The Hi-CLIMB seismic experiment (2002)(2003)(2004)(2005) operated 233 sites along an 800km long n... more The Hi-CLIMB seismic experiment (2002)(2003)(2004)(2005) operated 233 sites along an 800km long north-south array extending from the Himalayan foreland into the Central Tibetan Plateau and a flanking 350x350 km lateral array in southern Tibet and eastern Nepal. Data from the experiment's second phase (June 2004 to August 2005), when stations operated in Tibet, were used to locate earthquakes in southcentral Tibet, a region with no permanent seismic network and where little is known about its seismicity. The Antelope software package (Boulder Real Time Technologies) was used for automatic event detection and event-arrival association. The automated processing and arrival association produced a weekly average of roughly 1,700 declared events from local to teleseismic distances, totaling over 109,000 for the span of the project. The large database size rendered manual inspection unfeasible and automated post-processing modules were developed to weed out spurious detections and erroneous phase and event associations, which stemmed, e.g., from multiple coincident earthquakes within the array or misplaced seismicity from the great 2004 Sumatra earthquake. The resulting database contains more than 22,500 events within the local area, ~8,000 of those were located with 25 or more arrivals. Seismicity in this high-quality subset correlates well with mapped faults and structures observed in satellite imagery. The quality of the locations is confirmed by comparison with manuallylocated earthquakes, comparison with InSAR data, and by depth-distributions for a very-high subset of events that matches distributions observed by previous studies using manually-picked arrivals. Seismicity in south-central Tibet is intense north of the Yarlung-Tsangpo Suture. The majority of seismicity, 78% of events, occurred in the Lhasa Terrane mainly along north-south trending rifts. Seismicity in the Qiangtang Terrane accounts for more than 16% of activity. In the Tethyan I give many thanks to Andrew Meigs for his eager and optimistic response to my enthusiasm to undertake graduate studies in geosciences and for his continued encouragement through my studies. And, I thank both Andrew Meigs and Andrew Ross for serving on my committee. I appreciate Anne Trehu for her encouragement and the considerable effort she put into course instruction. To Jan Baur, who was in the trenches with me in Corvallis, I offer much appreciation for his sense of humor, the adventures, the excellent conversations, and the necessary soccer breaks.
Hydrothermal vents on mid-ocean ridges of the northeast Pacific Ocean are known to respond to sei... more Hydrothermal vents on mid-ocean ridges of the northeast Pacific Ocean are known to respond to seismic disturbances, with observed changes in vent temperature. But these disturbances resulted from submarine volcanic activity; until now, there have been no observations of the response of a vent system to non-magmatic, tectonic events. Here we report measurements of hydrothermal vent temperature from several vents on the Juan de Fuca ridge in June 1999, before, during and after an earthquake swarm of apparent tectonic origin. Vent fluid temperatures began to rise 4-11 days after the first earthquake. Following this initial increase, the vent temperatures oscillated for about a month before settling down to higher values. We also observed a tenfold increase in fluid output from the hydrothermal system over a period of at least 80 days, extending along the entire ridge segment. Such a large, segment-wide thermal response to relatively modest tectonic activity is surprising, and raises questions about the sources of excess heat and fluid, and the possible effect on vent biological communities.
Page 1. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, TO. B1, PAGES 441-460, JANUARY IO, I987 EARTHQU... more Page 1. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, TO. B1, PAGES 441-460, JANUARY IO, I987 EARTHQUAKES, GRAVITY, AND THE ORIGIN OF THE BALI BASIN: AN EXAMPLE OF A NASCENT CONTINENTAL ...
Bathymetric, hydro-acoustic, seismic, submersible, and gravity data are used to investigate the a... more Bathymetric, hydro-acoustic, seismic, submersible, and gravity data are used to investigate the active tectonics of the eastern Blanco Transform Fault Zone (BTFZ). The eastern BTFZ is dominated by the ~150 km long transform-parallel Blanco Ridge (BR) which is a right-lateral strike-slip fault bordered to the east and west by the Gorda and Cascadia Depressions. Acoustic locations, fault-parameter information, and slip vector
New analyses of teleseismic body waves from moderate earthquakes in western Argentina demonstrate... more New analyses of teleseismic body waves from moderate earthquakes in western Argentina demonstrate that active shortening of the Andean foreland occurs on reverse faults extending to 40-50 km depth. Existing crustal-scale models of foreland deformation invoke thinskinned fault geometries, which root into an east-dipping mid-crustal décollement. Whereas thinskinned thrust sheets dominate shallow-crustal structure, seismological and geological data illustrate that planar reverse faults and pure-shear deformation involving more than 75% of the crust characterizes this thickskinned structural province.
