José A. Álvarez-Gómez

Triggering of destructive earthquakes in El Salvador

We investigate the existence of a mechanism of static stress triggering driven by the interaction of normal faults in the Middle American subduction zone and strike-slip faults in the El Salvador volcanic arc. The local geology points to a large strike-slip fault zone, the El Salvador fault zone, as the source of several destructive earthquakes in El Salvador along the volcanic arc. We modeled the Coulomb failure stress (CFS) change produced by the June 1982 and January 2001 subduction events on planes parallel to the El Salvador fault zone. The results have broad implications for future risk management in the region, as they suggest a causative relationship between the position of the normal-slip events in the subduction zone and the strike-slip events in the volcanic arc. After the February 2001 event, an important area of the El Salvador fault zone was loaded with a positive change in Coulomb failure stress (.0.15 MPa). This scenario must be considered in the seismic hazard assessment studies that will be carried out in this area.

An Overview of the Damaging and Low Magnitude Mw 4.8 La Paca Earthquake on 29 January 2005: Context, Seismotectonics, and Seismic Risk Implications for Southeast Spain

This article presents an overview of the La Paca earthquake of magnitude mbLg 4.7, which occurred on 29 January 2005, with its epicenter located near the town of Avile´s in the Murcia region in southeast Spain. Despite its low magnitude, the earthquake caused important damage in two towns of the epicentral area, La Paca and Zarcilla de Ramos. These areas recorded intensities of VI–VII (European Macroseismic Scale, 1998) and sustained estimated economic losses amounting to 10 million €. Aftershocks continued for more than 2 weeks, producing considerable alarm in the population and mobilizing emergency services from the whole region. The La Paca seismic series is the third registered in the region in the past 8 years, being preceded by the Mula (1999) and southwest Bullas (2002) seismic series. These main events had also low magnitudes (mbLg 4.8) and caused damage levels similar to the 2005 earthquake. The case is an example of a moderate seismic zone where low-magnitude and frequent earthquakes have important implications on the seismic hazard and risk of the region. Although these are not the largest expected earthquakes, they have yielded important information for improving the knowledge of the seismic activity of the area. With this aim in mind, different topics have been analyzed from a multidisciplinary perspective, including seismicity, local tectonics and surface geology, focal mechanisms, macroseismic effects, and ground motion. Results indicate a local tectonic interpretation, consistent with a strike-slip focal mechanism, the confirmation of a triggering process between the 2002 and 2005 earthquakes, a geotechnical and ground-motion characterization for the damaged sites (supporting local amplification effects and estimated peak ground acceleration values of �0.1g), and an understanding of damage patterns in relation to local building trends. The results may be used as guidelines for future revisions of the Spanish Building Code (Norma de la Construccio´n Sismorresistente Espan˜ola [NCSE-02], 2002). The study results should contribute to risk mitigation in a region where strong-motion records from the maximum expected earthquakes are not available. This approach can be extended to other regions with similar seismic backgrounds and a lack of strong-motion records.

Approaches for tsunami risk assessment and application to the city of Cádiz, Spain

Tsunamis can represent a significant risk to the population and cause huge economic damage in many costal regions. In order to be able to identify risk hot spots and implement targeted risk reduction measures, decision makers need to have a clear picture of the risk situation in their countries or regions. This work reviews existing approaches for tsunami risk assessment and recommends a five-step process for assessing tsunami risk. As a case study, a qualitative risk assessment for a worst-case tsunami scenario was carried out to understand the tsunami risk to the population in Cádiz. Moreover, a sensitivity analysis of the tsunami hazard input parameters was performed as a strong influence of the variability of the input parameters on the resultant tsunami hazard and risk zonation maps was observed. The study shows that regardless of the assumptions made a non-negligible tsunami risk to Cádiz exists.

Geological and Seismological Analysis of the 13 February 2001 Mw 6.6 El Salvador Earthquake: Evidence for Surface Rupture and Implications for Seismic Hazard

The El Salvador earthquake of 13 February 2001 (Mw 6.6) caused tectonic rupture on the El Salvador fault zone (ESFZ). Right-lateral strike-slip surface rupture of the east–west trending fault zone had a maximum surface displacement of 0.60 m. No vertical component was observed. The earthquake resulted in widespread landslides in the epicentral area, where bedrock is composed of volcanic sediments, tephra, and weak ignimbrites. In the aftermath of the earthquake, widespread damage to houses and roads and the hazards posed by landslides captured the attention of responding agencies and scientists, and the presence of surface-fault rupture was overlooked. Additionally, the tectonic context in which the earthquake took place had not been clear until mapping of the ESFZ was completed for the present study.We identified several fault segments, the distribution of surface ruptures, the aftershock pattern, and fault-rupture scaling considerations that indicate the 21-km-long San Vicente segment ruptured in the 2001 event. Static Coulomb stress transfer models for the San Vicente rupture are consistent with both aftershock activity of the 2001 sequence and ongoing background seismicity in the region. At Mw 6.6, the 2001 earthquake was of only moderate magnitude, yet there was significant damage to the country’s infrastructure, including buildings and roads, and numerous deaths and injuries. Thus, earthquake hazard and risk in the vicinity of the ESFZ, which straddles the city of San Salvador with a population of >2 million, is high because even moderate-magnitude events can result in major damage, deaths, and injuries in the region.

