. In particular, the anisotropy we detect in the asthenosphere can account for the magnitude of SKS-wave splitting, with the fast wave-propagation directions inferred from SKS and surface-wave data also consistent, both parallel to the current plate motion.

Surface wave tomography of the western United States from ambient seismic noise: Rayleigh and Love wave phase velocity maps

SUMMARY We present the results of Rayleigh wave and Love wave phase velocity tomography in the western United States using ambient seismic noise observed at over 250 broad-band stations from the EarthScope/USArray Transportable Array and regional networks. All available threecomponent time-series for the 12-month span between 2005 November 1 and 2006 October 31 have been cross-correlated to yield estimated empirical Rayleigh and Love wave Green’s functions. The Love wave signals were observed with higher average signal-to-noise ratio (SNR) than Rayleigh wave signals and hence cannot be fully explained by the scattering of Rayleigh waves. Phase velocity dispersion curves for both Rayleigh and Love waves between 5 and 40 speriod were measured for each interstation path by applying frequency‐time analysis. The average uncertainty and systematic bias of the measurements are estimated using a method based on analysing thousands of nearly linearly aligned station-triplets. We find that empirical Green’s functions can be estimated accurately from the negative time derivative of the symmetric component ambient noise cross-correlation without explicit knowledge of the source distribution. The average traveltime uncertainty is less than 1 s at periods shorter than 24 s. We present Rayleigh and Love wave phase speed maps at periods of 8, 12, 16,and 20 s. The maps show clear correlations with major geological structures and qualitative agreement with previous results based on Rayleigh wave group speeds.

A Study of the Regional Variation of Low-Frequency around the Korean Peninsula

We studied regional ![Graphic][1] at 1 Hz (![Graphic][2] ) around the Korean Peninsula based on broadband vertical component seismic records of six Incorporated Research Institutions for Seismology (IRIS) Global Seismographic Network stations and 19 Korean stations of the Korea Institute of Geoscience and Mineral Resources (KIGAM). Using 177 seismic events with magnitudes between 5.3 and 5.7 and depths less than 50 km, the reversed two station method was applied, and 94 high quality interstation paths were selected from 869 possible pairs. These results show high ![Graphic][3] paths around the Sea of Japan (East Sea) reflecting the typical oceanic structure and low ![Graphic][4] paths around northeastern China related to inactive seismicity. Assigning these path values into 193 cells around South Korea with a size of 1° by 1°, we observed that the regional ![Graphic][5] decreased gradually from east to west between 2 and 1×10-3. [1]: /embed/inline-graphic-2.gif [2]: /embed/inline-graphic-3.gif [3]: /embed/inline-graphic-4.gif [4]: /embed/inline-graphic-5.gif [5]: /embed/inline-graphic-6.gif

Surface wave tomography of the western United States from ambient seismic noise: Rayleigh wave group velocity maps

We have applied ambient noise surface wave tomography to data that have emerged continuously from the EarthScope USArray Transportable Array (TA) between October 2004 and January 2007. Estimated Green's functions result by cross‐correlating noise records between every station‐pair in the network. The 340 stations yield a total of more than 55,000 interstation paths. Within the 5‐ to 50‐s period band, we measure the dispersion characteristics of Rayleigh waves using frequency‐time analysis. High‐resolution group velocity maps at 8‐, 16‐, 24‐, 30‐, and 40‐s periods are presented for the western United States. The footprint of the TA encloses a region with a resolution of about the average interstation spacing (∼70 km). Velocity anomalies in the group velocity maps correlate well with the dominant geological features of the western United States. Coherent velocity anomalies are associated with the Sierra Nevada, Peninsular, and Cascade Ranges, Great Valley, Salton Trough, and Columbia basins, the Columbia River flood basalts, the Snake River Plain and Yellowstone, and mantle wedge features associated with the subducting Juan de Fuca plate.

