Lg Phase Research Articles

Investigation of Site Response in Xinjiang, Northwestern China, from S–Lg Fourier Amplitude Spectra

ABSTRACT Abundant seismic waveforms have been collected in Xinjiang since 2007, following the deployment of permanent stations by the China National Seismic Network and the Xinjiang Seismic Network. We investigated regional attenuation of the S–Lg phase in the crustal waveguide and site response in Xinjiang using broadband recordings at 44 stations from 118 earthquakes with magnitudes of 4.4–7.0 occurring in Xinjiang and adjacent areas between January 2009 and February 2022. We used linear regression analysis of the Fourier acceleration amplitude spectra in 18 frequency bands from 0.1 to 24.86 Hz to estimate site response terms and distance-dependent anelastic attenuation quality factor Q in the crustal waveguide at regional distances. The regression site terms of stations located in the Tarim and Junggar Basins show significant amplification at low frequencies and strong attenuation at higher frequencies due to the presence of thick sedimentary deposits. We quantified the linear behavior of the logarithms of the high-frequency site terms versus frequency using the zero distance attenuation parameter κ0 (Anderson and Hough, 1984). A positive correlation was observed between κ0 and the thickness of the sedimentary layers in Xinjiang. We observed some consistent patterns in the behavior of the site response terms as a function of sediment thickness over the frequency range of 0.1–24.86 Hz. Linear functions of sediment thickness were used to model the site response in Xinjiang at each of the 18 frequencies. An alternative approach that adopted the κ0 model as a function of sediment thickness can be used to estimate site responses at high frequencies (≥5.66 Hz). The results of this study demonstrate the nature of site effects on earthquake ground motions caused by the thick sedimentary deposits in Xinjiang and provide a preliminary site response model as a function of sediment thickness for this region.

Two-Dimensional Attenuation and Velocity Tomography of Iran

Seismic bulletin data collected by the Iranian Seismological Center are used to image crust and mantle seismic attenuation, group velocity, and phase velocities for Lg, Pg, Sn, and Pn phases. This is possible because the peak amplitude time is picked, and amplitude measurements can be associated with the phase based on travel time plots. The group velocity is the apparent velocity of the maximum amplitude arrival and represents the combined effect of phase velocity and seismic scattering. Thus, it can be used in combination with the attenuation to identify where scattering attenuation is dominant. The Arabian–Iranian plate boundary separates low-velocity Zagros sediments from central Iran; however, in the mantle, it separates a high-velocity Arabian shield from central Iran. Scattering attenuation is low within the Arabian mantle and crust, and the Zagros sediments do not cause Lg or Pg attenuation. The Eocene Urumieh Dokhtar Magmatic Arc has high attenuation within both the crust and mantle, and while there is no partial melting in the crust, there may be some in the mantle. The northern Eocene Sistan Suture Zone shows particularly high attenuation that is accompanied by high scattering. It represents an incompletely closed ocean basin that has undergone intense alteration. The Alborz Mountains have high attenuation with some scattering.

Crustal attenuation from USArray ML amplitude tomography

SUMMARYSeismic attenuation across the US is estimated using station ML magnitude data from the USArray. Station magnitudes are recalibrated back to amplitude and back projected in a 2-D tomography. Data represent the amplitudes of the horizontal components of the Lg phase. The western US shows regions of very high attenuation and contrasts with the lesser attenuation of the eastern US. Individual attenuation anomalies can be clearly tied to regional geology. Station gains show broad regional variations that match geographic regions. Most of the high-attenuation areas are regions of high geothermal activity suggesting that intrinsic attenuation dominates over scattering attenuation. An exception is the central San Andreas Fault zone because it lacks any localized heat-flow anomaly. The US east of the Rocky Mountains is bland and contains none of the high-attenuation regions of the western US. Instead, the central US has low-attenuation patches that do not obviously correspond to geologic province. Sediments of the Gulf Coast Plain, Willison Basin and Michigan Basin do show up as intermediate attenuation while the Illinois Basin, Appalachian Basin and other basins are not apparent. In Alaska, attenuation is generally less than the western US, but still much greater than the eastern US. In southeast Alaska, the Wrangell Volcanic Field causes a sizeable high-attenuation zone. The volcanic Aleutian Mountains also have high attenuation. However, moderate to high attenuation also correlates with the tertiary sedimentary basins in Alaska. The North Slope Basin does not seem to attenuate. Thicker crust and mountain roots tend to show less attenuation, if anything, but this correspondence is most likely due to differences in temperature and seismic velocity. Heat, scattering and young sedimentary basins create seismic attenuation in the continental crust.

