Mauricio M. Mora

Seismicity related to the glacier of Cotopaxi Volcano, Ecuador

[1] Significant seismic activity is generally recorded on volcanoes covered by an icecap. This work was carried out in order to quantify the role of the glaciers in the generation of seismicity for Cotopaxi volcano. We compared the seismic activity registered on the glacier and on the rock near the snout of the north flank glacier. We focused on the analysis of low frequency events (<5 Hz) similar to volcanic LP events when recorded on rock base. The particle motion analysis helps to estimate source locations, which are distributed in crevasses areas. High incident angles suggest a superficial origin. These events are interpreted as icequakes for which we propose as source mechanism a fluid-driven crack model triggered by ice cracking or hydraulic transients. The low quality factor values estimated are consistent with the resonance of an ice crack filled with water. This work shows that low frequency icequakes can be confusingly taken as volcanic LP events.

Study of seismic site effects using H/V spectral ratios at Arenal Volcano, Costa Rica

By using data obtained with a linear array at Arenal volcano, we show that the H/V spectral ratio method can be profitably applied to detect site effects on volcanoes. Similar results are obtained when calculating spectral ratios with different types of seismo-volcanic signals (tremor, ambient noise, explosion quakes, LP events). We compare the H/V ratios with theoretical S-wave transfer functions calculated using velocity models obtained from seismic refraction studies. There is a good agreement when the H/V ratios display sharp peaks, indicating a close relationship between the ratios and the transfer function of the shallow structure. Furthermore, the main peaks of the spectral ratios are consistent with local amplification of seismic waves observed at the corresponding frequencies.

The steeply subducting edge of the Cocos Ridge: Evidence from receiver functions beneath the northern Talamanca Range, south-central Costa Rica

The deep structure of the south-central Costa Rican subduction zone has not been studied in great detail so far because large parts of the area are virtually inaccessible. We present a receiver function study along a transect of broadband seismometers through the northern flank of the Cordillera de Talamanca (south Costa Rica). Below Moho depths, the receiver functions image a dipping positive conversion signal. This is interpreted as the subducting Cocos Plate slab, compatible with the conversions in the individual receiver functions. In finite difference modeling, a dipping signal such as the one imaged can only be reproduced by a steeply (80°) dipping structure present at least until a depth of about 70–100 km; below this depth, the length of the slab cannot be determined because of possible scattering effects. The proposed position of the slab agrees with previous results from local seismicity, local earthquake tomography, and active seismic studies, while extending the slab location to greater depths and steeper dip angle. Along the trench, no marked change is observed in the receiver functions, suggesting that the steeply dipping slab continues until the northern flank of the Cordillera de Talamanca, in the transition region between the incoming seamount segment and Cocos Ridge. Considering the time predicted for the establishment of shallow angle underthrusting after the onset of ridge collision, the southern Costa Rican subduction zone may at present be undergoing a reconfiguration of subduction style, where the transition to shallow underthrusting may be underway but still incomplete.

Imaging crustal structure in south central Costa Rica with receiver functions

An array of broadband seismometers transecting the Talamanca Range in southern Costa Rica was operated from 2005 until 2007. In combination with data from a short‐period network near Quepos in central Costa Rica, this data is analyzed by the receiver function method to image the crustal structure in south‐central Costa Rica. Two strong positive signals are seen in the migrated images, interpreted as the Moho (at around 35 km depth) and an intra‐crustal discontinuity (15 km depth). A relatively flat crustal and Moho interface underneath the north‐east flank of the Talamanca Range can be followed for a lateral distance of about 50 km parallel to the trench, with only slight changes in the overall geometry. Closer to the coast, the topography of the discontinuities shows several features, most notably a deeper Moho underneath the Talamanca Mountain Range and volcanic arc. Under the highest part of the mountain ranges, the Moho reaches a depth of about 50 km, which indicates that the mountain ranges are approximately isostatically compensated. Local deviations from the crustal thickness expected for isostatic equilibrium occur under the active volcanic arc and in south Costa Rica. In the transition region between the active volcanic arc and the Talamanca Range, both the Moho and intracrustal discontinuity appear distorted, possibly related to the southern edge of the active volcanic zone and deformation within the southern part of the Central Costa Rica Deformed Belt. Near the volcanoes Irazu and Turrialba, a shallow converter occurs, correlating with a low‐velocity, low‐density body seen in tomography and gravimetry. Applying a grid search for the crustal interface depth and vp/vs ratio cannot constrain vp/vs values well, but points to generally low values (<1.7) in the upper crust. This is consistent with quartz‐rich rocks forming the mountain range.

