PL Waves Research Articles

On the retrieval of body waves from ambient noise based on regional seismic arrays

SUMMARY Ambient noise technology can efficiently extract surface wave signals from seismic background noise and has been extensively utilized in imaging lithospheric structures. However, retrieving crustal body wave signals, such as PmP or SmS phases, still poses a challenge. Only a limited number of reports have successfully extracted these regional-scale body wave signals from ambient noise in only a few limited study areas. It remains unclear why these signals are difficult to retrieve from ambient noise data. To investigate the mechanism of recovering body wave signals in noise cross-correlations, we calculate cross-correlation functions at four regions and observe the similarity of the recovered body waves. Through a series of synthetic simulations, we demonstrate that the appearance of body wave signals in noise cross-correlations is closely related to the distribution of noise sources. Among these signals, the post-critical SmS wave proves to be the most readily recoverable from ambient noise data, primarily stemming from distant sources. In contrast, the recovery of P-wave requires the array to be in proximity to the sources. Our experiments also reveal that the main origin of PL waves is the multiple reflections of S-waves propagating in the crust.

A seismic investigation of lithospheric velocities beneath the Chhotanagpur Plateau (India) using waveform modeling of shear-coupled PL waves and other phases

A seismic investigation of lithospheric velocities beneath the Chhotanagpur Plateau (India) using waveform modeling of shear-coupled PL waves and other phases

Regional characteristics of the lithospheric structure in the southeast of North China revealed by explosion seismology

Based on the blasting seismic detection data obtained in the southeast of North China in recent years, this paper comprehensively analyzes and studies the crust-mantle lithospheric structure and seismological characteristics of different tectonic regions, such as offshore basins, west Shandong uplift, Tanlu fault zone and Jiangsu-Shandong orogenic belt. The low-velocity Pg waves in Dongying depression and Northern Jiangsu basin reveal the unstable basement structure with extremely thick sediments. The travel time of Pg wave is characterized by relatively low propagation velocity and small crustal thickness of offshore continental margin; the first break time and high apparent velocity of Pg wave in west Shandong uplift indicate that the sedimentary basement is relatively thin. The Pm wave shows the characteristic of dominant wave in the first-order velocity discontinuity of the crust-mantle interface, which reflects the high crustal velocity and stable structure in west Shandong uplift. The Pm and Pl wave are obviously complicated, which can reflect the crust-mantle lithospheric structure of the transitional zone between Tanlu fault zone and Jiangsu-Shandong orogenic belt. The small time difference between Pn and PL waves can be regarded as the highly destructive seismological manifestation of Tanlu fault zone on the crust-lithosphere scale. Based on many geophysical phenomena such as electrical structure, density structure and terrestrial heat flow, it is believed that the lithospheric destruction degree of Tanlu fault zone and Jiangsu-Shandong orogenic belt was high during the destruction of the North China Craton.

The Significance of Long‐Period Ground Motion at Regional to Teleseismic Distances From the 610‐km Deep Mw 8.3 Sea of Okhotsk Earthquake of 24 May 2013

AbstractThe 24 May 2013 earthquake beneath the Sea of Okhotsk (610 km, Mw 8.3) produced significant ground motion across the whole span of the Japanese islands, from 1,300‐ to 4,200‐km epicentral distance. The largest shaking was concentrated along the back‐arc side of the subduction zone, which is the opposite of the normal pattern for deep earthquakes in the Pacific slab. Observations from the dense Hi‐net and F‐net arrays across Japan show that the largest shaking in northern Japan (near 2,000‐km epicentral distance) was caused by near‐caustic S waves, with triplication of upgoing and downgoing waves from the deep source and reflected waves from the 660‐km discontinuity. Three‐dimensional finite difference method simulations confirm that the antiwaveguide effect of the high‐wave speed slab is to push the zone of larger intensity 300 km farther to south than might be expected. The S wavefront distorted by the slab has near‐critical incidence at the free surface producing large sP and generating shear‐coupled PL (s‐PL) waves with period >3 s. With increasing epicentral distance the S incident angle exceeds critical, then total sS reflection creates large ground motion at large distance (>3,000 km) and even farther (>6,000 km) with sSS. The propagation of sS, sSS linking to sS‐PL, and sSS‐PL wave trains is very efficient in continental structures with thicker crust. The felt reports at large (4,000–8000 km) distances from the 2013 Sea of Okhotsk earthquake can be explained by lengthy, long‐period ground motion in the continental environment with amplification in sedimentary basins and in tall buildings.