Earthquake Magnitude and Lg Q Variations between the Grenville and Northern Appalachian Geologic Provinces of Eastern Canada

ABSTRACTThis article assesses the ability of regionally specific, frequency-dependent crustal attenuation (1/Q) to reduce mean magnitude discrepancies between seismic stations in the northern Appalachian and Grenville provinces (NAP and GP) of Canada. LgQ(f) is an important parameter in ground-motion models used in probabilistic seismic hazard analysis. Discrepancies in regional magnitude estimates have long been noted to exist between stations in the two provinces for common event origins. Such discrepancies could arise from systematic site condition variations between the geologic provinces or from varying crustal attenuative properties. To evaluate the effect of frequency-dependent anelastic attenuation, LgQ(f) on estimated magnitudes, we analyze Lg amplitudes from >6000 waveforms recorded by Grenville and northern Appalachian receivers from 420 natural earthquakes of MN magnitude 3–5.6. Waveform analysis is strictly limited to analyst-reviewed, vertical-component waveforms in which Lg is clearly identified, ensuring that the datasets exhibit dominant, high-frequency energy in the Lg velocity window. LgQ(f) is found to be higher in the GP than in the northern Appalachians. In the Grenville, Q(f)=761(±145)f0.25(±0.014), and in the northern Appalachians, attenuation is higher: Q(f)=506(±172)f0.33(±0.310). Earthquake magnitude determined using the peak amplitude of the Lg phase (mbLg) for eastern Canada is corrected to incorporate the frequency-dependent, regionally specific LgQ(f) determined in this study. Using the new LgQ(f) values diminishes and nearly resolves magnitude discrepancies between the provinces. Correcting regional magnitude discrepancies between provinces is critical for reliable regional seismic hazard estimates because magnitude error in a particular region could lead to increased uncertainty in seismic hazard models.

Crustal structure of western Tibet revealed by Lg attenuation tomography

The lateral Lg wave attenuation in western Tibet and surrounding regions is studied. A total of 706 Two-station method (TSM) and 533 Reverse two-station method (RTSM) pairs are generated from 282 regional earthquakes. The earthquakes are recorded at 48 seismic stations operated across western Kunlun and western Tibet regions. The Lg phases are extracted from the vertical component of seismograms with magnitude greater than 4.6 mb and focal depth less than 50 km to measure the TSM and RTSM Lg Q at 1 Hz (Q0). The LgQ0 tomography map is generated with grid-size 0.5∘×0.5∘ using the least-squares orthogonal factor decomposition (LSQR) algorithm. Lateral variation of Lg attenuation is found to exist across the region. The results provide new insights into the attenuation properties of the crust and its linkage with regional geodynamics. The low LgQ0<50 values are observed in TSM image along the Kunlun Suture, Tinashuihai terrane and in some pockets of north western side of the Lhasa terrane. The high LgQ0>200 is revealed in the central part of Lhasa terrane in western Tibet from both TSM and RTSM images. The high LgQ0 values complemented with the high Pn velocities beneath the central part of Lhasa terrane reveal the trace of underthrusting Indian lithosphere beneath the western Tibet.

