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La Paz, Bolivia

Ryan J.,University of Arizona | Beck S.,University of Arizona | Zandt G.,University of Arizona | Wagner L.,Carnegie Institution of Washington | And 2 more authors.
Tectonophysics | Year: 2016

The Central Andean Plateau (15°-27°S) is a high plateau in excess of 3. km elevation, associated with thickened crust along the western edge of the South America plate, in the convergent margin between the subducting Nazca plate and the Brazilian craton. We have calculated receiver functions using seismic data from a recent portable deployment of broadband seismometers in the Bolivian orocline (12°-21°S) region and combined them with waveforms from 38 other stations in the region to investigate crustal thickness and crust and mantle structures. Results from the receiver functions provide a more detailed map of crustal thickness than previously existed, and highlight mid-crustal features that match well with prior studies. The active volcanic arc and Altiplano have thick crust with Moho depths increasing from the central Altiplano (65. km) to the northern Altiplano (75. km). The Eastern Cordillera shows large along strike variations in crustal thickness. Along a densely sampled SW-NE profile through the Bolivian orocline there is a small region of thin crust beneath the high peaks of the Cordillera Real where the average elevations are near 4. km, and the Moho depth varies from 55 to 60. km, implying the crust is undercompensated by ~. 5. km. In comparison, a broader region of high elevations in the Eastern Cordillera to the southeast near ~. 20°S has a deeper Moho at ~. 65-70. km and appears close to isostatic equilibrium at the Moho. Assuming the modern-day pattern of high precipitation on the flanks of the Andean plateau has existed since the late Miocene, we suggest that climate induced exhumation can explain some of the variations in present day crustal structure across the Bolivian orocline. We also suggest that south of the orocline at ~. 20°S, the thicker and isostatically compensated crust is due to the absence of erosional exhumation and the occurrence of lithospheric delamination. © 2016 Elsevier B.V. Source


Pritchard M.E.,Cornell University | Henderson S.T.,Cornell University | Jay J.A.,Cornell University | Soler V.,CSIC - Institute of Natural Products and Agrobiology | And 12 more authors.
Journal of Volcanology and Geothermal Research | Year: 2014

We record non-eruptive background seismicity at eight potentially active volcanoes and one geothermal area in Chile and Bolivia for the first time in order to set a baseline for future episodes of unrest. We also compare seismicity to coincident new regional observations of ground deformation from InSAR and satellite observed thermal anomalies from the ASTER instrument. We deploy small temporary seismometer networks (1 to 5 stations each) of short and intermediate period instruments for 3-27. months at the nine areas between the years 2004 and 2012 at: Parinacota, Guallatiri, Isluga, Irruputuncu, Olca-Paruma, Ollagüe, Sol de Mañana, Putana, and Láscar. Despite the lack of shallow earthquakes in the global catalogs at these volcanoes, we find that all have volcano-tectonic events with at least 27 earthquake swarms - the most active are Putana, Guallatiri and Ollagüe. We find two examples where changes in seismicity are likely related to either deformation (in 2009 at Putana) or an increase in temperature (in 2012 at Isluga). Further, we document for the first time ground deformation at a Pliocene volcano called Sillajhuay, located in the Holocene volcano gap (i.e., 70. km from the nearest active volcano Isluga). We find that the four deforming volcanoes between 18 and 24°S are seismically active, but that seismic activity does not imply measurable ground deformation. Similarly, the seismically active volcanoes have satellite thermal hotspots, but there is no correlation between relative amounts of seismic activity and hotspot temperature. Because several of the volcanoes show variations in seismic activity, temperature, and deformation over the course of a few years unrelated to eruptions, decadal and longer observations are needed to constrain background activity in the central Andes. © 2014 Elsevier B.V. Source


Jay J.A.,Cornell University | Pritchard M.E.,Cornell University | West M.E.,University of Alaska Fairbanks | Christensen D.,University of Alaska Fairbanks | And 5 more authors.
Bulletin of Volcanology | Year: 2012

Using a network of 15 seismometers around the inflating Uturuncu Volcano from April 2009 to 2010, we find an average rate of about three local volcano-tectonic earthquakes per day, and swarms of 5-60 events a few times per month with local magnitudes ranging from -1.2 to 3.7. The earthquake depths are near sea level, more than 10 km above the geodetically inferred inflation source and the Altiplano Puna Magma Body. The Mw 8.8 Maule earthquake on 27 February 2010 triggered hundreds of earthquakes at Uturuncu with the onset of the Love and Rayleigh waves and again with the passage of the X2/X3 overtone phases of Rayleigh waves. This is one of the first incidences in which triggering has been observed from multiple surface wave trains. The earthquakes are oriented NW-SE similar to the regional faults and lineaments. The b value of the catalog is 0.49, consistent with a tectonic origin of the earthquakes. We perform ambient noise tomography using Love wave cross-correlations to image a low-velocity zone at 1.9 to 3.9 km depth below the surface centered slightly north of the summit. The low velocities are perhaps related to the hydrothermal system and the low-velocity zone is spatially correlated with earthquake locations. The earthquake rate appears to vary with time-a seismic deployment from 1996 to 1997 reveals 1-5 earthquakes per day, whereas 60 events/day were seen during 5 days using one seismometer in 2003. However, differences in analysis methods and magnitudes of completeness do not allow direct comparison of these seismicity rates. The rate of seismic activity at Uturuncu is higher than at other well-monitored inflating volcanoes during periods of repose. The frequent swarms and triggered earthquakes suggest the hydrothermal system is metastable. © 2011 Springer-Verlag. Source


Brooks B.A.,University of Hawaii at Manoa | Bevis M.,Ohio State University | Whipple K.,Arizona State University | Ramon Arrowsmith J.,Arizona State University | And 9 more authors.
Nature Geoscience | Year: 2011

Subduction of the Nazca plate beneath South America has driven the growth of the Andes Mountains. Subduction has routinely generated earthquakes larger than magnitude 8.0 along the western margin of the mountain belt, but the potential size of less frequent earthquakes in the eastern, backarc margin is unknown. Continued support of the high Andean Plateau at the centre of the Andes can be explained only if deformation of the backarc margin is ongoing. Here we present GPS data that record surface motions in the Subandean ranges that are part of the backarc margin. We find that the velocity of surface movement decreases sharply from west to east across the Subandean ranges. We suggest that a subhorizontal fault underlying the ranges slips freely at depth in the west, but is locked for up to 100-km in shallower sections further east. Analysis of fault scarps formed where the subhorizontal fault intersects the surface indicates that the fault has generated repeated large earthquakes. We suggest that rupture of the entire locked section of the fault could generate an earthquake of magnitude 8.7-8.9. We attribute the large seismic potential to the unusual width of the Subandean ranges, and suggest that deformation of the Subandean ranges, at a rate unmatched by erosion, causes the mountain range to widen. © 2011 Macmillan Publishers Limited. All rights reserved. Source

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