Significant P wave conversions from upgoing S waves generated by very deep earthquakes around Japan

An important component of the seismic wavefield at moderate epicentral distances from deep earthquakes comes from seismic waves that are radiated upwards from the source. For very deep events, there is a range of distances at which upgoing S can convert into P waves that travel in the crust or in the upper mantle as the sPn phase. For a 600-km-deep event, sPn becomes a precursor to S from about 8°, and can have significant amplitude if the source radiation pattern is favourable. These conversions to crustal P have a very similar travel time property to S, and interfere strongly with S to produce complex wavetrains on both vertical and radial components. Where the locus of conversion falls on thicker continental crust, S waves can be coupled into partially trapped P waves in the crust that produce a long-period shear-coupled PL (s-PL) wave. Such longer period phases generated by large, very deep earthquakes can make a major contribution to sustaining large ground motion for considerable distances from the source. Observations of three very deep (> 575 km) events around Japan demonstrate the range of propagation effects associated with S to P wave (sP) conversion that plays an important role in shaping the later part of the recorded seismograms. The influence of sP conversion on the observed seismograms and the development of the s-PL wave depend on the variation of crustal thickness along the path and epicentral distance, and particularly on the locus of the conversion zone and the properties of the crust at that location.

Unusual Strong Ground Motion Across Japan From the 680 km Deep 30 May 2015 Ogasawara Islands Earthquake

AbstractThe Mw 7.9 earthquake beneath the Ogasawara Islands on 2015 May 30 at 680 km depth (about 100 km deeper than other seismicity in the vicinity) produced significant shaking over a broad area of Japan in the epicentral distance range 1,000–2,000 km. Usually, deep earthquakes in the subducting Pacific slab develop a band of large ground motion along the east coast of northern Japan, due to efficient guiding of high‐frequency (>1 Hz) waves in the subducting slab. However, for this very deep earthquake, the large ground acceleration arises from relatively low‐frequency (<1 Hz) S wave pulses with a long tail of long‐period (<0.1 Hz) signals. The arrival of the slab‐guided high‐frequency signal was very late, and weak, compared with normal Pacific slab events. Numerical simulation of seismic wave propagation reveals that the observed low‐frequency pulses are the direct S wave from the very deep source followed by a large SV‐to‐P conversion at the free surface with near‐critical S wave incidence. These phases produce a long‐period shear‐coupled PL (S‐PL) wave by constructive interference between wide‐angle P and S reflections in the crust and incoming S wave to the crust. The S‐PL wave travels several hundred kilometers at regional distances with reinforcement from continuing S wave input from the very deep source. Such an unusual observation of strong low‐frequency ground motion was due to its very great source depth, outside the main slab, so the high‐frequency Pacific‐slab guided wave was weak.

Constraining shallow slip and tsunami excitation in megathrust ruptures using seismic and ocean acoustic waves recorded on ocean-bottom sensor networks

Great earthquakes along subduction-zone plate boundaries, like the 2011 magnitude 9.0 Tohoku-Oki, Japan, event, deform the seafloor to generate massive tsunamis. Tsunami wave heights near shore are greatest when excitation occurs far offshore near the trench, where water depths are greatest and fault slip is shallow. The Tohoku event, featuring over 30 m of slip near the trench, exemplifies this hazard. Unfortunately the rupture process that far offshore is poorly constrained with land-based geodetic and even most seafloor deformation measurements, and seismic inferences of shallow slip are often nonunique. Here we demonstrate, through dynamic rupture simulations of the Tohoku event, that long-period guided waves in the ocean (specifically, leaking oceanic P-wave modes known as PL waves) can resolve the shallow rupture process and tsunami excitation near the trench. With predicted pressure changes of ∼0.1–1 MPa along most of the seafloor landward of the trench, and periods of several seconds, these PL waves should be observable with ocean-bottom pressure sensors and/or seismometers. With cabled sensor networks like those being deployed offshore Japan and in other subduction zones, these waves could be used to rapidly quantify shallow slip and near-trench seafloor uplift and improve local tsunami early warning systems.

