Various velocity models have been built for Southeast Qinghai-Xizang Plateau with the purpose of revealing the internal dynamics and estimating local seismic hazards.In this study,we use a 3-D full-waveform modeling p...Various velocity models have been built for Southeast Qinghai-Xizang Plateau with the purpose of revealing the internal dynamics and estimating local seismic hazards.In this study,we use a 3-D full-waveform modeling package to systematically validate three published continental-scale velocity models,that is,Shen2016,FWEA18,and USTClitho1.0,leveraging the ample datasets in Southeast Qinghai-Xizang Plateau region.Travel time residuals and waveform similarities are measured between observed empirical Green’s functions and synthetic waveforms.The results show that the Shen2016 model,derived from traditional surface wave tomography,performs best in fitting Rayleigh waves in the Southeast Qinghai-Xizang Plateau,followed by FWEA18,built from full-waveform inversion of long-period body and surface waves.The USTClitho1.0 model,although inverted from body wave datasets,is comparable with FWEA18 in fitting Rayleigh waves.The results also show that all the models are faster than the ground-truth model and show relatively large travel-time residuals and poor waveform similarities at shorter period bands,possibly caused by small-scale structural heterogeneities in the shallower crust.We further invert the time residuals for spatial velocity residuals and reveal that all three models underestimate the amplitudes of high-and low-velocity anomalies.The underestimated amplitude is up to 4%,which is non-negligible considering that the overall amplitude of anomalies is only 5%−10%in the crust.These results suggest that datasets and the inversion method are both essential to building accurate models and further refinements of these models are necessary.展开更多
The recently built China Digital Seismic Network consists of the China National Digital Seismic Network （CNDSN）, 31 regional seismic networks and several small aperture arrays with more than 1 000 stations including...The recently built China Digital Seismic Network consists of the China National Digital Seismic Network （CNDSN）, 31 regional seismic networks and several small aperture arrays with more than 1 000 stations including 850＋ broadband stations. It forms a gigantic seismic array that provides an unprecedented opportunity to study the Earth＇s deep interior besides its routine task of seismic monitoring. Many modern seismic studies rely on rotation of vertical and horizontal components in order to separate different types of seismic waves. Knowledge of the orientations of the two horizontal components thus is important to perform a correction rotation. We analyzed particle motions of teleseismic P waves recorded by the network and used them to estimate the northcomponent azimuth of each station. An SNR-weighted-multi-event method was introduced to obtain component azimuths that best explain the P-wave particle motions of all the events recorded at a station. The method provides robust estimates including a measurement error calculated from background noise levels. We found that about one third of the stations have some sort of problems, including misorientation of the two horizontal components, mislabeling and polarity reversal in one or more components. These problems need to be taken into account for any rotation based seismic studies.展开更多
High-resolution seismic models of sediment basins are critical inputs for earthquake ground motion prediction and petroleum resource exploration.In this study we employed a newly developed technique that utilizes the ...High-resolution seismic models of sediment basins are critical inputs for earthquake ground motion prediction and petroleum resource exploration.In this study we employed a newly developed technique that utilizes the frequency-dependent nonlinear P-wave particle motion to estimate sedimentary structure beneath the Bohai Bay basin.A recent study suggests that the delay of the P wave on the horizontal component relative the vertical component and its variations over frequency are caused by interference of the direct P wave with waves generated at the sediment base.The frequency-dependent delay time can be used to constrain sediment thickness and seismic velocity beneath recording stations.We measured the particle motions of teleseismic P waves recorded by 249 broadband stations of the North China Array,which covers the western Bohai Bay basin and its surrounding areas.We found that the P waves of 90 stations inside the Bohai Bay basin and other local basins within the Taihang and Yanshan mountain ranges exhibit significant frequency-dependent nonlinear particle motions,and used the particle motion data to invert the sediment thickness(Z0)and surface S-wave velocity(β0).The estimated sediment thickness inside the Bohai Bay Basin varies from 1.02 km to 3.72 km,with an average of 3.20 km,which roughly agrees with previous active source studies.展开更多
