The Experts below are selected from a list of 3546 Experts worldwide ranked by ideXlab platform
Sharmin Shamsalsadati - One of the best experts on this subject based on the ideXlab platform.
-
near source Geometrical Spreading in the central virginia seismic zone determined from the aftershocks of the 2011 mineral virginia earthquake
Bulletin of the Seismological Society of America, 2016Co-Authors: Martin C. Chapman, Jacob N. Beale, Sharmin ShamsalsadatiAbstract:We used aftershocks of the 2011 Mineral, Virginia, earthquake to study Geometrical Spreading at hypocentral distances less than 60 km in the central Virginia seismic zone. Sixty‐nine aftershocks, occurring from 25 August 2011 through 24 December 2011, provided the data. We used the coda‐normalization method to estimate the attenuation coefficient associated with Geometrical Spreading. We filtered the time‐domain signals in several octave‐wide frequency bands and examined attenuation of peak S ‐wave amplitude in the 1.0–30.0 Hz frequency range. Amplitude was assumed to decrease as a function of hypocenter distance R according to R − γ . The coefficient of attenuation γ was examined for the three‐component S ‐wave amplitudes, with corrections for SH and SV radiation patterns. We observed no systematic frequency dependence of γ . The coefficient of attenuation for the radial and transverse components, assuming infinite quality factor Q , derived as a weighted mean over the entire range of frequencies (1–30 Hz), are both 1.51±0.05. The weighted mean value of the attenuation coefficient on the vertical component over the same range of frequencies is 1.45±0.05, slightly less than for the horizontal components. We corrected the data assuming three Q models. The estimated Geometrical Spreading coefficients are in the 1.30–1.46 range, depending on the assumed Q model and component, which is only slightly less than the estimates of γ determined assuming infinite Q . The estimated attenuation coefficients differ significantly from the value of 1.0 expected for a whole space. The results for the horizontal components are in agreement with previous full‐wavefield modeling. However, the observed vertical‐component attenuation is substantially less than that predicted by the synthetics. The depths of the earthquakes are less than 8 km, so these results may not be representative of Geometrical Spreading in parts of eastern North America where earthquakes occur at greater depths. Online Material: Table of earthquake hypocenters and focal mechanisms.
-
Near‐Source Geometrical Spreading in the Central Virginia Seismic Zone Determined from the Aftershocks of the 2011 Mineral, Virginia, Earthquake
Bulletin of the Seismological Society of America, 2016Co-Authors: Martin C. Chapman, Jacob N. Beale, Sharmin ShamsalsadatiAbstract:We used aftershocks of the 2011 Mineral, Virginia, earthquake to study Geometrical Spreading at hypocentral distances less than 60 km in the central Virginia seismic zone. Sixty‐nine aftershocks, occurring from 25 August 2011 through 24 December 2011, provided the data. We used the coda‐normalization method to estimate the attenuation coefficient associated with Geometrical Spreading. We filtered the time‐domain signals in several octave‐wide frequency bands and examined attenuation of peak S ‐wave amplitude in the 1.0–30.0 Hz frequency range. Amplitude was assumed to decrease as a function of hypocenter distance R according to R − γ . The coefficient of attenuation γ was examined for the three‐component S ‐wave amplitudes, with corrections for SH and SV radiation patterns. We observed no systematic frequency dependence of γ . The coefficient of attenuation for the radial and transverse components, assuming infinite quality factor Q , derived as a weighted mean over the entire range of frequencies (1–30 Hz), are both 1.51±0.05. The weighted mean value of the attenuation coefficient on the vertical component over the same range of frequencies is 1.45±0.05, slightly less than for the horizontal components. We corrected the data assuming three Q models. The estimated Geometrical Spreading coefficients are in the 1.30–1.46 range, depending on the assumed Q model and component, which is only slightly less than the estimates of γ determined assuming infinite Q . The estimated attenuation coefficients differ significantly from the value of 1.0 expected for a whole space. The results for the horizontal components are in agreement with previous full‐wavefield modeling. However, the observed vertical‐component attenuation is substantially less than that predicted by the synthetics. The depths of the earthquakes are less than 8 km, so these results may not be representative of Geometrical Spreading in parts of eastern North America where earthquakes occur at greater depths. Online Material: Table of earthquake hypocenters and focal mechanisms.
