Seismic Wave Propagation in South America (original) (raw)
Related papers
Journal of Geophysical Research, 2009
1] We measured shear wave splitting from SKS and SKKS data recorded by temporary stations deployed as part of the Broadband Onshore-Offshore Lithospheric Investigation of Venezuela and the Antilles Arc Region project and the national seismic network of Venezuela. Approximately 3000 station-event pairs yielded $300 with visible SKS and/or SKKS phases. We obtained 63 measurements at 39 of the 82 stations in the network using the method of Silver and Chan (1991) and conventional quality criteria. We combined our results with previous measurements made by . The most prominent feature in the data is an area of large (>2.0 s) lag times with roughly east-west fast axes in northeastern Venezuela. Mineral physics models show split times this large are difficult to explain with horizontal foliation, but are more feasible with anisotropy characterized by a coherent vertical foliation and an east-west fast axis extending over most of the upper 250 km of the mantle. We interpret the large split times in northeastern Venezuela as a consequence of eastward translation of the Atlantic slab, which has left a strong vertical foliation in its wake parallel to the plate boundary. The peak split times correspond closely with the point the slab intersects the base of the anisotropic asthenosphere at 250 km. Away from this area of large split times the measured times fall to more standard values, but an east-west fast axis still predominates. We suggest this is linked to the rapidly varying strain field at the southern edge of the Atlantic which quickly disrupts the coherent strain field that causes the very large split times in northeastern Venezuela.
Journal of Geophysical Research, 2009
We have measured shear wave velocity structure of the crust and upper mantle of the Caribbean-South American boundary region by analysis of fundamental mode Rayleigh waves in the 20-to 100-s period band recorded at the BOLIVAR/GEODINOS stations from 2003 to 2005. The model shows lateral variations that primarily correspond to tectonic provinces and boundaries. A clear linear velocity change parallels the plate bounding dextral strike-slip fault system along the northern coast of Venezuela, illustrating the ...
Shear-wave splitting in northeast Venezuela, Trinidad, and the eastern Caribbean
Physics of the Earth and Planetary Interiors, 1996
We present new measurements of splitting of core shear-wave phases (SKS, SKKS, PKS) recorded at seven stations in the eastern Caribbean region. Six stations in northeastern Venezuela and Trinidad constituted the SECaSA92 temporary broadband array. The seventh station, SJG, is installed on Puerto Rico. We also attempted to measure source-side splitting of S waves from earthquakes in the Lesser Antilles subducted slab, but with little success. Mean splitting parameters, fast polarization direction, ~b, and delay time, St, at the stations are consistently east-west trending and over 1 s, respectively. Delay times at the SECaSA92 array are amongst the highest yet measured, reaching 2.1 s and generally over 1.5 s. We note an approximately linear decrease in delay times here from maxima at the northernmost stations to a minimum at the southemmost. The SECaSA92 array is situated in the wide South America-Caribbean (SA-Ca) plate boundary zone, and SJG is within the North America-Caribbean (NA-Ca) plate boundary zone. In both cases, ~b trends are parallel to the plate boundary zones, locally. We interpret these results to indicate that the SA-Ca plate boundary deformation extends to depths of at least 200 km, and that South American crust, lithospheric mantle, and asthenosphere are coherently deformed. The extreme delay times we measure are probably the result of transpression in the SA-Ca plate boundary and overriding of the Lesser Antilles slab by South America: compressional flattening results in extremes of foliation development, and dextral shearing yields a strong lineation in upper-mantle olivine. By contrast, the delay time at SJG, 1.2 s, is near the global average and reflects the fact that the NA-Ca plate boundary zone is purely sinistral or includes minor transtension. We note that recent counterclockwise rotation of Puerto Rico does not apparently affect upper-mantle deformation beneath the island. We suggest that this rotation is crustal, driven by far-field sinistral shear, and partly decoupled from the mantle shear. Our results are consistent with the trench-parallel mantle flow hypothesis (Russo and Silver, 1994, Science, 263: 1105-1111) and indicate that asthenospheric mantle flow beneath the Caribbean plate is eastward, and probably drives the plate's eastward motion.
To investigate the subduction dynamics in northwestern South America, we measured SKS and slab-related local S splitting at 38 seismic stations. Comparison between the delay times of both phases shows that most of the SKS splitting is due to entrained mantle flow beneath the subducting Nazca and Caribbean slabs. On the other hand, the fast polarizations of local S-waves are consistently aligned with regional faults, which implies the existence of a lithosphere-confined anisotropy in the overriding plate, and that the mantle wedge is not contributing significantly to the splitting. Also, we identified a clear change in SKS fast directions at the trace of the Caldas Tear (58N), which represents a variation in the subduction style. To the north of 58N, fast directions are consistently parallel to the flat subduction of the Caribbean plate-Panama arc beneath South America, while to the south fast polarizations are subparallel to the Nazca-South America subduction direction. A new change in the SKS splitting pattern is detected at 2.88N, which is related to another variation in the subduction geometry marked by the presence of a lithosphere-scale tearing structure, named here as Malpelo Tear; in this region, NE-SW-oriented SKS fast directions are consistent with the general dip direction of the underthrusting of the Carnegie Ridge beneath South America. Further inland, this NE-SW-trending mantle flow continues beneath the Eastern Cordillera of Colombia and Merida Andes of Venezuela. Finally, our results suggest that the subslab mantle flow in northwestern South America is strongly controlled by the presence of lithospheric tearing structures.