We continuously monitor the long-term seismic velocity variation of one of the major ruptured faults of the devastating 2008 Mw7.9 Wenchuan earthquake in China from July 2009 to January 2012,jointly using accurately c...We continuously monitor the long-term seismic velocity variation of one of the major ruptured faults of the devastating 2008 Mw7.9 Wenchuan earthquake in China from July 2009 to January 2012,jointly using accurately controlled routinely operated signal system active source and seismic noise-based monitoring technique.Our measurements show that the temporal velocity change is not homogeneous and highly localized in the damaged fault zone and the adjacent areas.Velocity variations from the active and passive methods are quite consistent,which both are characterized by ±0.2 % seasonal variation,with peak and trough at winter and summer,respectively.The periodic velocity variation within fault zone exhibits remarkably positive correlation with barometric pressure with stress sensitivity in the order of 10-6Pa-1,suggesting that the plausible mechanism might be the crack density variation of the shallow subsurface medium of the damaged fault zone in response to the cyclic barometric pressure loading.展开更多
We examined the spatial variation of velocity structures around the 660-kin discontinuity at the western Pacific subduction zones by waveform modeling of triplication data. Data from two deep earthquakes beneath Izu-B...We examined the spatial variation of velocity structures around the 660-kin discontinuity at the western Pacific subduction zones by waveform modeling of triplication data. Data from two deep earthquakes beneath Izu-Bonin and Northeast China are used. Both events were well recorded by a dense broadband seismic network in China （CEArray）. The two events are located at approximately the same distance to the CEArray, yet significant differences are observed in their records： （1） the direct arrivals traveling above the 660-km discontinuity （AB branch） are seen in a different distance extent： -29° for the NE China event, -23° for Izu-Bonin event; （2） the direct （AB） and the refracted waves at the 660-km （CD branch） cross over at 19.5° and 17° for the NE China and the Izu-Bonin event, respectively. The best fitting model for the NE China event has a broad 660-km discontinuity and a constant high velocity layer upon it; while the Izu-Bonin model differs from the standard IASP91 model only with a high velocity layer above the 660-km discontinuity. Variations in velocity models can be roughly explained by subduction geometry.展开更多
Repeating microearthquakes were identified along the edge of the rupture area of the 2008 MW7.9 Wenchuan earthquake. Slip rates at depths derived from seismic moments and recurrence intervals are found to be systemati...Repeating microearthquakes were identified along the edge of the rupture area of the 2008 MW7.9 Wenchuan earthquake. Slip rates at depths derived from seismic moments and recurrence intervals are found to be systematically larger than those observed at surface. This large deep slip rate may explain the odds about the occurrence of this unanticipated event. Our observations here suggested that seismic hazard could be underestimated if surface measurements alone are employed.展开更多
SKS and SKKS waveforms from 16 events occurring between 2003 and 2005 in the Tonga Trench that were recorded by the BOLIVAR array are analyzed to determine the structure of the D＂ layer beneath the Galapagos Islands....SKS and SKKS waveforms from 16 events occurring between 2003 and 2005 in the Tonga Trench that were recorded by the BOLIVAR array are analyzed to determine the structure of the D＂ layer beneath the Galapagos Islands. 248 differential travel-time residuals of SKKS-SKS are measured and reveal a region of positive residuals of differential travel times in the northeast portion of the sampled region. Analyzing correlation statistics between the measured SKKS-SKS residuals and the observed absolute travel time delay of the individual SKS and SKKS phases for two events with high data quality, we determine that the residual differential travel time is due to excess low velocity along the SKKS raypaths. First order modeling of three potential low velocity structures, ultra-low velocity zones （ULVZ）, plume conduit, D＂ structure, indicates that the observed SKKS-SKS residuals can be best explained by a low velocity anomaly within the D＂ layer. To determine the presence of lower mantle anisotropy, amplitude ratios of the radial and transverse component of SKS and SKKS waveform are calculated and compared. Regions with significant presence of seismic anisotropy are interpreted as the edge of the flow field associated with a hypothetical mantle upwelling.展开更多
