Jorge Alonso-Henar

Neotectonic development of the El Salvador Fault Zone and implications for deformation in the Central America Volcanic Arc: Insights from 4-D analog modeling experiments

The El Salvador Fault Zone (ESFZ) is an active, approximately 150 km long and 20 km wide, nsegmented, dextral strike-slip fault zone within the Central American Volcanic Arc striking N100°E. Although nseveral studies have investigated the surface expression of the ESFZ, little is known about its structure at depth nand its kinematic evolution. Structural field data and mapping suggest a phase of extension, at some stage nduring the evolution of the ESFZ. This phase would explain dip-slip movements on structures that are currently nassociated with the active, dominantly strike slip and that do not fit with the current tectonic regime. Field nobservations suggest trenchward migration of the arc. Such an extension and trenchward migration of nthe volcanic arc could be related to slab rollback of the Cocos plate beneath the Chortis Block during the nMiocene/Pliocene. We carried out 4-D analog model experiments to test whether an early phase of extension nis required to form the present-day fault pattern in the ESFZ. Our experiments suggest that a two-phase ntectonic evolution best explains the ESFZ: an early pure extensional phase linked to a segmented volcanic arc nis necessary to form the main structures. This extensional phase is followed by a strike-slip dominated nregime, which results in intersegment areas with local transtension and segments with almost pure strike-slip nmotion. The results of our experiments combined with field data along the Central American Volcanic Arc nindicate that the slab rollback intensity beneath the Chortis Block is greater in Nicaragua and decreases nwestward to Guatemala.

The 1719 El Salvador Earthquake: An M>7.0 Event in the Central American Volcanic Arc?

In the regions where few field neotectonics and paleoseismic studies have been performed (e.g., Central America), the interpretation of the seismic sources responsible for the historical (preinstrumental) catastrophic earthquakes lies almost entirely in the spatial distribution of damage interpreted from historical sources, mainly fragmentary written documents. The occurrence of catastrophic earthquakes affecting this region justifies the necessity of a deeper analysis of the geologic implications of the more significant historical earthquakes in light of the new insights. Recent advances in the identification and dating of surface‐rupture evidences along the central El Salvador volcanic arc led us to revisit some historical evidence of damage along this region and to combine geologic (paleoseismic) evidence with damage distribution.nnAt least 11 destructive earthquakes have occurred in El Salvador since 1900 (White and Harlow, 1993; Fig.xa01). These events caused more than 3000 deaths as a consequence of strong ground motions and/or subsequent landslides (Bommer etxa0al. , 2002). The instrumental earthquake record shows that large ( M w>7) events occurred as reverse fault events along the interface between the subducted plate and the over‐riding continental plate or as normal‐faulting events within the subduction plate resulting from extensional forces generated by slab‐pull forces or by bending of the subduction plate (Alvarez‐Gomez, 2009). Onshore, instrumental earthquakes have been reported with moderate magnitudes ( M w 7.0 usually have been assigned to the subduction zone, whereas historical records with M w 7.0) associated with the rupture of the El Salvador fault zone …

Neogene-quaternary evolution from transpressional to transtensional tectonics in Northern Central America controlled by cocos: Caribbean subduction coupling change

Aims and MethodsThe structural evolution of the western boundary of the Chortis Block, particularly in the Central America Volcanic Arc crossing El Salvador is poorly known. We have done a kinematic analysis from seismic and fault slip data and combined our results with a review of previous regional studies. This approach allowed us to constrain the tectonic evolution and the processes that control the deformation in northern Central America.ResultsAlong the active volcanic arc we identified active transtensional deformation. On the other hand, we have identified two deformation phases in the back arc region: A first one of transpressional wrenching close to simple shearing (Miocene); and a second one characterized by almost E–W extension coincident with the extensional direction of the Honduras grabens opening. Our results reveal a change from transpressional to transtensional shearing coeval with a migration of the volcanism towards the trench in Late Miocene times.ImplicationsThe strain change could be related to a transition from coupled to decoupled interface on the Cocos—Caribbean subduction, which could be caused by a slab roll-back of the Cocos Plate beneath the Chortis Block. The combination of different degrees of coupling on the subduction interface, together with a constant relative eastward drift of the Caribbean Plate, control the deformation style along the western boundary of the Chortis Block.ResumenObjetivos y metodologíaLa evolución estructural del límite oeste del Bloque de Chortís, particularmente en la región del arco volcánico centroamericano cruzando El Salvador, ha sido muy poco estudiada. En este artículo presentamos los resultados de un análisis cinemático junto con una revisión de estudios regionales previos.ResultadosEsta aproximación nos ha permitido arrojar luz sobre la evolución tectónica y los procesos que controlan la deformación en el norte de Centroamérica. A lo largo del arco volcánico activo hemos identificado deformación transtensiva. Por otro lado, hemos identificado dos fases de deformación en la zona de retro-arco: una primera fase de cizalla transpresiva cercana a la cizalla simple (Mioceno); y una segunda fase caracterizada por una extensión cercana a la dirección E–W que coincide con la dirección de apertura de los grábenes de Honduras. Los resultados de nuestros análisis revelan un cambio de cizalla transpresiva a cizalla transtensiva coetáneo con la migración del arco volcánico hacia la fosa durante el Mioceno Superior.ImplicacionesEl cambio en el estilo de la deformación podría estar relacionado con un decrecimiento en el grado de acoplamiento de la inter-fase Cocos—Caribe, que podría deberse a un proceso de roll-back de la placa del Coco bajo el Bloque de Chortís. La combinación de diferentes grados de acoplamiento en la inter-fase de la subducción, junto con una deriva constante de la placa de Caribe, controlan el estilo de la deformación a lo largo del límite oeste del bloque de Chortís.

