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Sean C Solomon - One of the best experts on this subject based on the ideXlab platform.
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Microearthquake characteristics and crustal vp vs structure at the mid atlantic ridge 35 n
Journal of Geophysical Research, 2001Co-Authors: Andrew H Barclay, Douglas R Toomey, Sean C SolomonAbstract:We report on the results of a Microearthquake experiment conducted at the along-axis bathymetric high of the slow spreading ridge segment near 35°N on the Mid-Atlantic Ridge (MAR). Of a total of 255 Microearthquakes recorded during the 43-day experiment, 31 were located near the Oceanographer transform at the northern end of the segment, 79 occurred at the nontransform offset at the southern end of the segment, and 145 were at the segment center. At the segment center, earthquake epicenters lay within the median valley inner floor and formed a ∼12-km-long trend paralleling a steep scarp bounding the western wall of the inner valley; focal depths were 3-4 km below the seafloor. Most (80%) of the focal mechanisms for 32 segment center earthquakes are consistent with normal faulting on faults approximately parallel to the axial trend. From a joint inversion for hypocenters and P and S wave velocity structures, we determined a horizontally averaged V P /V S ratio that decreases from 2.9 in the shallowmost 300 m to 1.7 at 2-km depth, and we interpret this decrease as indicating a decreasing contribution of thin cracks to fracture porosity with depth. The maximum depth of seismicity, 4 km, is anomalously shallow compared with other MAR segments at which Microearthquake experiments have been carried out. Cross-axis relief is also anomalously low for this segment's center, and on the basis of this and other MAR Microearthquake experiments, there appears to be a correlation between cross-axis relief and maximum depth of seismicity. From the correlation of cross-axis relief and inferred crustal thickness we suggest a relationship between thick crust, high crustal temperatures, and low cross-axis relief, in qualitative agreement with thermomechanical models for the depth of the axial valley.
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Microearthquake characteristics and crustal velocity structure at 29 n on the mid atlantic ridge the architecture of a slow spreading segment
Journal of Geophysical Research, 1995Co-Authors: Cecily J Wolfe, G M Purdy, Douglas R Toomey, Sean C SolomonAbstract:We report the results of a Microearthquake and seismic tomography experiment conducted along the southern half of the Mid-Atlantic Ridge segment at 29°N and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes. The seismic velocity structure is obtained from two-dimensional (2-D) and three-dimensional (3-D) tomographic inversions of travel times from shots along an axial refraction line. Inversion solutions indicate that the velocity structure in the lower crust is heterogeneous, with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetrie high. The thickness of the lower crust at the segment end is asymmetric across axis, with thinner crust beneath the inside corner. The indicated variations in crustal thickness are consistent with those inferred from mantle Bouguer gravity anomalies. The Microearthquakes located along axis during the 41-day recording period cluster in three separate along-axis regions: (1) the southern segment end near 28°55′N, (2) the central along-axis topographic high at 29°11′N, near and north of the Broken Spur hydrothermal vent field, and (3) a region midway between, beneath a volcano near 29°02′N. The greatest level of Microearthquake activity was in a diffuse zone off axis beneath the inside corner of a nontransform offset. This pattern of off-axis Microearthquake activity, and the cross-axis asymmetry in crustal thickness at the segment end, support tectonic models in which normal faulting and consequent crustal thinning occur preferentially at inside corner regions. Anomalous focal mechanisms for Microearthquakes beneath the along-axis volcano and the significant seismicity beneath the axial volcanic ridge at the segment center, in contrast, may be the result of volcanic and hydrothermal processes, such as magma movement or thermal stresses generated near cooling plutons. A comparison of Microearthquake characteristics with residual gravity data and velocity structure leads to the hypothesis that Microearthquakes associated with areas of thin crust near the segment end and inside corner are dominantly tectonic in nature, whereas Microearthquakes associated with volcanic and hydrothermal processes are more likely to occur toward the segment center in areas of greater rates of magma supply and thicker crust. Along axis, well-resolved focal depths determined with a 3-D velocity model range from 3 to 6 km beneath the seafloor and do not shoal toward the segment center. These observations indicate that the thermal structure of the crust along this slow spreading ridge segment is not in steady state.
