Philippe Lesage

Seismicity related to the glacier of Cotopaxi Volcano, Ecuador

[1] Significant seismic activity is generally recorded on volcanoes covered by an icecap. This work was carried out in order to quantify the role of the glaciers in the generation of seismicity for Cotopaxi volcano. We compared the seismic activity registered on the glacier and on the rock near the snout of the north flank glacier. We focused on the analysis of low frequency events (<5 Hz) similar to volcanic LP events when recorded on rock base. The particle motion analysis helps to estimate source locations, which are distributed in crevasses areas. High incident angles suggest a superficial origin. These events are interpreted as icequakes for which we propose as source mechanism a fluid-driven crack model triggered by ice cracking or hydraulic transients. The low quality factor values estimated are consistent with the resonance of an ice crack filled with water. This work shows that low frequency icequakes can be confusingly taken as volcanic LP events.

Applications of autoregressive models and time–frequency analysis to the study of volcanic tremor and long-period events

Volcanic tremor and long-period (LP) events are characterized by sharp spectral peaks that generally result from resonance effects at the source and which concentrate most of the radiated energy. The understanding of these seismovolcanic phenomena requires good descriptions of the distribution in time and frequency of the different spectral components included in the signals, as well as a separation of the resonance effects from less energetic effects such as excitation and propagation. We address the issue of extracting from individual records information as detailed as possible on the physical processes involved at the source. We introduce and compare several time-frequency analysis methods, and we describe the application of autoregressive modeling and deconvolution methods to the characterization and separation of the main spectral components. We propose a signal analysis approach based on the joint use of a set of complementary methods, and we present applications to several examples of volcanic tremor and LP events. The time-frequency analysis of some of the LP events taken as examples reveals short-duration components at the seismogram onsets with energy concentrated at frequencies either higher or lower than the main resonance frequencies. These seismic phases are probably related to the excitation processes of the volcanic resonators. In several cases, the arrival of the main spectral peak has a delay of a few tenths of a second with respect to the first arrival. The residual signals obtained by deconvolving and eliminating the main spectral components contain information about the excitation, such as duration, delay, or frequency band. The residual signals are short for LP events, and continuous for volcanic tremor. The autoregressive modeling of the sample records gives precise estimations of the frequency and quality factor of the main spectral peaks. The measured parameters cover a wide range of values, which is consistent with the great variety of fluids filling resonating cavities in volcanoes.

Interactive Matlab software for the analysis of seismic volcanic signals

The computer program presented in this note applies methods commonly used in volcano seismology to the analysis of seismic data. It is complementary to the classic seismological software packages used to process tectonic earthquake seismograms. The programs six user-friendly interfaces provide a large set of tools for reading data in many different formats, for carrying out spectral analysis, autoregressive modelling and deconvolution, for calculating time-frequency representations and Real-time Seismic Amplitude Measurement (RSAM)-type time series, and for detecting volcanic tremors and discrete events in long-duration records. A separate version of the program facilitates signal classification when preparing training databases for automatic recognition systems.

Array analysis of the seismic wavefield of long‐period events and volcanic tremor at Arenal volcano, Costa Rica

We use wavefield decomposition methods (time-domain crosscorrelation and frequency-domain MUSIC) to analyze seismic data recorded by a dense, small-aperture array located 2 km West of Arenal volcano, Costa Rica, and operated during 2.5 days. The recorded wavefield is dominated by harmonic tremor and includes also spasmodic tremor and long-period (LP) events. We find that the initial stages of LP events are characterized by three different wave arrivals. These arrivals propagate with similar back-azimuths pointing to the volcano summit (~80◦N) and increasing apparent slownesses of 0.4, 1.1, and 1.7 s/km. Spasmodic tremors can not be regarded as coherent signals. On the contrary, harmonic tremors are highly coherent, characterized by the stability of the apparent slowness vector estimates. Apparent slownesses lay in the range 1-2 s/km. Back-azimuths point in the general direction of the volcano, but with a large variability (40-120◦N). Nevertheless, there are long-term variations and evidences of multiple simultaneous components in the harmonic tremor wavefield. These observations suggest that LP events and tremor are generated in a shallow source area near the volcano summit, although they do not share exactly the same source region or source processes. The tremor source is located in the shallowest part of the plumbing system, beneath the lava crust. This dynamic region is subject to complex fluctuations of the physical conditions. Degassing events at different locations of this region might generate variable seismic radiation patterns. The effects of topography and heterogeneous shallow structure of the volcano may amplify these variations and produce the wide directional span observed for volcanic tremor. On the other hand, the LP source seems to be more repeatable. LP events are likely triggered by fragmentation of the fluid flow in a slightly deeper portion of the volcanic conduits.

Time‐Dependent Spatial Amplitude Patterns of Harmonic Tremor at Arenal Volcano, Costa Rica: Seismic‐Wave Interferences?

Seismograms recorded at the receivers of a small‐aperture seismic array usually display very similar waveforms and amplitudes, as a consequence of their close proximity. During the analysis of the volcanic tremor wave field at Arenal volcano, Costa Rica, we detected significant differences in the amplitudes of harmonic tremor recorded at the stations of a small‐aperture (∼210  m) seismic array. The amplitude distributions are geometrically complex and characterized by strong gradients. They occur just for harmonic tremors; any other type of seismic event produces nearly uniform amplitudes across the array. This suggests some relation with harmonic frequency content. Moreover, the spatial amplitude patterns change with time. Some of these observations could be explained by an extreme combination of source, path, and site effects. But they also could be produced by interference of different components of the seismic wave field. We use numerical calculations to investigate the amplitude pattern generated by two interfering plane waves, and are able to reproduce the main features of the observed amplitude patterns. We propose mechanisms that might generate seismic wave fields with multiple components and conclude that interference can explain the complexity and variability of the harmonic tremor wave field at Arenal volcano. Online Material: Wave‐field animation.