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Scolamacchia T.,National Autonomous University of Mexico | Pullinger C.,Lageo SA | Caballero L.,National Autonomous University of Mexico | Montalvo F.,Servicio Nacional de Estudios Territoriales | And 2 more authors.
Journal of Volcanology and Geothermal Research | Year: 2010

We report the stratigraphic sequence of the 2005 eruption of Ilamatepec volcano together with sedimentological and chemical analyses of its products. Structural and textural characteristics of the deposits indicate that the eruption was driven by a small-volume rhyolitic intrusion at shallow levels, which resulted first in the collapse of the existing hydrothermally altered fan of previous deposits inside the crater lake, driving phreatic explosions with launching of blocks on ballistic trajectories; later the magma interacted with lake waters producing several hydromagmatic pyroclastic density currents (PDCs). These flows were energetic enough to knock down pine trees up to distances of 1.8 km from the crater in the E-NE sector of the volcano. Finally, ejection of ballistic blocks that landed on previously emplaced, wet pyroclastic density current deposits, caused the generation of a lahar that flowed down the steep eastern flank toward the El Jabillal gully. Subsequent lahars occurred as a result of intense rain caused by hurricane Stan. Radiocarbon ages on paleosols and charcoal fragments, separating previous volcanogenic sequences, indicate that similar eruptions have occurred more frequently in the past centuries, than previously thought. The new data confirms that Ilamatepec volcano is one of the most active volcanoes in El Salvador. Nevertheless, more detailed studies of the eruptive sequence of Ilamatepec volcano are mandatory to establish future eruptive patterns. © 2009 Elsevier B.V. All rights reserved. Source

Garibaldi N.,University of Wisconsin - Madison | Tikoff B.,University of Wisconsin - Madison | Hernandez W.,Servicio Nacional de Estudios Territoriales
Tectonophysics | Year: 2016

Dominantly westward movement of the El Salvador forearc at rates of 11. mm/yr is accommodated by a series of E-W to WNW oriented, dextral, strike-slip fault zones herein referred to as the El Salvador Fault System (ESFS). The geometry of the ESFS defines a series of extensional step-overs. Along the arc, basaltic volcanism in the stepovers is associated with NNW-oriented normal faults, whereas rhyolitic volcanism is associated with strike-slip fault zones of the ESFS.On the ESFS, the San Salvador Extensional Stepover (SSES) is bound to the south by the San Vicente fault zone, where the rhyolitic Ilopango caldera is located. In the SSES, tephras from Ilopango -the Tierra Blanca (TB) sequence- track long-term elongation. Older TB units (TB5-8) contain abundant normal faults; lying unconformably above these older TB units, younger TB members (TBJ, TB2-4) are generally unfaulted. Analyses of faults in TB5-8 indicate NE- to ENE-oriented elongation in the SSES. Deformation occurred between deposition of the TB4 and TB5 units, during quiescence of the Ilopango eruptive center. Using this temporal constraint, minimum elongation rates of 3.50×10-15 s-1, 2.06×10-14 s-1 and 4.42×10-14 s-1 were calculated for three traverses. From regional geodetic data and fault kinematics throughout El Salvador, we interpret the SSES as part of a series of pull-apart structures along the arc axis. The calculated paleostress orientations are consistent with a pull-apart geometry resulting from forearc movement.The extensional deformation occurs during a ~. 50. k.y. lull in rhyolitic activity, suggesting an interplay between magmatism and deformation within the arc. During significant rhyolitic volcanic activity, only minor elongation is observed in the SSES, despite ongoing translation of the Salvadoran forearc. We speculate that rhyolitic magmatism along upper crustal faults may facilitate strike-slip movement on the ESFS, rather than distributing deformation throughout the extensional stepover. © 2016 Elsevier B.V. Source

Canora C.,Complutense University of Madrid | Villamor P.,Institute of Geological & Nuclear Sciences | Martinez-Diaz J.J.,Complutense University of Madrid | Berryman K.R.,Institute of Geological & Nuclear Sciences | And 3 more authors.
Geologica Acta | Year: 2012

