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Item Restricted A mixed seismic–aseismic stress release episode in the Andean subduction zone(Nature Research, 2016) Villegas Lanza, Juan Carlos; Nocquet, J. M.; Rolandone, F.; Vallée, M.; Tavera, Hernando; Bondoux, Francis; Tran, T.; Martin, X.; Chlieh, MohamedIn subduction zones, stress is released by earthquakes and transient aseismic slip. The latter falls into two categories: slow slip and afterslip. Slow-slip events emerge spontaneously during the interseismic phase, and show a progressive acceleration of slip with a negligible contribution of synchronous tremors or microseismicity to the energy, or moment release. In contrast, afterslip occurs immediately after large and moderate earthquakes, decelerates over time, and releases between 20 and 400% of the moment released by the preceding earthquake. Here we use seismic and GPS data to identify transient aseismic slip that does not fit into either of these categories. We document a seismic–aseismic slip sequence which occurred at shallow depths along a weakly coupled part of the Andean subduction zone19 in northern Peru and lasted seven months. The sequence generated several moderate earthquakes that together account for about 25% of the total moment released during the full sequence, equivalent to magnitude 6.7. Transient slip immediately followed two of the earthquakes, with slip slowing at a logarithmic rate. Considered separately, the moment released by transient slip following the second earthquake was more than 1,000% of the moment released during the earthquake itself, a value incompatible with classical models of afterslip. Synchronous seismic swarms and aseismic slip may therefore define a stress-release process that is distinct from slow-slip events and afterslip.Item Restricted A new South American network to study the atmospheric electric field and its variations related to geophysical phenomena(Elsevier, 2014-12) Tacza, J.; Raulin, J. P.; Macotela, E.; Norabuena Ortiz, Edmundo; Fernández, G.; Correia, E.; Rycroft, M. J.; Harrison, R. G.In this paper we present the capability of a new network of field mill sensors to monitor the atmospheric electric field at various locations in South America; we also show some early results. The main objective of the new network is to obtain the characteristic Universal Time diurnal curve of the atmospheric electric field in fair weather, known as the Carnegie curve. The Carnegie curve is closely related to the current sources flowing in the Global Atmospheric Electric Circuit so that another goal is the study of this relationship on various time scales (transient/monthly/seasonal/annual). Also, by operating this new network, we may also study departures of the Carnegie curve from its long term average value related to various solar, geophysical and atmospheric phenomena such as the solar cycle, solar flares and energetic charged particles, galactic cosmic rays, seismic activity and specific meteorological events. We then expect to have a better understanding of the influence of these phenomena on the Global Atmospheric Electric Circuit and its time-varying behavior.Item Restricted A recent deep earthquake doublet in light of long-term evolution of Nazca subduction(Nature Research, 2017-03-31) Zahradnik, Jindrich; Čížková, Hana; Bina, Craig R.; Sokos, Efthimios N.; Jánský, Jirí; Tavera, Hernando; Carvalho, JoãoEarthquake faulting at ~600 km depth remains puzzling. Here we present a new kinematic interpretation of two Mw7.6 earthquakes of November 24, 2015. In contrast to teleseismic analysis of this doublet, we use regional seismic data providing robust two-point source models, further validated by regional back-projection and rupture-stop analysis. The doublet represents segmented rupture of a ∼30-year gap in a narrow, deep fault zone, fully consistent with the stress field derived from neighbouring 1976-2015 earthquakes. Seismic observations are interpreted using a geodynamic model of regional subduction, incorporating realistic rheology and major phase transitions, yielding a model slab that is nearly vertical in the deep-earthquake zone but stagnant below 660 km, consistent