Clear PKKP, a P wave reflects off the core-mantle boundary on the core side, is recorded by the transcontinental USArray from two deep earthquakes occurred in South America and Tonga, and one intermediatedepth earthqu...Clear PKKP, a P wave reflects off the core-mantle boundary on the core side, is recorded by the transcontinental USArray from two deep earthquakes occurred in South America and Tonga, and one intermediatedepth earthquake in the Hindu Kush region. We compare the PKKP waveforms with the direct P waves to investigate the fine structures near the core-mantle boundary, with a primary focus on the core side. We find no evidence for the existence of a sedimentary layer of lighter elements with a thickness above a few hundreds of meters beneath the reflection points of the two deep events, which are located at the Ninety-East Ridge and South Africa. On the other hand the PKKP wave duration of the Hindu Kush event is almost twice as long as that of the P wave, suggesting that multiple reflections may be occurring at the core-mantle boundary located beneath the Antarctic, which is located inside the so-called tangent cylinder of the outer core. The tangent cylinder is an imaginary cylindrical region suggested by geodynamics studies, which has different flow pattern and may have a higher concentration in lighter elements as compared to the rest of the outer core. One possible explanation of the elongated PKKP is a thin distinct layer with a thickness of a few kilometers at the top of the outer core, suggesting that precipitation of lighter elements may occur at the core-mantle boundary. Our data also indicate an extremely low Qp of 312, approximately 40% of the PREM average （N780）, within the large-scale low-velocity anomaly in the lowermost mantle beneath Pacific.展开更多
A seismic array is a set of seismographs being arranged in a regular geometric pattern to improve signal- to-noise ratio （SNR）. Seismic arrays at various length scales have been introduced since early 1960s and have...A seismic array is a set of seismographs being arranged in a regular geometric pattern to improve signal- to-noise ratio （SNR）. Seismic arrays at various length scales have been introduced since early 1960s and have been proved to be superior to single three-component stations for detecting and characterizing signals from earthquakes and explosions. The better detection capability of arrays is achieved with a so-called ＂beamforming＂ technique, which suppress noises while preserving signals, thus enhancing SNR. The ＂beamforming＂ technique also estimates the wave vector, i.e.,展开更多
A workshop on crustal structure and seismotectonics was held on the Chinese Teacher＇s Day, the September 10th of 2011, in the city Lanzhou, China. Scientists and graduate students from Chinese Academy of Sciences, Ch...A workshop on crustal structure and seismotectonics was held on the Chinese Teacher＇s Day, the September 10th of 2011, in the city Lanzhou, China. Scientists and graduate students from Chinese Academy of Sciences, China Earthquake Administration, Chinese Academy of Geological Sciences, and Japan Agency for Marine-Earth Science and Technology delivered over 20 oral presentations, with topics covering crustal and upper mantle structure, seismic anisotropy, recent earthquakes and seismotectonics,展开更多
Earth’s outer core is generally thought to be a well-mixed liquid consisting mostly of iron and a small amount of lighter elements.Recent seismic studies using Sm KS waves show that the top a few hundred kilometers o...Earth’s outer core is generally thought to be a well-mixed liquid consisting mostly of iron and a small amount of lighter elements.Recent seismic studies using Sm KS waves show that the top a few hundred kilometers of the outer core possess a P-wave velocity slightly lower than the PREM model,which cannot be explained by self-compression of a chemically homogeneous outer core.We investigated the Sm KS waveforms of a deep earthquake occurring beneath South America recorded by a large and dense seismic array in China,and measured the differential arrival times of the Sm KS pairs.We found significant waveform distortion of the Sm KS caused by postcritical refraction and reflection at the core-mantle boundary.This waveform distortion can introduce significant bias to the measured differential times,leading to incorrect estimate of P-wave velocity of the outer core.Whether stable stratification is occurring in outer core or not requires further seismic investigations.展开更多
Cratons formed due to the specific melting regime of the primitive mantle with elevated mantle temperature during Archean.However,each craton has undergone a distinct evolution history,and some have lost their stabili...Cratons formed due to the specific melting regime of the primitive mantle with elevated mantle temperature during Archean.However,each craton has undergone a distinct evolution history,and some have lost their stability.To investigate to what degree cratons in comparison with one another have been modified from their analogous initial form,we employed Sn-Pn differential(PSn) traveltimes to derive Vp/Vsratio,which is thought to be related to Mg# of the uppermantle.We assessed Pn,Sn,and PSn data using three datasets based on epicentral distance:(1) 2°–12°,(2) 2°–7°,and(3) 7°–12°.The results suggest that most cratons show comparable seismic properties with high velocities and low Vp/Vsratio,implying a highly depleted uppermost mantle that