We present results of travel time inversions of teleseismic P and S waves recorded at the SECaSA92 (Southeast Caribbean South America 1992) temporary broadband array in northeastern Venezuela and Trinidad. The inversions reveal the unusual structure of the southern termination of the Lesser Antilles subduction zone: A minimum 2% relatively high‐velocity anomaly trends WSW from the seismically defined Lesser Antilles slab beneath and NW of the Paria Peninsula to a point below the Venezuelan Serranía del Interior, well south of the Caribbean coast. Resolution tests utilizing actual ray geometries and densities of the source data indicate that the regional‐scale structure beneath the study area is reasonably well resolved. Thus a detached and detaching subducted South American slab appears to lie beneath continental South America. We infer that oceanic South American lithosphere has been overridden to a significant degree by continental South America. The detached slab now lying beneath continental South America was driven into its current position after detaching from the former eastward striking Mesozoic ocean‐continent passive margin as this margin entered the subduction zone. Because oceanic and continental South America are still attached without apparent relative motion between them along the Atlantic passive margin southeast of our study region, the slab must be the actively moving element during continental overriding. Thus the slab and its surrounding mantle (both Caribbean and South America) beneath northeastern South America are mobile and have moved ESE relative to the stable Guyana Shield craton.
Previous knowledge of the structure of the Cascadia subduction zone north of the Canada–United States border has been derived from a variety of geophysical studies that accurately delineated the downgoing Juan de Fuca plate from the offshore deformation front to depths of ~50–60 km beneath south-central Vancouver Island and the Georgia Strait. Little is known, however, of the structure of the Cascadia subduction zone farther westward and to greater depths in the upper mantle. We have assembled a set of some 1100 teleseismic traveltimes from events recorded on the Western Canadian Telemetered Network to augment a previously existing data set recorded on the Washington Regional Seismograph Network. The composite data set is inverted for upper mantle structure below Washington, Oregon, and southwestern British Columbia. We analyze the new northern portion of the model between 48.5–50°N and 118–127°W, which provides the first images of the deep slab structure in this region. The model is parameterized using splines under tension over a dense grid of knots. The nonlinearity of the inverse problem is treated by iteratively performing three-dimensional ray tracing and linear inversion. Resolution tests performed with a synthetic slab model indicate that the deep structure is resolved by the data north to at least 50°N. The inversions are characterized by a quasi-planar, high-velocity body inferred to represent the thermal and compositional anomaly of the subducted Juan de Fuca plate. This body exhibits velocity deviations of up to 3% from the background reference model and extends to depths of at least 400–500 km. The depth contours of the slab in the upper mantle mimic those of the shallow slab by changing strike, in the latitude range 48.0–48.5°N, from north–south in Washington to northwest–southeast in southern British Columbia. This forces the development of two arch-type structures: a main arch observed in previous studies trending east–west over Puget Sound and a possible second arch extending northeasterly from the Georgia Strait into the British Columbia interior. A steepening of the deep slab dip from British Columbia south towards Puget Sound and complexity in the evolution of the arches in depth may be the result of a change in plate motions at 3.5 Ma associated with the detachment of the Explorer plate.
An array of seismographs was deployed over the central Trans‐Hudson Orogen from July 1991 to January 1992 and from October 1994 to July 1996 with the objective of characterizing subcrustal lithospheric structure in a region of diamondiferous kimberlite occurrence using tomographic imaging techniques. The two‐dimensional array was located in south central Saskatchewan and consisted of 17 stations with an average spacing of 100 km. We obtained relative travel time residuals for 321 teleseismic events and inverted them for subcrustal velocity variations. The ray coverage affords resolution from 60 to 400 km depth. Our results reveal heterogeneities in mantle velocity that deviate by up to ±1.5% from the iasp91 Earth model. The most pronounced low‐velocity anomaly is quasi‐cylindrical, 120 km in diameter and extends to ∼220 km depth. This feature is partly surrounded by a region of high velocity which penetrates to slightly greater depths. Cretaceous diamondiferous kimberlites and high concentrations of kimberlitic minerals in glacial tills occur above or near the rims of the low velocity anomalies. In addition, correlations exist between a long‐wavelength gravity low and the high‐velocity region, as well as between high heat flow and low mantle velocities in the southern portions of the study area. Taken together, these observations are consistent with the interpretation of the imaged anomalies as due to thermomechanical erosion of the lithospheric keel of the Sask craton during the Cretaceous by plume activity or Rayleigh‐Taylor‐like instability within the asthenosphere. The diamondiferous kimberlites are viewed as a direct consequence of this process. Low levels of heterogeneity below 250 km depth are interpreted to be indicative of effective homogenization in a convecting asthenosphere.
