Felt earthquakes in Peru during 2011

Rapid subduction of the Nazca plate beneath South America (of around ~6cm/yr, Kendrick, et al., (2003)),  causes strong seismic coupling along the entire subduction interface plate, giving raise to the continuous occurrence of interplate and intraplate earthquakes of different seismic moment release and located at diverse depths. On a yearly basis the seismicity rate in Peru vary from hundreds up to thousands of  earthquakes, but from the the total, only a certain number of events are felt by the population. In Peru, the "Instituto Geofísico del Peru" (IGP) is the institution in charge of the monitoring and surveillance of the seismic activity in the national territory, and whenever a seismic event is felt by the population, the information of the source parameters as well as the intensity reports are given as fast as possible and provided to the population in order be informed.


During 2011 the seismological service of IGP reported the occurrence of 138 felt earthquakes in the peruvian territory. From this dataset the largest magnitude event ML=7.0 occurred the August 24th at 17:46 (GMT) in the Peruvian Amazon, more precisely at 44 km SE from the Pucallpa city with a depth=149Km (intermediate depth) and called by IGP "The Contamana Earthquake". The second largest magnitude event ML=6.7occurred the October 28th at 18:54 (GMT) causing severe shaking on the western central coast, the hypocenter was located 63km south of Ica city with a depth=35 km, reported intensities reached values up to VI in the modified Mercalli scale, despite its moderate magnitude, this earthquakes seems to have filled the small seismic gap between the Nasca 1996 (M7.6) and the Pisco 2007 (M8.0) earthquakes, this hypothesis was also suggested in the IGP report for this earthquake. The magnitudes for the other events are ranging between 3.9 up to 6.6 ML, and all them have caused from small to moderate intensities in the cities near to the epicenter. The Figure shows the location of all the earthquakes occurred during 2011, red circles correspond to superficial/shallow earthquakes (depths between 0-60km) and green circles to intermediate events (depths between 61-300km). The size of each circle describes the magnitude assigned to each event according to the legend.  

The map shows that the superficial events are mostly located between the coast-line and the Peru-Chile trench, although in southern and northern Peru the seismicity distribution is quite scarce compared to central Peru, this suggests that the interaction between these two plates in central Peru during this year was important,  moreover in less proportion, some superficial events are also located inland over the andean cordillera and eastward in the sub-andean region, suggesting the interaction of the active quaternary faults  (note that colors define the topography). Intermediate events are mainly distributed inland showing a sparse distribution in northern Peru but most concurrent in Central and Southern Peru, where the geometry of the plate changes from sub-horizontal to normal. Note that in southern Peru the shallow seismicity is not important. In this region a large earthquake Mw=8.4 occurred in Jun. 2001 releasing part of the stress accumulated since the greatest 1868 southern Peru Earthquake. If someone wants to compare with the earthquakes reported for 2010 follow this link.

posted by JCVillegas and SYauri

Mito? Pueden los animales "predecir" o "pre-sentir" los terremotos?

Un reciente estudio, indica que algunos animales pueden sentir cambios químicos en el agua subterránea antes de que se produzca la ruptura sísmica.

La reciente publicación de Grant and Freund, et al., 2011 (Journal of Env. Res. and Public Health) da cuenta que en el 2009, días antes de la ocurrencia del sismo (6.3 Mw) de L'Aquila-Italia del mismo año, una colonia de sapos abandonó el estanque donde habitaban. Ellos sugieren que este tipo de comportamiento en los animales debería ser incorporado en lo que se denomina "Pronostico Sísmico" o "Pronostico de Terremotos"

En esta investigación se describe el mecanismo por el cual las rocas al estar sometidas a un alto nivel de esfuerzos/presión en el interior de la tierra (corteza terrestre), liberan partículas cargadas que reaccionan con el agua subterránea. El incremento en la emisión y/o liberación de partículas y posterior reacción química se daría dias antes de que se produzca la ruptura de las rocas (es decir el sismo). Los autores indican que los animales que viven en o cerca de ambientes con agua subterránea son altamente sensibles a cualquier cambio químico que se produzca en este medio, por lo tanto, el comportamiento de abandono/escape de los animales de este tipo de entornos marcaria una probable señal de cercania a la liberación abrupta de la energia sismica.