... Northeast-trending dashed lines show Gillibrand Canyon (GC), San Fernando (SF), Pacoima, and ... more ... Northeast-trending dashed lines show Gillibrand Canyon (GC), San Fernando (SF), Pacoima, and Sunland segment boundaries (SB). T:Santa Susana fault tip; ORF:Oak Ridge fault; SCF-MS:San Cayetano fault, main strand; SCF-PS:San Cayetano fault, Piru strand. ...
INTRODUCTION Forearcs of many subduction zones undergo rapid arc-parallel extension, revealed by ... more INTRODUCTION Forearcs of many subduction zones undergo rapid arc-parallel extension, revealed by a sys- tematic rotation of interplate earthquake slip vec- tors away from expected plate convergence direc- tions (McCaffrey, 1996). In extreme cases, such as the Marianas, New Hebrides, ...
ABSTRACT Broadband body waves recorded at 15 digital seismic stations worldwide are used to study... more ABSTRACT Broadband body waves recorded at 15 digital seismic stations worldwide are used to study the rupture process of the May 23, 1989 Macquarie Ridge earthquake. The centroidal solution (strike 211°, dip 86°, rake 180°, and depth of 10 km below the seafloor) indicates shallow rupture with pure right-lateral strike-slip motion along the Pacific-Australia plate boundary, in agreement with motion predicted by plate tectonic models. The total seismic moment is 13.4x10²⁰ Nm, 80% of which was released in the first 24 s of the rupture process. Modeling favors a bilaterally propagating rupture with slightly different dip and rake for the northward and southward fault segments and similar moment release along both directions. The estimated fault length is quite short, about 90 km, and the derived stress drop of 180 bar and average displacement of 17 m are unusually high. The bathymetry in the epicentral region shows topographic segmentation of the ridge, possibly indicating fault segmentation which confines ruptures to short segments.
ABSTRACT That the Cascadia subduction zone ruptures in occasional large earthquakes on the megath... more ABSTRACT That the Cascadia subduction zone ruptures in occasional large earthquakes on the megathrust is now generally accepted. Recent paleoseismic work indicates that while the average rupture interval is ~560 years, the most common time period between events is 200-400 years, and the most recent event occurred on January 26, 1700. Instrumental seismicity, however, has not been documented for most of the shallow, nominally locked, part of the megathrust. One exception is a section of the continental margin offshore central Oregon where several small earthquakes are detected each year by the Pacific Northwest Seismic Network. During July and August, 2004, the region experienced two moderate and widely felt earthquakes with moment magnitude 4.9 and 4.8, respectively. Both events appear to have been thrust earthquakes with a shallow east-dipping nodal plane consistent with rupture along the megathrust. Crustal models based on active source seismic and gravity data indicate the presence of a buried basement ridge that abuts against crystalline rocks of the subduction backstop, which is composed of an anomalously thick oceanic accreted terrane in this region. We compare the seismicity and structure in this region to seismicity and structure in the shallow part of other subduction zones that have experienced large earthquakes in historic time and discuss the impact of upper and lower plate structure on subduction zone earthquakes. A better understanding of plate interactions here is needed to determine whether this is a likely nucleation point for the next great Cascadia earthquake.
ABSTRACT In the summer of 2004, two clusters of "repeating" earthquakes occurre... more ABSTRACT In the summer of 2004, two clusters of "repeating" earthquakes occurred beneath the continental shelf of the central Cascadia subduction zone near 44.5N, 124.5W where the subduction megathrust is thought to be locked or transitional. The largest event in each cluster reached moment magnitude M=4.8-4.9. Seismicity has continued since with small (M
The Blanco Transform Fault Zone (BTFZ), northeast Pacific Ocean, forms the ~350 km-long Pacific-J... more The Blanco Transform Fault Zone (BTFZ), northeast Pacific Ocean, forms the ~350 km-long Pacific-Juan de Fuca plate boundary between the Gorda and Juan de Fuca ridges. Near-by broadband seismic networks provide the unique framework for a detailed, long-term seismotectonic study of an entire oceanic transform fault (OTF) system. We use regional waveforms to determine 125 earthquake source parameters; combined with 28 Harvard moment-tensors, they represent the largest waveform derived OTF source parameter dataset. Joint epicenter determination removes the northeasterly routine location bias. Projecting seismicity onto the BTFZ, we map along-fault seismic slip rate variations. Earthquake source parameters and morphology indicate several transform segments separated by extensional step-overs. The eastern segment from Gorda Ridge to Gorda Depression is a pull-apart basin. The longest transform (~150 km) following Blanco Ridge from the Gorda to Cascadia depression is seismically very active, fully coupled, has a wider seismic zone (~9 km) than other transform segments and accommodates the largest (Mw 6.4-6.5) BTFZ earthquakes. Interpretation of Cascadia Depression as spreading ridge is supported by plate motion parallel normal faulting T-axes. Spreading is currently tectonic; 9 km deep earthquakes indicate a deep source for intermittent intrusives and rapid post- emplacement cooling. A short transform connects to the pull-apart Surveyor Depression. Widely spread seismicity along the western BTFZ reflects complex morphology indicating ongoing plate boundary reorganization along short, narrow-width sub-parallel faults that limit earthquake size to Mw <= 6.2. Seismic coupling is low in extensional (<=15%) compared to transform areas (35-100%) implying different mechanical properties. Centroid depth variations are consistent with seismic slip cut-off near 600°C.
