Abstract We present the results of mantle P velocity and attenuation tomography from the combined... more Abstract We present the results of mantle P velocity and attenuation tomography from the combined KRISP 1985 and KRISP 1990 teleseismic data sets. We incorporate the results from the KRISP 1990 refraction survey to remove crustal effects from the teleseismic data and thereby better image the lithospheric and asthenospheric structure in the mantle beneath the rift. We find a broad 6% low-velocity anomaly that extends beyond the limits of the rift, as defined by its bounding faults, and superimposed on this a narrower 6% low-velocity zone that is confined to the rift zone. The latter anomaly has a total contrast of 12% compared to nearby unrifted cratonic lithosphere The broad structure persists at all depths to 165 km, and is aligned with the rift, while the narrower structure varies with depth and does not extend along the entire length of the rift covered by our array. The attenuation signal is noisier than the travel time signal, but shows the greatest attenuation beneath the rift and at the southern and northern ends of our array.
Journal of Geophysical Research: Solid Earth, 2003
Inversion of teleseismic P wave travel time residuals collected along a 1280-km-long profile trav... more Inversion of teleseismic P wave travel time residuals collected along a 1280-km-long profile traversing the Baikal rift zone (BRZ) reveals the existence of an upwarped lithosphere/asthenosphere interface, which causes a travel time delay of about 1 s at the rift axis (''central high''). An area with early arrivals relative to the stable Siberian platform of up to 0.5 s is observed on each side of the rift, about 200 km from the rift axis (''flank lows''). While the location of the central high is approximately fixed in the vicinity of the rift axis, those of the flank lows vary as much as 200 km with the azimuth of the arriving rays. We use three techniques to invert the travel time residuals for velocity anomalies beneath the profile. Two of the techniques assume an isotropic velocity structure, and one of them considers a transversely isotropic velocity model with a vertical axis of symmetry. We use independent geophysical observations such as gravity, active source seismic exploration, and crustal thickness measurements to compare the applicability of the models. Other types of geophysical measurements suggest that the model involving transverse isotropy is a plausible one, which suggests that the central high and flank lows are caused by the combined effects of an upwarped asthenosphere with a 2.5% lateral velocity reduction, and a velocity increase due to transverse isotropy with a vertical axis of symmetry. We consider the anisotropy to be the result of the vertical component of a lithosphere/asthenosphere small-scale mantle convection system that is associated with the rifting.
Journal of Geophysical Research: Solid Earth, 1997
We present measurements of SKS splitting at 28 digital seismic stations and 35 analog stations in... more We present measurements of SKS splitting at 28 digital seismic stations and 35 analog stations in the Baikal rift zone, Siberia, and adjacent areas, and at 17 stations in the East African Rift in Kenya and compare them with previous measurements from the Rio Grande Rift of North America. Fast directions in the inner region of the Baikal rift zone are distributed in two orthogonal directions, NE and NW, approximately parallel and perpendicular to the NE strike of the rift. In the adjacent Siberian platform and northern Mongolian fold belt, only the rift‐orthogonal fast direction is observed. In southcentral Mongolia, the dominant fast direction changes to rift‐parallel again, although a small number of measurements are still rift‐orthogonal. For the axial zones of the East African and Rio Grande Rifts, fast directions are oriented on average NNE, that is, rotated clockwise from the N‐S trending rift. All three rifts are underlain by low‐velocity upper mantle as determined from telese...
Journal of Geophysical Research: Solid Earth, 1994
In the summer of 1991 we installed 27 seismic stations about lake Baikal, Siberia, aimed at obtai... more In the summer of 1991 we installed 27 seismic stations about lake Baikal, Siberia, aimed at obtaining accurately timed digital seismic data to investigate the deep structure and geodynamics of the Baikal rift zone and adjacent regions. Sixty‐six teleseismic events with high signal‐to‐noise ratio were recorded. Travel time and Q analysis of teleseisms characterize an upwarp of the lithosphere‐asthenosphere boundary under Baikal. Theoretical arrival times were calculated by using the International Association of Seismology and Physics of the Earth's interior 1991 Earth model, and travel time residuals were found by subtracting computed arrival times from observed ones. A three‐dimensional downward projection inversion method is used to invert the P wave velocity structure with constraints from deep seismic sounding data. Our results suggest that (1) the lithosphere‐asthenosphere transition upwarps beneath the rift zone, (2) the upwarp has an asymmetric shape, (3) the velocity cont...
