Seismic noise characterization at Volcan de Colima, Mexico: preliminary results

Colima Volcano also known as Volcan de Fuego de Colima is one of the most active volcanoes in Mexico. Colima Volcano is an andesitic stratovolcano 3860 m high, located in the western portion of the Mexican Volcanic Belt. Its most recent eruptive activity began 1997 and consists of Vulcanian explosions, piroclastic flows, gas-ash emissions, and dome construction and destruction episodes. Such activity still going on today, attracting worldwide interest in all scientific fields. Since June 2011 a temporal broadband seismic network, consisting of five stations equipped with Nanometrics Trillium 120 PA sensors, was installed on N, S, E, and W flanks of the volcano, at distances between 1.5 and 4 km from the crater. In volcanic areas microseismicity may cover a frequency range of 5 to 20 s. Since it is a persistent signal in time with amplitude fluctuations of more than twice the signal to noise ratio, in general may mask signals of volcanic origin. In the present work we describe the seismic activity recorded from Jun to Dec, 2011. We evaluate the noise levels at each station in terms of their relative amplitude with respect to the microseismic noise at the site. For this purpose, the minimum average noise (MAN), defined as the average of minimum amplitude of the power spectral density (PSD), in the microseismic bandwidth (5-20s) is considered. We first calculate the minimum PSD at each station per component, averaging 120 min time windows of records with apparent absence of volcanic activity, during 6-months. Then, we averaged the PSD of the three components to obtain the minimum average noise per site. To evaluate temporal variations of volcanic activity and its contribution to the microseismic band, we apply a spectral ratio technique between 120 min record windows containing volcanic signals and the MAN per site. In this manner, the minimum average noise represents the minimum energy at each observation point, and the spectral ratio provides a relative measurement of the volcanic activity contribution to the microseismic frequency bandwidth.