ABSTRACT The area near Maupin, Oregon has experienced over 300 earthquakes since December 2006. T... more ABSTRACT The area near Maupin, Oregon has experienced over 300 earthquakes since December 2006. The events, located by the Pacific Northwest Seismic Network (PNSN), occurred ~10 km SE of the town in central Oregon and ~50 km E-SE of Mount Hood. The temporal event pattern and lack of a distinct main shock are characteristic of an earthquake swarm with the event-size distribution indicating a low b-value similar to other non-volcanic swarms. Locations show a NW-SE trending, ~4x3 km cluster at apparent depths of 12-24 km. The largest events (Mw=3.8 and 3.9) on March 1, 2007 and July 14, 2008 occurred more than one year apart; 11 other events had a magnitude of 3 or greater. The larger events were felt locally. During the first 14 months EarthScope USArray seismic stations surrounded the swarm, providing a unique high-quality dataset. Waveform similarity at the closest USArray site G06A indicates hypocenters are much tighter than suggested by the PNSN distribution. Moment tensor inversion reveals nearly identical double- couple strike-slip mechanisms on a plane striking ~15° NW for the three largest 2007 events and the July 2008 event. The April 2008 Mw=3.3 event is rotated ~10° clockwise consistent with slight changes of G06A three-component waveforms relative to the other events. Preferred centroid depths are in the 15-20 km range. Historically, seismicity in the Pacific Northwest east of the Cascades is characterized by sporadic bursts of clustered seismicity with occasional M=6 earthquakes. The largest instrumentally recorded earthquake near Maupin (Mw=4.6) occurred 1976. An earlier swarm was observed 1987, but since then only ~2 events/yr occurred until the current swarm. In spite of recurrent seismicity, exposed surface rocks near Maupin are undeformed lava flows of the Columbia River Basalt Group and older John Day volcanics. The geologic map of Oregon shows a NW-trending dip slip fault near the epicenter area, inconsistent with moment tensor solutions. The cause for the swarm is currently unknown with hypotheses ranging from magmatic or fluid motion in the crust to tectonic activity along a weakly coupled microplate boundary.
Bulletin of the Seismological Society of America, 2017
We expand on the automatic polarization‐based S‐wave‐arrival detection technique of Cichowicz (19... more We expand on the automatic polarization‐based S‐wave‐arrival detection technique of Cichowicz (1993) and Diehl et al.(2009) to make it more effective for a broader range of distances and crustal structures. We use the characteristic function of Cichowicz (1993), which combines the polarization attributes of rectilinearity, directivity, and transverse‐to‐total‐energy ratio and, as does Diehl et al. (2009), modulates it with an amplitude weight function. Compared to Diehl, we increase the weight function exponent from 0.5 to 2, which stabilizes the characteristic function and emphasizes the Swave. We use the weight function by itself to determine if the signal‐to‐noise ratio is appropriate for picking and to determine a rough initial S pick. The pick is refined using the characteristic function. The picking algorithm works backward in time along the characteristic function from the initial pick until the characteristic function reaches a minimum. Using initial S picks based on the amplitude weight function, by increasing the weight function exponent and picking backward in time we avoid numerous incorrect early picks and drop traces with poor picking prospects. We apply the new approach to earthquakes in south‐central Tibet
Bulletin of the Seismological Society of America, Jun 1, 1985
... (1980) 10t This study 14 6.2 7.0 ISC 15 2.4 227 36 108 Kristy et al. ... structures at the so... more ... (1980) 10t This study 14 6.2 7.0 ISC 15 2.4 227 36 108 Kristy et al. ... structures at the source and receiver is approximated by a half-space with the P and S velocity ~ = 6.0 km sec -1 and/~ = 3.5 km sec -1, respectively, and the density p = 2.5 g cm -3. Mantle propagation effects are ...