Tomographic regionalization of crustal Lg Q in eastern Eurasia

We measure Q of crustal Lg waves in eastern Eurasia using the reliable two‐station methods. More than 5,000 spectral ratios are collected over 594 interstation paths, and are used to determine values of Lg Q0 and η (Q at 1 Hz and its frequency dependence) over these paths. These values are used to derive tomographic models of laterally varying Q0 and η with resolutions between 4 and 10°. The Q0 model contains values that vary between 100 and 900. Q0 are the lowest in the Tibetan plateau, increase to moderate levels towards the east and north, and reach maxima in the Siberian and eastern Europe Cratons. Q0 values correlate well with regional tectonics. Most η values are between −0.14 and 0.50, with a mean of 0.18. These values are lower than those obtained previously using either an Lg coda Q method, or a method of simultaneous inversion of source spectra and Q.

High-Resolution Surface-Wave Tomography from Ambient Seismic Noise

Cross-correlation of 1 month of ambient seismic noise recorded at USArray stations in California yields hundreds of short-period surface-wave group-speed measurements on interstation paths. We used these measurements to construct tomographic images of the principal geological units of California, with low-speed anomalies corresponding to the main sedimentary basins and high-speed anomalies corresponding to the igneous cores of the major mountain ranges. This method can improve the resolution and fidelity of crustal images obtained from surface-wave analyses.

Shear-wave velocity structure beneath North Island, New Zealand, from Rayleigh-wave interstation phase velocities

Fundamental-mode Rayleigh-wave dispersion velocities have been determined for interstation paths along the strike of the Hikurangi Margin in eastern North Island, New Zealand, for the period range 17 to 73 s. We invert the observed surface-wave dispersion velocities to show S-wave velocities of 4.2–4.4 km s−1 (Vp∼ 7.5–7.8 km s−1) within a subducting crust fixed at 12 km thickness, overlying an upper-mantle lid with S-wave velocities between 4.7 and 4.9 km s−1 (Vp∼ 8.3–8.7 km s−1), some 40 to 20 km, thick respectively. The inherent non-uniqueness in the inversion controls the surface-wave resolution at these depths to a minimum layer thickness and thickness increment of about 20 km. Earlier body-wave studies divide this upper-mantle lid into a 10 km thick layer of normal mantle velocities (Vp∼ 8.2 km s−1) overlying a very high-velocity layer (Vp∼8.9 km s−1). The implication from this surface-wave study is that this latter layer cannot be much thicker than about 10 km. The lithospheric thickness of the subducting Pacific plate is of the order of 30 to 50 km, which is at the lower end of the thicknesses found elsewhere for >100 Ma oceanic lithosphere. The depth to the S-wave upper-mantle low-velocity zone of approximately 60 to 80 km correlates with the maximum depth of seismicity along the belt. The thickness and S-wave velocity of the low-velocity zone is controlled by the inversion to be 60–80 km thick with a velocity of 4.6± 0.1 km s−1 (Vp∼ 8.2± 0.15 km s−1). Anisotropy of oriented olivine has recently been used to explain the presence of high velocities in the subducted slab's uppermost mantle in eastern North Island. However, large SKS splitting delays require the anisotropy to be more extensive in depth. We have carried out inversion of Rayleigh-wave interstation phase-velocity dispersion data assuming an anisotropic mantle with pseudo-hexagonal symmetry. Results indicate that mantle velocities derived from the isotropic inversion are overestimates of the S-wave velocity in the vertical plane of the anisotropic structure, perpendicular to the symmetry axis, by less than 3 per cent; this is within the error bounds of the isotropic inversion results. The LVZ is less well pronounced in anisotropic inversions.

Surface wave waveform inversions for local shear‐wave velocities under eastern Australia

The waveform inversion method developed by Kushnir et al. [1989] and expanded by Passier and Snieder [1995b] yields estimates of structure along short interstation paths and of average velocity gradients. In contrast, 3D tomographic inversions yield local estimates of the earth structure by combining the information contained in a large set of long propagation paths. Both approaches lead to estimates of the local earth structure, but the two methods are subject to different types of artifact. As a test case of the consistency of the two approaches we consider the recent S‐velocity model of the Australian region by Zielhuis and Van der Hilst [1996]. We invert 20 Rayleigh wave waveforms between 20 and 110 s recorded on the Australian continent, most of them by the mobile Skippy network. From the data, we directly extract measurements of the S‐velocity structure at various short interstation paths in Australia and obtain estimates of the average velocity gradient along long paths. The estimates of the local interstation structure and the estimates of the velocity gradient are consistent with the 3D model of Zielhuis and Van der Hilst [1996]. These results provide illustrative examples of how specific features of a complex velocity structure can be determined using only a few waveforms.