resembles the original residue from the partial melt extraction of the primitive mantle during the Archean.Conversely,the Eastern North China Craton(ENCC) displays the lowest P-and S-wave velocities,and noticeable high Vp/Vsratios in all datasets,implying a systematic difference with other cratons.This observation suggests a scenario of total removal of the depleted Archean mantle lithosphere beneath the ENCC.In contrast,the Ordos Block located at the western part of the North China Craton(WNCC) shows velocities and Vp/Vsratio comparable with those of the typical cratons,suggesting that it has still maintained its Archean mantle lithosphere.The Wyoming Craton has a high Vp/Vsratio similar to that of the ENCC and a high Pwave velocity comparable to that of the typical cratons.These features suggest that the Archean mantle lithosphere has been significantly modified rather than totally removed and replaced by a younger fertile mantle.The Indian Craton presents a low Vp/Vsratio and comparatively high velocities at shallow parts of the mantle lithosphere but a high Vp/Vsratio at deeper parts similar to that of the ENCC,suggesting a partial modification of the Indian Craton at deeper parts.展开更多
The perfectly matched layer(PML)boundary condition has been proven to be effective for attenuating reflections from model boundaries during wavefield simulation.As such,it has been widely used in time-domain finite-di...The perfectly matched layer(PML)boundary condition has been proven to be effective for attenuating reflections from model boundaries during wavefield simulation.As such,it has been widely used in time-domain finite-difference wavefield simulations.The conventional PML has poor performance for near grazing incident waves and low-frequency reflections.To overcome these limitations,a more complex frequency-shifted stretch(CSF)function is introduced,which is known as the CFSPML boundary condition and can be implemented in the time domain by a recursive convolution technique(CPML).When implementing the PML technique to second-order wave equations,all the existing methods involve adding auxiliary terms and rewriting the wave equations into new second-order partial differential equations that can be simulated by the finite-difference scheme,which may affect the efficiency of numerical simulation.In this paper,we propose a relatively simple and efficient approach to implement CPML for the second-order equation system,which solves the original wave equations numerically in the stretched coordinate.The spatial derivatives in the stretched coordinate are computed by adding a correction term to the regular derivatives.Once the first-order spatial derivatives are computed,we computed the second-order spatial derivatives in a similar way;therefore,we refer to the method as two-step CPML(TS-CPML).We apply the method to the second-order acoustic wave equation and a coupled second-order pseudo-acoustic TTI wave equation.Our simulations indicate that amplitudes of reflected waves are only about half of those computed with the traditional CPML method,suggesting that the proposed approach has computational advantages and therefore can be widely used for forwarding modeling and seismic imaging.展开更多
基金supported by the Special Fund of the Institute of Geophysics,China Earthquake Administration(Nos.DQJB23R28 and DQJB22K40)the National Natural Science Foundation of China(Nos.42304078,U1839210 and 42104043).
文摘Various velocity models have been built for Southeast Qinghai-Xizang Plateau with the purpose of revealing the internal dynamics and estimating local seismic hazards.In this study,we use a 3-D full-waveform modeling package to systematically validate three published continental-scale velocity models,that is,Shen2016,FWEA18,and USTClitho1.0,leveraging the ample datasets in Southeast Qinghai-Xizang Plateau region.Travel time residuals and waveform similarities are measured between observed empirical Green’s functions and synthetic waveforms.The results show that the Shen2016 model,derived from traditional surface wave tomography,performs best in fitting Rayleigh waves in the Southeast Qinghai-Xizang Plateau,followed by FWEA18,built from full-waveform inversion of long-period body and surface waves.The USTClitho1.0 model,although inverted from body wave datasets,is comparable with FWEA18 in fitting Rayleigh waves.The results also show that all the models are faster than the ground-truth model and show relatively large travel-time residuals and poor waveform similarities at shorter period bands,possibly caused by small-scale structural heterogeneities in the shallower crust.We further invert the time residuals for spatial velocity residuals and reveal that all three models underestimate the amplitudes of high-and low-velocity anomalies.The underestimated amplitude is up to 4%,which is non-negligible considering that the overall amplitude of anomalies is only 5%−10%in the crust.These results suggest that datasets and the inversion method are both essential to building accurate models and further refinements of these models are necessary.
