Lg Blockage Research Articles

A Model forLgPropagation in the Gulf Coastal Plain of the Southern United States

The southern United States experienced extensional tectonics during the early Mesozoic development of the Gulf of Mexico and subsequent accumulation of up to 12 km of marine sediment. Few seismic stations have operated in the Gulf Coastal Plain for extended periods of time, and data for constraining ground‐motion prediction models are sparse. Previous studies found that the region differs from other parts of eastern North America in terms of Lg attenuation. The Earthscope Transportable Array was in the south‐central United States in 2010–2012, during a time of increased seismicity. That circumstance provided data for investigating Lg propagation. Sixteen earthquakes with magnitudes in the 3.2–5.6 range occurred in Oklahoma, Arkansas, and Texas and were recorded to distances of 1000 km. Stations in the coastal plain of Texas, Arkansas, Louisiana, and Mississippi exhibit strong attenuation of the Lg phase, compared with stations located to the north in the Ouachita orogenic belt and cratonic platform. Q associated with the crustal waveguide underlying the Gulf Coastal Plain was estimated as Q =365 f 0.62. However, Lg blockage due to crustal thinning and anelastic attenuation due to the presence of several kilometers of sedimentary deposits occurs near the Gulf Coast. The average κ value south of latitude 33° N is 96±10 ms. A remarkable linear correlation exists between the attenuation parameter κ and the thickness of post‐Jurassic sediments in the study region, such that stations near the coast of Texas, Louisiana, and Mississippi, which overlie ∼12 km of sediments, exhibit values of κ ∼160 ms. This strong spatial dependence of attenuation has important implications and should be incorporated in the development of ground‐motion prediction models using the approach suggested here. Online Material: Figures showing maps of regression residuals and receiver terms for 12 frequency bands.

Average QLg, QSn, and observation of Lg blockage in the Continental Margin of Nova Scotia

AbstractThe term “Lg blockage” refers to the sudden disappearance of the Lg phase along a particular propagation path which is commonly seen at continental‐oceanic transition zones. In this paper we present observational evidence of Lg blockage across the continental margin of Nova Scotia in eastern Canada. Regional Lg and Sn spectra from 91 events with epicentral distances between 100 and 1200 km and magnitudes between 2.5 and 4.7 are inverted simultaneously for the source spectrum, site amplification, and average attenuation. The vertical displacement spectra were estimated between 0.9 and 10.75 Hz. The assumptions include a fixed frequency‐independent geometric spreading rate for Lg and a frequency‐dependent spreading model for the Sn. Estimates for the apparent regional attenuations are QLg (f) = 615(±25) f0.35(±0.04) and QSn (f) = 404(±23) f0.45(±0.03). Results from this study provide an accurate parameterization of observed amplitude spectra and are valuable for representing wave propagation in the region. Based on the observation of a strong trade‐off between Sn and Lg amplitudes which have different attenuation characteristics, we conclude any attenuation study based on measuring amplitude of a package of several different phases, without taking into consideration the propagation characteristics of individual waveforms at the region of study, may bias the estimation of average regional Q.

LG PHASE EFFICIENCY PROPAGATION AND THE INTENSITY OF GROUND SHAKING IN THE NORTH WESTERN PART OF SYRIA

The high level of seismicity at the regional seismic sources around Syria and the good distribution of the 57 seismic network stations in Syria and surrounding areas (Turkey, Cyprus, Jordan) allowed a detailed characterization of regional Lg phase propagation and definition of the parts where this phase is clearly seen, well developed and zones of Lg blockage by determining the values of Lg Q0 (Q at 1 Hz) over these paths using the standard two-station method. Since the Lg phase for distances larger than 150 km carries substantial energy for large events [1], and the differences in efficiency of propagation influences the intensity of ground shaking [2]. Thus, in this study, were correlated the good paths of lg phase with the intensity contours of the 1822, 1872 historical large determined, earthquake located at the north western part of Syria [3] and the pick ground acceleration recorded at Syrian strong motion stations of the recent event (22 June 1997) with magnitude 4.9 located in the same region of historical events [4],[5]. It is found that the trend of north east direction extended for the historical intensity contours and the high pick ground acceleration observed for the recent event (22 June 1997) are taking the same trend as Lg efficient propagation in that Area.