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

Short Note: SeriesBuster: a Matlab program to extract spatio-temporal series from an earthquake database

SeriesBuster is a Matlab program made to extract seismic series attending to the temporal and spatial relations among records from earthquake catalogs. The program allows to compare earthquake records making use of a number of variables: maximum distance, maximum and minimum temporal difference, focal depth, or any other feature marked by a flag, by means of a set of filters and functions (Fig. 1). SeriesBuster works comparing all possible relations in all records from the catalog in order to ensure that no possible seismic series is missed and that the produced ones are complete. The user has the option of eliminating from the output file those series composed by less events than a minimum established. SeriesBuster shows an interactive histogram of the distribution of the series versus the number of events in order to facilitate the user’s decision. SeriesBuster can work with either geographic or cartesian coordinates. The former must be in decimal degrees format. The output file contains two added columns. The first one refers to the serial day from a reference

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.

New insights on the seismogenic potential of the Eastern Betic Shear Zone (SE Iberia): Quaternary activity and paleoseismicity of the SW segment of the Carrascoy Fault Zone

The Carrascoy Fault (CAF) is one of the main active faults that form part of the Eastern Betic Shear Zone, a 450 km fault system that accommodates most of the convergence between the Eurasian (Iberia) and Nubian plates in the Betic Cordillera, south Spain. Although the CAF represents a major earthquake threat to the nearby City of Murcia, studies on its Quaternary tectonics and seismogenic potential are scarce to date. We present evidence that supports the division of the CAF into two overlapping segments with contrasting tectonic structure, Quaternary activity, and landform control: a SW segment, characterized by a broad fold-and-thrust zone similar to the forebergs defined in the Gobi-Altai region, and a NE segment, characterized by a sharp mountain front controlled by strike-slip tectonics. We attribute the differentiation into these two segments to the stresses associated with topography, which in turn is a consequence of the shortening component, at the middle Pleistocene, after circa 217.4 ka. For the SW segment we infer the occurrence of 9 to 11, Mw 6.7 paleoearthquakes in the last 30.2 kyr, and a slip rate of 0.37 ± 0.08 m/kyr. We date the occurrence of the last surface rupture event after 2750 B.P., and we estimate an average recurrence period of major events of 3.3 ± 0.7 kyr.

Effects of propofol anesthesia on nonlinear properties of EEG: Lyapunov exponents and short-term predictability

This paper focuses on the analysis of the short term predictability of the EEG signal after the estimation of the maximum Lyapunov exponent (MLE) of the chaotic attractor associated to the EEG signal. After clinical research and ethical committee approval and signed informed consent, EEG data was collected from 6 patients scheduled for surgery under general anesthesia and 7 ones scheduled for ambulatory endoscopic procedures. One differential channel of EEG (+ in mastoids M1/M2, - in the middle-line of the forehead FPz/AFz and referenced to F7/F8) was amplified with a gain of 10.000 (Biopac MP100-EEG100B), low-pass filtered at 300 Hz (2nd order Butterwoth) and acquired at 2.5 KHz. Data were digitally filtered (50 Hz comb filter and 120-taps linear phase FIR lowpass filter at 85 Hz) and then decimated to 250 Hz prior to analysis under OpenTstool. In order to reconstruct the associated chaotic attractor, two different approaches were followed. First, classical methods were used to find time-lag and embedding dimension. Also, a parametric swept was used for tau=1,2,3...,20 and d=2,3...6. Once the attractor was reconstructed, MLE was estimated as the finite growth rate proposed by Wessel et al., for different window lengths. Surrogate data sets were generated for validation purposes. After analyzing the data with different window lengths (250, 500, 750, 1000, 1500 and 2000 samples), finite growth rates were found to decrease with anesthesia. Embedding after formal methods did NOT offer the best discrimination among different anesthesia levels. Discrimination was very effective even with very short time series (250 samples - 1s). These results seem to indicate that finite growth rates are able of detecting the changes on brain dynamics induced by anaesthetic agents.

Effects of propofol anesthesia on nonlinear properties of EEG: Time-lag and embedding dimension

Depth of anesthesia monitors frequently use linear features. Though some non-linear methods such as spectral entropy and bispectrum analysis are also used; the application of methods derived after Chaos Theory is still pending. This paper focuses on the analysis of the embedding parameters required for the reconstruction of the chaotic atractor associated to the EEG signal. After clinical research and ethical committee approval and signed informed consent, EEG data was collected from 6 patients scheduled for surgery under general anesthesia and 7 ones scheduled for ambulatory endoscopic procedures. One differential channel of EEG (+ in mastoids M1/M2, - in the middle-line of the forehead FPz/AFz and referenced to F7/F8) was amplified with a gain of 10000 (Biopac MP100-EEG100B), low-pass filtered at 300 Hz (2nd order Butterwoth) and acquired at 2.5 KHz. Data were digitally filtered (50 Hz comb filter and 120-taps linear phase FIR lowpass filter at 85 Hz) and decimated to 250 Hz prior to analysis under OpenTstool. Time-lag was evaluated through autocorrelation and auto-mutual information functions. False nearest neighbors, and Higuchi Fractal Dimension were used for the study of the embedding dimension. After analyzing the data with different window lengths (250, 500, 1000 and 2000 samples), time lag was found to decrease from the basal level with anesthesia levels. Embedding dimension after Cao’s method was not found to vary, due to the short length of the time series. Higuchi fractal dimension was able to show a slight dimensionality reduction with anesthesia. These results seem to indicate a decrease on the data-rate of the brain channel due to the reduction of the number of active neurons and the decrease of the neural conduction velocity induced by the anaesthetic agent used (propofol) at the GABAA receptors.