Martin C. Chapman - One of the best experts on this subject based on the ideXlab platform.
-
near source Geometrical Spreading in the central virginia seismic zone determined from the aftershocks of the 2011 mineral virginia earthquake
Bulletin of the Seismological Society of America, 2016Co-Authors: Martin C. Chapman, Jacob N. Beale, Sharmin ShamsalsadatiAbstract:We used aftershocks of the 2011 Mineral, Virginia, earthquake to study Geometrical Spreading at hypocentral distances less than 60 km in the central Virginia seismic zone. Sixty‐nine aftershocks, occurring from 25 August 2011 through 24 December 2011, provided the data. We used the coda‐normalization method to estimate the attenuation coefficient associated with Geometrical Spreading. We filtered the time‐domain signals in several octave‐wide frequency bands and examined attenuation of peak S ‐wave amplitude in the 1.0–30.0 Hz frequency range. Amplitude was assumed to decrease as a function of hypocenter distance R according to R − γ . The coefficient of attenuation γ was examined for the three‐component S ‐wave amplitudes, with corrections for SH and SV radiation patterns. We observed no systematic frequency dependence of γ . The coefficient of attenuation for the radial and transverse components, assuming infinite quality factor Q , derived as a weighted mean over the entire range of frequencies (1–30 Hz), are both 1.51±0.05. The weighted mean value of the attenuation coefficient on the vertical component over the same range of frequencies is 1.45±0.05, slightly less than for the horizontal components. We corrected the data assuming three Q models. The estimated Geometrical Spreading coefficients are in the 1.30–1.46 range, depending on the assumed Q model and component, which is only slightly less than the estimates of γ determined assuming infinite Q . The estimated attenuation coefficients differ significantly from the value of 1.0 expected for a whole space. The results for the horizontal components are in agreement with previous full‐wavefield modeling. However, the observed vertical‐component attenuation is substantially less than that predicted by the synthetics. The depths of the earthquakes are less than 8 km, so these results may not be representative of Geometrical Spreading in parts of eastern North America where earthquakes occur at greater depths. Online Material: Table of earthquake hypocenters and focal mechanisms.
-
Near‐Source Geometrical Spreading in the Central Virginia Seismic Zone Determined from the Aftershocks of the 2011 Mineral, Virginia, Earthquake
Bulletin of the Seismological Society of America, 2016Co-Authors: Martin C. Chapman, Jacob N. Beale, Sharmin ShamsalsadatiAbstract:We used aftershocks of the 2011 Mineral, Virginia, earthquake to study Geometrical Spreading at hypocentral distances less than 60 km in the central Virginia seismic zone. Sixty‐nine aftershocks, occurring from 25 August 2011 through 24 December 2011, provided the data. We used the coda‐normalization method to estimate the attenuation coefficient associated with Geometrical Spreading. We filtered the time‐domain signals in several octave‐wide frequency bands and examined attenuation of peak S ‐wave amplitude in the 1.0–30.0 Hz frequency range. Amplitude was assumed to decrease as a function of hypocenter distance R according to R − γ . The coefficient of attenuation γ was examined for the three‐component S ‐wave amplitudes, with corrections for SH and SV radiation patterns. We observed no systematic frequency dependence of γ . The coefficient of attenuation for the radial and transverse components, assuming infinite quality factor Q , derived as a weighted mean over the entire range of frequencies (1–30 Hz), are both 1.51±0.05. The weighted mean value of the attenuation coefficient on the vertical component over the same range of frequencies is 1.45±0.05, slightly less than for the horizontal components. We corrected the data assuming three Q models. The estimated Geometrical Spreading coefficients are in the 1.30–1.46 range, depending on the assumed Q model and component, which is only slightly less than the estimates of γ determined assuming infinite Q . The estimated attenuation coefficients differ significantly from the value of 1.0 expected for a whole space. The results for the horizontal components are in agreement with previous full‐wavefield modeling. However, the observed vertical‐component attenuation is substantially less than that predicted by the synthetics. The depths of the earthquakes are less than 8 km, so these results may not be representative of Geometrical Spreading in parts of eastern North America where earthquakes occur at greater depths. Online Material: Table of earthquake hypocenters and focal mechanisms.