The Journal of the Acoustical Society of America, 1996
Body wave arrivals at SOSUS hydrophone arrays are used to determine the seismic velocity structur... more Body wave arrivals at SOSUS hydrophone arrays are used to determine the seismic velocity structure of the Juan de Fuca ridge and Cascadia subduction zone regions. Since 1991 NOAA has been recording data from the Navy’s SOSUS hydrophone arrays. This has provided a unique set of continuously recorded hydrophone data from oceanic stations. Body wave or P‐phase arrivals from large earthquakes are detected by the SOSUS hydrophones. Using body wave arrivals Pn velocities of 8.0 km/s are measured for paths purely beneath the oceanic plates and for paths originating beneath the North American continental plate the Pn velocity is 7.7 km/s. For paths along the subduction zone velocities of 7.6–7.7 km/s are measured. The data from the SOSUS arrays are combined with data from the Pacific Northwest Seismic Network (PNSN), a network of seismic stations in Washington, Oregon and northern California, to construct a tomographic image of the crust and uppermost mantle for the Cascadia subduction zone. The model extends fro...
The Journal of the Acoustical Society of America, 1996
Since 1991, NOAA/PMEL has routinely detected Tertiary waves from NE Pacific ocean earthquakes usi... more Since 1991, NOAA/PMEL has routinely detected Tertiary waves from NE Pacific ocean earthquakes using the U.S. Navy’s SOSUS hydrophone arrays. Tertiary (T) waves are seismically generated acoustic waves (between 5–50 Hz) that propagate over great distances in the oceanic sound channel with little loss in signal strength. Low attenuation, combined with the availability of detailed ocean acoustic velocity models, have made T waves extremely useful for monitoring seafloor earthquakes;T waves allow for highly accurate epicentral locations, and a reduction of the detection threshold by almost two orders of magnitude. However, it is still not well understood how seismic energy generated at the earthquake source propagates through the seafloor–ocean interface and couples to the sound channel. Hence, the relationship between earthquake source parameters and the T‐wave amplitude recorded at the hydrophone are not directly known. The goal of this paper is to empirically compare earthquake magnitude, seismic moment, a...
Journal of Geophysical Research: Solid Earth, 1999
P wave arrivals recorded by the U.S. Navy's SOund SUrveillance System (SOSUS) hydrophone arrays w... more P wave arrivals recorded by the U.S. Navy's SOund SUrveillance System (SOSUS) hydrophone arrays were used to estimate earthquake detection thresholds and Pn velocities in the northeast Pacific Ocean. The Navy hydrophones have been used successfully to detect and locate oceanic earthquakes using their waterborne acoustic tertiary (T) waves; however,' use of these hydrophones for seismic body wave detection allows regional seismic analyses to be extended to the oceanic environment. The P wave detection threshold of the SOSUS hydrophones was quantified using the epicentral distance and magnitude of 250 northeast Pacific Ocean earthquakes. Earthquakes with body wave magnitudes as low as 2 have detectable P wave arrivals at epicentral distances of _•500 km. Earthquakes with rno between 3.5 and 5 were detected •050% of the time at distances of 100-1500 km, while events with rno • 5 were all detected, even out to distances of 1000-1500 km. Both P and T wave hydrophone arrival times were used to estimate the epicenters of 100 earthquakes. The peak amplitude of the T wave coda and the onset of the P wave were used as the earthquake arrival times to estimate event locations. T wave arrival time residuals have a Gaussian distribution with zero mean, which implies that using T wave peak amplitude is consistent with using the P wave onset as the arrival time. There are typically _•6 stations used to derive a T wave based location, hence location error ellipses are not well constrained. A Monte Carlo technique was employed to estimate T wave event location uncertainty. • T wave locations have error bars of .ol km in latitude and longitude when •3 hydrophones are used for a location estimate. The detected P wave arrivals and earthquake locations were used to measure Pn velocities. Pn velocity values of 7.9 q-0.1 and 8.0 q-0.1 km/s were found for the Pacific and Juan de Fuca plates, respectively. A Pn velocity of 7.5 q-0.1 km/s was measured for rays traveling northward from the Mendocino Triple Junction along the Cascadia subduction zone. A Pn velocity of 7.7 4-0.3 km/s was estimated for ray paths originating onshore western North America and traveling to the offshore hydrophones. 