Global propagation of body waves revealed by cross‐correlation analysis of seismic hum

Seismic interferometry has now been applied to the exploration of the Earth's interior at scales ranging from local to global. Most studies have used surface‐wave propagation. Recently, some studies have focused on body wave propagation on local and regional scales but not on a global scale. In this study, we succeed in extracting global body wave propagation(of P, PP, PKP, S, SS, ScS, P′P′, etc. waves) using seismic hum with frequency‐wave number filtering in the range of 5 to 40 mHz. Although the observed body wave propagation is similar to that of the corresponding components of Green's functions, there are two differences between them: the lack of reflection phases in the observation and the dominance of shear‐coupled PL waves in the observation. These differences originate from the dominance of shear‐traction sources on the Earth's surface, which causes the breakdown of equipartition among modes with different radial orders. For further studies of body wave exploration by seismic interferometry, these differences should be considered.

Follow-up study on multifocal electroretinogram of acute and convalescence stage of Vogt-Koyanagi-Harada syndrome

Background Researches showed that multifocal electroretinogram （mfERG） is able to assess the retinal function in the eyes with acute Vogt-Koyanagi-Harada （VKH） syndrome. But the mfERG characteristics of convalescence stage of VKH are still below clear. Objective Present study was to compare and follow up the variation process of visual acuity and mfERG in acute and recovery stages of VKH syndrome. Methods This was a clinic-based retrospective study. Visual acuity, mfERG and fundus fluorescence angiography （FFA） were recorded from 35 eyes of 18 acute VKH cases. The period of follow-up in recovery stage lasted about 18 months with the repetitive recording results for 4 times. Results In this study,the visual acuity range in acute stage VKH was 0.01 to 1.0, and 91.4% （32/35 eyes） was below 0.6. Compared with normal control group, the visual acuity wassignificantly decreased （ P〈0. O1 ）. The response densities （amplitudes） of N,, PI waves of the first-order kernel were significantly lowed in all the 6 rings,and the implicit times of 1-4 rings of both waves were significantly prolonged in acute VKH eyes（P〈0.05 ）. The abnormalities of retinal function showed a regional difference at the posterior pole retina with the dominant change in the first ring,showing a cutting off 78% in the PI amplitude. The abnormal degree of mfERG was more serious as the the increase of retinal eccentricity. In 2 months of convalescence after glucocorticosteroids therapy, the range of visual acuity were 0. 1-1.2, and the amplitudes of N1 ,Pl of 1-2 rings were greatly elevated in comparison with acute on-set （P〈0.05）. However, there was still a remarkable difference in the amplitudes of from 1 through 6 rings,comparing with normal. The response density of Pl wave from whole recording region was only 44% of normal. Though the visual acuity was stable during the follow-up duration, a decreasing tendency in NI and PI amplitudes were seen. The implicit times of both wave shortened only in 1-3 rings in recovery stages of VKH （ P 〈 0. 05 ）. Conclusion VKH syndrome cause serious damage of posterior retinal function. Macular region is the site with greater retinal functional lesion and restore before and after medication. This hardly recovery of retinal function can last over one and half year, even satisfied visual acuity is stable after proper treatment. Key words: Vogt-Koyanagi-Harada syndrome; Visual acuity; Multifocal electroretinogram

Observations of shear-coupledPLwaves from shallow earthquakes in the Sumatra and Java subduction zones