-
Modeling Geometrical Spreading and the Relative Amplitudes of Vertical and Horizontal High‐Frequency Ground Motions in Eastern North America
Bulletin of the Seismological Society of America, 2012Co-Authors: Martin C. Chapman, R. W. GodbeeAbstract: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.
-
modeling Geometrical Spreading and the relative amplitudes of vertical and horizontal high frequency ground motions in eastern north america
Bulletin of the Seismological Society of America, 2012Co-Authors: Martin C. Chapman, R. W. GodbeeAbstract: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.
Charles A. Langston - One of the best experts on this subject based on the ideXlab platform.
-
wave gradiometry for usarray rayleigh waves
Journal of Geophysical Research, 2009Co-Authors: Chuntao Liang, Charles A. LangstonAbstract:[1] Wave gradiometry (WG) is a new array data processing technique to extract phase velocity, wave directionality, Geometrical Spreading, and radiation pattern from spatial gradients of waveforms. A weighted inversion method and a reducing velocity method are introduced to compute spatial gradients accurately for irregular arrays. Numerical experiments are conducted to test techniques and to evaluate the parameters determined from the WG method. We apply this method to USArray data for the western United States. In this study, Rayleigh waves from nine earthquakes with varying azimuths are analyzed. The stability of this method is shown by the similarity between the results from two nearly collocated earthquakes from the Kurile Islands. The error check shows the WG results are stable for ambient noise level as high as 10%. Phase velocities determined by WG and two station (TS) methods are statistically consistent, while these determined from beam forming method are systematically higher for wavelength larger than one quarter of the array diameter. Our results show that, first, the average phase velocities of Rayleigh waves range from 3.8 to 4.1 km/s for periods from 60 s to 150 s. This is consistent with average earth models. The prominent feature on the phase velocity map is that the Basin and Range province is dominated by velocity lows while the west coast of the United States and the north and northeastern Snake River plain are dominated by velocity highs. The Snake River plain appears to be a primary tectonic boundary. Second, azimuthal variations represent the accumulated wave directionality changes along the raypath. A velocity contrast of 0.25 km/s across the oceanic-continental lithosphere boundary along the west coast of the United States is needed to explain the negative azimuth variations. Third, Geometrical Spreading is slightly anticorrelated with phase velocity, which may suggest that amplitude variations in radial directions are subject to surface wave focusing and defocusing. Fourth, similar to the wave directionality, radiation pattern variations also exhibit strong path dependence. Further theoretical and experimental studies will be conducted to understand the two amplitude parameters: Geometrical Spreading and radiation pattern and their relations with the local geophysical properties.
-
Wave Gradiometry in the Time Domain
Bulletin of the Seismological Society of America, 2007Co-Authors: Charles A. LangstonAbstract:A time-domain approach for solving for the change in Geometrical Spreading and horizontal wave slowness in wave gradiometry is presented based on the use of the analytic signal. The horizontal displacement gradient of a wave is linearly related to the displacement and its time derivative. The coefficients of this relationship give the change of Geometrical Spreading, the change in radiation pattern, and horizontal slowness. The new time-domain technique incorporates estimates of the instantaneous amplitude and frequency of the three time series to solve uniquely for the wave-field coefficients. The analysis is simpler and more suited to fast array processing of displacement gradient data sets compared with a spectral ratio method.
Shahram Pezeshk - One of the best experts on this subject based on the ideXlab platform.