基金supported by NSF grant EAR-063566(F.N.)National Natural Science Foundation of China grant 40774042(J.L.)
文摘The recently built China Digital Seismic Network consists of the China National Digital Seismic Network （CNDSN）, 31 regional seismic networks and several small aperture arrays with more than 1 000 stations including 850＋ broadband stations. It forms a gigantic seismic array that provides an unprecedented opportunity to study the Earth＇s deep interior besides its routine task of seismic monitoring. Many modern seismic studies rely on rotation of vertical and horizontal components in order to separate different types of seismic waves. Knowledge of the orientations of the two horizontal components thus is important to perform a correction rotation. We analyzed particle motions of teleseismic P waves recorded by the network and used them to estimate the northcomponent azimuth of each station. An SNR-weighted-multi-event method was introduced to obtain component azimuths that best explain the P-wave particle motions of all the events recorded at a station. The method provides robust estimates including a measurement error calculated from background noise levels. We found that about one third of the stations have some sort of problems, including misorientation of the two horizontal components, mislabeling and polarity reversal in one or more components. These problems need to be taken into account for any rotation based seismic studies.
文摘High-resolution seismic models of sediment basins are critical inputs for earthquake ground motion prediction and petroleum resource exploration.In this study we employed a newly developed technique that utilizes the frequency-dependent nonlinear P-wave particle motion to estimate sedimentary structure beneath the Bohai Bay basin.A recent study suggests that the delay of the P wave on the horizontal component relative the vertical component and its variations over frequency are caused by interference of the direct P wave with waves generated at the sediment base.The frequency-dependent delay time can be used to constrain sediment thickness and seismic velocity beneath recording stations.We measured the particle motions of teleseismic P waves recorded by 249 broadband stations of the North China Array,which covers the western Bohai Bay basin and its surrounding areas.We found that the P waves of 90 stations inside the Bohai Bay basin and other local basins within the Taihang and Yanshan mountain ranges exhibit significant frequency-dependent nonlinear particle motions,and used the particle motion data to invert the sediment thickness(Z0)and surface S-wave velocity(β0).The estimated sediment thickness inside the Bohai Bay Basin varies from 1.02 km to 3.72 km,with an average of 3.20 km,which roughly agrees with previous active source studies.
基金supported by the National Natural Science Foundation of China with Grant No.41174040the Wenchuan earthquake Fault Scientific Drilling project
文摘We continuously monitor the long-term seismic velocity variation of one of the major ruptured faults of the devastating 2008 Mw7.9 Wenchuan earthquake in China from July 2009 to January 2012,jointly using accurately controlled routinely operated signal system active source and seismic noise-based monitoring technique.Our measurements show that the temporal velocity change is not homogeneous and highly localized in the damaged fault zone and the adjacent areas.Velocity variations from the active and passive methods are quite consistent,which both are characterized by ±0.2 % seasonal variation,with peak and trough at winter and summer,respectively.The periodic velocity variation within fault zone exhibits remarkably positive correlation with barometric pressure with stress sensitivity in the order of 10-6Pa-1,suggesting that the plausible mechanism might be the crack density variation of the shallow subsurface medium of the damaged fault zone in response to the cyclic barometric pressure loading.
基金supported by National Natural Science Foundation of China under grant 40874095 and NSF under grant EAR-063566
文摘We examined the spatial variation of velocity structures around the 660-kin discontinuity at the western Pacific subduction zones by waveform modeling of triplication data. Data from two deep earthquakes beneath Izu-Bonin and Northeast China are used. Both events were well recorded by a dense broadband seismic network in China （CEArray）. The two events are located at approximately the same distance to the CEArray, yet significant differences are observed in their records： （1） the direct arrivals traveling above the 660-km discontinuity （AB branch） are seen in a different distance extent： -29° for the NE China event, -23° for Izu-Bonin event; （2） the direct （AB） and the refracted waves at the 660-km （CD branch） cross over at 19.5° and 17° for the NE China and the Izu-Bonin event, respectively. The best fitting model for the NE China event has a broad 660-km discontinuity and a constant high velocity layer upon it; while the Izu-Bonin model differs from the standard IASP91 model only with a high velocity layer above the 660-km discontinuity. Variations in velocity models can be roughly explained by subduction geometry.