Laterally heterogeneous scattering explains Lg blockage in the Pyrenees

We present an algorithm to simulate the propagation of seismic energy in a model that contains both deterministic structure and statistically described small‐scale heterogeneities. With this algorithm, we study the propagation of seismic waves through the Pyrenees. Using a series of seismograms, the variations of the attenuation properties of the crust along the Pyrenean axis are illustrated. The western Pyrenees exhibit strong attenuation of crustal phases, a phenomenon known as Lg blockage. Previous studies demonstrated that the large‐scale velocity structure of the western Pyrenees cannot explain the strong lateral variations of attenuation along the Pyrenean range. In this paper we present a model that is able to explain the principal observations. We propose that the Lg blockage is caused by a perturbed region of the crust with scattering and anelastic properties that differ markedly from the surrounding crust. Numerical simulations of the elastic radiative transfer equation in a layered medium show that it is possible to model the observed Lg blockage with increased scattering in the perturbed body. In order to explore the relative importance of scattering and attenuation we performed an inversion of the data using a genetic algorithm. Our study reveals that stronger intrinsic attenuation in the perturbed body improves the fit to the data but that intrinsic attenuation alone cannot explain the observations. Our work puts forward the role of multiple scattering in the attenuation mechanism of crustal phases.

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

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

Regional seismic wave propagation (Lg and Sn) and Pn attenuation in the Arabian Plate and surrounding regions

Continuous recordings of 17 broadband and short-period digital seismic stations from a newly established seismological network in Saudi Arabia, along with digital recordings from the broadband stations of the GSN, MEDNET, GEOFON, a temporary array in Saudi Arabia, and temporary short period stations in Oman, were analysed to study the lithospheric structure of the Arabian Plate and surrounding regions. The Arabian Plate is surrounded by a variety of types of plate boundaries: continental collision (Zagros Belt and Bitlis Suture), continental transform (Dead Sea fault system), young seafloor spreading (Red Sea and the Gulf of Aden) and oceanic transform (Owen fracture zone). Also, there are many intraplate Cenozoic processes such as volcanic eruptions, faulting and folding that are taking place. We used this massive waveform database of more than 6200 regional seismograms to map zones of blockage, inefficient and efficient propagation of the Lg and Sn phases in the Middle East and East Africa. We observed Lg blockage across the Bitlis Suture and the Zagros fold and thrust belt, corresponding to the boundary between the Arabian and Eurasian plates. This is probably due to a major lateral change in the Lg crustal waveguide. We also observed inefficient Lg propagation along the Oman mountains. Blockage and inefficient Sn propagation is observed along and for a considerable distance to the east of the Dead Sea fault system and in the northern portion of the Arabian Plate (south of the Bitlis Suture). These mapped zones of high Sn attenuation, moreover, closely coincide with extensive Neogene and Quaternary volcanic activity. We have also carefully mapped the boundaries of the Sn blockage within the Turkish and Iranian plateaus. Furthermore, we observed Sn blockage across the Owen fracture zone and across some segments of the Red Sea. These regions of high Sn attenuation most probably have anomalously hot and possibly thin lithospheric mantle (i.e. mantle lid). A surprising result is the efficient propagation of Sn across a segment of the Red Sea, an indication that active seafloor spreading is not continuous along the axis of the Red Sea. We also investigated the attenuation of Pn phase (QPn) for 1–2 Hz along the Red Sea, the Dead Sea fault system, within the Arabian Shield and in the Arabian Platform. Consistent with the Sn attenuation, we observed low QPn values of 22 and 15 along the western coast of the Arabian Plate and along the Dead Sea fault system, respectively, for a frequency of 1.5 Hz. Higher QPn values of the order of 400 were observed within the Arabian Shield and Platform for the same frequency. Our results based on Sn and Pn observations along the western and northern portions of the Arabian Plate imply the presence of a major anomalously hot and thinned lithosphere in these regions that may be caused by the extensive upper mantle anomaly that appears to span most of East Africa and western Arabia.