Geophysical Journal International, 1986
Fundamental-mode Rayleigh and Love waves generated by several earthquakes situated along great-circle paths between pairs of seismograph stations have been analysed to obtain coefficients of attenuation, group velocities, phase velocities, and specific quality factors in the period range 18-80s in two regions of the South American continent. One set of paths crosses the shield region which lies on the eastern coast and another set traverses the mountainous region inland. The average attenuation coefficient values are clearly higher in the tectonically active western region throughout the entire period range than in the eastern or shield region. Inversion of the attenuation data yielded shear wave internal friction (8') models as a function of depth in the crust and upper mantle in both regions. A low-Q zone below the lithosphere is prominent in both regions. The results show that substantial variations of Qp occur in the two regions of South America. The Qp values were found to be inversely related to the heat flow values or to the temperature.
Pure and Applied Geophysics, 1997
On May 25th, 1992, an M s =6.9 earthquake occurred off the southwestern tip of Cuba, along the boundary between the Caribbean and North American plates. This earthquake was the largest to strike southern Cuba since 1917 and the largest ever recorded in that region by global seismic networks. It is therefore a key element for our understanding of the tectonic and kinematic regime along the northern Caribbean plate boundary. In order to test the previously proposed source parameters of the Cabo Cruz earthquake and to better constrain its focal mechanism, we derived a new set of source parameters from unfiltered broad-band teleseismic records. We used a hybrid ray tracing method that allows us to take into account propagation effects of seismic waves in a realistic crustal model around the source. Our solution is consistent with the long-period focal mechanism solution of Virieux et al. (1992). Our solution also models the higher frequency crustal and water layer phases. The primarily strike-slip focal mechanism has a small thrust component. Its shows an east-west trending nodal plane dipping 55°to the north that we interpret as the rupture plane since it corresponds to the geometry of the major active fault in that area. The displacement on this plane is a left-lateral strike-slip combined with a small amount of southward thrust. The result is in good agreement with the active tectonic structures observed along the Oriente fault south of Cuba. The small thrust component demonstrates that, contrary to prior belief, the transpressive regime extends along this whole segment of the Caribbean/North American plate boundary. Together with historical seismicity, it suggests that most of the stress accumulated by the Caribbean/North American plate motion is released seismically along the southern Cuban margin during relatively few but large earthquakes.
Characterizing the Caribbean–South American plate boundary at 64°W using wide-angle seismic data
Journal of Geophysical Research, 2008
We present wide-angle velocity modeling results from profile 64°W of the Broadband Ocean-Land Investigation of Venezuela and the Antilles arc Region (BOLIVAR) project. Line 64W is a 460-km-long, approximately north-south, onshore-offshore reflection/ refraction transect located approximately at 64°W longitude. The profile extends across the transform plate boundary between the southeastern Caribbean (CAR) and South American (SA) plates. East of the profile the plate boundary bends to the north, and SA subducts beneath CAR. We utilize first-arrival tomography to resolve a smooth velocity field for the sediments and upper/middle crust and then use a layered approach to resolve a sharp velocity contrast for the Moho, simultaneously inverting refracted Pn and reflected PmP arrivals. We image crustal and upper mantle structure across the plate boundary zone. We interpret that the strike-slip system that accommodates relative motion between CAR and SA extends near vertically through the entire crust and offsets the Moho. We see no evidence supporting a major component of convergence, and rather than a wide boundary zone of overlapping lithospheric plates, we interpret the plate boundary to be confined to the 33-km-wide, near-vertical strike-slip system. Previously interpreted thrust faults flanking the strike-slip system appear to be confined to the upper/ middle crust and may be related to the detachment of subducting South American lithosphere at the southern terminus of the Lesser Antilles subduction zone east of 64°W.
AAPG Memoir 108, 2015
The offshore South Caribbean deformed belt (SCDB) is a 100-km-wide (62 mi), late Cenozoic sedimentary accretionary prism formed where the Caribbean plate is obliquely subducted beneath northern South America. Progradation of deltaic deposits of the 1500-km-long (932 mi) Magdalena River over the SCDB and tectonic deformation of the deltaic sedimentary rocks has created one of the youngest (last 10 Ma) and thickest (5-18 km [3-5 mi]) accretionary prisms in the world. We use three types of data (deep-penetration, seismic-reflection profiles, gravity modeling collinear with the seismic lines, and structural restorations) to describe the late Miocene to Recent thrust kinematics of the 10-to 18-km-thick (6.2-11.1 mi) SCDB formed above the subducting Caribbean Oceanic Plateau whose crust varies in thickness from 17 km (10.6 mi) in the southern part of the 180-km-long (111 mi) study area to 8 km (4.9 mi) in the north. In the southern area of thicker subducted plateau crust, the structural style is characterized by Neogene growth strata defining a major thrusted sequence that is backthrust in a landward direction, deformed by reactivation of preexisting faults as out-of-sequence thrust (OOST) faults, common shale diapirism, and active, margin-parallel strike-slip faults that accommodate the oblique-slip component of subduction. In the northern area of thinner oceanic crust (6-8 km [3.7-4.9 mi]), the structural style includes seaward-verging, imbricated thrust fans with less prominent backthrusting and strike-slip faulting, and more prominent shale diapirism. Proposed controls for the observed structural differences between the northern and southern areas include (1) more buoyancy of the subducted plate in the southern area due to its greater crustal thickness, and (2) the presence of more overpressured and ductile deformed shale layers with associated shale diapirs in the north.