Pero este no es el unico caso, ya en anteriores oportunidades se observó que el comportamiento de algunos animales (anfibios, perros, pescados, aves...) se veía alterado antes de la ocurrencia de grandes sismos. Por ejemplo, en 1975 en Haicheng, China, muchas personas observaron que un gran número de serpientes comenzaban a salir de sus madrigueras un mes antes de que su ciudad fuera echada abajo por un fuerte sismo (7.3 Mw). Este caso es particular ya que este hecho se dio en época de invierno cuando dichos animales normalmente están hibernando. Su repentina salida podría considerarse un acto suicida ya que las temperatura están por debajo de cero, pero quizás la razón de su escape sea la misma del escenario de L'Aquila 2009.

Investigadores de la NASA han demostrado que cuando las rocas están sometidas a un alto nivel de esfuerzos, estas liberan particulas cargadas en forma de flujo, y este al alcanzar la superficie y entrar en contacto con el agua/aire reccionarían en un compuesto denominado Hidrogeno-Peroxido. No descartó que este flujo de particulas cargadas podría alcanzar niveles mas altos como la troposfera y/o ionosfera donde ya se ha registrado (con medidas GPS, magnetometría, otros) que se producen ciertas perturbaciones antes de la ocurrencia de grandes sismos.

Software: GeoMapApp useful geosciences mapping free software

GeoMapApp is an interactive portal displays whit a customized interface that allow showing earthquake epicentral locations Different parameters can be modified and animations saved saved in file .mov format. Created by the Marine Geoscience Data System group at Lamont-Doherty Earth Observatory of Columbia University. For more information about GeoMapApp see http://www.geomapapp.org

VIDEO: The 2010 Maule-Chile GREAT earthquake and its sequence of aftersocks

This video from Nathan Becker a specialist in tsunamis at the PTWC Hawaii, shows the seismic history of the great megathrust 2010 Maule earthquake, starting the January first. As it's known the 8.9 Mw mainshock occurred the February 27 and it was followed by a series of aftershocks with low to moderate magnitudes. Largest aftershock M=7.2 occurred the March 11. The animation shows the history during six months

insights into the mysteries of materials and dynamics in the Earth's deep interior

Unprecedented insights into the mysteries of materials and dynamics in the Earth's deep interior. Professor K. Hirose, from the Department of Earth and Planetary Sciences of the Tokyo Institute of Technology, explains recent experiments using ultra high pressure and temperature for clarification of materials and their dynamics inside the Earth. Their results are very interesting and will contribute to the understanding of the dynamics and internal process of the material in the earth's interior.

Geodynamic and seismotectonic context of Peru

The subduction of the Nazca plate beneath South America constitutes the major tectonic characteristic in Peru. According to magnetic and geodetic data, the Nazca plate is continuously moving north-eastward (azimuth 78 degress )with an average velocity of 6 cm/yr (DeMets et al., 1990; Norabuena et al., 1999). This rapid convergence causes a high seismic coupling along the subduction interface giving raise to the occurrence of interplate and intraplate earthquakes with different magnitudes and depths. As a result of this process, different geomorphological structures and tectocni features have been created such as: the Peru-Chile trench, the Andean cordillera, the volcanic chain and fault systems. In case of the fault systems those are also sources of seismicity but of lower magnitudes. The figure shows all these features.

-  The Peru-Chile trench: is a structure emplaced parallel to the Peruvian coast with a distance of about 160 km from the coast, bordering the contact between the Nazca plate and the South America plate.