A brittle failure model for long‐period seismic events recorded at Turrialba Volcano, Costa Rica

A temporary seismic network, consisting of 23 broadband and six short-period stations, was installed in a dense network at Turrialba Volcano, Costa Rica, between 8 March and 4 May 2011. During this time 513 long-period (LP) events were observed. Due to their pulse-like waveforms, the hypothesis that the events are generated by a slow-failure mechanism, based on a recent new model by Bean et al. (2014), is tested. A significant number (107) of the LPs are jointly inverted for their source locations and mechanisms, using full-waveform moment tensor inversion. The locations are mostly shallow, with depths < 800 m below the active Southwest Crater. The results of the decompositions of the obtained moment tensor solutions show complex source mechanisms, composed of high proportions of isotropic and low, but seemingly significant, proportions of compensated linear vector dipole and double-couple components. It is demonstrated that this can be explained as mode I tensile fracturing with a strong shear component. The source mechanism is further investigated by exploring scaling laws within the data. The LPs recorded follow relationships very similar to those of conventional earthquakes, exhibiting frequency-magnitude and corner frequency versus magnitude relationships that can be explained by brittle failure. All of these observations indicate that a slow-failure source model can successfully describe the generation of short-duration LP events at Turrialba Volcano.

Eruptive activity at Turrialba volcano (Costa Rica): Inferences from 3He/4He in fumarole gases and chemistry of the products ejected during 2014 and 2015

A new period of eruptive activity started at Turrialba volcano, Costa Rica, in 2010 after almost 150 years of quiescence. This activity has been characterized by sporadic explosions whose frequency clearly increased since October 2014. This study aimed to identify the mechanisms that triggered the resumption of this eruptive activity and characterize the evolution of the phenomena over the past 2 years. We integrate 3He/4He data available on fumarole gases collected in the summit area of Turrialba between 1999 and 2011 with new measurements made on samples collected between September 2014 and February 2016. The results of a petrological investigation of the products that erupted between October 2014 and May 2015 are also presented. We infer that the resumption of eruptive activity in 2010 was triggered by a replenishment of the plumbing system of Turrialba by a new batch of magma. This is supported by the increase in 3He/4He values observed since 2005 at the crater fumaroles and by comparable high values in September 2014, just before the onset of the new eruptive phase. The presence of a number of fresh and juvenile glassy shards in the erupted products increased between October 2014 and May 2015, suggesting the involvement of new magma with a composition similar to that erupted in 1864–1866. We conclude that the increase in 3He/4He at the summit fumaroles since October 2015 represents strong evidence of a new phase of magma replenishment, which implies that the level of activity remains high at the volcano. This article is protected by copyright. All rights reserved.

Array analysis of the seismic wavefield of long‐period events and volcanic tremor at Arenal volcano, Costa Rica

We use wavefield decomposition methods (time-domain crosscorrelation and frequency-domain MUSIC) to analyze seismic data recorded by a dense, small-aperture array located 2 km West of Arenal volcano, Costa Rica, and operated during 2.5 days. The recorded wavefield is dominated by harmonic tremor and includes also spasmodic tremor and long-period (LP) events. We find that the initial stages of LP events are characterized by three different wave arrivals. These arrivals propagate with similar back-azimuths pointing to the volcano summit (~80◦N) and increasing apparent slownesses of 0.4, 1.1, and 1.7 s/km. Spasmodic tremors can not be regarded as coherent signals. On the contrary, harmonic tremors are highly coherent, characterized by the stability of the apparent slowness vector estimates. Apparent slownesses lay in the range 1-2 s/km. Back-azimuths point in the general direction of the volcano, but with a large variability (40-120◦N). Nevertheless, there are long-term variations and evidences of multiple simultaneous components in the harmonic tremor wavefield. These observations suggest that LP events and tremor are generated in a shallow source area near the volcano summit, although they do not share exactly the same source region or source processes. The tremor source is located in the shallowest part of the plumbing system, beneath the lava crust. This dynamic region is subject to complex fluctuations of the physical conditions. Degassing events at different locations of this region might generate variable seismic radiation patterns. The effects of topography and heterogeneous shallow structure of the volcano may amplify these variations and produce the wide directional span observed for volcanic tremor. On the other hand, the LP source seems to be more repeatable. LP events are likely triggered by fragmentation of the fluid flow in a slightly deeper portion of the volcanic conduits.

Time‐Dependent Spatial Amplitude Patterns of Harmonic Tremor at Arenal Volcano, Costa Rica: Seismic‐Wave Interferences?