The Explorer region offshore western Canada is a tectonically complex area surrounded by the Paci... more The Explorer region offshore western Canada is a tectonically complex area surrounded by the Pacific, North America, and Juan de Fuca plates. Existing tectonic models for the region differ fundamentally. Proposed plate configurations range from multiple independent plate fragments to an Explorer plate now fused to North America along the continental margin and cut by Pacific-North America transform faults in the west. We present new seismological data constraining the region's current tectonics. We use three-component regional waveforms to determine the source parameters of 84 earthquakes with magnitude greater than 4. Combined with 34 Harvard centroid moment tensor solutions, they represent the region's largest earthquake source parameter data set obtained by robust waveform modeling techniques. In addition, we perform joint epicenter determination to relocate larger earthquakes recorded since 1918. The source parameters and improved locations provide a consistent tectonic picture. Earthquake slip vector azimuths along the Pacific plate boundary change smoothly and are significantly less northerly oriented than the Pacific-North America plate motion direction, requiring an independent Explorer plate. The present-day Pacific-Explorer boundary is formed by transform faults subparallel to the Revere-Dellwood-Wilson fault. Plate motion vectors indicate that the Winona block is part of the Explorer plate. Current Explorer motion is more northerly than indicated by magnetic anomalies prior to 2 Ma, implying a recent change, possibly coinciding with a northwestward ridge jump near Explorer plate's northern end transferring the Winona block from the Pacific to the Explorer plate. In response to these plate motion changes the region north of the western Sovanco fracture zone was assimilated into the Pacific plate. The region around the eastern Sovanco fracture zone, characterized by broadly distributed seismicity, is composed of well-defined sets of conjugate faults bounding rotating crustal blocks. Earthquake fault strikes agree with the dominant northwest-southeast fault sets; however, the conjugate sets must be also active to fully accommodate present-day Explorer plate motion. The SW portion of the strike-slip Nootka fault zone, the Explorer-Juan de Fuca plate boundary, is well defined by focused seismicity; however, its full extent under Nootka Island remains unresolved. The Explorer-North America boundary shows sporadic low-magnitude seismicity. Our Explorer-North America rotation pole predicts convergence varying from negligible at the boundary's northwest end to $2 cm/yr at the SE end. This convergence can be accommodated either by subduction or by crustal thickening extending to the North American continent. We favor subduction based on low deformation rates observed by onshore GPS sites. The present Explorer plate system configuration is a result of stepwise reorientation of the Explorer ridge system, each step successively reducing the subduction rate relative to North America.
The actively subsiding North China sedimentary basin is associated with an unusually high level o... more The actively subsiding North China sedimentary basin is associated with an unusually high level of seismic activity. This oil-and gas-producing basin has been the site of nine large (M > 7), destructive earthquakes since 1600 A.D. An analysis of faulting during the Tangshan earthquake sequence, which includes some of the largest shocks that have occurred in this basin during the past 400 years, showed that the dominant pattern of deformation during this sequence was associated with displacement on right-stepping, right-lateral strike-slip faults. A large amount of subsidence (~1.0-1.5 m) occurred in pull-apart regions between steps of the northnortheast (NNE) trending strike-slip faults. The directivity of the P wave radiation pattern and the distribution of aftershocks of the Bohai Gulf earthquake of July 18, 1969, also indicate strike-slip faulting on planes of similar NNE trend. Together with the four largest shocks of the Hsingtai sequence in 1966, earthquakes with right-lateral slip on NNE trending faults account for about two-thirds of the seismic moment released in this region for the past 100 years. The average regional strain due to seismic slip has a dominant component of dextral simple shear (on NNE striking planes) that is approximately twice as large as north-south extension or east-west shortening. Based on a combination of published field and borehole data, the dominance of horizontal dextral simple shear over subsidence seems to have begun no later than the mid-Pliocene and the total amount of extension over the entire basin is only about 20-30%. This pattern of deformation, the spatially variable heat flow values, the scattered locations of rapid deposition of Quatemary sediments, and the fluctuating Copyright 1988 by the American Geophysical Union. Paper number 8T0341.