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Microearthquakes on and near the East Pacific Rise, 9°–10°N
Geophysical Research Letters, 1992Co-Authors: William S. D. Wilcock, G M Purdy, Sean C Solomon, David L. Dubois, Douglas R ToomeyAbstract:Records from a seismic network deployed as part of an active tomography experiment at 9°30′N on the East Pacific Rise (EPR) provide an opportunity to characterize local Microearthquake activity over an 8-day period. With the exception of the region around the 9°03′ overlapping spreading center (OSC), no events were located on the rise axis. One Microearthquake, located from P and S-wave arrival times, lies adjacent to the network, 18 km to the west of the rise axis. Five more events, located from P-wave and T-phase data, are to the south of the network. Three cluster around the western arm of the 9°03′ OSC, while two are located 25–30 km to the east of the OSC. The seismic moments of the Microearthquakes range from 5 × 1019 to 4 × 1020 dyn cm, several orders of magnitude larger than those reported in other studies of shallow events along the axis of the EPR. These results suggest that overlapping spreading centers may be the loci of substantial Microearthquake activity and that at this location the normal faults that form on the young flanks of the EPR are active off-axis to distances of at least 20 km.
Andrew H Barclay - One of the best experts on this subject based on the ideXlab platform.
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upper crustal seismic velocity structure and Microearthquake depths at the endeavour segment juan de fuca ridge
Geochemistry Geophysics Geosystems, 2004Co-Authors: Andrew H Barclay, William S. D. WilcockAbstract:[1] We present the results of a study to invert Microearthquake and explosive shot data from the Endeavour segment of the intermediate-spreading Juan de Fuca Ridge. The average isotropic P wave velocity structure, derived from the shot data, in the uppermost 1.5 km of the oceanic crust is characterized by an increase with age of ∼8% from the axis to at least 0.5 Ma, that is attributed to the sealing of layer 2A porosity by hydrothermal processes. Superimposed on this variation are axis-parallel, 2-km-wide, alternating bands of high and low velocity with a peak-to-peak variation of 5–12%. High and low velocities away from the axis correspond to bathymetric trenches and ridges, respectively and are likely due to variations in layer 2A thickness. P wave azimuthal anisotropy is present in the data that is best fit with a model of 9% anisotropy at 750 m depth, decreasing to 1% at 3 km depth and is likely due to the preferential alignment of vertical cracks and fissures in the along-axis direction. Anisotropy and velocity heterogeneity are coupled; anisotropy alone may explain the form but not the magnitude of the axis-parallel bands. There are strong trade-offs between the hypocentral depths of Microearthquakes and the P and S wave velocity structures. Changing the mean hypocentral depth by up to 0.5 km leads to only modest increases in the travel time RMS but the resulting velocity models appear more feasible when the earthquakes are forced deeper than when they are forced shallower.
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Microearthquake patterns following the 1998 eruption of axial volcano juan de fuca ridge mechanical relaxation and thermal strain
Journal of Geophysical Research, 2004Co-Authors: Robert A Sohn, Andrew H Barclay, Spahr C WebbAbstract:[1] Ocean bottom seismic networks deployed following the 1998 eruption of Axial seamount reveal an evolving pattern of Microearthquake activity associated with subsurface magmatism and thermal strain. Seismicity rates decay steadily over 15 months of observation (February 8, 1998, to April 30, 1999), consistent with a trend toward thermal and mechanical equilibrium in the shallow crust after the magmatic event. Immediately after the eruption, seismicity rates were high for about 60 days in the southeast corner of the caldera where lava flows from the 1998 eruption were mapped. A small burst of seismic activity was observed on the southeast shoulder of the volcano from 100 to 150 days after the eruption. These events, which are characterized by slip on nearly vertical faults in the shallow crust, extend about 6 km from the southeast corner of the caldera and overlie a mid-crustal low-velocity zone. After this episode, seismicity rates remain low until the end of the observation period, 455 days after the eruption. Shallow (∼0.7 km depth) events, consistent with thermal contraction and volume changes of ∼2 × 10−3 m3 in ∼5 m3 sources, are observed in individual clusters beneath hydrothermal vents within the 1998 lava flow at the southeast edge of the caldera. Microearthquakes observed during the last 70 days of observation are distributed around the central caldera, most likely representing small amounts of subsidence on caldera faults during the final stages of equilibration following melt withdrawal associated with the 1998 eruption.