1. Introduction In 1991, the National Oceanic and Atmospheric Administration (NOAA) was granted access to the U.S. Navy's SOund SUrveillance System (SOSUS) in the northeast Pacific Ocean for use in monitoring the Juan Copyright 1999 by the American Geophysical Union. Paper number 1999JB900112. 0148-0227 / 99 / 1999JB 900112509.00 de Fuca Ridge spreading center for low-level seismic activity. Although SOSUS is designed for tracking vessels in the open ocean, NOAA developed a supplementary recording system to allow monitoring and localization of microseismicity along the seafloor spreading centers of the northeast Pacific Ocean [Fox et al., 1994]. The earthquake location method relies on the detection of hydroacoustic waves (Tertiary or T waves) that propagate within the oceanic water column. The presence of a layer of slow sound speed (sound channel) through-13,061
The lithosphere beneath a continental rift should be significantly modified due to extension. To ... more The lithosphere beneath a continental rift should be significantly modified due to extension. To image the lithosphere beneath the Rio Grande rift (RGR), we analyzed teleseismic travel time delays of both P and S wave arrivals and solved for the attenuation of P and S waves for four seismic experimems spanning the Rio Grande rift. Two tomographic inversions of the
[1] We compare new results on S-wave delays and P wave tomography to characterize the rising limb... more [1] We compare new results on S-wave delays and P wave tomography to characterize the rising limb and melt zone of an inferred mantle convection cell beneath the Kenya dome. These results are extended to the Nyiragongo and Ethiopia domes using long wavelength gravity and topography. We suggest that the east African rift results from separation of deeper mantle upwelling into three currents that impinge on and erode the base of the lithosphere. Their thermal buoyancy drives the domal uplift, whereas brittle failure of the upper lithosphere forms the rift grabens.
Abstract We present the results of mantle P velocity and attenuation tomography from the combined... more Abstract We present the results of mantle P velocity and attenuation tomography from the combined KRISP 1985 and KRISP 1990 teleseismic data sets. We incorporate the results from the KRISP 1990 refraction survey to remove crustal effects from the teleseismic data and thereby better image the lithospheric and asthenospheric structure in the mantle beneath the rift. We find a broad 6% low-velocity anomaly that extends beyond the limits of the rift, as defined by its bounding faults, and superimposed on this a narrower 6% low-velocity zone that is confined to the rift zone. The latter anomaly has a total contrast of 12% compared to nearby unrifted cratonic lithosphere The broad structure persists at all depths to 165 km, and is aligned with the rift, while the narrower structure varies with depth and does not extend along the entire length of the rift covered by our array. The attenuation signal is noisier than the travel time signal, but shows the greatest attenuation beneath the rift and at the southern and northern ends of our array.
Journal of Geophysical Research: Solid Earth, 2003
Inversion of teleseismic P wave travel time residuals collected along a 1280-km-long profile trav... more Inversion of teleseismic P wave travel time residuals collected along a 1280-km-long profile traversing the Baikal rift zone (BRZ) reveals the existence of an upwarped lithosphere/asthenosphere interface, which causes a travel time delay of about 1 s at the rift axis (''central high''). An area with early arrivals relative to the stable Siberian platform of up to 0.5 s is observed on each side of the rift, about 200 km from the rift axis (''flank lows''). While the location of the central high is approximately fixed in the vicinity of the rift axis, those of the flank lows vary as much as 200 km with the azimuth of the arriving rays. We use three techniques to invert the travel time residuals for velocity anomalies beneath the profile. Two of the techniques assume an isotropic velocity structure, and one of them considers a transversely isotropic velocity model with a vertical axis of symmetry. We use independent geophysical observations such as gravity, active source seismic exploration, and crustal thickness measurements to compare the applicability of the models. Other types of geophysical measurements suggest that the model involving transverse isotropy is a plausible one, which suggests that the central high and flank lows are caused by the combined effects of an upwarped asthenosphere with a 2.5% lateral velocity reduction, and a velocity increase due to transverse isotropy with a vertical axis of symmetry. We consider the anisotropy to be the result of the vertical component of a lithosphere/asthenosphere small-scale mantle convection system that is associated with the rifting.