Reasons for crack closure and possibilities for detection and sizing by NDE : 39928 Deuster, G. International Journal of Pressure Vessels and Piping, Vol. 35, pp. 173–188 (1988). (Proceedings of 5th International Seminar on Nondestructive Examination in Relation to Structural Integrity, Davos, Switzerland, 26–27 August 1987)

Models of the velocity structure of volcanoes can help define possible magma pathways and contribute to calculating more accurate earthquake locations, which can help with monitoring volcanic activity. However, shear-wave velocity of volcanoes is difficult to determine from traditional seismic techniques, such as local earthquake tomography (LET) or refraction/reflection surveys. Here we use the recently developed technique of noise cross correlation of continuous seismic data to investigate the subsurface shear-wave velocity structure of the Tongariro Volcanic Centre (TgVC) of New Zealand, focusing on the active Ruapehu and Tongariro Volcanoes. We observe both the fundamental and first higher-order modes of Rayleigh and Love waves within our noise dataset, made from stacks of 15 min cross-correlation functions. We manually pick group velocity dispersion curves from over 1900 correlation functions, of which we consider 1373 to be high quality. We subsequently invert a subset of the fundamental mode Rayleigh- and Love-wave dispersion curves both independently and jointly for one dimensional shear-wave velocity (Vs) profiles at Ruapehu and Tongariro Volcanoes. Vs increases very slowly at a rate of approximately 0.2 km/s per km depth beneath Ruapehu, suggesting that progressive hydrothermal alteration mitigates the effects of compaction driven velocity increases. At Tongariro, we observe larger Vs increases with depth, which we interpret as different layers within Tongariro's volcanic system above altered basement greywacke. Slow Vs, on the order of 1–2 km/s, are compatible with P-wave velocities (using a Vp/Vs ratio of 1.7) from existing velocity profiles of areas within the TgVC, and the observations of worldwide studies of shallow volcanic systems that used ambient noise cross-correlation methods.Most of the measured group velocities of fundamental mode Love-waves across the TgVC are 0.1–0.4 km/s slower than those of fundamental mode Rayleigh-waves in the frequency range of 0.25–1 Hz. First-higher mode Love-waves are similarly slower than first-higher mode Rayleigh waves. This is incompatible with synthetic dispersion curves we calculate using isotropic, layered velocity models appropriate for Ruapehu and Tongariro, in which Love waves travel more quickly than Rayleigh waves of the same period. The Love-Rayleigh discrepancy is likely due to structures such as dykes or cracks in the vertical plane having increased influence on surface-wave propagation. However, several measurements at Ruapehu have Love-wave group velocities that are faster than Rayleigh-wave group velocities. The differences between the Love- and Rayleigh-wave dispersion curves also vary with the azimuth of the interstation path across Ruapehu and Tongariro Volcanoes. Significant azimuthal dependence of both Love and Rayleigh-wave velocities are also observed. This suggests azimuthal anisotropy within the volcanic structures, which coupled with radial anisotropy, makes the Vs structures of Ruapehu and Tongariro Volcanoes anisotropic with orthorhombic or lower order symmetry. We suggest that further work to determine three-dimensional volcanic structures should include provisions for such anisotropy.

Detection sensitivity in thick assemblies (in French) : 39927 Odorico, J.; Lecuru, D. Societe Nationale Industrielle Aerospatiale, Suresnes (France), N88-23221/0/GAR, 60 pp. (Aug. 1987)