Rotational motions from the 2016, Central Italy seismic sequence, as observed by an underground ring laser gyroscope

We present the analysis of rotational and translational ground motions from earthquakes recorded during October-November 2016, in association with the Central Italy seismic sequence. We use co-located measurements of the vertical ground rotation rate from a large ring laser gyroscope and the three components of ground velocity from a broad-band seismometer. Both instruments are positioned in a deep underground environment, within the Gran Sasso National Laboratories of the Istituto Nazionale di Fisica Nucleare. We collected dozens of events spanning the 3.5-5.9 magnitude range and epicentral distances between 30 and 70 km. This data set constitutes an unprecedented observation of the vertical rotational motions associated with an intense seismic sequence at local distance. Under the plane-wave approximation we process the data set in order to get an experimental estimation of the events backazimuth. Peak values of rotation rate (PRR) and horizontal acceleration (PGA) are markedly correlated, according to a scaling constant which is consistent with previous measurements from different earthquake sequences. We used a prediction model in use for Italy to calculate the expected PGA at the recording site, obtaining consequently predictions for PRR. Within the modelling uncertainties, predicted rotations are consistent with the observed ones, suggesting the possibility of establishing specific attenuation models for ground rotations, like the scaling of peak velocity and peak acceleration in empirical ground-motion prediction relationships. In a second step, after identifying the direction of the incoming wavefield, we extract phase-velocity data using the spectral ratio of the translational and rotational components. This analysis is performed over time windows associated with the P-coda, S-coda and Lg phase. Results are consistent with independent estimates of shear wave velocities in the shallow crust of the Central Apennines.

High-frequency ground motion from Australian earthquakes

ABSTRACTThe dominant high-frequency component of ground motion for most seismic events in Australia comes from the crustal Lg phase that can commonly propagate efficiently to considerable distances across the continent. Using results from recent tomography for the attenuation of Lg, we are able to build the pattern of transmission of ground motion from events within the continent. The effects are readily displayed in terms of a distribution of effective magnitude. The same approach can also be used to look at the cumulative effects across the continent from 1840 to date as a useful complement to other ways of assessing earthquake potential.

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.

Regional seismic wave propagation (Lg & Sn phases) in the Amerasia Basin and High Arctic

Observation of Lg seismic waves at regional distances has long been considered indicative of continental crust that is 30–40 km thick. This study updates an earlier assessment of Lg propagation efficiency to characterize continental or non-continental crust and related structures across the Amerasia Basin and surrounding continental areas of the high Arctic. Recent refraction surveys and receiver function studies provide crustal thickness estimates of 18–41 km for comparison. Among 7000 candidate earthquake–station pairs considered, no classic Lg phases (0.14–2 Hz) are observed to cross the Amerasia Basin (Canada Basin, Alpha-Mendeleev Ridge) efficiently, but lower frequency (0.035–0.17 Hz) arrivals with group velocities intermediate between Lg and Sn (sometimes called early Lg) are observed along many ray paths crossing the basin. The characteristic frequencies of these observed arrivals match well with those of synthetic waves propagated within models with thinned or pinched continental crust, such as crust characteristic of the North Sea, and thus suggest that most parts of the Amerasia Basin have a crustal thickness intermediate between that typical of thin continental and oceanic crust.

An Inversion of Lg-Wave Attenuation and Site Response from Seismic Spectral Ratios in the Eastern China Region

Abstract Both Lg ‐wave attenuation and site responses are determined in the eastern China region, the region ranges from 26° N to 33° N and 117° E to 124° E. We analyze over 1360 three‐component seismograms from 92 local and regional earthquakes, the seismograms are recorded in the distance range of 100–600 km. The earthquakes occurred between 2002 and 2012 in the region and varied in magnitude between 2.0 and 4.7 ( M L ). A combined inversion method, which can simultaneously estimate attenuation and site response from seismic spectral ratios, is implemented in the study. Modeling errors are determined using a delete‐j jackknife resampling technique. Fourier spectral amplitudes are measured from Lg phases windowed between the group velocity limits 2.6 and 3.6 km/s. The inversion are performed in a frequency range of 1–6 Hz with an interval of 0.2 Hz, The frequency‐dependent quality factor is modeled in the form of Q = Q 0 f η . The eastern China region displays high Q 0 and a weaker frequency dependence characterized by the functions Q =335±16 f 0.45±0.02 , Q =411±21 f 0.26±0.01 , and Q =432±26 f 0.24±0.01 for vertical, north–south, and east–west components, respectively, the values are similar to the average for other stable regions of the globe. Site responses from 32 stations are estimated in the region, most stations do not show obvious amplification. The site responses are mainly smooth, flat, and less than 5 in the frequency range of 1–6 Hz.