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Microearthquake characteristics and crustal vp vs structure at the mid atlantic ridge 35 n
Journal of Geophysical Research, 2001Co-Authors: Andrew H Barclay, Douglas R Toomey, Sean C SolomonAbstract:We report on the results of a Microearthquake experiment conducted at the along-axis bathymetric high of the slow spreading ridge segment near 35°N on the Mid-Atlantic Ridge (MAR). Of a total of 255 Microearthquakes recorded during the 43-day experiment, 31 were located near the Oceanographer transform at the northern end of the segment, 79 occurred at the nontransform offset at the southern end of the segment, and 145 were at the segment center. At the segment center, earthquake epicenters lay within the median valley inner floor and formed a ∼12-km-long trend paralleling a steep scarp bounding the western wall of the inner valley; focal depths were 3-4 km below the seafloor. Most (80%) of the focal mechanisms for 32 segment center earthquakes are consistent with normal faulting on faults approximately parallel to the axial trend. From a joint inversion for hypocenters and P and S wave velocity structures, we determined a horizontally averaged V P /V S ratio that decreases from 2.9 in the shallowmost 300 m to 1.7 at 2-km depth, and we interpret this decrease as indicating a decreasing contribution of thin cracks to fracture porosity with depth. The maximum depth of seismicity, 4 km, is anomalously shallow compared with other MAR segments at which Microearthquake experiments have been carried out. Cross-axis relief is also anomalously low for this segment's center, and on the basis of this and other MAR Microearthquake experiments, there appears to be a correlation between cross-axis relief and maximum depth of seismicity. From the correlation of cross-axis relief and inferred crustal thickness we suggest a relationship between thick crust, high crustal temperatures, and low cross-axis relief, in qualitative agreement with thermomechanical models for the depth of the axial valley.
Robert A Sohn - One of the best experts on this subject based on the ideXlab platform.
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Microearthquake evidence for reaction driven cracking within the trans atlantic geotraverse active hydrothermal deposit
Journal of Geophysical Research, 2014Co-Authors: Claire W Pontbriand, Robert A SohnAbstract:We detected 32,078 very small, local Microearthquakes (average ML = −1) during a 9 month deployment of five ocean bottom seismometers on the periphery of the Trans-Atlantic Geotraverse active mound. Seismicity rates were constant without any main shock-aftershock behavior at ~243 events per day at the beginning of the experiment, 128 events per day after an instrument failed, and 97 events per day at the end of the experiment when whale calls increased background noise levels. The Microearthquake seismograms are characterized by durations of <1 s and most have single-phase P wave arrivals (i.e., no S arrivals). We accurately located 6207 of the earthquakes, with hypocenters clustered within a narrow depth interval from ~50 to 125 m below seafloor on the south and west flanks of the deposit. We model the Microearthquakes as reaction-driven fracturing events caused by anhydrite deposition in the secondary circulation system of the hydrothermal mound and show that under reasonable modeling assumptions an average event represents a volume increase of 31–58 cm3, yielding an annual (seismogenic) anhydrite deposition rate of 27–51 m3.
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Microearthquake patterns following the 1998 eruption of axial volcano juan de fuca ridge mechanical relaxation and thermal strain
Journal of Geophysical Research, 2004Co-Authors: Robert A Sohn, Andrew H Barclay, Spahr C WebbAbstract:[1] Ocean bottom seismic networks deployed following the 1998 eruption of Axial seamount reveal an evolving pattern of Microearthquake activity associated with subsurface magmatism and thermal strain. Seismicity rates decay steadily over 15 months of observation (February 8, 1998, to April 30, 1999), consistent with a trend toward thermal and mechanical equilibrium in the shallow crust after the magmatic event. Immediately after the eruption, seismicity rates were high for about 60 days in the southeast corner of the caldera where lava flows from the 1998 eruption were mapped. A small burst of seismic activity was observed on the southeast shoulder of the volcano from 100 to 150 days after the eruption. These events, which are characterized by slip on nearly vertical faults in the shallow crust, extend about 6 km from the southeast corner of the caldera and overlie a mid-crustal low-velocity zone. After this episode, seismicity rates remain low until the end of the observation period, 455 days after the eruption. Shallow (∼0.7 km depth) events, consistent with thermal contraction and volume changes of ∼2 × 10−3 m3 in ∼5 m3 sources, are observed in individual clusters beneath hydrothermal vents within the 1998 lava flow at the southeast edge of the caldera. Microearthquakes observed during the last 70 days of observation are distributed around the central caldera, most likely representing small amounts of subsidence on caldera faults during the final stages of equilibration following melt withdrawal associated with the 1998 eruption.