The El Salvador earthquake of February 13 th 2001 (Mw 6.6) was associated with the tectonic rupture of the El Salvador Fault Zone. Paleoseismic studies of the El Salvador Fault Zone undertaken after this earthquake provide a basis for examining the longer history of surface rupturing earthquakes on the fault. Trenching at five sites along the San Vicente segment, a 21km-long and up to 2km-wide central section of the El Salvador Fault Zone, shows that surface fault rupture has occurred at least seven times during the past 8ka. Single-event displacements identified at each trench vary from several decimetres to at least 3.7m. Fault trace mapping, geomorphic analysis, and paleoseismic studies indicate a maximum magnitude for the El Salvador Fault Zone is c. M w 7.6, with a recurrence interval of around 800yr. Earthquakes of M w 6.6 or smaller, such as the February 2001 event are unlikely to be identified in the paleoseismic trenches, so our observations represent the minimum number of moderate to large earthquakes that have occurred on this part of the El Salvador Fault Zone. We observe significant variability in single-event displacement in the trenches, which we interpret as possible cascade rupture of several segments of the El Salvador Fault Zone. Combining displacements of river courses and the timing of events revealed in the trenches, we calculate a slip rate of c. 4mm/yr for El Salvador Fault Zone, identifying the fault zone as a major tectonic feature of the region, and a major source of seismic hazard and risk in El Salvador. Source

Lexa J.,Slovak Academy of Sciences | Sebesta J.,Czech Geological Survey | Chavez J.A.,Oficina de Planificacion del Area Metropolitana de San Salvador OPAMSS | Chavez J.A.,Czech Technical University | And 2 more authors.
Journal of Geosciences | Year: 2011