The Hi-CLIMB seismic experiment (2002)(2003)(2004)(2005) operated 233 sites along an 800km long n... more The Hi-CLIMB seismic experiment (2002)(2003)(2004)(2005) operated 233 sites along an 800km long north-south array extending from the Himalayan foreland into the Central Tibetan Plateau and a flanking 350x350 km lateral array in southern Tibet and eastern Nepal. Data from the experiment's second phase (June 2004 to August 2005), when stations operated in Tibet, were used to locate earthquakes in southcentral Tibet, a region with no permanent seismic network and where little is known about its seismicity. The Antelope software package (Boulder Real Time Technologies) was used for automatic event detection and event-arrival association. The automated processing and arrival association produced a weekly average of roughly 1,700 declared events from local to teleseismic distances, totaling over 109,000 for the span of the project. The large database size rendered manual inspection unfeasible and automated post-processing modules were developed to weed out spurious detections and erroneous phase and event associations, which stemmed, e.g., from multiple coincident earthquakes within the array or misplaced seismicity from the great 2004 Sumatra earthquake. The resulting database contains more than 22,500 events within the local area, ~8,000 of those were located with 25 or more arrivals. Seismicity in this high-quality subset correlates well with mapped faults and structures observed in satellite imagery. The quality of the locations is confirmed by comparison with manuallylocated earthquakes, comparison with InSAR data, and by depth-distributions for a very-high subset of events that matches distributions observed by previous studies using manually-picked arrivals. Seismicity in south-central Tibet is intense north of the Yarlung-Tsangpo Suture. The majority of seismicity, 78% of events, occurred in the Lhasa Terrane mainly along north-south trending rifts. Seismicity in the Qiangtang Terrane accounts for more than 16% of activity. In the Tethyan I give many thanks to Andrew Meigs for his eager and optimistic response to my enthusiasm to undertake graduate studies in geosciences and for his continued encouragement through my studies. And, I thank both Andrew Meigs and Andrew Ross for serving on my committee. I appreciate Anne Trehu for her encouragement and the considerable effort she put into course instruction. To Jan Baur, who was in the trenches with me in Corvallis, I offer much appreciation for his sense of humor, the adventures, the excellent conversations, and the necessary soccer breaks.
Hydrothermal vents on mid-ocean ridges of the northeast Pacific Ocean are known to respond to sei... more Hydrothermal vents on mid-ocean ridges of the northeast Pacific Ocean are known to respond to seismic disturbances, with observed changes in vent temperature. But these disturbances resulted from submarine volcanic activity; until now, there have been no observations of the response of a vent system to non-magmatic, tectonic events. Here we report measurements of hydrothermal vent temperature from several vents on the Juan de Fuca ridge in June 1999, before, during and after an earthquake swarm of apparent tectonic origin. Vent fluid temperatures began to rise 4-11 days after the first earthquake. Following this initial increase, the vent temperatures oscillated for about a month before settling down to higher values. We also observed a tenfold increase in fluid output from the hydrothermal system over a period of at least 80 days, extending along the entire ridge segment. Such a large, segment-wide thermal response to relatively modest tectonic activity is surprising, and raises questions about the sources of excess heat and fluid, and the possible effect on vent biological communities.
Page 1. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, TO. B1, PAGES 441-460, JANUARY IO, I987 EARTHQU... more Page 1. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, TO. B1, PAGES 441-460, JANUARY IO, I987 EARTHQUAKES, GRAVITY, AND THE ORIGIN OF THE BALI BASIN: AN EXAMPLE OF A NASCENT CONTINENTAL ...
Bathymetric, hydro-acoustic, seismic, submersible, and gravity data are used to investigate the a... more Bathymetric, hydro-acoustic, seismic, submersible, and gravity data are used to investigate the active tectonics of the eastern Blanco Transform Fault Zone (BTFZ). The eastern BTFZ is dominated by the ~150 km long transform-parallel Blanco Ridge (BR) which is a right-lateral strike-slip fault bordered to the east and west by the Gorda and Cascadia Depressions. Acoustic locations, fault-parameter information, and slip vector
New analyses of teleseismic body waves from moderate earthquakes in western Argentina demonstrate... more New analyses of teleseismic body waves from moderate earthquakes in western Argentina demonstrate that active shortening of the Andean foreland occurs on reverse faults extending to 40-50 km depth. Existing crustal-scale models of foreland deformation invoke thinskinned fault geometries, which root into an east-dipping mid-crustal décollement. Whereas thinskinned thrust sheets dominate shallow-crustal structure, seismological and geological data illustrate that planar reverse faults and pure-shear deformation involving more than 75% of the crust characterizes this thickskinned structural province.
