Shallow active seismicity and neotectonic structures reveal important changes in the degree of in... more Shallow active seismicity and neotectonic structures reveal important changes in the degree of interplate and intraplate coupling along the convergent Hellenic plate boundary from Crete to Rhodes. The onshore/offshore Pliocene-Holocene surface geology of the Hellenic forearc records three different deformation states: 1) A western segment (western Crete) where incipient continent-continent collision produces shortening under strong interplate coupling; 2) a central segment (central-eastern Crete) partly coupled to Africa where oblique convergence is partitioned into sinistral strike slip and orthogonal shortening which is confined to the accretionary wedge; and 3) an eastern trantensional segment (Rhodes), mechanically decoupled from African oblique convergence, instead reflecting slab rollback and Aegea's southward motion relative to Anatolia. Such along-strike heterogeneity of neotectonic structures suggests each segment should also display distinct crustal-scale stress patte...
Many studies have shown that typical oceanic lithosphere is underlain by a well-developed astheno... more Many studies have shown that typical oceanic lithosphere is underlain by a well-developed asthenosphere characterized by slow seismic velocities from ~100 to 250 km depth. However, the fate of the oceanic asthenosphere at subduction zones is poorly understood. I show here using shear-wave splitting of S waves emanating from earthquakes in the Juan de Fuca slab that upper mantle asthenospheric anisotropy beneath the slab is consistent with the presence of two distinct subducted asthenospheric layers, one with fast shear trends parallel to the subduction trench, and a second, deeper layer with fast upper mantle fabrics parallel to the motion of the Juan de Fuca plate with respect to the deeper mantle. The consistent orientation of unsubducted Pacifi c asthenospheric anisotropy in the direction of current plate motion implies that the trench-parallel, subslab anisotropy develops when the lithosphere subducts.
We present a teleseismic travel-time inversion for upper-mantle velocity structure beneath the Ch... more We present a teleseismic travel-time inversion for upper-mantle velocity structure beneath the Chile triple junction region. Data were recorded at 46 seismic stations deployed in southern Chile from December 2004 to February 2007 (for details, see the Chile Ridge Subduction Project, http://seismology.geology.ufl.edu/chile). The area covered by the network (42-46 degrees South and 72-78 degrees West) lies above the projected position
Facilitate a deeper understanding of modern The Mongolian lithosphere was formed from multiple pl... more Facilitate a deeper understanding of modern The Mongolian lithosphere was formed from multiple plate collisions in We use local seismicity recorded by a 72 station array from the Central Mongolia Seismic Experiment to the Neoproterozoic-Early Paleozoic associated With the Central Asian characterize seismicity and 3D velocity structure in the Hangay. We start by using the REST Autopicker3 to Orogenic Belt (Figure 1) develop a catalog of P and S arrival times and event locations. The catalog includes 24,585 events (318,104 P and 203,550 S picks), 8,378 of which are depth S 50 and have at least 15 defined phases.
Although the well-developed Cascades continental volcanic arc provides exceptional evidence of th... more Although the well-developed Cascades continental volcanic arc provides exceptional evidence of the subduction of the Juan de Fuca plate beneath western North America, active seismicity, often the clearest gauge of large-scale horizontal convergence, is relatively infrequent along the Juan de Fuca subduction zone, particularly at subcrustal depths. Thus, the February 28, 2001 Nisqually earthquake, located around 50 km beneath the
We determined receiver functions (RFs) at six permanent Advanced National Seismic System (ANSS) s... more We determined receiver functions (RFs) at six permanent Advanced National Seismic System (ANSS) stations to examine crust and upper mantle structure of the Wyoming craton (WC) and Medicine Hat block (MHB). The Deep Probe & SAREX projects (Henstock et al., 1998; Clowes et al., 2002; Gorman et al., 2002) used active source seismics to model a high velocity crustal layer
... Geophys. J. Int. 99, 549559. Vinnik, LE, Farra, V., Romanowicz, BA, 1989b. Azimuthal anisotro... more ... Geophys. J. Int. 99, 549559. Vinnik, LE, Farra, V., Romanowicz, BA, 1989b. Azimuthal anisotropy in the Earth from observations of SKS at GEO SCOPE and NARS broadband stations. Bull. Seismol. Soc. Am. 79, 15421558. Vinnik, LP, Makeyeva, LI, Usenko, A.Yu., 1992. ...