We relocated the six large-magnitude (5.2 < Mw < 6.2) earthquakes of the destructive, tsunamigenic Aysen seismic swarm, which occurred from 2007 January–October in Patagonian Chile. We used P and SH arrival times from near-source stations of a temporary seismic network fortuitously deployed in the area when the swarm began, and also traveltimes to stations of the permanent global networks, to locate the 2007 January 23, Mw 5.2 earthquake, the first of the six large magnitude events. This earthquake's hypocentre lies at shallow depth (<10 km) on the eastern strand of a major intraarc shear zone, the dextral Liquiñe-Ofqui fault zone. Using the hypocentre of the January 23 earthquake as a fixed location, we relocated the five other large magnitude Aysen earthquakes by joint hypocentral determination. Four of these five events also occurred at shallow depth on the eastern strand Liquiñe-Ofqui fault, whereas the 2007 April 2, earthquake occurred some 45 km to the west on the Aysen fault, a strike-slip duplex fault that segments the area between the eastern and western Liquiñe-Ofqui fault strands. The five earthquakes on the Liquiñe-Ofqui fault were all produced by dextral slip on ∼N–S nodal planes approximately parallel to the mapped trace of the fault. The April 2 earthquake resulted from normal slip on the Aysen fault. Modelling of Coulomb stress changes on the nodal planes of the April 2 earthquake shows that the cumulative slip on the Liquiñe-Ofqui fault strand could have triggered the April 2 earthquake. Similarly, the April 2 earthquake may have triggered the Mw 6.2 April 21 earthquake, which caused mass wasting into Aysen Fjord, generating a destructive tsunami. The system of channels and fjords in the study region is a major shipping route around South America, and therefore careful evaluation of the seismic hazard is warranted.
We use P wave tomography and receiver function analysis to place new constraints on the nature of the thermal anomaly in the upper mantle beneath the Arabian Shield. A broad, low velocity anomaly is found in the upper mantle characterized by a strong lateral velocity gradient, with a peak to peak anomaly of at least 4–6% extending from the Red Sea eastward into the interior of the shield. The lowest velocities are found under the region adjacent to the Red Sea where elevations are more than 1 km higher than elsewhere in the Arabian Shield. We infer that large lateral temperature variations exist beneath the Arabian Shield associated with the higher elevations near the Red Sea. Receiver function stacks of P to S conversions from the 410 and 660 km discontinuities do not indicate thinning of the transition zone, suggesting that the broad, low velocity anomaly is likely confined to depths shallower than 410 km.
On June 9, 1994 the M w 8.3 Bolivia earthquake (636 km depth) occurred in a region which had not experienced significant, deep seismicity for at least 30 years. The mainshock and aftershocks were recorded in Bolivia on the BANJO and SEDA broadband seismic arrays and on the San Calixto Network. We used the joint hypocenter determination method to determine the relative location of the aftershocks. We have identified no foreshocks and 89 aftershocks ( m > 2.2) for the 20‐day period following the mainshock. The frequency of aftershock occurrence decreased rapidly, with only one or two aftershocks per day occuring after day two. The temporal decay of aftershock activity is similar to shallow aftershock sequences, but the number of aftershocks is two orders of magnitude less. Additionally, a m b ∼6, apparently triggered earthquake occurred just 10 minutes after the mainshock about 330 km east‐southeast of the mainshock at a depth of 671 km. The aftershock sequence occurred north and east of the mainshock and extends to a depth of 665 km. The aftershocks define a slab striking N68°W and dipping 45°NE. The strike, dip, and location of the aftershock zone are consistent with this seismicity being confined within the downward extension of the subducted Nazca plate. The location and orientation of the aftershock sequence indicate that the subducted Nazca plate bends between the NNW striking zone of deep seismicity in western Brazil and the N‐S striking zone of seismicity in central Bolivia. A tear in the deep slab is not necessitated by the data. A subset of the aftershock hypocenters cluster along a subhorizontal plane near the depth of the mainshock, favoring a horizontal fault plane. The horizontal dimensions of the mainshock [ Beck et al., this issue; Silver et al., 1995] and slab defined by the aftershocks are approximately equal, indicating that the mainshock ruptured through the slab.