SUMMARY S phases generated by shallow earthquakes in the Sumatra subduction zone and recorded at the Geoscope station PAF on the Kerguelen Isles in the southern Indian Ocean, are often followed by a long and particularly large wave train on the longitudinal component. The wave train has a dominant period of 25‐30 s and lasts usually for more than 100 s. It shows slight normal dispersion, and has a prograde elliptical motion in the vertical propagation plane. These characteristics suggest that the observed waves are shear-coupled PL (SPL) waves. Similar wave trains are not observed for events at comparable distances in other source regions. A computational scheme, based on the reflectivity method and allowing for different crust and upper mantle structures beneath source and receiver, has been used to model the SPL waveforms for six of the events located beneath the accretionary domain and outer high of the Sumatra subduction zone. For the source area, a model representing the structure seaward of the trench has been used, comprising a 5.5 km water layer and 600 m of sediments. By placing the sources in the sedimentary layer, the SPL wave train on the longitudinal component is reproduced quite well. These findings suggest that wave energy is guided into the water and sedimentary layers seaward of the trench, where P and S wave reverberations are generated and leak as SV waves into the mantle. The guidance can be explained by trapping of wave energy within the wedge shaped accretionary domain and outer high, or in oceanic sediments which are subducted with the oceanic plate.

PL波の群速度に基づく地殻速度構造の推定について

Group velocities of PL waves recorded at Osakayama and Amagase Observatories of Kyoto University for earthquakes occurring in the east and northeast regions of Japan show a considerable regional difference with respect to propagation paths. The group velocities for propagation paths through the northern region (Tohoku and northern Chubu Districts) are much lower than those for propagation paths through the southern region of Honshu Island (Kanto and Southern Chubu Districts).Comparison between the observed and theoretical group velocities shows that the low group velocities for the propagation paths through the northern region should be attributed to a significant decrease in the velocity in the lower-crust. The theoretical group velocities were calculated by using OLIVER and MAJOR'S (1960) method, since our examination showed that this method was more appropriate than that of SU and DORMAN (1965).It is shown that a surface layer with low velocity, such as a sedimentary layer, could create a condition favorable for generation of large-amplitude PL waves. It is also shown that, at least in the range of period between 15 and 30 sec., group velocities of fundamental PL waves are more sensitive to shear velocity than to compressional velocity. There remains a problem, however, that it is difficult to explain dispersions both of Rayleigh waves and PL waves by one and the same structural model.

Study of elastoplastic deformation for cylindrical shells with axial shock loading

i. E. I. Shemyakin, "The Lamb problem for a medium with elastic post-action," Dokl. Akad. Nauk SSSR, 104, No. 2 (1955). 2. I. A. Viktorov, Surface Acoustic Waves in Solids [in Russian], Nauka, Moscow (1981). 3. Y. N. Tsai and H. Kolsky, "Surface wave propagation for linear viscoelastic," J. Mech. Phys. Solids, 16, No. 2 (1968). 4. K. Aki and P. Richards, Quantitative Seismology: Theory and Methods [Russian translation], Vol. i, Mir, Moscow (1983). 5. M. A. Lavrent'ev and B. V. Shabat, Methods of Theory of Functions of a Complex Variable [in Russian], Nauka, Moscow (1973). 6. N. I. Onis'ko and E. I. Shemyakin, "Motion of the free surface of a homogeneous soil during underground explosion," Zh. Prikl. Mekh. Tekh. Fiz., No. 4 (1961). 7. R. A. Phinney, "Leaking modes in crystal waveguide. Part i. The oceanic PL wave," J. Geophys. Res., 66, 1445 (1961). 8. A. P. Prudnikov, Yu. A. Brychkov, and O. I. Marichev, Integrals and Series, Gordon and Breach, New York (1986). 9. V. N. Nikolaevskii, K. S. Basniev, A. G. Gorbunov, and G. A. Zotov, Mechanics of Saturated Porous Media [in Russian], Nedra, Moscow (1970). i0. A. A. Oliner (ed.), Acoustic Surface Waves, Springer-Verlag (1978). ii. L. A. Vainshtein, "Pulse propagation," Usp. Fiz. Nauk, 118, No. 2 (1976). 12. S. Z. Dunin and G. A. Maksimov, "Acoustic waves in dissipative media," Preprint MIFI No. 039-85, Moscow (1985).