Journal of Geophysical Research: Solid Earth, 1998
We determined shear wave splitting parameters at four island sites in French Polynesia: Tiputa (T... more We determined shear wave splitting parameters at four island sites in French Polynesia: Tiputa (TPT) on Rangiroa in the Tuamotu archipelago; Papeete (PPT) on Tahiti in the Society Islands; Tubuai (TBI) in the Cook-Austral island chain; and Rikitea (RKT) on Mangareva in the Gambier Islands. We also examined splitting at Pitcairn (PTCN) on Pitcairn Island; because of the short time of operation of PTCN, our results there are preliminary. We find substantial differences in splitting, most likely caused by variable upper mantle deformation beneath the five stations. At TPT the fast split shear wave (•b) direction is N66øW:t:4 ø, parallel to the current Pacific-hotspots relative motion (APM) vector; the delay time between fast and slow waves is 1.3 :t: 0.2 s. At PPT, on Tahiti, we could detect no splitting despite many clear $KS observations. At TBI, on Tubuai we detected splitting with a delay time of 1.1 :t: 0.1 s and a •b direction midway between the local APM direction and the fossil spreading direction (N86øW:t:2 ø), as locally indicated by the nearby Austral Fracture Zone. At RKT in the Gambier Islands, •b trends N53øW:t:6 ø, 16 ø clockwise of the local APM azimuth, and delay time at RKT is 1.1 :t: 0.1 s. Results at PTCN include •b near N38øW:t:9 ø and a delay time of 1.1 :t: 0.3 s. These different results imply variable upper mantle deformation beneath the five sites. We interpret splitting at TPT and, possibly, RKT as indicative of asthenospheric flow or shear in the APM direction beneath the stations. At PPT, azimuthal isotropy indicates deformed upper mantle with a vertical symmetry axis, or absence of strong or consistently oriented mantle deformation fabric beneath Tahiti. Either effect could be related to recent hotspot magmatism on Tahiti. At TBI, splitting may be complicated by juxtaposition of different lithospheric thicknesses along the nearby Austral Fracture Zone, resulting in perturbation of asthenospheric flow. The absence of splitting related to fossil spreading in French Polynesia indicates that upper mantle deformation processes postdating lithosphere formation are important at all four sites within that region. The •b azimuth at PTCN does not align with either the fossil direction or the APM direction, but our best individual determination of splitting parameters at this station lies within 10 ø of the local APM at Pitcairn Island.
The form of asthenospheric flow beneath subducted lithospheric slabs can be discerned using split... more The form of asthenospheric flow beneath subducted lithospheric slabs can be discerned using splitting of shear waves emanating from earthquakes in the slabs themselves. However, the subducted Pacific plate's systematic change in convergence direction from nearly orthogonal to highly oblique from east to west along the Aleutians island arc is a potential complication to the sub-slab mantle flow. S waves from earthquakes within the Pacific slab reveal details of the upper mantle flow field below and in the vicinity of the slab. Slab earthquakes large enough to be well recorded (M > 5.7, typically), and deep enough to separate S from pS and sS (40 km or more), are suitable for such study, and, for events between 1990 and 2010, recording stations are mostly well-distributed azimuthally about the source event. The S waves were recorded at seismic stations at teleseismic distances from the events, and were corrected for known sub-station seismic anisotropy. Thus, the shear wave sp...