... Northeast-trending dashed lines show Gillibrand Canyon (GC), San Fernando (SF), Pacoima, and ... more ... Northeast-trending dashed lines show Gillibrand Canyon (GC), San Fernando (SF), Pacoima, and Sunland segment boundaries (SB). T:Santa Susana fault tip; ORF:Oak Ridge fault; SCF-MS:San Cayetano fault, main strand; SCF-PS:San Cayetano fault, Piru strand. ...
INTRODUCTION Forearcs of many subduction zones undergo rapid arc-parallel extension, revealed by ... more INTRODUCTION Forearcs of many subduction zones undergo rapid arc-parallel extension, revealed by a sys- tematic rotation of interplate earthquake slip vec- tors away from expected plate convergence direc- tions (McCaffrey, 1996). In extreme cases, such as the Marianas, New Hebrides, ...
ABSTRACT Broadband body waves recorded at 15 digital seismic stations worldwide are used to study... more ABSTRACT Broadband body waves recorded at 15 digital seismic stations worldwide are used to study the rupture process of the May 23, 1989 Macquarie Ridge earthquake. The centroidal solution (strike 211°, dip 86°, rake 180°, and depth of 10 km below the seafloor) indicates shallow rupture with pure right-lateral strike-slip motion along the Pacific-Australia plate boundary, in agreement with motion predicted by plate tectonic models. The total seismic moment is 13.4x10²⁰ Nm, 80% of which was released in the first 24 s of the rupture process. Modeling favors a bilaterally propagating rupture with slightly different dip and rake for the northward and southward fault segments and similar moment release along both directions. The estimated fault length is quite short, about 90 km, and the derived stress drop of 180 bar and average displacement of 17 m are unusually high. The bathymetry in the epicentral region shows topographic segmentation of the ridge, possibly indicating fault segmentation which confines ruptures to short segments.
ABSTRACT That the Cascadia subduction zone ruptures in occasional large earthquakes on the megath... more ABSTRACT That the Cascadia subduction zone ruptures in occasional large earthquakes on the megathrust is now generally accepted. Recent paleoseismic work indicates that while the average rupture interval is ~560 years, the most common time period between events is 200-400 years, and the most recent event occurred on January 26, 1700. Instrumental seismicity, however, has not been documented for most of the shallow, nominally locked, part of the megathrust. One exception is a section of the continental margin offshore central Oregon where several small earthquakes are detected each year by the Pacific Northwest Seismic Network. During July and August, 2004, the region experienced two moderate and widely felt earthquakes with moment magnitude 4.9 and 4.8, respectively. Both events appear to have been thrust earthquakes with a shallow east-dipping nodal plane consistent with rupture along the megathrust. Crustal models based on active source seismic and gravity data indicate the presence of a buried basement ridge that abuts against crystalline rocks of the subduction backstop, which is composed of an anomalously thick oceanic accreted terrane in this region. We compare the seismicity and structure in this region to seismicity and structure in the shallow part of other subduction zones that have experienced large earthquakes in historic time and discuss the impact of upper and lower plate structure on subduction zone earthquakes. A better understanding of plate interactions here is needed to determine whether this is a likely nucleation point for the next great Cascadia earthquake.
ABSTRACT In the summer of 2004, two clusters of &quot;repeating&quot; earthquakes occurre... more ABSTRACT In the summer of 2004, two clusters of &quot;repeating&quot; earthquakes occurred beneath the continental shelf of the central Cascadia subduction zone near 44.5N, 124.5W where the subduction megathrust is thought to be locked or transitional. The largest event in each cluster reached moment magnitude M=4.8-4.9. Seismicity has continued since with small (M
The Blanco Transform Fault Zone (BTFZ), northeast Pacific Ocean, forms the ~350 km-long Pacific-J... more The Blanco Transform Fault Zone (BTFZ), northeast Pacific Ocean, forms the ~350 km-long Pacific-Juan de Fuca plate boundary between the Gorda and Juan de Fuca ridges. Near-by broadband seismic networks provide the unique framework for a detailed, long-term seismotectonic study of an entire oceanic transform fault (OTF) system. We use regional waveforms to determine 125 earthquake source parameters; combined with 28 Harvard moment-tensors, they represent the largest waveform derived OTF source parameter dataset. Joint epicenter determination removes the northeasterly routine location bias. Projecting seismicity onto the BTFZ, we map along-fault seismic slip rate variations. Earthquake source parameters and morphology indicate several transform segments separated by extensional step-overs. The eastern segment from Gorda Ridge to Gorda Depression is a pull-apart basin. The longest transform (~150 km) following Blanco Ridge from the Gorda to Cascadia depression is seismically very active, fully coupled, has a wider seismic zone (~9 km) than other transform segments and accommodates the largest (Mw 6.4-6.5) BTFZ earthquakes. Interpretation of Cascadia Depression as spreading ridge is supported by plate motion parallel normal faulting T-axes. Spreading is currently tectonic; 9 km deep earthquakes indicate a deep source for intermittent intrusives and rapid post- emplacement cooling. A short transform connects to the pull-apart Surveyor Depression. Widely spread seismicity along the western BTFZ reflects complex morphology indicating ongoing plate boundary reorganization along short, narrow-width sub-parallel faults that limit earthquake size to Mw <= 6.2. Seismic coupling is low in extensional (<=15%) compared to transform areas (35-100%) implying different mechanical properties. Centroid depth variations are consistent with seismic slip cut-off near 600°C.
