International audienc

[1] We systematically used two approaches to analyze broadband seismic signals for monitoring active volcanoes: one is waveform inversion of very-long-period (VLP) signals assuming possible source mechanisms; the other is a source location method of long-period (LP) events and tremor using their amplitudes. The deterministic approach of the waveform inversion is useful to constrain the source mechanism and location but is basically only applicable to VLP signals with periods longer than a few seconds. The source location method assumes isotropic radiation of S waves and uses seismic amplitudes corrected for site amplifications. This simple approach provides reasonable source locations for various seismic signals such as a VLP event accompanying LP signals, an explosion event, and tremor associated with lahars and pyroclastic flows observed at five or fewer stations. Our results indicate that a frequency band of about 5-12 Hz and a Q factor of about 60 are appropriate for the determination of the source locations. In this frequency band the assumption of isotropic radiation may become valid because of the path effect caused by the scattering of seismic waves. The source location method may be categorized as a stochastic approach based on the nature of scattering waves. Systematic use of these two approaches provides a way to better utilize broadband seismic signals observed at a limited number of stations for improved monitoring of active volcanoes.Citation: Kumagai, H., M. Nakano, T. Maeda, H. Yepes, P. Palacios, M. Ruiz, S. Arrais, M. Vaca, I. Molina, and T. Yamashima (2010), Broadband seismic monitoring of active volcanoes using deterministic and stochastic approaches,

International audienceBuilding a unified and homogeneous earthquake catalog is a preliminary step for estimating probabilistic seismic hazard in a country. Ecuador, a territory of ∼600 km×500 km, is characterized by an active seismicity, both in the shallow crust and in the subduction zone. Several international and local earthquake catalogs are available, covering different time and spatial windows, characterized by different magnitude types and uncertainties. After a careful analysis of each catalog, in particular for completeness and uncertainty levels, we propose a priority scheme for merging the instrumental catalogs. Moreover, several historical earthquakes are analyzed to estimate epicentral location and magnitude, completing the solutions obtained in a previous publication. Once the historical earthquakes are appended to the instrumental catalog, the resulting catalog covers five centuries in the Cordillera region. Next, homogenization of magnitudes and removal of aftershocks is performed; different options are studied and the impact on the recurrence curve is evaluated. For the Cordillera region within −2.5° and 1° latitude, the average occurrence of an earthquake with Mw≥6.0 is 10-20 years based on the historical catalog

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