Browsing by Autor "Terry C. Wallace"

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Anomalous crust of the Bolivian Altiplano, central Andes: Constraints from broadband regional seismic waveforms
(American Geophysical Union, 1996) G. Zandt; S. L. Beck; S. R. Ruppert; Charles J. Ammon; Don Rock; E. Minaya; Terry C. Wallace; Paul G. Silver
A one‐year deployment of broadband seismographs in the Bolivian Altiplano recorded numerous intermediate‐depth earthquakes at near‐regional distances. We modeled the associated broadband waveforms of two earthquakes to estimate an average crustal structure for the Altiplano. The resulting model is characterized by an anomalously low mean P velocity of 6.0 km/s, a low Poisson's ratio of 0.25, and a crustal thickness of 65 km. The combination of the low mean velocity and low Poisson's ratio can be explained only by a predominantly quartz‐rich, felsic bulk composition. This constraint precludes significant volumes of magmatic addition from the mantle contributing to the great thickness of the Altiplano crust, but is consistent with thickening by compressive shortening concentrated in a weak felsic layer.
Crustal-thickness variations in the central Andes
(Geological Society of America, 1996) S. L. Beck; G. Zandt; Stephen C. Myers; Terry C. Wallace; Paul G. Silver; Lawrence A. Drake
Research Article| May 01, 1996 Crustal-thickness variations in the central Andes Susan L. Beck; Susan L. Beck 1SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar George Zandt; George Zandt 2IGPP, Lawrence Livermore National Laboratory, Livermore, California, and SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Stephen C. Myers; Stephen C. Myers 1SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Terry C. Wallace; Terry C. Wallace 1SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Paul G. Silver; Paul G. Silver 3Carnegie Institution of Washington, Washington, D.C. Search for other works by this author on: GSW Google Scholar Lawrence Drake Lawrence Drake 4Observatorio San Calixto, La Paz, Bolivia Search for other works by this author on: GSW Google Scholar Author and Article Information Susan L. Beck 1SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 George Zandt 2IGPP, Lawrence Livermore National Laboratory, Livermore, California, and SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Stephen C. Myers 1SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Terry C. Wallace 1SASO and Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Paul G. Silver 3Carnegie Institution of Washington, Washington, D.C. Lawrence Drake 4Observatorio San Calixto, La Paz, Bolivia Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1996) 24 (5): 407–410. https://doi.org/10.1130/0091-7613(1996)024<0407:CTVITC>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Susan L. Beck, George Zandt, Stephen C. Myers, Terry C. Wallace, Paul G. Silver, Lawrence Drake; Crustal-thickness variations in the central Andes. Geology 1996;; 24 (5): 407–410. doi: https://doi.org/10.1130/0091-7613(1996)024<0407:CTVITC>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We estimated the crustal thickness along an east-west transect across the Andes at lat 20°S and along a north-south transect along the eastern edge of the Altiplano from data recorded on two arrays of portable broadband seismic stations (BANJO and SEDA). Waveforms of deep regional events in the downgoing Nazca slab and teleseismic earthquakes were processed to isolate the P-to-S converted phases from the Moho in order to compute the crustal thickness. We found crustal-thickness variations of nearly 40 km across the Andes. Maximum crustal thicknesses of 70–74 km under the Western Cordillera and the Eastern Cordillera thin to 32–38 km 200 km east of the Andes in the Chaco Plain. The central Altiplano at 20°S has crustal thicknesses of 60 to 65 km. The crust also appears to thicken from north (16°S, 55–60 km) to south (20°S, 70–74 km) along the Eastern Cordillera. The Subandean zone crust has intermediate thicknesses of 43 to 47 km. Crustal-thickness predictions for the Andes based on Airy-type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation. In contrast, at the boundary between the Eastern Cordillera and the Subandean zone and in the Chaco Plain, the crust is thinner than predicted, suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere. With additional constraints, we conclude that the observation of Airy-type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting Nazca plate and the cratonic lithosphere of the Brazilian craton. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Implications of spatial and temporal development of the aftershock sequence for the <i>M<sub>w</sub></i> 8.3 June 9, 1994 Deep Bolivian Earthquake
(American Geophysical Union, 1995) Stephen C. Myers; Terry C. Wallace; S. L. Beck; Paul G. Silver; G. Zandt; J. C. VanDecar; E. Minaya
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.