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hydrothermal Microearthquake swarms beneath active vents at middle valley northern juan de fuca ridge
Journal of Geophysical Research, 2003Co-Authors: Charles E Golden, Spahr C Webb, Robert A SohnAbstract:[1] Over 3000 local and regional earthquakes were recorded by a compact network of eight ocean bottom seismographs (500 m instrument spacing) from August 1996 to January 1997 in Middle Valley, a sediment-covered rift valley on the northern Juan de Fuca Ridge. Thirteen swarms of small-magnitude Microearthquakes (−1.2 < Mw < 0.2) were detected beneath Dead Dog vent field, a major hydrothermal area in Middle Valley with exit fluid temperatures near 270°C. High precision relative positions for 304 events within swarms were determined using waveform cross-correlation techniques. The events were relocated into small, spatially distinct clusters. The intensity of the swarms is correlated with high heat flow with the largest swarm positioned 1.3 km beneath the Dead Dog vents. Smaller clusters of earthquakes are located up to hundreds of meters outside the vent field. The results suggest that the observed seismicity in the Dead Dog region is triggered by thermal strain (contraction) in the hydrothermal reaction zone as fluids extract heat from hot basement rock. Microearthquake swarms appear to be concentrated in regions where faulting has promoted seawater penetration through the sediment layer, cooling the crust, and yielding larger strain rates than those produced by seafloor spreading.
J. R. Kayal - One of the best experts on this subject based on the ideXlab platform.
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Microearthquake seismology and seismotectonics of south asia
2008Co-Authors: J. R. KayalAbstract:Earthquakes and Seismic Waves.- Microearthquake Recording and Data Analysis.- Dynamics of Faulting and Fault Plane Solution.- Himalayas, Pamir-Hindu Kush and Foredeep Region.- Northeast India, Myanmar, Bangladesh and Andaman-Sumatra Region.- Seismotectonics of Peninsular India and Sri Lanka.
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Microearthquake seismology and seismotectonics of South Asia
Microearthquake Seismology and Seismotectonics of South Asia, 2008Co-Authors: J. R. KayalAbstract:This volume is the outcome of about 30 years of research in the field of earthquake seismology in various parts of South Asia. It comprehensively deals with plate tectonics and seismic waves in general and earthquake monitoring by permanent and temporary networks for active fault mapping, aftershock, swarm and induced seismicity investigations in interplate collision and subduction zones and in intraplate shield region. Data acquisition as well as data analysis are well presented for clear understanding of interpretation with limitations of the techniques. State-of-the-art techniques in earthquake location/relocation, fault plane solution, waveform inversion, seismic tomography, fractals etc. are discussed, and the results are interpreted in terms of seismic source processes in the region. A large quantity of seismic data in the Himalaya from western syntaxis, Pamir-Hindu Kush to western (Garhwal) Himalaya, central (Nepal) Himalaya, eastern (Sikkim and Bhutan) Himalaya to the eastern syntaxis and Arunachal Himalaya are analysed to understand seismotectonics of the Himalayan collision zone. The atypical continental plate (Indian continental plate) subduction beneath the Indo-Burma ranges and typical oceanic plate (Indian Oceanic plate) subduction beneath the Andaman-Sumatra arc in Southeast Asia are also examined with the available data. Temporary and permanent Microearthquake network data in the peninsular India shield region and in Bangladesh and Sri Lanka shed light on the intraplate tectonics in South Asia. "Professor Kayal is to be congratulated for this very compendium, which is firmly based on his more than 25 years experience as a Microearthquake seismologist and researcher. The book will serve multiple readerships. It is a comprehensive and up-to-date textbook for students interested in field seismology. It also provides a synthesis of the seismotectonics and seismic hazard of the Indian subcontinent and India-Eurasia collision zone."--Euan Smith, Professor of Geophysics, Victoria University of Wellington "Dr. J.R. Kayal has been leading Geological Survey of India for over two decades in operating seismic stations in the complex Indian-Eurasian collision zone. Dr. Kayal has put together his vast experience in this useful book. This timely compilation addresses several aspects of Microearthquake studies as well as their application to comprehending complex seismotectonics of South Asia. Dr. Kayal deserves compliments for this beautiful compilation."--Prof. Harsh K. Gupta, Raja Ramanna Fellow, National Geophysical Research Institute, Hyderabad, India "We normally hear of earthquakes in terms of the destruction that they cause. However, the study of Microearthquakes serves a very practical purpose. It helps to define the seismotectonic framework which includes the determination of the stress field causing the earthquake, the geometry of the seismogenic faults and the style of faulting. These are essential parameters in both the assessment of seismic hazards and in the study of rectonics of the region. With illustrative examples from various seismotectonic regions in India, Dr. Kayal has written an excellent, hands-on book, for both the investigator and the user of these data." -Pradeep Talwani, Professor of Geophysics and Director of the South Carolina Seismic Network, University of South Carolina, Columbia, SC, USA. Earthquakes and Seismic Waves -- Microearthquake Recording and Data Analysis -- Dynamics of Faulting and Fault Plane Solution -- Himalayas, Pamir-Hindu Kush and Foredeep Region -- Northeast India, Myanmar, Bangladesh and Andaman-Sumatra Region -- Seismotectonics of Peninsular India and Sri Lanka.