We have carried out geoslogical studies including mapping at the scale 1: 50 000 in the southern part of the San Salvador Metropolitan Area to support urban planning and natural hazard mitigation. The study area extends over the Cordillera del Bálsamo, marginal fault system and southern part of the Central Graben between the active San Salvador volcano and Ilopango caldera. It represents a segment in the Central American Volcanic Front. Volcanic rocks of the Late Miocene to recent age, classified as the Bálsamo, Cuscatlán and San Salvador formations, occur in the area. Remnants of two large basaltic andesite to andesite stratovolcanoes, Panchimalco and Jayaque, represent the Bálsamo Formation. They show periclinal dips and facies zoning from lava flows and coarse epiclastic volcanic breccias of the proximal zone through epiclastic volcanic breccias/conglomerates of the medial zone to epiclastic volcanic conglomerates and sandstones of the distal zone. Their ages are 7.2-6.1 Ma and 2.6-1.5 Ma respectively. The Cuscatlán Formation comprises the Jayaque and Santo Tomás calderas, the andesitic-dacitic Ilopango and Jayaque ignimbrites (1.9-1.4 Ma) in the SW and SE parts of the area, the Ilopango andesitic volcano (1.5-0.8 Ma), the Loma Larga basaltic volcano (0.8-0.5 Ma), the Planes de Renderos caldera, the dacite-andesite San Jacinto extrusive domes and effusive cone (0.4-0.25 Ma), the San José tuff/scoria cone, the Ilopango caldera extrusive domes (0.25-0.05 Ma), the Antiguo Cuscatlán scoria cone (0.2-0.08 Ma) and older tephra deposits of the Coatepeque and Ilopango calderas exposed along marginal faults of the Central Graben. The San Salvador Formation occurs as tephra cover along the crest of the Cordillera del Bálsamo where it rests on laterites atop the Bálsamo Formation and in the Central Graben. Tephra units belong to the Coatepeque caldera (Arce and Congo), San Salvador volcano (Apopa, G1 and G2) and Ilopango caldera (Tierra Blanca 1-4) spanning 70-1 ka. Tephra units are separated by palaeosols and aeolian dusty deposits. Las amenazas naturales afectan al territorio de El Salvador en toda su extensión de manera constante. Se ha llevado a cabo un mapeo geológico en la parte Sur del Área Metropolitana de San Salvador (AMSS), asimismo se han evaluado las amenazas naturales potenciales que pueden afectar a la zona. El área de estudio se extiende sobre la Cordillera del Bálsamo, el sistema de fallas marginales y en la parte Sur del Graben Central entre los volcanes activos de San Salvador y la Caldera de Ilopango; representando un segmento del frente volcánico de Centro América. Las rocas volcánicas del Mioceno tardío hasta de edad reciente que pertenecen a las Formaciones Bálsamo, Cuscatlán y San Salvador conforman la geología del área. Los remanentes de dos extensos estratovolcanes basálticos-andesíticos hasta andesíticos, Panchimalco y Jayaque, representan la Formación Bálsamo. Estos presentan un buzamiento periclinal y zonas con facies que van desde flujos de lava y brechas epiclásticas volcánicas gruesas de la zona proximal, brechas epiclásticas volcánicas/conglomerados de la zona media hasta los conglomerados volcánicos epiclásticos y areniscas de la zona distal. Su edad están entre los 7.2-6.1 Ma y 2.6-1.5 Ma respectivamente. La Formación Cuscatlán está representada por las Calderas Jayaque y Santo Tomás, las ignimbritas andesíticas/dacíticas de Ilopango y las ignimbritas de Jayaque (1.9-1.4 Ma) en la parte SO y SE del área, el volcán andesítico de Ilopango (1.5-0.8 Ma), el volcán basáltico Loma Larga (0.8-0.5 Ma), la Caldera Planes de Renderos, los domos extrusivos dacíticos/andesíticos de San Jacinto y el cono efusivo (0.4-0.25 Ma), el cono de toba/escoria de San José, los domos extrusivos de la Caldera de Ilopango (0.25-0.05 Ma), el cono de escoria de Antiguo Cuscatlán (0.2-0.08 Ma) y los depósitos de tefra inferiores de las calderas de Coatepeque e Ilopango expuestos a lo largo de las fallas marginales del Graben Central. La Formación San Salvador está presente como una cubierta de tefra que cubre la cresta de la Cordillera del Bálsamo a lo largo de su extensión, donde yace en lateritas sobre la Formación Bálsamo y en el Graben Central. Las unidades de tefra que pertenecen a la Caldera Coatepeque (Arce y Congo) el Volcán de San Salvador (Apopa, G1 y G2) y la Caldera de Ilopango (Tierra Blanca 1-4) cubren un periodo de tiempo entre 70-1 ka. Las unidades de tefra están separadas por horizontes de suelos fósiles y depósitos de polvo eólicos. Source

Geirsson H.,Pennsylvania State University | Geirsson H.,European Center for Geodynamics and Seismology | Lafemina P.C.,Pennsylvania State University | Demets C.,University of Wisconsin - Madison | And 7 more authors.
Geophysical Journal International | Year: 2015

Subduction zones exhibit variable degrees of interseismic coupling as resolved by inversions of geodetic data and analyses of seismic energy release. The degree to which a plate boundary fault is coupled can have profound effects on its seismogenic behaviour. Here we use GPS measurements to estimate co- and post-seismic deformation from the 2012 August 27, Mw7.3 megathrust earthquake offshore El Salvador, which was a tsunami earthquake. Inversions of estimated coseismic displacements are in agreement with published seismically derived source models, which indicate shallow (<20 km depth) rupture of the plate interface. Measured post-seismic deformation in the first year following the earthquake exceeds the coseismic deformation. Our analysis indicates that the post-seismic deformation is dominated by afterslip, as opposed to viscous relaxation, and we estimate a post-seismic moment release one to eight times greater than the coseismic moment during the first 500 d, depending on the relative location of coseismic versus post-seismic slip on the plate interface. We suggest that the excessive post-seismic motion is characteristic for the El Salvador-Nicaragua segment of the Central American margin and may be a characteristic of margins hosting tsunami earthquakes. © The Authors 2015. Published by Oxford University Press on behalf of The Royal Astronomical Society. All rights reserved. Source

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