with tomographic imaging. Geodynamically modelled stresses match the seismically inferred stress field, where the steeply down-dip orientation of compressive stress axes at ∼600 km arises from combined viscous and buoyant forces resisting slab penetration into the lower mantle and deformation associated with slab buckling and stagnation. Observed fault-rupture geometry, demonstrated likelihood of seismic triggering, and high model temperatures in young subducted lithosphere, together favour nanometric crystallisation (and associated grain-boundary sliding) attending high-pressure dehydration as a likely seismogenic mechanism, unless a segment of much older lithosphere is present at depth.Item Restricted A report on the 24 August 2011 Mw 7.0 Contamana, Peru, intermediate-depth earthquake(Seismological Society of America, 2012-11-01) Tavera, HernandoThe seismic activity in Peru has its origin in the convergence process between the Nazca and the South American plates. Such convergence takes place at an average velocity on the order of 7–8 cm/yr (DeMets et al., 1980; Norabuena et al., 1999). This process is responsible for the largest damaging shallow interplate underthrusting earthquakes, the intraplate plate events in the downgoing Nazca slab, and the shallow intraplate crustal events in the overriding South American plate. The interplate events, representing slip between the plates, are the largest earthquakes and can cause considerable damage along the coast.Item Restricted A strong-motion database from the Peru–Chile subduction zone(Springer, 2011) Arango, Maria C.; Strasser, Fleur O.; Bommer, Julian J.; Boroschek, Ruben; Comte, Diana; Tavera, HernandoEarthquake hazard along the Peru – Chile subduction zone is amongst the highest in the world. The development of a database of subduction-zone strong-motion recordings is, therefore, of great importance for ground-motion prediction in this region. Accelerograms recorded by the different networks operators in Peru and Chile have been compiled and processed in a uniform manner, and information on the source parameters of the causative earthquakes, fault-plane geometries and local site conditions at the recording stations has been collected and reviewed to obtain high-quality metadata. The compiled database consists of 98 triaxial ground-motion recordings from 15 subduction-type events with moment magnitudes ranging from 6.3 to 8.4, recorded at 59 different sites in Peru and Chile, between 1966 and 2007. While the database presented in this study is not sufficient for the derivation of a new predictive equation for ground motions from subduction events in the Peru–Chile region, it significantly expands the global database of strongmotion data and associated metadata that can be used in the derivation of predictive equations for subduction environments. Additionally, the compiled database will allow the assessment of existing predictive models for subduction-type events in terms of their suitability for the Peru– Chile region, which directly influences seismic hazard assessment in this region.Item Restricted Active tectonics of Peru: heterogeneous interseismic coupling along the Nazca megathrust, rigid motion of the Peruvian Sliver, and Subandean shortening accommodation(American Geophysical Union, 2016-10) Villegas Lanza, Juan Carlos; Chlieh, Mohamed; Cavalié, O.; Tavera, Hernando; Baby, P.; Chire Chira, J.; Nocquet, J.‐M.Over 100 GPS sites measured in 2008–2013 in Peru provide new insights into the present‐day crustal deformation of the 2200 km long Peruvian margin. This margin is squeezed between the eastward subduction of the oceanic Nazca Plate at the South America trench axis and the westward continental subduction of the South American Plate beneath the Eastern Cordillera and Subandean orogenic wedge. Continental active faults and GPS data reveal the rigid motion of a Peruvian Forearc Sliver that extends from the oceanic trench axis to the Western‐Eastern Cordilleras boundary and moves southeastward at 4–5 mm/yr relative to a stable South America reference frame. GPS data indicate that the Subandean shortening increases southward by 2 to 4 mm/yr. In a Peruvian Sliver