ABSTRACT The area near Maupin, Oregon has experienced over 300 earthquakes since December 2006. T... more ABSTRACT The area near Maupin, Oregon has experienced over 300 earthquakes since December 2006. The events, located by the Pacific Northwest Seismic Network (PNSN), occurred ~10 km SE of the town in central Oregon and ~50 km E-SE of Mount Hood. The temporal event pattern and lack of a distinct main shock are characteristic of an earthquake swarm with the event-size distribution indicating a low b-value similar to other non-volcanic swarms. Locations show a NW-SE trending, ~4x3 km cluster at apparent depths of 12-24 km. The largest events (Mw=3.8 and 3.9) on March 1, 2007 and July 14, 2008 occurred more than one year apart; 11 other events had a magnitude of 3 or greater. The larger events were felt locally. During the first 14 months EarthScope USArray seismic stations surrounded the swarm, providing a unique high-quality dataset. Waveform similarity at the closest USArray site G06A indicates hypocenters are much tighter than suggested by the PNSN distribution. Moment tensor inversion reveals nearly identical double- couple strike-slip mechanisms on a plane striking ~15° NW for the three largest 2007 events and the July 2008 event. The April 2008 Mw=3.3 event is rotated ~10° clockwise consistent with slight changes of G06A three-component waveforms relative to the other events. Preferred centroid depths are in the 15-20 km range. Historically, seismicity in the Pacific Northwest east of the Cascades is characterized by sporadic bursts of clustered seismicity with occasional M=6 earthquakes. The largest instrumentally recorded earthquake near Maupin (Mw=4.6) occurred 1976. An earlier swarm was observed 1987, but since then only ~2 events/yr occurred until the current swarm. In spite of recurrent seismicity, exposed surface rocks near Maupin are undeformed lava flows of the Columbia River Basalt Group and older John Day volcanics. The geologic map of Oregon shows a NW-trending dip slip fault near the epicenter area, inconsistent with moment tensor solutions. The cause for the swarm is currently unknown with hypotheses ranging from magmatic or fluid motion in the crust to tectonic activity along a weakly coupled microplate boundary.
Bulletin of the Seismological Society of America, 2017
We expand on the automatic polarization‐based S‐wave‐arrival detection technique of Cichowicz (19... more We expand on the automatic polarization‐based S‐wave‐arrival detection technique of Cichowicz (1993) and Diehl et al.(2009) to make it more effective for a broader range of distances and crustal structures. We use the characteristic function of Cichowicz (1993), which combines the polarization attributes of rectilinearity, directivity, and transverse‐to‐total‐energy ratio and, as does Diehl et al. (2009), modulates it with an amplitude weight function. Compared to Diehl, we increase the weight function exponent from 0.5 to 2, which stabilizes the characteristic function and emphasizes the Swave. We use the weight function by itself to determine if the signal‐to‐noise ratio is appropriate for picking and to determine a rough initial S pick. The pick is refined using the characteristic function. The picking algorithm works backward in time along the characteristic function from the initial pick until the characteristic function reaches a minimum. Using initial S picks based on the amplitude weight function, by increasing the weight function exponent and picking backward in time we avoid numerous incorrect early picks and drop traces with poor picking prospects. We apply the new approach to earthquakes in south‐central Tibet
Bulletin of the Seismological Society of America, Jun 1, 1985
... (1980) 10t This study 14 6.2 7.0 ISC 15 2.4 227 36 108 Kristy et al. ... structures at the so... more ... (1980) 10t This study 14 6.2 7.0 ISC 15 2.4 227 36 108 Kristy et al. ... structures at the source and receiver is approximated by a half-space with the P and S velocity ~ = 6.0 km sec -1 and/~ = 3.5 km sec -1, respectively, and the density p = 2.5 g cm -3. Mantle propagation effects are ...