We present analyses of episodic non-volcanic seismic tremor recorded at the Chile Ridge Subductio... more We present analyses of episodic non-volcanic seismic tremor recorded at the Chile Ridge Subduction Project temporary seismic network, operated jointly by the University of Florida and the Universidad de Chile, Santiago. The 57 station network was deployed to study subduction of the Chile Ridge spreading center in the Chile Triple Junction region. Tremor activity during the first project year (2004-5)
Shallow active seismicity and neotectonic structures reveal important changes in the degree of in... more Shallow active seismicity and neotectonic structures reveal important changes in the degree of interplate and intraplate coupling along the convergent Hellenic plate boundary from Crete to Rhodes. The onshore/offshore Pliocene-Holocene surface geology of the Hellenic forearc records three different deformation states: 1) A western segment (western Crete) where incipient continent-continent collision produces shortening under strong interplate coupling; 2) a central segment (central-eastern Crete) partly coupled to Africa where oblique convergence is partitioned into sinistral strike slip and orthogonal shortening which is confined to the accretionary wedge; and 3) an eastern trantensional segment (Rhodes), mechanically decoupled from African oblique convergence, instead reflecting slab rollback and Aegea's southward motion relative to Anatolia. Such along-strike heterogeneity of neotectonic structures suggests each segment should also display distinct crustal-scale stress patte...
Many studies have shown that typical oceanic lithosphere is underlain by a well-developed astheno... more Many studies have shown that typical oceanic lithosphere is underlain by a well-developed asthenosphere characterized by slow seismic velocities from ~100 to 250 km depth. However, the fate of the oceanic asthenosphere at subduction zones is poorly understood. I show here using shear-wave splitting of S waves emanating from earthquakes in the Juan de Fuca slab that upper mantle asthenospheric anisotropy beneath the slab is consistent with the presence of two distinct subducted asthenospheric layers, one with fast shear trends parallel to the subduction trench, and a second, deeper layer with fast upper mantle fabrics parallel to the motion of the Juan de Fuca plate with respect to the deeper mantle. The consistent orientation of unsubducted Pacifi c asthenospheric anisotropy in the direction of current plate motion implies that the trench-parallel, subslab anisotropy develops when the lithosphere subducts.
We present a teleseismic travel-time inversion for upper-mantle velocity structure beneath the Ch... more We present a teleseismic travel-time inversion for upper-mantle velocity structure beneath the Chile triple junction region. Data were recorded at 46 seismic stations deployed in southern Chile from December 2004 to February 2007 (for details, see the Chile Ridge Subduction Project, http://seismology.geology.ufl.edu/chile). The area covered by the network (42-46 degrees South and 72-78 degrees West) lies above the projected position
Facilitate a deeper understanding of modern The Mongolian lithosphere was formed from multiple pl... more Facilitate a deeper understanding of modern The Mongolian lithosphere was formed from multiple plate collisions in We use local seismicity recorded by a 72 station array from the Central Mongolia Seismic Experiment to the Neoproterozoic-Early Paleozoic associated With the Central Asian characterize seismicity and 3D velocity structure in the Hangay. We start by using the REST Autopicker3 to Orogenic Belt (Figure 1) develop a catalog of P and S arrival times and event locations. The catalog includes 24,585 events (318,104 P and 203,550 S picks), 8,378 of which are depth S 50 and have at least 15 defined phases.
Although the well-developed Cascades continental volcanic arc provides exceptional evidence of th... more Although the well-developed Cascades continental volcanic arc provides exceptional evidence of the subduction of the Juan de Fuca plate beneath western North America, active seismicity, often the clearest gauge of large-scale horizontal convergence, is relatively infrequent along the Juan de Fuca subduction zone, particularly at subcrustal depths. Thus, the February 28, 2001 Nisqually earthquake, located around 50 km beneath the
We determined receiver functions (RFs) at six permanent Advanced National Seismic System (ANSS) s... more We determined receiver functions (RFs) at six permanent Advanced National Seismic System (ANSS) stations to examine crust and upper mantle structure of the Wyoming craton (WC) and Medicine Hat block (MHB). The Deep Probe & SAREX projects (Henstock et al., 1998; Clowes et al., 2002; Gorman et al., 2002) used active source seismics to model a high velocity crustal layer
... Geophys. J. Int. 99, 549559. Vinnik, LE, Farra, V., Romanowicz, BA, 1989b. Azimuthal anisotro... more ... Geophys. J. Int. 99, 549559. Vinnik, LE, Farra, V., Romanowicz, BA, 1989b. Azimuthal anisotropy in the Earth from observations of SKS at GEO SCOPE and NARS broadband stations. Bull. Seismol. Soc. Am. 79, 15421558. Vinnik, LP, Makeyeva, LI, Usenko, A.Yu., 1992. ...