reference frame, the residual GPS data indicate that the interseismic coupling along the Nazca megathrust is highly heterogeneous. Coupling in northern Peru is shallow and coincides with the site of previous moderate‐sized and shallow tsunami‐earthquakes. Deep coupling occurs in central and southern Peru, where repeated large and great megathrust earthquakes have occurred. The strong correlation between highly coupled areas and large ruptures suggests that seismic asperities are persistent features of the megathrust. Creeping segments appear at the extremities of great ruptures and where oceanic fracture zones and ridges enter the subduction zone, suggesting that these subducting structures play a major role in the seismic segmentation of the Peruvian margin. In central Peru, we estimate a recurrence time of 305 ± 40 years to reproduce the great 1746 Mw~8.8 Lima‐Callao earthquake.Item Open Access Actividad sismovolcánica asociada a la intranquilidad del volcán Sabancaya observada entre febrero y julio de 2013(Sociedad Geológica del Perú, 2014) Puma Sacsi, Nino; Torres Aguilar, José Luis; Jay, Jennifer; Delgado, Francisco; Pritchard, Matthew; Macedo Sánchez, Orlando EfraínEl volcán Sabancaya está ubicado a 80 km al NNO de la ciudad de Arequipa (15°47’ S; 71°72’W; 5976 msnm), en el sur del Perú. Es un estrato-‐volcán andesítico de edad holocénica reciente y forma parte del complejo volcánico conformado por los volcanes Ampato, Sabancaya y Hualca–Hualca. Según los registros históricos, el Sabancaya erupcionó en 1750 y 1784-‐1785; entre 1990 y 1998 presentó una última erupción que alcanzó un VEI 2. Luego de 15 años de tranquilidad, este volcán está mostrando nuevos signos de actividad desde el 22/02/2013, con alta sismicidad y emisiones fumarólicas de colores blanquecinos y azulinos al nivel del cráter, muchas veces intensas y densas, que se elevan a alturas de hasta 3 km. Atendiendo a esta situación, el OVA-IGP inmediatamente instaló una red de 6 estaciones portátiles (5 de GURALP-‐6TD, banda ancha, y 1 Lennartz 3DLite, periodo corto con digitalizador CMG-‐DM24). Adicionalmente, a partir del 24 de marzo de 2013, entró en operación la red telemétrica Sabancaya (RESSAB), la cual consta de 3 estaciones: SABA, CAJA, y PATA, equipadas con sensores de banda ancha GURALP 40T y digitalizadores Reftek130.Item Restricted Advances in scientific understanding of the Central Volcanic Zone of the Andes: a review of contributing factors(Springer, 2022-02-12) Aguilera, Felipe; Apaza, Fredy; Del Carpio Calienes, José Alberto; Grosse, Pablo; Jiménez, Néstor; Ureta, Gabriel; Inostroza, Manuel; Báez, Walter; Layana, Susana; Gonzalez, Cristóbal; Rivera, Marco; Ortega, Mayra; Gonzalez, Rodrigo; Iriarte, RodrigoThe Central Volcanic Zone of the Andes (CVZA) has been the focus of volcanological research for decades, becoming a very important site to understand a number of volcanic processes. Despite most of the research in the CVZA being carried out by foreign scientists, the last two decades have seen a significant increase in contributions by regional researchers. This surge has been facilitated by the creation of new volcanic observatories, improvement of the monitoring networks, creation of postgraduate programs where new local volcanologists are trained, creation of specialized research nuclei or groups, and increasing investment in research. This article presents a review of the evolution of the contributions of the regional volcanological community to the knowledge of the CVZA in the last 20 years (2000–2019), both from research and monitoring institutions in Peru, Bolivia, Argentina, and Chile. Based on updates made by the regional groups, a new list of active/potentially active volcanoes of the CVZA is presented, as is a complete database for article published on the CVZA. We find that a significant motivator has been regional volcanic unrest that has triggered new investment. Perú is the country with the highest investment in monitoring and research and is the best instrumented, Argentina is the country with the highest number of local participation in published papers in the domain of volcanology and magmatic systems, and Chilean volcanoes