Characteristics of Lg attenuation in the Tibetan Plateau

Lg, a regional seismic wave comprised of multiple shear wave reverberations trapped in the crustal waveguide, is important for magnitude estimation and source discrimination for monitoring nuclear testing treaties. In stable continental regions, Lg propagates with a group velocity of about 3.5 km/s and can often be observed at distances up to 4000 km. To better understand the absence of high‐frequency Lg arrivals for paths traversing the northern boundary of the Tibetan Plateau, we investigate spatial variations of broadband (0.15–5.0 Hz) energy in the Lg group velocity window (3.6–3.0 km/s) using regional waveforms recorded at the Chinese Digital Seismic Network station WMQ. Vertical component seismograms are analyzed for 90 events with magnitudes of 4.4 ≤ mb ≤ 6.4 that occurred between 1987 and 1999 in the Tibetan Plateau and around its margins. The Lg amplitude spectra for events located near the northern margin of the plateau have apparent corner frequencies of 1–2 Hz, nearly identical to those for comparable size events at similar distances outside the plateau. High‐frequency (>1 Hz) Lg energy recorded at WMQ decreases rapidly as a function of source distance into the plateau. A path length of 300–400 km within the northern Plateau suffices to eliminate 2–5 Hz Lg energy. For events in southern Tibet with paths crossing the central portion of the Tibetan Plateau, almost total Lg extinction occurs, even for energy in the low‐frequency band of 0.2–1 Hz. Corresponding apparent corner frequencies of the “Lg” amplitude spectra range between 0.2 and 0.4 Hz. The corner frequency shift is found to vary systematically with path length across the plateau. Linear regressions demonstrate that the shift in apparent corner frequency of Lg amplitude spectra is negatively correlated with features of the Tibetan Plateau, such as mean elevation along the paths or travel distance within the plateau above specified elevation thresholds. The systematic variations in the amplitude and frequency content of energy in the Lg window as a function of path length within the plateau indicate that strong crustal attenuation plays an important role in Lg extinction for paths traversing central and northern Tibet, superimposed on any structural blockage effects associated with abrupt thinning of the crust near the northern boundary of the plateau. Spectral ratios of many event pairs along great circle paths give estimates of frequency‐dependent Lg attenuation for paths crossing western, central, and eastern sectors of Tibet. The region of strong Sn attenuation in northern central Tibet also has strong Lg attenuation with QLg for 1 Hz on the order of Q0 = 80–90, while in southern central Tibet, Q0 increases to about 316, and in eastern and western Tibet, Q0 is on the order of 120–200 for paths traversing the entire plateau. The strong Lg attenuation in northern central Tibet is responsible for the so‐called Lg blockage and may be associated with partial melting in the crustal low‐velocity layer in northern Tibet. Our QLg values for Tibet are significantly lower than most earlier estimates, primarily as a result of not excluding blocked observations along with allowing for lateral variations within Tibet.

Lg Q in the Eastern Tibetan Plateau

Lg spectra are collected from the 1991–1992 Tibetan Plateau Passive Experiment to measure Q values. Using a standard two-station method that virtually eliminates source and site effects, I obtain a model of Q 0 = (126 ± 9) and η = (0.37 ± 0.02) in a frequency range between 0.2 and 3.6 Hz, where Q 0 and η are Lg Q at 1 Hz and its power-law frequency dependence, respectively. The estimated Q 0 value is among the lowest ever reported for continental areas; it qualitatively supports the observation by McNamara et al. (1996) that Lg can not be observed inside the plateau beyond about 700 km, a limiting distance that is much shorter than those in the other low Q 0 (∼200) regions, such as Iran and the western United States. The low Q 0 value may be the cause of Lg blockage across the northern boundary of the plateau and may indicate abnormally high temperature and fluid content in the Tibetan crust. Quantitatively, the estimated Q 0 value is lower by a factor of 3 than the value of 366 estimated by McNamara et al. (1996), who used data from the same experiment. Since there are several differences in the data processing and inversion procedures used in this and the previous studies, I investigated the effects of these differences on the Q estimates. I conclude that the most probable cause of the discrepancy is in the different inverse methods used. The previous inversion solved for a large number of free parameters that include the source and site terms. In this study only two free parameters ( Q 0 and η) are solved for, thus avoiding the instability caused by parameter trade-offs.