Volcanic tremor at Pavlof Volcano, Alaska, October 1973?April 1986

In thirteen years (1973–1986) of seismic monitoring of Pavlof Volcano, 488 episodes of volcanic tremor have been recorded, only 26 of which have been previously described in the literature. This paper tabulates and describes all the tremor episodes and reports on the results of all analyses to date. Pavlof tremor durations range from 2 minutes to greater than 1 week; episodes accompanying magmatic eruptions have durations greater than 1 hour, and sustained amplitudes of greater than 6 mmP-P (=54 nanometers at 1.5 Hz) on station PVV, 8.5 km from the vent. Digital data provide much better amplitude resolution than helicorders do. Helicorders, however, provide continuous coverage, whereas digital data are intermittent. Correlations of tremor with visual eruption observations shows that tremor amplitudes are roughly correlated with heights of lava fountains, but the correlation of tremor amplitudes with plume heights is more problematic. Fast Fourier Transform (FFT) spectra show that Pavlof tremor is quite statinary for the entire time period, 1973–1983. All principal spectral peaks lie between 0.8 and 3.0 Hz, and may be caused by resonance of magma and gas, and resonance of the volcanic pile. Preliminary analysis of 2-and 3-component data shows thatP, S, PL, and Rayleigh waves may be present in Pavlof volcanic tremor. Other waveforms can be misidentified as tremor, most commonly those caused by storms orS-waves of regional earthquakes. A strategy is proposed to distinguish tremor from noise using automatic seismic data acquisition and analysis systems. Pavlof's volcanic tremor is briefly compared with a preliminary sample of over 1100 cases of tremor from 84 volcanoes worldwide. Finally, several recommendations for monitoring and reporting volcanic tremor are discussed.

Propagation of PL and implications for the structure of Tibet

PL is a long‐period (20 s or more) wave train beginning just after the P arrival in seismograms and continuing until the S arrival or the Rayleigh wave. The wave train can be observed at epicentral distances of about 5°–20° with instruments sensitive to these low frequencies. PL propagates as a partially trapped P‐SV wave in the crust; S wave energy is lost to the mantle during propagation making PL a “leaky mode.” We study PL propagation for a variety of earth models using the synthetic seismogram algorithm wave number integration and find that the vertical travel time in the crust is the most important parameter controlling PL's oscillation period. This period can vary by more than a factor of two between oceanic and continental paths. P‐SV leaky mode propagation includes many different modes; the low‐frequency motion termed “PL” is only the first, or fundamental mode, in this family. A second, higher‐frequency mode roughly equal in amplitude to the fundamental appears for models without a net positive velocity gradient in the crust. We use these results to match an observed PL wave train whose propagation path consisted almost entirely of the Tibetan Plateau. By considering first and second PL mode behavior we find that the Tibetan crust is about 85 km thick, with an uncertainty of about 20 km, and possesses a significant (≥0.01 s−1) positive velocity gradient. The presence of a large, low‐velocity zone in the lower Tibetan crust thus seems unlikely. Much higher‐frequency PL modes also appear in the Green's functions for layer over half space models and appear to be responsible for the high‐frequency Pg phase, making PL and Pg different members of the same type of leaky mode propagation.

Propagation of dispersed compressional and Rayleigh waves on the Texas coastal plain

During a refraction profile on the Texas coastal plain, a strong surface wave with predominant frequencies between 2 and 10 Hz and group velocities near 1.9 km/sec was observed to ranges as great as 64 km. This dispersed wave, with velocities near the compressional wave speed of near‐surface sediments, corresponds to the “leaky” compressional or PL wave. PL dispersion can be predicted from the theory of the normal modes of a layered liquid medium. Efficient propagation of the PL wave is related to the high Poisson ratio of the unconsolidated sediments in the shallow subsurface, and additional examples from the published literature show that the PL wave is commonly excited by shallow sources both on land and offshore. In addition to the PL waves, dispersed waves with group velocities between 0.3 and 0.7 km/sec were observed at ranges less than 10 km. These are identified as Rayleigh waves (LR). Smoothly varying P and S velocity structures for the upper 1 km are obtained by fitting theoretical dispersion curves to the observed PL and LR data.