An Investigation of MbLg Versus Mw for Eastern North America

Research Article| May 01, 2014 An Investigation of MbLg Versus Mw for Eastern North America Christopher Rigsby; Christopher Rigsby aDepartment of Earth and Atmospheric Sciences, Saint Louis University, 3642 Lindell Boulevard, St. Louis, Missouri 63108 U.S.A.rbh@eas.slu.edu Search for other works by this author on: GSW Google Scholar Robert B. Herrmann; Robert B. Herrmann aDepartment of Earth and Atmospheric Sciences, Saint Louis University, 3642 Lindell Boulevard, St. Louis, Missouri 63108 U.S.A.rbh@eas.slu.edu Search for other works by this author on: GSW Google Scholar Harley Benz Harley Benz bNational Earthquake Information Center, U.S. Geological Survey, Denver Federal Center, Box 25046, MS 966, Denver, Colorado 80225 U.S.A.benz@usgs.gov Search for other works by this author on: GSW Google Scholar Author and Article Information Christopher Rigsby aDepartment of Earth and Atmospheric Sciences, Saint Louis University, 3642 Lindell Boulevard, St. Louis, Missouri 63108 U.S.A.rbh@eas.slu.edu Robert B. Herrmann aDepartment of Earth and Atmospheric Sciences, Saint Louis University, 3642 Lindell Boulevard, St. Louis, Missouri 63108 U.S.A.rbh@eas.slu.edu Harley Benz bNational Earthquake Information Center, U.S. Geological Survey, Denver Federal Center, Box 25046, MS 966, Denver, Colorado 80225 U.S.A.benz@usgs.gov Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2014 by the Seismological Society of America Seismological Research Letters (2014) 85 (3): 625–630. https://doi.org/10.1785/0220130138 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Christopher Rigsby, Robert B. Herrmann, Harley Benz; An Investigation of MbLg Versus Mw for Eastern North America. Seismological Research Letters 2014;; 85 (3): 625–630. doi: https://doi.org/10.1785/0220130138 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search Since the introduction of the concept of magnitude to seismology by Richter (1935), many different magnitude scales have been proposed. The United States Geological Survey National Earthquake Information Center (NEIC) routinely uses many of these, including moment magnitude, Mw (Hanks and Kanamori, 1979); energy magnitude, Me (Choy and Boatwright, 1995); surface‐wave magnitude, Ms (International Association of Seismology and Physics of the Earth’s Interior, IASPEI); short‐period teleseismic P‐wave magnitude, mb (Gutenberg and Richter, 1956); local magnitude, ML (Richter, 1935; Hutton and Boore, 1987); surface‐wave magnitude... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The 12 February 2013 North Korean Underground Nuclear Test

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Modeling Geometrical Spreading and the Relative Amplitudes of Vertical and Horizontal High-Frequency Ground Motions in Eastern North America