Journal of Geophysical Research: Solid Earth, 1997
We present measurements of SKS splitting at 28 digital seismic stations and 35 analog stations in... more We present measurements of SKS splitting at 28 digital seismic stations and 35 analog stations in the Baikal rift zone, Siberia, and adjacent areas, and at 17 stations in the East African Rift in Kenya and compare them with previous measurements from the Rio Grande Rift of North America. Fast directions in the inner region of the Baikal rift zone are distributed in two orthogonal directions, NE and NW, approximately parallel and perpendicular to the NE strike of the rift. In the adjacent Siberian platform and northern Mongolian fold belt, only the rift‐orthogonal fast direction is observed. In southcentral Mongolia, the dominant fast direction changes to rift‐parallel again, although a small number of measurements are still rift‐orthogonal. For the axial zones of the East African and Rio Grande Rifts, fast directions are oriented on average NNE, that is, rotated clockwise from the N‐S trending rift. All three rifts are underlain by low‐velocity upper mantle as determined from telese...
Journal of Geophysical Research: Solid Earth, 1994
In the summer of 1991 we installed 27 seismic stations about lake Baikal, Siberia, aimed at obtai... more In the summer of 1991 we installed 27 seismic stations about lake Baikal, Siberia, aimed at obtaining accurately timed digital seismic data to investigate the deep structure and geodynamics of the Baikal rift zone and adjacent regions. Sixty‐six teleseismic events with high signal‐to‐noise ratio were recorded. Travel time and Q analysis of teleseisms characterize an upwarp of the lithosphere‐asthenosphere boundary under Baikal. Theoretical arrival times were calculated by using the International Association of Seismology and Physics of the Earth's interior 1991 Earth model, and travel time residuals were found by subtracting computed arrival times from observed ones. A three‐dimensional downward projection inversion method is used to invert the P wave velocity structure with constraints from deep seismic sounding data. Our results suggest that (1) the lithosphere‐asthenosphere transition upwarps beneath the rift zone, (2) the upwarp has an asymmetric shape, (3) the velocity cont...
The Journal of the Acoustical Society of America, 1996
Body wave arrivals at SOSUS hydrophone arrays are used to determine the seismic velocity structur... more Body wave arrivals at SOSUS hydrophone arrays are used to determine the seismic velocity structure of the Juan de Fuca ridge and Cascadia subduction zone regions. Since 1991 NOAA has been recording data from the Navy’s SOSUS hydrophone arrays. This has provided a unique set of continuously recorded hydrophone data from oceanic stations. Body wave or P‐phase arrivals from large earthquakes are detected by the SOSUS hydrophones. Using body wave arrivals Pn velocities of 8.0 km/s are measured for paths purely beneath the oceanic plates and for paths originating beneath the North American continental plate the Pn velocity is 7.7 km/s. For paths along the subduction zone velocities of 7.6–7.7 km/s are measured. The data from the SOSUS arrays are combined with data from the Pacific Northwest Seismic Network (PNSN), a network of seismic stations in Washington, Oregon and northern California, to construct a tomographic image of the crust and uppermost mantle for the Cascadia subduction zone. The model extends fro...
The Journal of the Acoustical Society of America, 1996
Since 1991, NOAA/PMEL has routinely detected Tertiary waves from NE Pacific ocean earthquakes usi... more Since 1991, NOAA/PMEL has routinely detected Tertiary waves from NE Pacific ocean earthquakes using the U.S. Navy’s SOSUS hydrophone arrays. Tertiary (T) waves are seismically generated acoustic waves (between 5–50 Hz) that propagate over great distances in the oceanic sound channel with little loss in signal strength. Low attenuation, combined with the availability of detailed ocean acoustic velocity models, have made T waves extremely useful for monitoring seafloor earthquakes;T waves allow for highly accurate epicentral locations, and a reduction of the detection threshold by almost two orders of magnitude. However, it is still not well understood how seismic energy generated at the earthquake source propagates through the seafloor–ocean interface and couples to the sound channel. Hence, the relationship between earthquake source parameters and the T‐wave amplitude recorded at the hydrophone are not directly known. The goal of this paper is to empirically compare earthquake magnitude, seismic moment, a...