Variations in Regional Phase Propagation in the Area around Japan

The high level of seismicity and dense network of short-period stations in Japan allows a detailed characterization of regional phase propagation. There are substantial differences depending on the location of the source and stations. Despite the structural complexity, Lg is clearly seen in many parts of Japan. Lg is particularly well developed in the western part of the main island, Honshu, and will propagates through some volcanic zones with loss of high-frequency components. There are also zones of Lg blockage in northeastern Honshu and for subduction zone events with significant oceanic paths for which the mantle phases Pn and Sn are particularly clear. The oceanic region in the Sea of Japan blocks Lg propagation paths for events in mainland Asia at many stations, but there are clear Lg propagation corridors through Korea to stations in the west (Kyushu, western Honshu) and in the north into Hokkaido. The Lg phase carries substantial energy for large events, and the differences in efficiency of propagation influences the intensity of ground shaking. Thus the 1995 Hyogo-ken Nanbu (Kobe) earthquake shows intensity contours extended to the west in the region of efficient Lg-wave propagation. The Rg phase and the fundamental mode of Love waves are very significant for the shallow 2000 Tottori-ken Seibu event, and the effect of the combination of Lg with these slightly lower frequency surface waves would help to explain the discrepancy between the Japan Meteoro- logical Agency magnitude from regional stations (Mj 7.3) and the moment magnitude from distant observations (M w 6.6).

A Hybrid BE-GS Method for Modeling Regional Wave Propagation

— We present a hybrid boundary-element (BE) and generalized screen propagator (GSP) method for the 2-D SH problem to model the combined effects of arbitrarily irregular topography, large-scale crustal variation, and the associated small-scale heterogeneities on regional wave propagation. We develop a boundary connection technique to couple the wave fields calculated by the BE method with those of the GSP method. Its validity is tested by numerical experiments. For a long crustal waveguide, the relatively short sections with severe surface topography can be modeled by the time-consuming BE method to high frequencies, and the exterior field in the relatively weak heterogeneous media of large volume can be calculated by the GSP method. For the waveguide with severe topography, the BE method can be used section by section via the boundary connection technique to model the combined effects of rough topography and large-scale structural variation on Lg wave propagation at extended regional distances.¶Numerical comparisons with independent methods showed that the hybrid method is relatively accurate for Lg simulation. We apply the hybrid method to Lg wave propagation in two real crustal waveguides in the Tibet region; one with Lg blockage and another without blockage. We found that the most characteristic effect from the irregular topography is the strong scattering by the topographic structures. The scattering by local irregular topographies leads to anomalous near-receive effects and tends to remove energy from the guided waves, which causes decay of amplitude and waveform distortion. It can be expected that rough surface topography and random heterogeneities with scale length close to the dominant wavelength will be very efficient in attenuating regional waves. The dramatic lateral variation of the topography-Moho large-scale structure combined with the small-scale rough topography and random heterogeneities could be the cause of Lg anomalous attenuation and blockage observed in this region. More quantitative assessment of the topographic effects must be conducted in the future.