are the focus of the highest number of articles published. The current situation and general projections for the next decade (2020–2030) are also presented for each country, where we believe that the over the next 10 years, will be increased the monitoring and research capabilities, improved the scientific knowledge with more participation of regional institutions, and strengthen the collaboration and integrated work between CVZA countries, especially in border volcanoes.Item Open Access Ambient noise tomography across the Central Andes(Oxford University Press, 2013-09) Ward, Kevin M.; Porter, Ryan C.; Zandt, George; Beck, Susan L.; Wagner, Lara S.; Minaya, Estela; Tavera, HernandoThe Central Andes of southern Peru, Bolivia, Argentina and Chile (between 12°S and 42°S) comprise the largest orogenic plateau in the world associated with abundant arc volcanism, the Central Andean Plateau, as well as multiple segments of flat-slab subduction making this part of the Earth a unique place to study various aspects of active plate tectonics. The goal of this continental-scale ambient noise tomography study is to incorporate broad-band seismic data from 20 seismic networks deployed incrementally in the Central Andes from 1994 May to 2012 August, to image the vertically polarized shear wave velocity (Vsv) structure of the South American Cordillera. Using dispersion measurements calculated from the cross-correlation of 330 broad-band seismic stations, we construct Rayleigh wave phase velocity maps in the period range of 8–40 s and invert these for the shear wave velocity (Vsv) structure of the Andean crust. We provide a dispersion misfit map as well as uncertainty envelopes for our Vsv model and observe striking first-order correlations with our shallow results (∼5 km) and the morphotectonic provinces as well as subtler geological features indicating our results are robust. Our results reveal for the first time the full extent of the mid-crustal Andean low-velocity zone that we tentatively interpret as the signature of a very large volume Neogene batholith. This study demonstrates the efficacy of integrating seismic data from numerous regional broad-band seismic networks to approximate the high-resolution coverage previously only available though larger networks such as the EarthScope USArray Transportable Array in the United States.Item Restricted Ambient noise tomography of Misti volcano, Peru(Elsevier, 2022) Cabrera-Pérez, Iván; Centeno Quico, Riky; Soubestre, Jean; D'Auria, Luca; Rivera, Marco; Machacca, RogerTo better understand the recent internal structure of Misti volcano, we determined a 3D S-wave velocity model applying Ambient Noise Tomography (ANT). We used data from 23 broadband and short-period seismic stations temporarily installed at Misti volcano between March and December 2011. This dataset allowed us to obtain empirical Green's functions by cross-correlating seismic ambient noise signals. Then, we retrieved 104 dispersion curves using the frequency-time analysis (FTAN) and, through a non-linear multiscale inversion, we obtained nine 2-D Rayleigh waves group velocity maps for periods in the range 0.7 s - 2 s. Finally, we carried out the depth inversion through a Bayesian transdimensional inversion to obtain a 3-D S-wave velocity model down to 3 km depth. Our study highlights five relevant seismic velocity anomalies. We observed the presence of three high-velocity zones located in the west-northwest, southwest and southeast parts of the crater, that could be related to intrusive bodies possibly associated with the formation of Misti volcano. We also observed two low-velocity anomalies in the volcano's western and central parts, which coincide with previous studies' findings and are related to fractured and weakened materials associated with the external caldera collapse and recent eruption episodes.Item Open Access An 8 month slow slip event triggers progressive nucleation of the 2014 Chile megathrust(American Geophysical Union, 2017-05-16) Socquet, Anne; Piña Valdes, Jesús; Jara, Jorge; Cotton, Fabrice; Walpersdorf, Andrea; Cotte, Nathalie; Specht, Sebastian; Ortega‐Culaciati, Francisco; Carrizo, Daniel; Norabuena