-
Near-source attenuation of high-frequency body waves beneath the New Madrid Seismic Zone
Journal of Seismology, 2018Co-Authors: Shahram Pezeshk, Farhad Sedaghati, Nima NazemiAbstract:Attenuation characteristics in the New Madrid Seismic Zone (NMSZ) are estimated from 157 local seismograph recordings out of 46 earthquakes of 2.6 ≤ M ≤ 4.1 with hypocentral distances up to 60 km and focal depths down to 25 km. Digital waveform seismograms were obtained from local earthquakes in the NMSZ recorded by the Center for Earthquake Research and Information (CERI) at the University of Memphis. Using the coda normalization method, we tried to determine Q values and Geometrical Spreading exponents at 13 center frequencies. The scatter of the data and trade-off between the Geometrical Spreading and the quality factor did not allow us to simultaneously derive both these parameters from inversion. Assuming 1/ R ^1.0 as the Geometrical Spreading function in the NMSZ, the Q _P and Q _S estimates increase with increasing frequency from 354 and 426 at 4 Hz to 729 and 1091 at 24 Hz, respectively. Fitting a power law equation to the Q estimates, we found the attenuation models for the P waves and S waves in the frequency range of 4 to 24 Hz as Q _P = (115.80 ± 1.36) f ^(0.495 ± 0.129) and Q _S = (161.34 ± 1.73) f ^(0.613 ± 0.067), respectively. We did not consider Q estimates from the coda normalization method for frequencies less than 4 Hz in the regression analysis since the decay of coda amplitude was not observed at most bandpass filtered seismograms for these frequencies. Q _S/ Q _P > 1, for 4 ≤ f ≤ 24 Hz as well as strong intrinsic attenuation, suggest that the crust beneath the NMSZ is partially fluid-saturated. Further, high scattering attenuation indicates the presence of a high level of small-scale heterogeneities inside the crust in this region.
-
estimation of the coda wave attenuation and Geometrical Spreading in the new madrid seismic zone
Bulletin of the Seismological Society of America, 2016Co-Authors: Farhad Sedaghati, Shahram PezeshkAbstract:Using the single backscattering method, coda quality factor functions through coda window lengths of 20, 30, 40, 50, and 60 s have been estimated for the New Madrid seismic zone (NMSZ). Furthermore, Geometrical Spreading functions for distances less than 60 km have been determined in this region at different center frequencies exploiting the coda normalization method. A total of 284 triaxial seismograms with good signal‐to‐noise ratios (SNR>5) from broadband stations located in the NMSZ were used. The database consisted of records from 57 local earthquakes with moment magnitudes of 2.6–4.1, and hypocentral distances less than 200 km. Q ‐factor values were evaluated at five frequency bands with central frequencies of 1.5, 3, 6, 12, and 24 Hz. Vertical components were utilized to estimate vertical coda Q ‐factor values. Horizontal coda Q ‐factor values were determined using the average amount of the Q ‐factor values estimated from two orthogonal horizontal components. The coda Q ‐factor increases with increasing of the coda window length implying that with increasing the depth, the coda Q ‐factor increases. The intermediate values of the Q ‐factor and intermediate values of the frequency dependency indicate that the Earth’s crust and upper mantle beneath the entire NMSZ is tectonically a moderate region with a moderate to relatively high degree of heterogeneities. The Geometrical Spreading factors of S ‐wave amplitudes are frequency dependent and determined to be −0.761, −0.991, −1.271, −1.182, and −1.066 for center frequencies of 1.5, 3, 6, 12, and 24 Hz, respectively, at hypocentral distances of 10–60 km. The Geometrical Spreading factors for lower frequencies are not recommended to be used due to the greater impact of the radiation pattern and directivity effect on low frequencies, as well as the greater sensitivity of band‐pass‐filtered seismograms of small earthquakes to the noise in low frequencies.