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Microearthquake activity in some parts of the himalaya and the tectonic model
Tectonophysics, 2001Co-Authors: J. R. KayalAbstract:Abstract Microearthquake data from temporary/permanent networks in different parts of the Himalaya shed new light on understanding the earthquake generating processes and their relation to tectonic models of the region. The Microearthquake activity in Arunachal Pradesh, northeastern Himalaya, is found to be pronounced at the Main Boundary Thrust (MBT) and to its south; the subcrustal earthquakes (depth 50–80 km) occur much below the plane of detachment of the tectonic models proposed by Seeber et al., 1981 , Ni and Barazangi, 1984 . The MBT is not the seismogenic fault; the earthquakes are generated by strike–slip movement on deep seated hidden faults, transverse to the MBT. The high seismic activity in the Shillong Plateau, about 200 km south of the MBT in the northeast region, is due to the influence of Himalayan collision tectonics to the north and Burmese arc subduction tectonics to the east. The activity in the Plateau is not directly related to the Himalayan thrust belt or seismic belt. These are mostly crustal earthquakes (depth 10–30 km), and are caused by local active faults/lineaments. In the eastern Himalaya, in the Sikkim and Darjeeling area, the seismic activity is found to be clustered mostly to the north of the MBT. The earthquakes occur at a depth range 0–50 km; the majority of them occur below the detachment plane by thrust-faulting. In the central part, in the Nepal Himalaya, lateral variations of the seismic activity are observed, which represent lateral segmentation of the MBT by transverse tectonic features. In the western Himalaya, however, the tectonic models fit well with the Microearthquake data. In the Himachal Pradesh of the western Himalaya, the Microearthquakes are mostly recorded in the MBT zone, and the hypocentres (depth 0–20 km) are confined above the plane of detachment or on the Basement Thrust. The earthquakes mostly occur to the south of the Main Central Thrust (MCT), which suggests that the MCT is not seismogenic; it is rather a dormant fault. No single tectonic model explains the Himalayan earthquakes.
Douglas R Toomey - One of the best experts on this subject based on the ideXlab platform.
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Microearthquake characteristics and crustal vp vs structure at the mid atlantic ridge 35 n
Journal of Geophysical Research, 2001Co-Authors: Andrew H Barclay, Douglas R Toomey, Sean C SolomonAbstract:We report on the results of a Microearthquake experiment conducted at the along-axis bathymetric high of the slow spreading ridge segment near 35°N on the Mid-Atlantic Ridge (MAR). Of a total of 255 Microearthquakes recorded during the 43-day experiment, 31 were located near the Oceanographer transform at the northern end of the segment, 79 occurred at the nontransform offset at the southern end of the segment, and 145 were at the segment center. At the segment center, earthquake epicenters lay within the median valley inner floor and formed a ∼12-km-long trend paralleling a steep scarp bounding the western wall of the inner valley; focal depths were 3-4 km below the seafloor. Most (80%) of the focal mechanisms for 32 segment center earthquakes are consistent with normal faulting on faults approximately parallel to the axial trend. From a joint inversion for hypocenters and P and S wave velocity structures, we determined a horizontally averaged V P /V S ratio that decreases from 2.9 in the shallowmost 300 m to 1.7 at 2-km depth, and we interpret this decrease as indicating a decreasing contribution of thin cracks to fracture porosity with depth. The maximum depth of seismicity, 4 km, is anomalously shallow compared with other MAR segments at which Microearthquake experiments have been carried out. Cross-axis relief is also anomalously low for this segment's center, and on the basis of this and other MAR Microearthquake experiments, there appears to be a correlation between cross-axis relief and maximum depth of seismicity. From the correlation of cross-axis relief and inferred crustal thickness we suggest a relationship between thick crust, high crustal temperatures, and low cross-axis relief, in qualitative agreement with thermomechanical models for the depth of the axial valley.