Ortiz, EdmundoThe mechanisms leading to large earthquakes are poorly understood and documented. Here we characterize the long‐term precursory phase of the 1 April 2014 Mw8.1 North Chile megathrust. We show that a group of coastal GPS stations accelerated westward 8 months before the main shock, corresponding to a Mw6.5 slow slip event on the subduction interface, 80% of which was aseismic. Concurrent interface foreshocks underwent a diminution of their radiation at high frequency, as shown by the temporal evolution of Fourier spectra and residuals with respect to ground motions predicted by recent subduction models. Such ground motions change suggests that in response to the slow sliding of the subduction interface, seismic ruptures are progressively becoming smoother and/or slower. The gradual propagation of seismic ruptures beyond seismic asperities into surrounding metastable areas could explain these observations and might be the precursory mechanism eventually leading to the main shock.Item Restricted An evaluation of the applicability of current ground-motion models to the South and Central American subduction zones(Seismological Society of America, 2012-02) Arango, M. C.; Strasser, F. O.; Bommer, J. J.; Cepeda, J. M.; Boroschek, R.; Hernández, D. A.; Tavera, HernandoThe applicability of existing ground‐motion prediction equations (GMPEs) for subduction‐zone earthquakes is an important issue to address in the assessment of the seismic hazard affecting the Peru–Chile and Central American regions. Few predictive equations exist that are derived from local data, and these do not generally meet the quality criteria required for use in modern seismic hazard analyses. This paper investigates the applicability of a set of global and regional subduction ground‐motion models to the Peru–Chile and Central American subduction zones, distinguishing between interface and intraslab events, in light of recently compiled ground‐motion data from these regions. Strong‐motion recordings and associated metadata compiled by Arango, Strasser, Bommer, Boroschek, et al. (2011) and Arango, Strasser, Bommer, Hernandez, et al. (2011) have been used to assess the performance of the candidate equations following the maximum‐likelihood approach of Scherbaum et al. (2004) and its extension to normalized intraevent and interevent residual distributions developed by Stafford et al. (2008). The results of this study are discussed in terms of the transportability of GMPEs for subduction‐zone environments from one region to another, with a view to providing guidance for developing ground‐motion logic trees for seismic hazard analysis in these regions.Item Open Access Análisis de la actividad sísmica en la región del volcán Sabancaya (Arequipa)(Sociedad Geológica del Perú, 2001) Antayhua Vera, Yanet Teresa; Tavera, Hernando; Bernal Esquia, Yesenia IsabelEl volcán Sabancaya forma parte del gran Complejo Volcánico Ampato-Sabancaya-Hualca Hualca y se ubica a 30 km al SW de la localidad de Chivay ya 80 km de la ciudad de Arequipa. Este volcán entra en reactivación en 1986 después de 200 años con intensa actividad fumarólica acompañado de importante actividad sísmica. En abril de 1990, el Instituto Geofísico del Perú instala seis estaciones sísmicas temporales alrededor del volcán Sabancaya que funcionaron de manera irregular hasta 1993, fecha en la cual se instala la Red Sísmica Telemétrica (RSTS) compuesta por 3 estaciones de período corto y que estuvo operativa hasta finales de 1995. La RSTS registro 212 sismos de tipo tectónico, los mismos que se distribuyen sobre fallas y lineamientos presentes en el área de Pampa Sepina ubicada a 5km en dirección NE del volcán Sabancaya. Los sismos presentan magnitudes ML menores a 3.0 y profundidades máximas de 24 km. La información geológica, las observaciones visuales del proceso eruptivo del volcán y las características de la sismicidad, sugieren que la cámara magmática del volcán Sabancaya habría sido de menor volumen y que la deformación superficial presente en Pampa Sepina asociada a la importante actividad sísmica registrada entre 1993y 1995,tendría su origen en los esfuerzos resultantes de la presión que ejerció el