基金supported by China Earthquake Administration under grants 200708008 and IES02092405
文摘Repeating microearthquakes were identified along the edge of the rupture area of the 2008 MW7.9 Wenchuan earthquake. Slip rates at depths derived from seismic moments and recurrence intervals are found to be systematically larger than those observed at surface. This large deep slip rate may explain the odds about the occurrence of this unanticipated event. Our observations here suggested that seismic hazard could be underestimated if surface measurements alone are employed.
文摘SKS and SKKS waveforms from 16 events occurring between 2003 and 2005 in the Tonga Trench that were recorded by the BOLIVAR array are analyzed to determine the structure of the D＂ layer beneath the Galapagos Islands. 248 differential travel-time residuals of SKKS-SKS are measured and reveal a region of positive residuals of differential travel times in the northeast portion of the sampled region. Analyzing correlation statistics between the measured SKKS-SKS residuals and the observed absolute travel time delay of the individual SKS and SKKS phases for two events with high data quality, we determine that the residual differential travel time is due to excess low velocity along the SKKS raypaths. First order modeling of three potential low velocity structures, ultra-low velocity zones （ULVZ）, plume conduit, D＂ structure, indicates that the observed SKKS-SKS residuals can be best explained by a low velocity anomaly within the D＂ layer. To determine the presence of lower mantle anisotropy, amplitude ratios of the radial and transverse component of SKS and SKKS waveform are calculated and compared. Regions with significant presence of seismic anisotropy are interpreted as the edge of the flow field associated with a hypothetical mantle upwelling.
文摘Clear PKKP, a P wave reflects off the core-mantle boundary on the core side, is recorded by the transcontinental USArray from two deep earthquakes occurred in South America and Tonga, and one intermediatedepth earthquake in the Hindu Kush region. We compare the PKKP waveforms with the direct P waves to investigate the fine structures near the core-mantle boundary, with a primary focus on the core side. We find no evidence for the existence of a sedimentary layer of lighter elements with a thickness above a few hundreds of meters beneath the reflection points of the two deep events, which are located at the Ninety-East Ridge and South Africa. On the other hand the PKKP wave duration of the Hindu Kush event is almost twice as long as that of the P wave, suggesting that multiple reflections may be occurring at the core-mantle boundary located beneath the Antarctic, which is located inside the so-called tangent cylinder of the outer core. The tangent cylinder is an imaginary cylindrical region suggested by geodynamics studies, which has different flow pattern and may have a higher concentration in lighter elements as compared to the rest of the outer core. One possible explanation of the elongated PKKP is a thin distinct layer with a thickness of a few kilometers at the top of the outer core, suggesting that precipitation of lighter elements may occur at the core-mantle boundary. Our data also indicate an extremely low Qp of 312, approximately 40% of the PREM average （N780）, within the large-scale low-velocity anomaly in the lowermost mantle beneath Pacific.
文摘A seismic array is a set of seismographs being arranged in a regular geometric pattern to improve signal- to-noise ratio （SNR）. Seismic arrays at various length scales have been introduced since early 1960s and have been proved to be superior to single three-component stations for detecting and characterizing signals from earthquakes and explosions. The better detection capability of arrays is achieved with a so-called ＂beamforming＂ technique, which suppress noises while preserving signals, thus enhancing SNR. The ＂beamforming＂ technique also estimates the wave vector, i.e.,
基金support by basic research project of Institute of Earthquake Science,China Earthquake Administration(No.2011IESLZ05 and No.2009-11&21)
文摘A workshop on crustal structure and seismotectonics was held on the Chinese Teacher＇s Day, the September 10th of 2011, in the city Lanzhou, China. Scientists and graduate students from Chinese Academy of Sciences, China Earthquake Administration, Chinese Academy of Geological Sciences, and Japan Agency for Marine-Earth Science and Technology delivered over 20 oral presentations, with topics covering crustal and upper mantle structure, seismic anisotropy, recent earthquakes and seismotectonics,
基金jointly supported by the National Key Research and Development Program of China(2017YFC1500303)the National Natural Science Foundation of China(41630209)
文摘Earth’s outer core is generally thought to be a well-mixed liquid consisting mostly of iron and a small amount of lighter elements.Recent seismic studies using Sm KS waves show that the top a few hundred kilometers of the outer core possess a P-wave velocity slightly lower than the PREM model,which cannot be explained by self-compression of a chemically homogeneous outer core.We investigated the Sm KS waveforms of a deep earthquake occurring beneath South America recorded by a large and dense seismic array in China,and measured the differential arrival times of the Sm KS pairs.We found significant waveform distortion of the Sm KS caused by postcritical refraction and reflection at the core-mantle boundary.This waveform distortion can introduce significant bias to the measured differential times,leading to incorrect estimate of P-wave velocity of the outer core.Whether stable stratification is occurring in outer core or not requires further seismic investigations.