Main crustal seismic sources in El Salvador

We present a map and a data set containing information about intra-plate seismic sources in El Salvador. These are the results of the field campaigns and data analysis carried out by the research group of Planetary Geodinamics, Active Tectonics and Related Risks from Complutense University of Madrid during the last 12 years. We include two maps, the first map contains 1405 fault traces with evidences of Quaternary activity derived form morphometric, paleoseismological and geomorphological analysis together with field data mapping carried in El Salvador. The second map is a synthesis of the 29 intra-plate seismic sources selected from the quaternary faults map. The geometry of these sources was simplified and we also include a table where some available data of the proposed sources are included, such as their name, orientation, length and slip-rate. For further interpretation and discussion of these sources see (Alonso-Henar et al., 2018) [1, doi.org/10.1016/j.enggeo.2018.06.015].

Geological evidences of surface rupture related to a seventeenth century destructive earthquake in Betic Cordillera (SE Spain): constraining the seismic hazard of the Alhama de Murcia fault

Constraining the date of the last major event occurred in a fault is of paramount importance in probabilistic seismic hazard assessment when time-dependent models are considered. Eight of the twelve destructive earthquakes occurred in the eastern Betic Cordillera since sixteenth century, are located less than 10xa0km away from the Alhama de Murcia fault (AMF). Up to now, it has not been identified any geological evidence on the ground surface to associate these events with the activity of specific fault sections of the AMF. In this work we present the first geological evidence of the catastrophic 1674 event occurred at Lorca (SE Spain). The excavations carried out at La Torrecilla Creek exposed archaeological remains from the Islamic period (VIII–XIII centuries in this region) affected by 55u2009±u200920xa0cm offset by the AMF fault. This event reached intensity VIII and produced 30 fatalities at Lorca for an estimated population of 7300 inhabitants. This supports the occurrence of earthquakes with surface rupture in the historical epoch on the Alhama de Murcia fault and reinforces the results obtained in previous paleoseismological work. The theoretical scenarios of maximum magnitudes and recurrence time obtained by combining this historical event with the fault slip rate allow us to conclude that the seismic hazard associated with maximum magnitude events in this section could be high. In addition, the static Coulomb stress transferred to the Góñar–Lorca section by the 2011 (Mw 5.2) Lorca earthquake may have significantly increased the hazard.ResumenAsignar la fecha del último terremoto importante ocurrido en un segmento de falla es de gran importancia para las estimaciones probabilistas de peligrosidad sísmica, especialmente cuando se consideran modelos dependientes del tiempo. Ocho de los doce terremotos destructivos ocurridos en la Cordillera Bética Oriental desde el siglo XVI están localizados a menos de 10 km de la falla de Alhama de Murcia (AMF). Hasta el momento no se han identificado evidencias geológicas concretas en superficie que permitan asociar estos eventos a segmentos específicos de falla. En este trabajo presentamos la primera evidencia geológica del terremoto catastrófico de 1674 en Lorca (SE de España). Las excavaciones llevadas a cabo en la rambla de La Torrecilla han expuesto restos arqueológicos de época Árabe afectados por un desplazamiento de la AMF de 55±20 cm. Este evento alcanzó una intensidad VIII y produjo 30 víctimas mortales en Lorca, que en aquella época tenía 7300 habitantes. Esto apoya la ocurrencia de terremotos con ruptura superficial en época histórica en la falla de Alhama de Murcia y refuerza los resultados obtenidos en trabajos paleosismológicos previos. Los escenarios teóricos de magnitudes máximas e intervalos de recurrencia obtenidos, combinando este evento histórico con la velocidad de movimiento de la falla, nos lleva a concluir que la amenaza sísmica asociada con los eventos de magnitud máxima en esta sección de la AMF puede ser alta. Además, el esfuerzo de fractura de Coulomb transferido a la sección Góñar-Lorca por el terremoto de Lorca de Mw 5.2 de 2011 puede haber aumentado significativamente la amenaza.