magma sobre las capas superficiales durante su propagación antes de llegar a la cámara magmática. Durante el período de funcionamiento de la RSTS, no se registro sismos tectónicos con posible origen en el interior del cono volcánico.Item Open Access Análisis de los niveles de ruido sísmico en las estaciones de banda ancha de la Red Sísmica Nacional - Perú(Sociedad Geológica del Perú, 2006) Cutipa Vargas, Graciano Elard; Tavera, Hernando; Bernal Esquia, Yesenia IsabelEl nivel de ruido presente en las estaciones sísmicas de banda ancha de la Red Sísmica Nacional (RSN) a cargo del Instituto Geofísico del Perú (IGP) ha sido estudiado para cuantificar la calidad de la señal sísmica que se registra en el rango de frecuencias de 0.01 a 10 Hz. Para tal fin, se aplica el método de Densidad de Potencia Espectral (DPE) y se utiliza ventanas traslapadas de 7.45 minutos de linngitud de registro. Los resultados muestran que en el rango de frecuencias de 1 a 10 Hz, las estaciones sísmicas de CUS, YLA y CHA por encontrarse en roca, alejadas de fuentes naturales o culturales (actividades del hombre, carreteras, ciudades, etc.) presentan un bajo nivel de ruido sísmico. Contrariamente, las estaciones CTH, PUC y CAJ al estar operando sobre material aluvial y tufo volcánico, y cercanas a fuentes de ruido cultural, presentan altos niveles de ruido con diferencias del orden de 25dB. Para frecuencias intermedias (0.1 a 1 Hz.), los niveles de ruido se encuentran próximos al mínimo establecido por Peterson (1993); mientras que para las bajas frecuencias (menores a 0.1 Hz) se observan fuertes incrementos en el nivel de ruido debido a las fluctuaciones locales de la presión atmosférica con diferencias de hasta 15dB.Item Open Access Análisis de los procesos de ruptura de los sismos ocurridos en 1990 y 1991 en el Valle del Alto Mayo (Moyobamba-Perú)(Sociedad Geológica del Perú, 2001) Tavera, Hernando; Buforn, Elisa; Bernal Esquia, Yesenia Isabel; Antayhua Vera, Yanet TeresaEl proceso de ruptura de los sismos ocurridos el 30 de mayo de 1990 y 4, 5 de abril de 1991 en el Valle del Alto Mayo (VAM) es analizado, a fin de conocer las características de la importante deformación superficial que se produce en esta zona. El VAM se ubica en la Zona Subandina de la región Norte de Perú y es la fuente sismogénica continental de mayor importancia por su alto índice de sismicidad y deformación. Los parámetros focales de los sismos son obtenidos a partir de la polaridad de la onda P y modelado de ondas de volumen a distancias telesismicas (registros sísmicos de banda ancha). Los resultados muestran mecanismos focales de tipo inverso con planos nodales, en promedio, orientados en dirección paralela a la Cordillera Andina y ejes de presión (P) orientados en dirección NE-SW y NW-SE que sugieren la presencia de procesos complejos de deformación asociados probablemente a la curvatura de la Cordillera Andina a la latitud de 60S (deflexion de Cajamarca) y a la subsidencia del Escudo Brasileño. Los registros de estos sismos son complejos y su modelado ha permitido definir la presencia de funciones temporales para la fuente sísmica (STF) que se caracterizan por presentar una serie de dos y tres pulsos asociados a igual número de rupturas aleatorias en periodos de tiempo menores a 10 segundos. El sismo del 5 de abril, fue generado por dos rupturas importantes sobre el mismo plano de falla, pero con diferente ángulo de deslizamiento. Los focos sísmicos se distribuyen, en profundidad, sobre una línea con pendiente de 35° hacia el Oeste y que tiende a ser horizontal si la profundidad de los sismos aumenta. Esta característica permite configurar la geometría de una falla de tipo Iístrica, propuesto por muchos autores para explicar el estilo de deformación en la Zona Subandina. Finalmente, se presenta un modelo sismotectónico que explicaría el origen de los sismos que se producen en esta zona debido a la subcidencia del Escudo Brasileño bajo la Cordillera Oriental.Item Open Access Análisis de los sismos superficiales de Chacapampa-Huasicancha (Junín) de julio y agosto de 2003 (4,7 y 4,2 MW): región central de Perú(Sociedad Geológica del Perú, 2005) Tavera, Hernando; Vilcapoma, Luis; Fernández, Efraín; Antayhua