Sedimentary Basins and the Blockage of Lg Wave Propagation in the Continents

— The phenomenon of “Lg blockage,” where Lg is strongly attenuated by crustal heterogeneities, poses a serious problem to CTBT monitoring because Lg is an important seismic phase for discrimination. This paper examines blockage in three continental regions where the Lg blockages may be caused by large, enclosed sedimentary basins along the propagation path. The Barents Sea Basin blocks Lg propagation across the Barents Sea from the Russian nuclear test sites at Novaya Zemlya to Scandinavian stations. Also, “early Lg” waves are observed in Sn codas on NORSAR, NORESS, and ARCESS recordings of Novaya Zemlya explosions where direct Lg is blocked. Early Lg waves may have resulted from Sn-to-Lg mode conversion at the contact between the Barents Basin and the Kola Peninsula. The Northern and Southern Caspian Sea Basins also block Lg waves from PNEs and earthquakes, perhaps due to thick, low-velocity, low-Q sediments replacing the granitic layer rocks in the crust. Lg blockage has also been observed in the Western Mediterranean/Levantine Basin due to low-Q sediments and crustal thinning. A “basin capture” model is proposed to explain Lg blockage in sedimentary basins. In this model, shear waves that reverberate in the crust and constitute the Lg wave train are captured, delayed, and attenuated by thick, low-velocity sediments that replace the “granitic” layer rocks of the upper crust along part of the propagation path. Sn waves, which propagate below the basin, would not be blocked and in fact, the blocked Lg waves may be diverted downward into Sn waves by the low velocity sediments in the basin.

Mapping Crustal Heterogeneity Using Lg Propagation Efficiency Throughout the Middle East, Mediterranean, Southern Europe and Northern Africa

— In this paper we describe a technique for mapping the lateral variation of Lg characteristics such as Lg blockage, efficient Lg propagation, and regions of very high attenuation in the Middle East, North Africa, Europe and the Mediterranean regions. Lg is used in a variety of seismological applications from magnitude estimation to identification of nuclear explosions for monitoring compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). These applications can give significantly biased results if the Lg phase is reduced or blocked by discontinuous structure or thin crust. Mapping these structures using quantitative techniques for determining Lg amplitude attenuation can break down when the phase is below background noise. In such cases Lg blockage and inefficient propagation zones are often mapped out by hand. With our approach, we attempt to visually simplify this information by imaging crustal structure anomalies that significantly diminish the amplitude of Lg. The visualization of such anomalies is achieved by defining a grid of cells that covers the entire region of interest. We trace Lg rays for each event/station pair, which is simply the great circle path, and attribute to each cell a value equal to the maximum value of the Lg/P-coda amplitude ratio for all paths traversing that particular cell. The resulting map, from this empirical approach, is easily interpreted in terms of crustal structure and can successfully image small blockage features often missed by analysis of raypaths alone. This map can then be used to screen out events with blocked Lg prior to performing Q tomography, and to avoid using Lg-based methods of event identification for the CTBT in regions where they cannot work.¶For this study we applied our technique to one of the most tectonically complex regions on the earth. Nearly 9000 earthquake/station raypaths, traversing the vast region comprised of the Middle East, Mediterranean, Southern Europe and Northern Africa, have been analyzed. We measured the amplitude of Lg relative to the P-coda and mapped the lateral variation of Lg propagation efficiency. With the relatively dense coverage provided by the numerous crossing paths we are able to map out the pattern of crustal heterogeneity that gives rise to the observed character of Lg propagation. We observe that the propagation characteristics of Lg within the region of interest are very complicated but are readily correlated with the different tectonic environments within the region. For example, clear strong Lg arrivals are observed for paths crossing the stable continental interiors of Northern Africa and the Arabian Shield. In contrast, weakened to absent Lg is observed for paths crossing much of the Middle East, and Lg is absent for paths traversing the Mediterranean. Regions that block Lg transmission within the Middle East are very localized and include the Caspian Sea, the Iranian Plateau and the Red Sea. Resolution is variable throughout the region and strongly depends on the distribution of seismicity and recording stations. Lg propagation is best resolved within the Middle East where regions of crustal heterogeneity on the order of 100 km are imaged (e.g., South Caspian Sea and Red Sea). Crustal heterogeneity is resolvable but is poorest in seismically quiescent Northern Africa.