Models of the velocity structure of volcanoes can help define possible magma pathways and contribute to calculating more accurate earthquake locations, which can help with monitoring volcanic activity. However, shear-wave velocity of volcanoes is difficult to determine from traditional seismic techniques, such as local earthquake tomography (LET) or refraction/reflection surveys. Here we use the recently developed technique of noise cross correlation of continuous seismic data to investigate the subsurface shear-wave velocity structure of the Tongariro Volcanic Centre (TgVC) of New Zealand, focusing on the active Ruapehu and Tongariro Volcanoes. We observe both the fundamental and first higher-order modes of Rayleigh and Love waves within our noise dataset, made from stacks of 15 min cross-correlation functions. We manually pick group velocity dispersion curves from over 1900 correlation functions, of which we consider 1373 to be high quality. We subsequently invert a subset of the fundamental mode Rayleigh- and Love-wave dispersion curves both independently and jointly for one dimensional shear-wave velocity (Vs) profiles at Ruapehu and Tongariro Volcanoes. Vs increases very slowly at a rate of approximately 0.2 km/s per km depth beneath Ruapehu, suggesting that progressive hydrothermal alteration mitigates the effects of compaction driven velocity increases. At Tongariro, we observe larger Vs increases with depth, which we interpret as different layers within Tongariro's volcanic system above altered basement greywacke. Slow Vs, on the order of 1–2 km/s, are compatible with P-wave velocities (using a Vp/Vs ratio of 1.7) from existing velocity profiles of areas within the TgVC, and the observations of worldwide studies of shallow volcanic systems that used ambient noise cross-correlation methods.Most of the measured group velocities of fundamental mode Love-waves across the TgVC are 0.1–0.4 km/s slower than those of fundamental mode Rayleigh-waves in the frequency range of 0.25–1 Hz. First-higher mode Love-waves are similarly slower than first-higher mode Rayleigh waves. This is incompatible with synthetic dispersion curves we calculate using isotropic, layered velocity models appropriate for Ruapehu and Tongariro, in which Love waves travel more quickly than Rayleigh waves of the same period. The Love-Rayleigh discrepancy is likely due to structures such as dykes or cracks in the vertical plane having increased influence on surface-wave propagation. However, several measurements at Ruapehu have Love-wave group velocities that are faster than Rayleigh-wave group velocities. The differences between the Love- and Rayleigh-wave dispersion curves also vary with the azimuth of the interstation path across Ruapehu and Tongariro Volcanoes. Significant azimuthal dependence of both Love and Rayleigh-wave velocities are also observed. This suggests azimuthal anisotropy within the volcanic structures, which coupled with radial anisotropy, makes the Vs structures of Ruapehu and Tongariro Volcanoes anisotropic with orthorhombic or lower order symmetry. We suggest that further work to determine three-dimensional volcanic structures should include provisions for such anisotropy.

A novel technique for measuringLgattenuation—results from Eastern Canada between 1 to 10 hz

AbstractSpectral amplitudes of regionally recorded Lg waves are studied in detail between 0.6 and 10 Hz, using vertical-component, velocity seismograms of the Eastern Canada Telemetered Network stations and a supplementary Seismic Research Observatory-type station at Glen Almond (GAC), Québec. We find that the site responses vary among these stations by more than a factor of 3 within the frequency range of interest. Furthermore, they are found to be strongly frequency dependent. Consequently, it is essential that they be taken into consideration in studies of Lg wave attenuation and Lg source spectra of regionally recorded seismic events.We present a new method of measuring interstation surface wave attenuation, which is closely related to the conventional two-station method. While retaining all the desirable features of the conventional two-station method, the new technique, which we will call the “reversed two-station method,” allows simple, direct (one-parameter) determination of the Lg wave attenuation from sparse spectral data in a manner unaffected by station site effects and associated instrument error.The reversed two-station method is successfully tested over weakly attenuating, short (53 to 210 km) interstation paths in eastern Canada, a difficult experimental condition by normal standards. The Lg attenuation coefficient (0.6 to 10 Hz) in eastern Canada is found to be frequency dependent and of the form γ(f) = 0.0008 f0.81 km−1. At higher frequencies, the Lg attenuation appears to be essentially frequency independent. This latter finding is preliminarily interpreted as evidence that regionally recorded Lg waves in the Canadian Shield are, as in the case of Lg waves propagating through the structurally complex Appalachian Province, contaminated by the high-frequency coda of Sn waves. The Lg contamination over the shield paths becomes severe starting at 14 Hz, twice the frequency above which the Lg signal propagating over the Appalachian Province becomes completely dominated by non-Lg arrivals.