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Alternative method helps relocalize seisms with 80% less uncertainness

Localizing seisms in volcanos such as the Galeras Volcano (Province of Nariño) carried out by seismologic and volcanology observatories have error margins which can be up to several miles.

Bogotá D. C., 07 de mayo de 2014Agencia de Noticias UN-

The Galeras volcano in the Province of Nariño was the center of Miguel Lizarazo’s research. Source: www.narino.gov.co

Image of the siesmogenic sources used during the research project. Photo: Courtesy of Miguel Lizarazo

The Galeras volcano is study object of several geophysics and seismology researchers. Photo: Courtesy of Miguel Lizarazo

Relocalization map. Photo: Courtesy of Miguel Lizarazo

Massive relocalization model. Photo: Courtesy of Miguel Lizarazo

Miguel Lizarazo UNal Geophysics M. Sc. Photo: Victor Holguin/Unimedios

John Jairo Sánchez, UNal Geosciences Department Professor and Researcher.

In face of this scenario the UNal Geophysics Master’s Program recently implemented a method which relocated the seismic sources and reduced uncertainness up to 80% with respect to previous locations at the Galeras Volcano. 

This means that seisms localized with traditional methods which were off by miles could be relocalized and in some cases to just feet, said Miguel Lizarazo a Geophysics M.Sc. and who also carried out the research project at the Galeras Volcano. 

UNal Geosciences Department Professor and Researcher John Jairo Sánchez, says they have used an ample variety of seism localization techniques at the Galeras and other volcanoes around the world and that preliminary localizations have always had significant mistakes. 

The work carried out by Lizarazo focused on identifying seism families and calculating delays between absolute journey times by using a statistical method known as crossed-correlation and using data gathered by sensors deployed around the Urcunin caldera, a mountain which was a volcano millions a years and which gave birth to the Galeras volcano. 

The results of the research revealed a 1.6 km (1 mile) fault which marks-off the caldera and a 1.8 km (1.1 mile) ringed fault neighboring the crater. They decreased uncertainness of the spatial hypocentral parameters between 56.84 and 87.48%. 

The strategy implemented in this project is known as the “Double- Difference Method”, created at the beginning of the Twenty-first Century but applied for the first time in Colombia.

To begin the process it was necessary to use preliminary data provided by the Volcanology and Seismology Observatory of Pasto (Province of Nariño) and ascribed to the Colombian Geological Survey. 

This process compares seism pairs with neighboring events. Movement wave forms are processed and according to the similarity of their seismograms (ground motion records carried out by seismograph) they can establish the closeness in space and its focal mechanism. With this they create, what is known as seism families. 

Thanks to this feature they calculate the double-difference from crossed-correlation, which compares wave forms amongst similar families. This helps to specify how near seism sources are and to relocate them using a software program known as HypoDD. 

“A seism family whose seismograms shows correlations equal or greater than 0.85 (when they reach 1, signals are identical) and which been identified in several stations enables reasonably interpreting that the seismic source is common for all its members as the high correlation rates indicate that they were generated close in space,”  said Lizarazo. 

He also added that double-difference calculations amongst said families provide relative seism localizations: geographical coordinates, depth, and source time. 

“When seisms are located with such precision (we lowered uncertainness levels of up to 80%) we can establish with greater certainness in space the sources and structures which produce seismicity, such as faults and structures identified,” he added. 

Despite calculations being implemented around the Galeras volcano, applicability of this seism relocalization model could be used in diverse tectonic areas. 

For instance in areas of great seismic activity such as the Province of Santander, the method used by the researchers could be used to determine which areas produce greater risk for construction in terms of locating siesmogenic areas with greater precision. This should be supplemented with geological studies. 

“On the other hand from the general oil and mining standpoint, establishing fault localization with greater accuracy is important for the geology and development of the country,” said Lizarazo.

(Por: Fin/DSGM/CAPG/SUP
)
N.° 717

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