The North Sea Lg-blockage puzzle

SUMMARY The North Sea Lg blockage for wave paths across crustal graben structures is a well-established observational fact. Analysis of such observations implies that Lg blockage takes place in graben areas associated with sedimentary basin formation and crustal thinning. These intriguing observations have triggered many theoretical studies aimed at highlighting specific Lg loss mechanisms, albeit so far with only moderate success. Our approach to this problem is to simulate seismic wavefield propagation through the crustal waveguide using 2-D finite-difference techniques. The graben structures are known in detail from oil exploration works in the North Sea, which has enabled us to use realistic crustal models in our Lg synthetics. In the most extreme model tested, the crystalline crust thickness beneath the graben amounted to only 5 km, while the overlying sedimentary pile is nearly 10 km thick. At the base of the crust in the graben area the Moho is elevated nearly 10 km. This model has similarities to the oceanic crustal waveguide, where total Lg blockage is claimed for path lengths exceeding 100 km. The synthetic wavefields are displayed in terms of snapshots, semblance velocity analysis and time-space rms amplitudes. The dominant structural Lg loss mechanisms are the delay of the Lg waves in the thick sediments, Lg-to-Rg conversions (scattering) by lateral heterogeneities in the sediments, and S-wave leakage out of the crustal wave-guide and into the upper mantle. A fraction of these upper-mantle S waves return to the crust and appear as Sn coda. Observationally, strong Sn phases of long duration are often associated with weak Lg phases and vice versa. Our synthetics produced Lg amplitude decay amounting at most to 6-10 dB, while observational data imply blockage amounting to 15-20 dB. The latter is equivalent to a Pn-Lg magnitude difference of nearly one magnitude unit. The main outcome of this study is therefore that Lg-wave propagation is very robust and that a dominant blockage effect associated with intrinsic attenuation, that is Q values of the order of 100 at 2 Hz for a path length of minimum 100 km, is necessary to conform to observations.

Regional seismic phases across the Ligurian Sea: Lg blockage and oceanic propagation

The Ligurian Sea (West Mediterranean) is an example of a narrow oceanic basin which stops Lg wave propagation. Locating the exact point of Lg extinction was one of the goals of the SISBALIG II seismological experiment, which deployed both inland stations in Provence and Corsica and Ocean Bottom Seismometers (OBS) in the Ligurian Sea. A precise analysis of all available regional seismological data using energy diagrams reveals the role of the Provence margin in the Lg phase extinction for seismic events occurring in the southwestern Alps. The extinction is not a progressive phenomenon along oceanic travel path but occurs in a narrow zone of about 20 km width in the vicinity of the Provence margin. The extinction is accompanied by the generation of two types of diffracted waves: the mantle phase Sn ( V s > 3.5 km s −1), and very low velocity S waves appearing both near the Provence margin and in the middle of the Ligurian basin. We performed numerical simulation of SH wave propagation in two-dimensional (2D) media to investigate the mechanisms of Lg extinction. The simulations confirm that simple crustal thinning at the continent-ocean boundary alone cannot cause the observed extinction of Lg waves, although the change in Moho depth does induce the diffraction of a part of the incident Lg wavetrain into mantle waves, giving rise to the Sn phase on the other side of the basin. The existence of soft oceanic sediments is a very important factor in explaining the extinction, as a large part of the Lg energy is transferred into slow S waves trapped in the sediments. Moreover, comparison of synthetic seismograms and observed data shows that the observed two packets of low-velocity waves are an indication of the asymmetry of the Ligurian basin, with the existence of two shallow basins with very low S velocity (1.0 km s −1) near the Provence margin and in the middle of the Ligurian Sea. However, the attenuation of Lg waves in the vicinity of the Provence margin is more abrupt in the data than in the simulations. This discrepancy could result from the complex geometry of the continent-ocean transition zone and its possibly very diffractive nature, two factors that could add to Lg extinction at the margin and are impossible to take into account in models.