The Explorer region offshore western Canada is a tectonically complex area surrounded by the Paci... more The Explorer region offshore western Canada is a tectonically complex area surrounded by the Pacific, North America, and Juan de Fuca plates. Existing tectonic models for the region differ fundamentally. Proposed plate configurations range from multiple independent plate fragments to an Explorer plate now fused to North America along the continental margin and cut by Pacific-North America transform faults in the west. We present new seismological data constraining the region's current tectonics. We use three-component regional waveforms to determine the source parameters of 84 earthquakes with magnitude greater than 4. Combined with 34 Harvard centroid moment tensor solutions, they represent the region's largest earthquake source parameter data set obtained by robust waveform modeling techniques. In addition, we perform joint epicenter determination to relocate larger earthquakes recorded since 1918. The source parameters and improved locations provide a consistent tectonic picture. Earthquake slip vector azimuths along the Pacific plate boundary change smoothly and are significantly less northerly oriented than the Pacific-North America plate motion direction, requiring an independent Explorer plate. The present-day Pacific-Explorer boundary is formed by transform faults subparallel to the Revere-Dellwood-Wilson fault. Plate motion vectors indicate that the Winona block is part of the Explorer plate. Current Explorer motion is more northerly than indicated by magnetic anomalies prior to 2 Ma, implying a recent change, possibly coinciding with a northwestward ridge jump near Explorer plate's northern end transferring the Winona block from the Pacific to the Explorer plate. In response to these plate motion changes the region north of the western Sovanco fracture zone was assimilated into the Pacific plate. The region around the eastern Sovanco fracture zone, characterized by broadly distributed seismicity, is composed of well-defined sets of conjugate faults bounding rotating crustal blocks. Earthquake fault strikes agree with the dominant northwest-southeast fault sets; however, the conjugate sets must be also active to fully accommodate present-day Explorer plate motion. The SW portion of the strike-slip Nootka fault zone, the Explorer-Juan de Fuca plate boundary, is well defined by focused seismicity; however, its full extent under Nootka Island remains unresolved. The Explorer-North America boundary shows sporadic low-magnitude seismicity. Our Explorer-North America rotation pole predicts convergence varying from negligible at the boundary's northwest end to $2 cm/yr at the SE end. This convergence can be accommodated either by subduction or by crustal thickening extending to the North American continent. We favor subduction based on low deformation rates observed by onshore GPS sites. The present Explorer plate system configuration is a result of stepwise reorientation of the Explorer ridge system, each step successively reducing the subduction rate relative to North America.
The actively subsiding North China sedimentary basin is associated with an unusually high level o... more The actively subsiding North China sedimentary basin is associated with an unusually high level of seismic activity. This oil-and gas-producing basin has been the site of nine large (M > 7), destructive earthquakes since 1600 A.D. An analysis of faulting during the Tangshan earthquake sequence, which includes some of the largest shocks that have occurred in this basin during the past 400 years, showed that the dominant pattern of deformation during this sequence was associated with displacement on right-stepping, right-lateral strike-slip faults. A large amount of subsidence (~1.0-1.5 m) occurred in pull-apart regions between steps of the northnortheast (NNE) trending strike-slip faults. The directivity of the P wave radiation pattern and the distribution of aftershocks of the Bohai Gulf earthquake of July 18, 1969, also indicate strike-slip faulting on planes of similar NNE trend. Together with the four largest shocks of the Hsingtai sequence in 1966, earthquakes with right-lateral slip on NNE trending faults account for about two-thirds of the seismic moment released in this region for the past 100 years. The average regional strain due to seismic slip has a dominant component of dextral simple shear (on NNE striking planes) that is approximately twice as large as north-south extension or east-west shortening. Based on a combination of published field and borehole data, the dominance of horizontal dextral simple shear over subsidence seems to have begun no later than the mid-Pliocene and the total amount of extension over the entire basin is only about 20-30%. This pattern of deformation, the spatially variable heat flow values, the scattered locations of rapid deposition of Quatemary sediments, and the fluctuating Copyright 1988 by the American Geophysical Union. Paper number 8T0341.
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