Vera, Yanet Teresa; Salas, HenryEl 23 de Julio y 8 de Agosto de 2003, ocurren en la Alta Cordillera de la región central de Perú dos sismos de foco superficial y magnitudes Mw de 4.7 y 4.2 que produjeron intensidades máximas de V y IV (MM) en las localidades de Chacapampa, Huasicancha y Carhuacallanga ubicadas a escasos 8 km de los epicentros. Estos sismos ocurren en una zona en donde la actividad sísmica es muy baja y por lo tanto, no existen suficientes antecedentes del estado actual de la deformación superficial. El uso de información sísmica local proveniente de la RSN-IGP, ha permitido determinar con precisión los parámetros epicentrales de estos sismos y ajustar la profundidad de sus focos en 11 y 1 7 km. Los mecanismos focales obtenidos a partir de la polaridad de la onda P, indican fallas inversas con planos nodales orientados en dirección NNOSSE y eje de presión (P) en dirección ENE-OSO. A partir del espectro de amplitud del desplazamiento de la onda S se obtiene para el sismo del 23 de Julio un momento sísmico de 1.62x10²³ dinas-cm y para el sismo del 8 de Agosto de 2.25x10²² dinas-cm. Los radios de fractura oscilan entre 1.6 y 1.7 km. Ambos sismos sugieren el desarrollo de procesos de deformación local por compresión, cerca de una falla indiferenciada de 4 km de longitud ubicada al Este de sus epicentros.Item Open Access Análisis del mecanismo del sismo de foco profundo del 20 de Junio de 2003 (Límite Perú-Brasil)(Sociedad Geológica del Perú, 2003) Tavera, Hernando; Manrique, María; Salas, Henry; Fernández, EfraínEn Sudamérica, los sismos de foco profundo se concentran cerca de los límites entre Chile y Argentina, y Perú con Bolivia, Brasil y Colombia. La frecuencia de estos sismos es mayor en el límite Perú-Brasil y a la fecha el sismo de mayor magnitud (Mw=8.3) se ha producido en el límite Perú-Bolivia. El sismo del 20 de junio de 2003 ocurrió cerca del límite Perú-Brasil (Mw=6.8-7.0) y generó intensidades del orden de IV (MM) en la ciudad de Pucallpa ubicada a una distancia epicentral de 320 Km en dirección Oeste. La distribución espacial de los sismos con foco profundo ocurridos entre 1995 y 2003 con magnitudes Mw6.0, sugiere una migración de los sismos de Sur a Norte y la presencia de hasta 4 zonas que aun no han experimentado ruptura. Las formas de onda y el modelado de los registros de onda P utilizando estaciones de la red IRIS, indican la ocurrencia de hasta tres eventos separados del primero por 7 y 10 segundos, además de sugerir la propagación de la ruptura en dirección SE. El mecanismo de foco corresponde a esfuerzos extensionales que se desarrollan en dirección paralela a la convergencia de placas y con una componente compresiva casi vertical. El momento sísmico escalar es del orden de 2.2x10¹⁹ Nm, equivalente a una magnitud de Mw=6.8.Item Open Access Análisis y evaluación del sismo de Calacoa (Omate-Moquegua) del 6 de mayo de 1999 (Mw= 4.0)(Sociedad Geológica del Perú, 2001) Aguilar, Victor; Tavera, Hernando; Bernal Esquia, Yesenia Isabel; Palza, Héctor; Kosaka, RobertoEn este estudio se analiza y evalúa los parámetros hipocentrales del 'Sismo de Calacoa' ocurrido el 6 de mayo de 1999 (Mw=4.0), sus características y los efectos del mismo. La información a utilizarse proviene de una red sísmica temporal compuesta por tres estaciones portátiles del Instituto Geofísico de la UNSA (IGUNSA) y cuatro digitales de la Red Símica Nacional (RSN) a cargo del Instituto Geofísico del Perú (IGP). El sismo de Calacoa ha sido localizado a 7 Km al NE de la localidad del mismo nombre y sobre la proyección en esa dirección, de la falla de Calacoa. El foco del sismo presenta una profundidad de 6.5 Km y una magnitud de 4.0 Mw. La intensidad máxima de IV-V ha sido evaluada en las localidades de Calacoa, Cuchumbaya, Bellavista y Quebaya. A partir del análisis espectral se ha obtenido un momento sísmico escalar de 2x10²² dina-cm y un radio de fractura de 1000 metros. La distribución espacial y en profundidad del sismo de Calacoa y de algunas réplicas, sugiere un área de ruptura de 8x13 Km; sin embargo, no se ha observado en superficie ninguna traza de falla. El sismo de Calacoa y sus réplicas, tuvieron