-
Estimation of the Coda‐Wave Attenuation and Geometrical Spreading in the New Madrid Seismic Zone
Bulletin of the Seismological Society of America, 2016Co-Authors: Farhad Sedaghati, Shahram PezeshkAbstract:Using the single backscattering method, coda quality factor functions through coda window lengths of 20, 30, 40, 50, and 60 s have been estimated for the New Madrid seismic zone (NMSZ). Furthermore, Geometrical Spreading functions for distances less than 60 km have been determined in this region at different center frequencies exploiting the coda normalization method. A total of 284 triaxial seismograms with good signal‐to‐noise ratios (SNR>5) from broadband stations located in the NMSZ were used. The database consisted of records from 57 local earthquakes with moment magnitudes of 2.6–4.1, and hypocentral distances less than 200 km. Q ‐factor values were evaluated at five frequency bands with central frequencies of 1.5, 3, 6, 12, and 24 Hz. Vertical components were utilized to estimate vertical coda Q ‐factor values. Horizontal coda Q ‐factor values were determined using the average amount of the Q ‐factor values estimated from two orthogonal horizontal components. The coda Q ‐factor increases with increasing of the coda window length implying that with increasing the depth, the coda Q ‐factor increases. The intermediate values of the Q ‐factor and intermediate values of the frequency dependency indicate that the Earth’s crust and upper mantle beneath the entire NMSZ is tectonically a moderate region with a moderate to relatively high degree of heterogeneities. The Geometrical Spreading factors of S ‐wave amplitudes are frequency dependent and determined to be −0.761, −0.991, −1.271, −1.182, and −1.066 for center frequencies of 1.5, 3, 6, 12, and 24 Hz, respectively, at hypocentral distances of 10–60 km. The Geometrical Spreading factors for lower frequencies are not recommended to be used due to the greater impact of the radiation pattern and directivity effect on low frequencies, as well as the greater sensitivity of band‐pass‐filtered seismograms of small earthquakes to the noise in low frequencies.
-
Investigation of Geometrical Spreading and Quality Factor Functions in the New Madrid Seismic Zone
Bulletin of the Seismological Society of America, 2010Co-Authors: Arash Zandieh, Shahram PezeshkAbstract:The accuracy and applicability of Geometrical Spreading and quality factor functions are investigated for the New Madrid seismic zone (NMSZ) using recorded small and moderate earthquakes. These functions represent the path effect in frequency domain. The database used in this study consists of 500 broadband seismograms from 63 eventsofmagnitudeMw 2.5to5.2,recordedbytheCenterforEarthquakeResearchand Information (CERI) at the University of Memphis. The hypocentral distances range from 10 to 400 km. All the broadband stations are located within the Mississippi embayment with different site conditions. The vertical components of the records areprocessedandusedtodefinethepatheffectterminfrequencyrangeof0.2to30Hz. A hinged-trilinear Geometrical Spreading and frequency-dependent quality factor functions are used to describe the path term. The regression analysis using a genetic algorithm(GA)indicatesthatatdistanceslessthan70kmthespectralamplitudesdecay as R � 1 ; between 70 and 140 km spectral amplitudes increase with distance and the geometric Spreading is defined as R � 0:25 ; beyond 140 km, the attenuation is described by R � 0:5 . The quality factor function is expressed as Q � 614f 0:32 for frequencies greater than 1 Hz after the regression analysis. For the broader range offrequency used in this study (0.2 to 30 Hz), the Q function is described by a third-degree polynomial described as logQf �� 2:898 � 0:464logf � 1:238� logf� 2 � 0:540� logf� 3 . The results of this study are compared with those of Atkinson (2004) and Samiezade-Yazd etal.(1997).Thepathtermobtainedinthisstudycanbeusedinthestochasticmethodto predict ground motions in the NMSZ and eastern North America (ENA).
Juan Madrid - One of the best experts on this subject based on the ideXlab platform.
-
A Geometrical approach to time evolving wave fronts
Geophysical Journal International, 2008Co-Authors: Juan MadridAbstract:SUMMARY The parameter that defines the ray tracing equations in the direct Geometrical approach is the product of the radius of curvature of the wave front by the velocity on the wave front (RV). To show this, we derive motion equations for the centre and the radius of curvature of an expanding wave front. The continuity of RV along rays implies Snell's Law. For constant velocities the equation for the radius of curvature reduces to the original Huygens' Principle. The variable RV can be computed during ray tracing and used to determine the local radius of curvature, which in turn can be used in Geometrical Spreading, amplitude corrections and structure interpretation.