Why the Lg phase does not traverse oceanic crust

It has long been recognized that Lg waves are not observed on paths traversing oceanic crust, but this has not yet been fully explained. Using normal- mode analysis and finite-difference simulations, we demonstrate that (1) the overall thickness of the crustal wave guide affects the number of normal modes in a given frequency range; in general, thinner crust accommodates fewer modes; (2) 6-km- thick oceanic crust does not allow Lg to develop as a significant phase in the frequency band 0.3 to 2 Hz because of the limited number of modes that exist; (3) in continental crust thicker than 15 km, there are usually sufficient modes that Lg is stable; (4) the shallow sediment layer plays important roles in crustal-guided wave propagation, trapping energy near the surface, separating Lg and Rg waves; (5) a 100-km-long segment of oceanic structure on a mixed ocean/continent path can block P-SV type Lg propagation. The primary reason why Lg does not travel through oceanic crust thus lies in the structure of the crustal wave guide, with the decisive factor being the crustal thickness. The detailed shape of ocean-to-continent crustal transitions can influence Lg blockage, but the general inefficiency of Lg propagation in the oceanic structure is the dominant effect.

Investigation of teleseismic Lg blockage and scattering using regional arrays

Abstract Incoherent beamforming and continuous frequency-wavenumber “polar-scan” analysis have been applied to study the characteristics of P coda and Lg waves recorded at the regional arrays NORESS and ARCESS from presumed explosions on the northwestern Russian platform and at test sites of Novaya Zemlya and eastern Kazakh. Also, recordings at the Graefenburg array, near the new GERESS regional array, of Novaya Zemlya events and an eastern European platform (White Sea) event have been compared with the Norwegian array recordings of the same events. The White Sea explosion, at distances of 14° and 22° from NORESS and Graefenburg, respectively, exhibits Lg rms amplitudes on incoherent beams that exceed those of Sn . However, for explosions at Novaya Zemlya at 20° from NORESS and eastern Kazakh at 30° from ARCESS, the Lg waves have much smaller amplitudes than Sn . In the case of Novaya Zemlya, the Lg wave is almost completely blocked at both NORESS and ARCESS, but is well recorded at Graefenburg. This reduction of the Lg amplitudes relative to those from events at longer distances cannot be accounted for by anelastic attenuation and spreading. Comparison of propagation paths suggests that the reduction in amplitudes may be due to blockage and scattering of Lg in the Barents Sea. Analysis of Sn coda waves on incoherent beams and continuous frequency-wavenumber analysis “polar scans” suggests that they can result from Lg to Sn scattering from the coastline between the Barents Sea and Scandinavia. Recent geologic studies have revealed significant variations in crustal structure in the Barents Sea basin, which may cause the blockage of Lg waves from Novaya Zemlya. In the case of eastern Kazakh explosions, ARCESS incoherent beams show much reduced Lg amplitudes relative to the P coda waves compared with NORESS, even though the distance to ARCESS (31°) is less than that to NORESS (38°). This reduction in Lg amplitude at both NORESS and ARCESS is accompanied by enhancement of P and Sn coda. The timing of the coda waves and the propagation paths suggest that Lg waves may be partially blocked and scattered by the Ural Mountains and that the P and Sn coda waves may result from Lg -to- Pn , Pn -to- Sn , Sn -to- Lg , and Lg -to- Sn conversions from lateral heterogeneities in the crust beneath the Urals. The blockage is greater for the Eastern Kazakh-to-ARCESS path because it crosses the middle part of the Urals whereas the Eastern Kazakh-to-NORESS path crosses the southern part of the Urals chain. These blockage effects seem to result from sudden lateral variations in shallow crustal structure, such as variations in sediment thicknesses in the sedimentary basins of the Russian platform and adjacent to the Urals. These blockage effects can be predicted from geologic studies and need to be accounted for when using Lg amplitudes to estimate absolute yields of explosions and for event identification.