Seismograms recorded at the receivers of a small‐aperture seismic array usually display very similar waveforms and amplitudes, as a consequence of their close proximity. During the analysis of the volcanic tremor wave field at Arenal volcano, Costa Rica, we detected significant differences in the amplitudes of harmonic tremor recorded at the stations of a small‐aperture (∼210  m) seismic array. The amplitude distributions are geometrically complex and characterized by strong gradients. They occur just for harmonic tremors; any other type of seismic event produces nearly uniform amplitudes across the array. This suggests some relation with harmonic frequency content. Moreover, the spatial amplitude patterns change with time. Some of these observations could be explained by an extreme combination of source, path, and site effects. But they also could be produced by interference of different components of the seismic wave field. We use numerical calculations to investigate the amplitude pattern generated by two interfering plane waves, and are able to reproduce the main features of the observed amplitude patterns. We propose mechanisms that might generate seismic wave fields with multiple components and conclude that interference can explain the complexity and variability of the harmonic tremor wave field at Arenal volcano. Online Material: Wave‐field animation.

El sismo de Capellades del 2016 y su secuencia sísmica: Manifestación de fallamiento de rumbo en el arco volcánico de Costa Rica

espanolEl 30 de noviembre del 2016 a las 18:25 (00:25 UTC del 1 de diciembre) ocurrio un sismo de Mw 5,5 a una profundidad de 2,7 km, 4 km al norte de Capellades de Alvarado, Costa Rica. Fue el evento principal de una secuencia con precursores y replicas, localizada a 5 km de los volcanes activos Irazu y Turrialba. Este sismo es el mas reciente de una lista de terremotos originados en las fallas que cortan la Cordillera Volcanica Central, la cual representa el limite norte del area mas poblada del pais. Usando principalmente los registros instrumentales de la Red Sismologica Nacional (RSN), en este trabajo se presenta un analisis sismologico de la secuencia y se determinan la ubicacion y las caracteristicas de la falla que origino esta sismicidad. Adicionalmente, se describe el entorno geologico-tectonico de su origen y sus efectos. La secuencia de sismos muestra un claro alineamiento de 8 km de longitud y rumbo nor-noroeste, entre los volcanes Irazu y Turrialba. La interpretacion conjunta de la relocalizacion de la secuencia, el tensor de momento del sismo principal y los mecanismos focales de 17 eventos permitio determinar que la falla de origen es casi vertical y de tipo de desplazamiento de rumbo dextral, lo cual es congruente con los sistemas de fallamiento activo de la zona. Esta falla no habia sido reconocida previamente y se ha denominado Liebres en este estudio. El sismo principal fue sentido en casi todo el pais, con una intensidad maxima de VI+. Este sismo ha sido el de mayor magnitud en el sector oriental de la Cordillera Volcanica Central desde el terremoto de Patillos de 1952 (Ms 5,9) y el primer sismo de Mw > 5,0 registrado por la RSN en el edifiio volcanico del Turrialba. A pesar de la cercania con ese volcan, que ha presentado erupciones periodicas desde el 2010, no se observaron efectos eruptivos inmediatos. EnglishOn 30 November 2016 at 18:25 (1 st December at 00:25, UTC time) a Mw 5 5 earthquake occurred at 2.7 km depth, 4 km north of the town Capellades de Alvarado, Costa Rica. It was the main shock of an earthquake sequence including foreshocks and aftershocks, located 5 km from the active Irazu and Turrialba volcanoes. This is the most recent of a series of damaging earthquakes originated in the faults crossing the Central Volcanic Range, which constitutes the northern boundary of the most populated area of the country. Using mainly the seismic records from the National Seismological Network (RSN), we present in this study a seismological analysis of the earthquake sequence and the location and characteristics of the fault that originated this seismicity. Additionally, we describe the geotectonic context of the fault and the Capellades earthquake effects. The earthquake sequence shows a clear 8-km long alignment striking nor-northwest between Irazu and Turrialba volcanoes. The joint interpretation of the earthquake relocation, the main-shock moment tensor solution, and the focal mechanisms of 17 events allows for determining the source in a nearly vertical strike-slip fault, in agreement with regional active fault systems. This structure had not been recognized previously and has been named Liebres Fault in this study. The main shock was felt in most of the country, with a maximum intensity of VI+. This earthquake has been the largest in the eastern part of the Central Volcanic Range since the 1952 Patillos earthquake (Ms 5.9) and the fist Mw > 5.0 earthquake recorded by the RSN in the Turrialba volcano edifie. Despite the proximity to this active volcano, which has been erupting periodically since 2010, there were no immediate eruptive effects.