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Microearthquake characteristics and crustal velocity structure at 29 n on the mid atlantic ridge the architecture of a slow spreading segment
Journal of Geophysical Research, 1995Co-Authors: Cecily J Wolfe, G M Purdy, Douglas R Toomey, Sean C SolomonAbstract:We report the results of a Microearthquake and seismic tomography experiment conducted along the southern half of the Mid-Atlantic Ridge segment at 29°N and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes. The seismic velocity structure is obtained from two-dimensional (2-D) and three-dimensional (3-D) tomographic inversions of travel times from shots along an axial refraction line. Inversion solutions indicate that the velocity structure in the lower crust is heterogeneous, with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetrie high. The thickness of the lower crust at the segment end is asymmetric across axis, with thinner crust beneath the inside corner. The indicated variations in crustal thickness are consistent with those inferred from mantle Bouguer gravity anomalies. The Microearthquakes located along axis during the 41-day recording period cluster in three separate along-axis regions: (1) the southern segment end near 28°55′N, (2) the central along-axis topographic high at 29°11′N, near and north of the Broken Spur hydrothermal vent field, and (3) a region midway between, beneath a volcano near 29°02′N. The greatest level of Microearthquake activity was in a diffuse zone off axis beneath the inside corner of a nontransform offset. This pattern of off-axis Microearthquake activity, and the cross-axis asymmetry in crustal thickness at the segment end, support tectonic models in which normal faulting and consequent crustal thinning occur preferentially at inside corner regions. Anomalous focal mechanisms for Microearthquakes beneath the along-axis volcano and the significant seismicity beneath the axial volcanic ridge at the segment center, in contrast, may be the result of volcanic and hydrothermal processes, such as magma movement or thermal stresses generated near cooling plutons. A comparison of Microearthquake characteristics with residual gravity data and velocity structure leads to the hypothesis that Microearthquakes associated with areas of thin crust near the segment end and inside corner are dominantly tectonic in nature, whereas Microearthquakes associated with volcanic and hydrothermal processes are more likely to occur toward the segment center in areas of greater rates of magma supply and thicker crust. Along axis, well-resolved focal depths determined with a 3-D velocity model range from 3 to 6 km beneath the seafloor and do not shoal toward the segment center. These observations indicate that the thermal structure of the crust along this slow spreading ridge segment is not in steady state.
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Microearthquakes on and near the East Pacific Rise, 9°–10°N
Geophysical Research Letters, 1992Co-Authors: William S. D. Wilcock, G M Purdy, Sean C Solomon, David L. Dubois, Douglas R ToomeyAbstract:Records from a seismic network deployed as part of an active tomography experiment at 9°30′N on the East Pacific Rise (EPR) provide an opportunity to characterize local Microearthquake activity over an 8-day period. With the exception of the region around the 9°03′ overlapping spreading center (OSC), no events were located on the rise axis. One Microearthquake, located from P and S-wave arrival times, lies adjacent to the network, 18 km to the west of the rise axis. Five more events, located from P-wave and T-phase data, are to the south of the network. Three cluster around the western arm of the 9°03′ OSC, while two are located 25–30 km to the east of the OSC. The seismic moments of the Microearthquakes range from 5 × 1019 to 4 × 1020 dyn cm, several orders of magnitude larger than those reported in other studies of shallow events along the axis of the EPR. These results suggest that overlapping spreading centers may be the loci of substantial Microearthquake activity and that at this location the normal faults that form on the young flanks of the EPR are active off-axis to distances of at least 20 km.