Modeling the long-period body waves from shallow earthquakes at regional ranges

A procedure for modeling P and PL waves recorded on long-period WWSSN instruments at ranges 1° to 12° is presented. Following the experience gained by modeling explosions (Helmberger, 1972), we demonstrate that these long-period phases are adequately treated by a single crustal layer for most of Western United States. After generating the Green's functions at the various ranges for the three fundamental dislocation types, we need only construct linear combinations of these vectors to represent any arbitrary oriented earthquake. The waveform patterns produced from the various fault types are quite diagnostic with the dip-slip orientations showing a strong ringing nature which is caused by the vertical SV lobes. To test the usefulness of this technique, we construct synthetics for some well-studied west coast earthquakes where the orientation, time history, and moment have been determined independently. Comparing the predicted seismograms with observations, we find good agreement in waveshapes and amplitudes.

Low-velocity zone variations in the southwestern United States

Time differences between arrivals of P and PL waves on 55 seismograms from 13 events were measured and compared with model study values. Variations are mapped as a point midway between station and event for each measurement. The results are presented as a contour map of large-scale lateral variations in the upper mantle beneath the western United States.

Seismic Source Descriptions of Underground Explosions and a Depth Discriminate

Synthetic seismograms of both body waves and Rayleigh waves are used to determine the radiation field of a few large contained underground explosions. A number of possible source descriptions are investigated. A reduced displacement potential of the form, o(t) = o_0t^ξ exp(-ηt), fits the long- and short-period data. The source parameters appropriate for the Boxcar event are ξ = 0·5 and η = 0·15. Synthetic PL and Rayleigh waves are compared with observations from a number of different size events to determine the dependence of η on yield. The amplitude of the long period synthetic body wave responses at ranges greater than about 12° increases rapidly as the source depth is increased. Thus the difference in spectral properties of explosions and earthquakes can be largely explained by the depth effect. The theoretical ratio SP/LP, that is the short period divided by the long-period amplitude, is computed from 12 to 25° for the Johnson upper mantle model and the Boxcar source. A study of an earthquake which cannot be distinguished from an explosion using the m_b vs. M_s criterion is investigated by the SP/LP discriminate.

The generation and properties of shear-coupled PL waves

abstract The dispersion of shear-coupled PL waves, PL(S) and PL(SS), has been studied for several paths on the Canadian Shield over the distance range 33.4° to 115.5° with the twofold aim of elucidating the manner in which these waves are generated and of examining their value in investigating the structure of the crust and upper mantle. In many cases these waves are composed of several wave trains quasi-stationary in period. These observations can be explained by considering the large amplitudes associated with cusps in the S-wave travel-time curve and the adjacent flat portion of the S-wave apparent velocity curve produced by velocity discontinuities in the upper mantle; little-dispersed PL(S) trains appear to be generated in regions where S waves of very high amplitudes impinge on the crust. In addition, the recovery of the periods of these individual wave trains in the PL(S) record can provide information on the crustal structure in a relatively localized region and on the ray parameter close to each cusp.

PL Waves and Crustal Structure in Canada

A study of the group velocity of PL for about fifty paths in Canada has been made. It is difficult to measure the dispersion of PL for long periods because two Airy phases arrive in the beginning of the wave train. It is also concluded that like Rayleigh waves PL waves cannot really give more than an S-velocity distribution because the partial derivatives in SV are too large compared to those in P for the period range where a reliable estimate of the dispersion can be obtained. The different dispersion curves are interpreted by looking for lateral variations of PL dispersion. As these curves have only one or two degrees of freedom, we label a curve with an index of dispersion. As in Santo's studies, this index is attributed to each region crossed by fitting the propagation times for a given period. Diagrams are then used giving the variation of the index with the average S velocity and the depth of the Moho. The structures found by this rather simple method are well correlated with tectonic regions and gravity measurements.