Journal of Geophysical Research: Solid Earth, 1999
P wave arrivals recorded by the U.S. Navy's SOund SUrveillance System (SOSUS) hydrophone arrays w... more P wave arrivals recorded by the U.S. Navy's SOund SUrveillance System (SOSUS) hydrophone arrays were used to estimate earthquake detection thresholds and Pn velocities in the northeast Pacific Ocean. The Navy hydrophones have been used successfully to detect and locate oceanic earthquakes using their waterborne acoustic tertiary (T) waves; however,' use of these hydrophones for seismic body wave detection allows regional seismic analyses to be extended to the oceanic environment. The P wave detection threshold of the SOSUS hydrophones was quantified using the epicentral distance and magnitude of 250 northeast Pacific Ocean earthquakes. Earthquakes with body wave magnitudes as low as 2 have detectable P wave arrivals at epicentral distances of _•500 km. Earthquakes with rno between 3.5 and 5 were detected •050% of the time at distances of 100-1500 km, while events with rno • 5 were all detected, even out to distances of 1000-1500 km. Both P and T wave hydrophone arrival times were used to estimate the epicenters of 100 earthquakes. The peak amplitude of the T wave coda and the onset of the P wave were used as the earthquake arrival times to estimate event locations. T wave arrival time residuals have a Gaussian distribution with zero mean, which implies that using T wave peak amplitude is consistent with using the P wave onset as the arrival time. There are typically _•6 stations used to derive a T wave based location, hence location error ellipses are not well constrained. A Monte Carlo technique was employed to estimate T wave event location uncertainty. • T wave locations have error bars of .ol km in latitude and longitude when •3 hydrophones are used for a location estimate. The detected P wave arrivals and earthquake locations were used to measure Pn velocities. Pn velocity values of 7.9 q-0.1 and 8.0 q-0.1 km/s were found for the Pacific and Juan de Fuca plates, respectively. A Pn velocity of 7.5 q-0.1 km/s was measured for rays traveling northward from the Mendocino Triple Junction along the Cascadia subduction zone. A Pn velocity of 7.7 4-0.3 km/s was estimated for ray paths originating onshore western North America and traveling to the offshore hydrophones. 1. Introduction In 1991, the National Oceanic and Atmospheric Administration (NOAA) was granted access to the U.S. Navy's SOund SUrveillance System (SOSUS) in the northeast Pacific Ocean for use in monitoring the Juan Copyright 1999 by the American Geophysical Union. Paper number 1999JB900112. 0148-0227 / 99 / 1999JB 900112509.00 de Fuca Ridge spreading center for low-level seismic activity. Although SOSUS is designed for tracking vessels in the open ocean, NOAA developed a supplementary recording system to allow monitoring and localization of microseismicity along the seafloor spreading centers of the northeast Pacific Ocean [Fox et al., 1994]. The earthquake location method relies on the detection of hydroacoustic waves (Tertiary or T waves) that propagate within the oceanic water column. The presence of a layer of slow sound speed (sound channel) through-13,061
The lithosphere beneath a continental rift should be significantly modified due to extension. To ... more The lithosphere beneath a continental rift should be significantly modified due to extension. To image the lithosphere beneath the Rio Grande rift (RGR), we analyzed teleseismic travel time delays of both P and S wave arrivals and solved for the attenuation of P and S waves for four seismic experimems spanning the Rio Grande rift. Two tomographic inversions of the
[1] We compare new results on S-wave delays and P wave tomography to characterize the rising limb... more [1] We compare new results on S-wave delays and P wave tomography to characterize the rising limb and melt zone of an inferred mantle convection cell beneath the Kenya dome. These results are extended to the Nyiragongo and Ethiopia domes using long wavelength gravity and topography. We suggest that the east African rift results from separation of deeper mantle upwelling into three currents that impinge on and erode the base of the lithosphere. Their thermal buoyancy drives the domal uplift, whereas brittle failure of the upper lithosphere forms the rift grabens.
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Papers by Philip Slack