Abstract Horizontally layered velocity models were used with point‐source and finite‐fault sources to investigate geometrical spreading and the relative amplitudes of vertical and horizontal ground acceleration within 120 km of the source. Full‐wave‐field simulations were done for a range of focal depths and for strike‐slip and reverse focal mechanisms. The attenuation of the geometric mean of randomly oriented horizontal‐component maximum acceleration amplitudes, averaged over all azimuths, significantly exceeds the theoretical geometrical spreading for far‐field body waves in a homogeneous whole space for hypocentral distances less than approximately 60 km. The behavior of the vertical component is different from the horizontal: vertical attenuation near the epicenter is greater and is more dependent on source mechanism and depth. Because of the rapid near‐source decay of the direct S wave, reflections from the mid‐lower crust and Moho control the maximum amplitude of the vertical‐component acceleration in the 60–120‐km hypocenter distance range, resulting in a flattening of the vertical amplitude‐distance relation. Near‐source vertical maximum amplitudes averaged over all source–receiver azimuths tend to be less than the geometric mean horizontal amplitude for strike‐slip focal mechanisms, but, near the source for reverse faults, the azimuthally averaged vertical‐component amplitude exceeds that of the geometric mean horizontal. The modeling indicates that similar vertical‐ and horizontal‐component geometrical spreading and approximately constant horizontal/vertical amplitude ratios observed in connection with the Lg phase at distances greater than approximately 100 km in eastern North America may not hold at smaller distances. Ground‐motion prediction models for the vertical component near the source may need to incorporate strong geometrical spreading and dependence on radiation pattern.

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.

Improvements to Seismic Monitoring of the European Arctic Using Three-Component Array Processing at SPITS

Abstract The detectability of low magnitude seismic events in the European Arctic is determined primarily by the small-aperture International Monitoring System arrays ARCES and SPITS. In August 2004, the SPITS array was upgraded to a broadband array with an increase in the sampling rate from 40 to 80 Hz. Most important, however, for the detection and location of small-magnitude seismic events was the deployment of three-component instruments at six of the nine sites. Detection and correct classification of secondary phases are of paramount importance for events observed by only a small number of stations at regional distances; and, in the absence of the strong Lg phases typically observed for continental propagation paths, multiple three-component stations were deemed necessary to exploit the higher S -phase amplitudes anticipated on the horizontal sensors. We demonstrate improved signal-to-noise ratios (SNRs) for S phases on horizontal beams for several events close to Novaya Zemlya. Horizontal component f - k analysis improves direction estimates and phase classification for low-SNR signals. We demonstrate secondary phases that are misidentified by vertical-only f - k analysis but which are correctly classified by three-component array processing. A significant problem with array processing at SPITS is the overlap in slowness space of regional P and S phases. Phase identification is improved greatly by comparing the coherence between vertical traces with the coherence between horizontal traces. Considerations in the routine array processing of SPITS data are reviewed, including the need for elevation corrections in slowness estimation and the need to take into account azimuth-dependent variation of apparent velocity estimates for regional phases.

Reliable Lg Arrival-Time Picks and Potential for Enhanced Epicenter Location

Lg waves are generally the most prominent phase in seismic records at local and regional distances. They are dispersive and have complex waveforms. Their propagation characteristics, such as phase and group velocity, are not as easily related to formal crustal model features, as are P and S velocities. However, slowly propagating Lg waves may provide important travel-time observations in the context of epicenter location, but this potential has not been realized. The main reason is that Lg phases arrive in the coda of S waves and that the original waveforms are complex. The latter was resolved by replacing waveforms with their envelopes using the short-term average (STA) transform. We address the Lg arrival-time picking problem by associating time picks with the Lg amplitude maxima ( Z component) in the record envelope. Validations of Lg onsets are not easy due to the lack of accurate Lg travel-time curves. Instead Lg times are converted to group velocities and station-wise consistency of velocities for closely spaced earthquake sources is required. With rare exceptions these group velocities agree within ![Graphic][1] . We also tested the potential of our Lg arrival times for the epicenters location using the grid search scheme of Rodi (2006). On a stand alone basis the locations were almost identical to the International Seismological Centre (ISC) and the National Earthquake Information Center (NEIC) epicenter solutions. A practical problem with introducing joint P - Lg locations is that these phases do not have similar wave paths, and besides Lg travel times are not focal depth dependent. The latter result stems from our Lg synthetic record analysis. We conclude that the Lg arrival-time picking procedure developed is robust and convenient for near real time analysis. Because of its slow propagation velocities between 2.8 and 3.7 km/s, use of Lg arrival times can enhance focal parameter estimates reported by seismological centers around the world. [1]: /embed/inline-graphic-1.gif