-  The Andean cordillera: the most obvious tectonic feature product of the continuous interaction of the Nazca plate, covers an area of 7,500 km, with heights of 6,000 m and higher. The volcanic chain is located in the southern part of Peru from 14º N to 25º S in Chile. Finally, fault systems are result of the deformation in the continental crust due to subduction process.

Different authors have proposed that the subduction zone could be divided into three zones according to its seismic activity (North, Central and South), this zones might be separated by two prominent geomorphologic features on the subduction plate: the Mendana fracture zone (10ºS) and the Nazca ridge (15ºS). The Nazca ridge location is considered as a barrier of rupture propagation as observed in the last Pisco earthquake (Mw8.0) in 2007 (Perfettini et al., 2010; Sladen et al., 2010). However, Okal et al. (2006), using tsunami simulations for the 1868 and 1687 events suggest that the Nazca ridge appears more like a hurdle than a barrier serving as a rupture barrier for certain events such as the 1604 event for the south and 1746 event to the north. Moreover, this feature is considered as a transition between a flat subduction zone to the north and a subduction steeply dipping to the south, with different azimuth of the shore line and seismicity activity (Okal et al., 2006; Sladen et al., 2010). The Mendana fracture zone is considered as a transition zone with a difference crustal age of about 10 Myr between north of the Mendana fracture zone and off the central and south of Peru (Muller et al., 1997).

- The volcanic chain: Located in the south of Peru from 14º S to 25º S in Chile. This chain is distributed along the Andean cordillera following an aparent linearity on NW-SE direction. Among the main volcanoes located in southern Peru: Coropuna (6425 m), Sabancaya (5795 m), Misti (5825 m), Ubinas (5672 m.), Chachani (3745 m.), Huaynaputina (4800 m.), Tutupaca (5806 m.), Yucamane (5508 m.). In northern and central Peru volcanic activity have disappeared approximately 8 Ma, due to changes in the form of the subduction process (Moroco 1980).

- The fault systems: are result of constant deformation process in the continental crust due to the subduction process. These faults are present in great number, from North to South along Sub-Andean zone and the eastern side of the Andes Mountains creating major folds in the contact with the Brazilian Shield. The main fault systems located in Peru are: Alto Mayo (AM), Satipo (SA), Madre de Dios (MD),  in less proportion located on the high Cordillera and the Altiplano: the Cordillera Blanca (CB), Huaytapallana (HU), Tambomachay (TM). All fault systems owe their origin to a heterogeneous distribution of tensional and compressional efforts within thecontinent (James, 1978).

VIDEO: El gran terremoto de Tohoku: premonitores, sismo principal y réplicas

Si bien el sismo de Tohoku ocurrido el 11 de marzo de 2011alcanzó una gran magnitud 9.1Mw, este sismo fue precedido por un precursor de magnitud 7.2Mw el día 9 de Marzo. La animación posteada por Nathan B. (investigador del Pacific Tsunami Warning Center) muestra en 4 minutos (equivalentes al periodo de enero a Septiembre) la distribución espacio-temporal de los eventos premonitores, evento principal y réplicas de este gran sismo. Al observar la actividad sismica durante estos nueve meses se aprecia que durante los primeros meses la actividad sismica de fondo es normal o típica de la región, luego al producirse el evento premonitor le suceden el sismo principal y la serie de replicas que terminan por completar y estabilizar el área de ruptura del sismo. De este contexto se puede desprender que el evento premonitor del 9 de Marzo corresponde a lo que los autores que estudian el ciclo sismico denominan etapa de carga o fase pre-sismica, luego el sismo principal del 11 de Marzo corresponde a la etapa en la que se libera la energia sismica usualmente denominada fase co-sismica y finalmente la serie de réplicas que al parecer a la fecha aún no cesan corresponde a la etapa de relajación denominada fase post-sismica.

En el video cada segundo corresponde aproximadamente a 1 día, y la magnitud de los sismos se presenta en escala logaritmica.