基金supported by the National Natural Science Foundation of China (Grant Nos. 41630209, U2039203, 42130306)。
文摘Cratons formed due to the specific melting regime of the primitive mantle with elevated mantle temperature during Archean.However,each craton has undergone a distinct evolution history,and some have lost their stability.To investigate to what degree cratons in comparison with one another have been modified from their analogous initial form,we employed Sn-Pn differential(PSn) traveltimes to derive Vp/Vsratio,which is thought to be related to Mg# of the uppermantle.We assessed Pn,Sn,and PSn data using three datasets based on epicentral distance:(1) 2°–12°,(2) 2°–7°,and(3) 7°–12°.The results suggest that most cratons show comparable seismic properties with high velocities and low Vp/Vsratio,implying a highly depleted uppermost mantle that resembles the original residue from the partial melt extraction of the primitive mantle during the Archean.Conversely,the Eastern North China Craton(ENCC) displays the lowest P-and S-wave velocities,and noticeable high Vp/Vsratios in all datasets,implying a systematic difference with other cratons.This observation suggests a scenario of total removal of the depleted Archean mantle lithosphere beneath the ENCC.In contrast,the Ordos Block located at the western part of the North China Craton(WNCC) shows velocities and Vp/Vsratio comparable with those of the typical cratons,suggesting that it has still maintained its Archean mantle lithosphere.The Wyoming Craton has a high Vp/Vsratio similar to that of the ENCC and a high Pwave velocity comparable to that of the typical cratons.These features suggest that the Archean mantle lithosphere has been significantly modified rather than totally removed and replaced by a younger fertile mantle.The Indian Craton presents a low Vp/Vsratio and comparatively high velocities at shallow parts of the mantle lithosphere but a high Vp/Vsratio at deeper parts similar to that of the ENCC,suggesting a partial modification of the Indian Craton at deeper parts.
基金supported by the National Natural Science Foundation of China(Grant No.41630209)。
文摘The perfectly matched layer(PML)boundary condition has been proven to be effective for attenuating reflections from model boundaries during wavefield simulation.As such,it has been widely used in time-domain finite-difference wavefield simulations.The conventional PML has poor performance for near grazing incident waves and low-frequency reflections.To overcome these limitations,a more complex frequency-shifted stretch(CSF)function is introduced,which is known as the CFSPML boundary condition and can be implemented in the time domain by a recursive convolution technique(CPML).When implementing the PML technique to second-order wave equations,all the existing methods involve adding auxiliary terms and rewriting the wave equations into new second-order partial differential equations that can be simulated by the finite-difference scheme,which may affect the efficiency of numerical simulation.In this paper,we propose a relatively simple and efficient approach to implement CPML for the second-order equation system,which solves the original wave equations numerically in the stretched coordinate.The spatial derivatives in the stretched coordinate are computed by adding a correction term to the regular derivatives.Once the first-order spatial derivatives are computed,we computed the second-order spatial derivatives in a similar way;therefore,we refer to the method as two-step CPML(TS-CPML).We apply the method to the second-order acoustic wave equation and a coupled second-order pseudo-acoustic TTI wave equation.Our simulations indicate that amplitudes of reflected waves are only about half of those computed with the traditional CPML method,suggesting that the proposed approach has computational advantages and therefore can be widely used for forwarding modeling and seismic imaging.