Journal of Geophysical Research: Solid Earth, 1998
We determined shear wave splitting parameters at four island sites in French Polynesia: Tiputa (T... more We determined shear wave splitting parameters at four island sites in French Polynesia: Tiputa (TPT) on Rangiroa in the Tuamotu archipelago; Papeete (PPT) on Tahiti in the Society Islands; Tubuai (TBI) in the Cook-Austral island chain; and Rikitea (RKT) on Mangareva in the Gambier Islands. We also examined splitting at Pitcairn (PTCN) on Pitcairn Island; because of the short time of operation of PTCN, our results there are preliminary. We find substantial differences in splitting, most likely caused by variable upper mantle deformation beneath the five stations. At TPT the fast split shear wave (•b) direction is N66øW:t:4 ø, parallel to the current Pacific-hotspots relative motion (APM) vector; the delay time between fast and slow waves is 1.3 :t: 0.2 s. At PPT, on Tahiti, we could detect no splitting despite many clear $KS observations. At TBI, on Tubuai we detected splitting with a delay time of 1.1 :t: 0.1 s and a •b direction midway between the local APM direction and the fossil spreading direction (N86øW:t:2 ø), as locally indicated by the nearby Austral Fracture Zone. At RKT in the Gambier Islands, •b trends N53øW:t:6 ø, 16 ø clockwise of the local APM azimuth, and delay time at RKT is 1.1 :t: 0.1 s. Results at PTCN include •b near N38øW:t:9 ø and a delay time of 1.1 :t: 0.3 s. These different results imply variable upper mantle deformation beneath the five sites. We interpret splitting at TPT and, possibly, RKT as indicative of asthenospheric flow or shear in the APM direction beneath the stations. At PPT, azimuthal isotropy indicates deformed upper mantle with a vertical symmetry axis, or absence of strong or consistently oriented mantle deformation fabric beneath Tahiti. Either effect could be related to recent hotspot magmatism on Tahiti. At TBI, splitting may be complicated by juxtaposition of different lithospheric thicknesses along the nearby Austral Fracture Zone, resulting in perturbation of asthenospheric flow. The absence of splitting related to fossil spreading in French Polynesia indicates that upper mantle deformation processes postdating lithosphere formation are important at all four sites within that region. The •b azimuth at PTCN does not align with either the fossil direction or the APM direction, but our best individual determination of splitting parameters at this station lies within 10 ø of the local APM at Pitcairn Island.
The form of asthenospheric flow beneath subducted lithospheric slabs can be discerned using split... more The form of asthenospheric flow beneath subducted lithospheric slabs can be discerned using splitting of shear waves emanating from earthquakes in the slabs themselves. However, the subducted Pacific plate's systematic change in convergence direction from nearly orthogonal to highly oblique from east to west along the Aleutians island arc is a potential complication to the sub-slab mantle flow. S waves from earthquakes within the Pacific slab reveal details of the upper mantle flow field below and in the vicinity of the slab. Slab earthquakes large enough to be well recorded (M > 5.7, typically), and deep enough to separate S from pS and sS (40 km or more), are suitable for such study, and, for events between 1990 and 2010, recording stations are mostly well-distributed azimuthally about the source event. The S waves were recorded at seismic stations at teleseismic distances from the events, and were corrected for known sub-station seismic anisotropy. Thus, the shear wave sp...
We present analyses of episodic non-volcanic seismic tremor recorded at the Chile Ridge Subductio... more We present analyses of episodic non-volcanic seismic tremor recorded at the Chile Ridge Subduction Project temporary seismic network, operated jointly by the University of Florida and the Universidad de Chile, Santiago. The 57 station network was deployed to study subduction of the Chile Ridge spreading center in the Chile Triple Junction region. Tremor activity during the first project year (2004-5)
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