82 research outputs found
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Earthquakes : caught in the act
Faults break under the stress of plate tectonic forces, but the processes immediately preceding rupture are enigmatic. Monitoring of a remote oceanic fault that breaks
regularly indicates that rupture is controlled by physical properties of the fault
zone.KEYWORDS: structural geology, seismology, tectonics and geodynamic
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Down-dip geometry and depth extent of normal faults in the Aegean-evidence from earthquakes
Automatic regional moment tensor inversion in the European-Mediterranean region
We produce fast and automatic moment tensor solutions for all moderate to strong earthquakes in the European-Mediterranean region. The procedure automatically screens near real-time earthquake alerts provided by a large number of agencies. Each event with magnitude M â„ 4.7 triggers an automatic request for near real-time data at several national and international data centres. Moment tensor inversion is performed using complete regional long-period (50-100 s) waveforms. Initially the data are inverted for a fixed depth to remove traces with a low signal-to-noise ratio. The remaining data are then inverted for several trial depths to find the best-fitting depth. Solutions are produced within 90 min of an earthquake. We analyse the results for the period 2000 April to 2002 April to evaluate the performance of the procedure. For quality assessment, we compared the results with the independent Swiss regional moment tensor catalogue (SRMT), and divided the 87 moment tensor solutions into three groups: 38 A-quality solutions with well-resolved Mw, depth and focal mechanism; 21 B-quality solutions with well-resolved Mw; and 28 unreliable C-quality solutions. The non-homogeneous station and event distributions, varying noise level, and inaccurate earthquake locations affected solution quality. For larger events (Mw â„ 5.5) we consistently obtained A-quality solutions. For Mw = 4.5-5.5 we obtained A- and B-quality solutions. Solutions that pass empirical rules mimicking the a posteriori quality for our data set are automatically disseminate
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Segmentation of the Blanco Transform Fault Zone from earthquake analysis: Complex tectonics of an oceanic transform fault
The Blanco Transform Fault Zone (BTFZ) forms the ~350 km long PacificâJuan de
Fuca plate boundary between the Gorda and Juan de Fuca ridges. Nearby broadband seismic networks provide a unique framework for a detailed, long-term seismotectonic study of an entire oceanic transform fault (OTF) system. We use regional waveforms to determine 129 earthquake source parameters; combined with 28 Harvard moment tensors, they represent the largest waveform derived OTF source parameter data set. Joint epicenter determination removes the northeasterly routine location bias. Projecting seismicity onto the BTFZ, we determine 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, seismically fully coupled, has a wider seismic zone (~9 km) than other BTFZ transform segments and accommodates the largest (Mw 6.4â6.5) 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 postemplacement 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 subparallel faults. Seismic coupling is low in extensional (~15%) compared to transform areas (35â100%), implying different mechanical properties. Centroid depth variations are consistent with seismic slip cutoff near 600°C.Keywords: Oceanic transform faults, Blanco Transform Fault ZoneKeywords: Oceanic transform faults, Blanco Transform Fault Zon
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Geometry of continental normal faults : seismological constraints
Teleseismic body waves from large earthquakes are used to study the downdip
geometry of continental normal faults in the Aegean. Waveform modeling technique together with rigorous statistical tests are applied to put firm bounds on the amount of downdip curvature of these faults and the role of coseismic slip on a basal detachment. Synthetic modeling shows that good azimuthal station coverage and inclusion of SH waves are necessary to resolve fault curvature. The data indicate ruptures of the Aegean events occurred on planar faults extending across the entire brittle portion of the crust. No seismogenic low-angle detachment faulting at the base of the upper crust was detected for these events. Decoupling of the brittle upper crust from the plastic lower crust probably occurs aseismically in a ductile fashion.Copyrighted by American Geophysical Union
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Rupture process of the MacQuarie Ridge earthquake of May 23, 1989
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 13.4x10ÂČâ° 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
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The 1993 Klamath Falls, Oregon, earthquake sequence: Source mechanisms from regional data
We use regional broadband seismograms to obtain
seismic moment-tensor solutions of the two September 20, 1993,
Mw =6, Klamath Falls, Oregon earthquakes, their foreshock and
largest aftershocks (MD>3.5). Several sub-groups with internally
consistent solutions indicate activity on several fault segments
and faults. From the estimated moment-tensors and depths of the
main shocks and from the aftershock distribution we deduce that
both main shocks occurred on an east-dipping normal fault, possibly
related to the Lake of the Woods fault system. Rotation of
T-axes between the two main shocks is consistent with the two
dominant trends of the aftershocks and mapped faults. We propose
that a change in fault strike acted as temporary barrier separating
the rupture of the main shocks. Empirical Green's function
analysis shows that the first main event had a longer rupture duration
(half-duration 1.7 s) than the second (1.2 s). In December,
vigorous shallow activity commenced near Klamath Lake's western
shore, 5-10 km east of the primary aftershock zone. It appears
a Mw=5.5 aftershock occurring the day before, though
within the primary aftershock zone, triggered the activity
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A seasonally modulated earthquake swarm near Maupin, Oregon
From December 2006 to November 2011, the Pacific Northwest Seismic Network (PNSN)
reported 467 earthquakes in a swarm 60 km east of Mt Hood near the town of Maupin, Oregon.