su origen en los procesos de deformación superficial del tipo extensivo, los mismo que frecuentemente se producen en las zonas altas de la Cordillera Andina.Item Restricted Analysis of dynamics of vulcanian activity of Ubinas volcano, using multicomponent seismic antennas(Elsevier, 2014-01-15) Inza Callupe, Lamberto Adolfo; Métaxian, J. P.; Mars, J. I.; Bean, C. J.; O'Brien, G. S.; Macedo Sánchez, Orlando Efraín; Zandomeneghi, D.A series of 16 vulcanian explosions occurred at Ubinas volcano between May 24 and June 14, 2009. The intervals between explosions were from 2.1 h to more than 6 days (mean interval, 33 h). Considering only the first nine explosions, the average time interval was 7.8 h. Most of the explosions occurred after a short time interval (< 8 h) and had low energy, which suggests that the refilling time was not sufficient for large accumulation of gas. A tremor episode followed 75% of the explosions, which coincided with pulses of ash emission. The durations of the tremors following the explosions were longer for the two highest energy explosions. To better understand the physical processes associated with these eruptive events, we localized the sources of explosions using two seismic antennas that were composed of three-component 10 and 12 sensors. We used the high-resolution MUSIC-3C algorithm to estimate the slowness vector for the first waves that composed the explosion signals recorded by the two antennas assuming propagation in a homogeneous medium. The initial part of the explosions was dominated by two frequencies, at 1.1 Hz and 1.5 Hz, for which we identified two separated sources located at 4810 m and 3890 m +/− 390 altitude, respectively. The position of these two sources was the same for the full 16 explosions. This implies the reproduction of similar mechanisms in the conduit. Based on the eruptive mechanisms proposed for other volcanoes of the same type, we interpret the position of these two sources as the limits of the conduit portion that was involved in the fragmentation process. Seismic data and ground deformation recorded simultaneously less than 2 km from the crater showed a decompression movement 2 s prior to each explosion. This movement can be interpreted as gas leakage at the level of the cap before its destruction. The pressure drop generated in the conduit could be the cause of the fragmentation process that propagated deeper. Based on these observations, we interpret the position of the highest source as the part of the conduit under the cap, and the deeper source as the limit of the fragmentation zone.Item Restricted Angola seismicity(Springer Verlag, 2018-05-30) Pereira Neto, Francisco António; Sand França, George; Condori Quispe, Cristobal; Sant’Anna Marotta, Giuliano; Chimpliganond, Cristiano NaibertThis work describes the development of the Angolan earthquake catalog and seismicity distribution in the Southwestern African Plate, in Angola. This region is one of the least seismically active, even for stable continental regions (SCRs) in the world. The maximum known earthquake had a magnitude of 6.0 Ms, while events with magnitudes of 4.5 have return period of about 10 years. Events with magnitude 5 and above occur with return period of about 20 years. Five seismic zones can be confirmed in Angola, within and along craton edges and in the sedimentary basins including offshore. Overall, the exposed cratonic regions tend to have more earthquakes compared to other regions such as sedimentary basins. Earthquakes tend to occur in Archaic rocks, especially inside preexisting weakness zones and in tectonic-magmatic reactivation zones of Mesozoic and Meso-Cenozoic, associated with the installation of a wide variety of intrusive rocks, strongly marked by intense tectonism. This fact can be explained by the models of preexisting weakness zones and stress concentration near intersecting structures. The Angolan passive margin is also a new region where seismic activity occurs. Although clear differences are found between different areas along the passive margin, in the middle near Porto Amboim city, seismic activity is more frequent compared with northwestern and southwestern regions.