한반도 지역의 저주파 Lg파 감쇠상수

Lg amplitude decrease becomes very important when harzadous earthquakes occurred in neighbor nations such as Japan and China because the Lg phase is the largest seismic phase in Korea. The values of Lg are correlated with seismic activities from the RTSM for IRIS data with long pair distance of stations and events. The Japanese paths crossing the East Sea (Sea of Japan) show high values related with seismicity and oceanic crust. The paths of Shanghai having the moderate seismicity show the second highest values followed by the Japanese region, while the paths between Manchuria and the Korean Peninsula exhibit low values associated with the low seismicity of the regions.

Calibration of Acoustic Gauge in the Field Using Seismic Lg Phase and Coupled High-Frequency Local Infrasound

Abstract Local microphone field calibration can be performed using seismic signals recorded by a well calibrated seismometer. When the 29 May 2004 regional earthquake ( M w 5.1) occurred off the coast of Korea, the Pn , Pg , and Lg seismic phases produced high-frequency (1–4 Hz) local infrasound signals at the Chulwon Seismo-Acoustic Array (CHNAR), an array deployed in the Republic of Korea. Instrument-corrected waveforms of seismic signals are compared to those of local infrasound signals using time-varying coherence estimates to identify the time window and bandwidth for the calibration process. Based on this assessment, the first 6.4 sec of the Lg phase and the coupled infrasound signals from 1 to 4 Hz are used. Envelope functions for the instrument-corrected Lg phase and local infrasound signal are calculated and integrated in the time domain. The transfer function from ground velocity to atmospheric pressure perturbation is estimated in the time domain using the ratio of the integrated envelope functions. The observed transfer functions estimated in this way are compared to the theoretical transfer function based on a model relating ground velocity to pressure perturbation, which is dependent on the density of air and speed of sound at the surface. A maximum difference of 4 dB is apparent between observed transfer functions and theory for highly correlated seismic and coupled infrasound signals (>0.8). Six factors are considered as contributors to these differences, including the porous hose array attached to each acoustic sensor for noise reduction, variations in the density of air or speed of sound, local perturbations in air pressure, differences in ground velocity at each sensor, and differences in acoustic instrument sensitivity. Variations of the acoustic gauge’s sensitivities at CHNAR are identified using a laboratory calibration procedure and documented as the major contributor to the difference between observed and theoretical transfer function at CHNAR.

Lg Attenuation in a Region with Both Continental and Oceanic Environments

Abstract The crustally-guided shear wave, Lg , is typically the strongest phase at regional distances. Lg phases are analyzed often for estimation of magnitudes of regional events. The variation of Lg in regions with both continental and oceanic environments has been rarely investigated. We investigate Lg attenuation in a plate-margin area around Korea and Japan that is encompassed by seas and oceans. The mean quality factor of Lg at 1 Hz ( Q 0 ) is 498. The geometrical-spreading exponent term is estimated as 0.75. These observations characterize the study region as a high attenuation region compared to typical intraplate continental regions. Low- Q regions are widely developed over the Japanese islands, with some spotted high- Q regions. Relatively high Q is observed in most regions of the Korean Peninsula except a southeastern part where a Cretaceous volcanic-sediment basin exists. The high- Q values are close to the typical Q value in continental crust. Significantly low Q of 100 or less is observed in most oceanic regions including the East Sea (Sea of Japan), the Pacific Ocean, and the South China Sea. The high attenuation of Lg in oceanic regions suggests possible underestimation of magnitudes of oceanic events. Thus, it appears that proper correction of Lg amplitude is highly desired for accurate estimation of magnitudes of regional oceanic events.