The swarm included 20 M[subscript D] â„ 3.0 events, which account for over 80 per cent of the cumulative
seismic moment release of the sequence. Relocation of 45 M[subscript D] â„ 2.5 earthquakes and moment
tensor analysis of nine 3.3 †M[subscript w] †3.9 earthquakes reveals right-lateral strike-slip motion
on a north-northwest trending, 70° west dipping, 1 kmÂČ active fault patch at about 17 km
depth. The swarm started at the southern end of the patch and migrated to the northwest at an
average rate of 1â2 m dâ»Âč during the first 18 months. Event migration was interrupted briefly
in late 2007 when the swarm encountered a 10° fault bend acting as geometrical barrier. The
slow migration rate suggests a pore pressure diffusion process. We speculate that the swarm
was triggered by flow into the fault zone from upwards-migrating, subduction-derived fluids.
Superimposed on the swarm is seasonal modulation of seismicity, with the highest rates in
spring, which coincides with the maximum snow load in the nearby Cascade Mountains. The
resulting surface load variation of about 4 Ă 10ÂčÂč N kmâ»Âč arc length causes 1 cm annual vertical
displacements at GPS sites in the Cascades and appears sufficient to modulate seismicity by
varying normal stresses at the fault and fluid flow rates into the fault zone.This is the publisherâs final pdf. The published article is copyrighted by the author(s) and published by Oxford University Press on behalf of The Royal Astronomical Society. The published article can be found at: http://gji.oxfordjournals.org/.Keywords: Earthquake dynamics, Fractures and faults, Earthquake source observations, Continental tectonics: strike-slip and transform, Dynamics and mechanics of faultingKeywords: Earthquake dynamics, Fractures and faults, Earthquake source observations, Continental tectonics: strike-slip and transform, Dynamics and mechanics of faultin
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Evidence for low-angle normal faulting in the Pumqu-Xianza Rift, Tibet
Low-angle normal faulting is widely discussed as a possible mechanism for continental extension, however, unambiguous evidence for seismogenic low-angle normal faulting is lacking. Here, we investigate seismicity along a short segment of the Pumqu-Xianza Rift (PXR) in southern Tibet, where the HiCLIMB seismic array recorded over 500 earthquakes between 2004 July and 2005 August. Hypocentres of the 40 best recorded earthquakes are approximately 20â25 km west of the rift and tightly clustered at about 10 km depth, consistent with moment tensor depths of the 11 largest (3.4 â€M[subscript w]†4.5) earthquakes. Events in this group have NâS striking normal faulting mechanisms with low-angle (29°) west dipping fault planes. Rupture along a west dipping, low-angle, planar normal fault (the eastern PXR boundary fault) is consistent with event hypocentres, fault dip from moment tensors, and prominent surface morphology. The dip of 29° is at the low end of physically possible values assuming normal frictional behaviour and state of stress. We suggest it is possible for a planar, low-angle fault to nucleate seismically at a low angle at depth in the presence of basal shear and work its way aseismically through the brittle crust to the surface with the aid of lubricating minerals.This is the publisherâs final pdf. The article is copyrighted by the Royal Astronomical Society and published by John Wiley & Sons, Inc. It can be found at: http://www.blackwellpublishing.com/journal.asp?ref=0956-540x.Keywords: Dynamics and mechanics of faulting, extensional, Seismicity and tectonics, Continental tectonicsKeywords: Dynamics and mechanics of faulting, extensional, Seismicity and tectonics, Continental tectonic
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Seismic anisotropy beneath the Juan de Fuca plate system: Evidence for heterogeneous mantle flow
Here we use SKS shear wave splitting observations from ocean-bottom seismometer data to infer patterns of mantle deformation beneath the Juan de Fuca plate and its adjoining boundaries. Our results indicate that the asthenosphere beneath the Juan de Fuca plate responds largely to absolute plate motion with an anisotropic layer developing rapidly near the ridge and persisting into the subduction zone. Geographically restricted deviations from this pattern indicate the presence of secondary processes. At discrete plate boundaries, such as the Blanco transform fault, seismic anisotropy is attributed to relative plate motion within a narrow zone (<50 km). Beneath the deforming southern Gorda plate regionâa diffuse plate boundaryâsplitting observations similarly suggest deformation dominated by relative motion between the rigid Juan de Fuca and Pacific plates but distributed over a broad zone (âŒ200 km). Our results are inconsistent with toroidal flow around the southern edge of the subducting slab due to rollback, as suggested by onshore studies. Instead, reorganization of upper mantle flow associated with plate fragmentation seems to dominate the anisotropic signature of southern Cascadia
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