We use the spatial autocorrelation (SPAC) method with improved inversion algorithms to estimate the Love and Rayleigh dispersion curves at two sites at the West and Northeast flanks of Arenal volcano, Costa Rica. At the West flank site, the Rayleigh waves phase velocities vary from 765 m s(-1) at 1 Hz to 300 m s(-1) at 12 Hz and those of Love waves between 780 and 295 m s(-1) in the same frequency band. At the Northeast flank site, the Rayleigh wave velocities range from 1386 to 300 in s(-1) and those of Love from 1983 to 315 m s(-1). From dispersion curves we derive shallow (< 400 m) P and S waves velocity models. 2D velocity models down to a depth of 150 m are also obtained by seismic refraction surveys along two radial profiles on the tephra apron at West and East flanks. They present strong vertical and lateral variations in the velocity and thickness of the layers.trong variations in amplitude of the spectral peaks are observed for the seismic events along two radial arrays. These site effects are analysed using the HN spectral ratio method and S-wave theoretical transfer functions. Results show that the wave amplifications are related to resonance effects of shallow structure (< 150 m) and occur only where impedance contrast with the deeper layers is strong enough. In contrast, almost no site effect are detected at the Masaya shield volcano, Nicaragua, where the structure is more homogeneous and mainly composed of lava flows.When a resonance of the shallow layers occurs, the correlation coefficients between close stations increase at the corresponding frequency. The site effects may thus produce spurious results with the SPAC method. The HN spectral ratio, used in complement of the SPAC method, can help detecting the site effects and testing the plane layer hypothesis. Furthermore, the theoretical transfer functions calculated for the estimated velocity models is also useful to validate the models. (c) 2005 Elsevier B.V. All rights reserved.

By using data obtained with a linear array at Arenal volcano, we show that the H/V spectral ratio method can be profitably applied to detect site effects on volcanoes. Similar results are obtained when calculating spectral ratios with different types of seismo‐volcanic signals (tremor, ambient noise, explosion quakes, LP events). We compare the H/V ratios with theoretical S‐wave transfer functions calculated using velocity models obtained from seismic refraction studies. There is a good agreement when the H/V ratios display sharp peaks, indicating a close relationship between the ratios and the transfer function of the shallow structure. Furthermore, the main peaks of the spectral ratios are consistent with local amplification of seismic waves observed at the corresponding frequencies.

The Masaya Volcano, Nicaragua, is a basaltic caldera in a subduction zone. The permanent source of the volcanic tremor was located inside Santiago crater, at the lava lake's position and 400 m below the NE rim, and therefore corresponds to superficial magma activity. We used two tripartite arrays (90 m side), one semicircular array (r=120 m) in 1992, and two semicircular arrays (r=60 m) and a 2500 m long linear array radiating out from the source and on the flank of the crater in 1993. We used both a cross‐spectrum method and a correlation method to determine the wave delay time between the reference station and the other stations of an array and to quantify the wave field. Using the delays therefore by intersecting the back azimuth wave directions from the arrays, we could pinpoint the source. Additionally, the correlation coefficients obtained as functions of frequency for the three components of motion confirm the inferred position of the source of tremor. The tremor's wave field is composed of comparable quantities of dispersed Rayleigh and Love surface waves, whose phase velocities lie in the ranges 730–1240 m/s at 2 Hz and 330–550 m/s at 6 Hz. The dispersive phase velocities were inverted to obtain crustal structures with a minimal number of layers. The resulting velocity models are similar for the northern and southern parts of the volcano. After geometrical spreading corrections, Q2Hz=14 and Q3Hz=31 were determined along the northern linear array. The typical low velocities and low Q corresponding to the cone structure and are similar to those of other basaltic volcanoes like Puu Oo, Hawaii, and Klyuchevskoy, Kamchatka.

The explosive activities of the volcanoes Stromboli (Aeolian Islands, Italy) and Yasur (Tanna Island, Vanuatu) produce low-frequency seismic signals. Several types of such signals were identified both on Stromboli (Types Is and IIs) and on Yasur (Types Iy, IIy, and IIIy). The dominant frequencies of these signals fall generally between 1 and 3 Hz although sometimes frequencies in the range 3-6 Hz also exist. The two volcanoes display similar seismic characteristics: the seismic signal associated with the strombolian explosion is accompanied by a forerunner signal that occurs several seconds before eruption and which corresponds to the time separating the formation of the gas pocket at different levels in the magmatic column and its reaching the surface.The similar spectral characteristics of the volcanic background seismic noise and the discrete signals suggest a common source.

Digital recordings of ground motion during a seismic operation organized at Stromboli volcano (Italy) from May 31, 1986 to June 26, 1986 have been studied. The amplitude of background seismic noise is greater at the summit than at the base of the volcano and the spectra calculated on different days are dominated by at least two frequencies: 2.0 and 6.0 Hz with subdominant frequencies at 4.1, 4.8 and 8 Hz. According to their waveform and their spectral content, three types of discrete volcanic signals are distinguishable. The first type is characterized by a dominant frequency at 2 Hz and a subdominant frequency at 4.1 Hz. The second type also has a dominant peak frequency at 2 Hz with two important subdominant frequencies at 5 and 8 Hz. The type 3 has a sharp dominant frequency at 6 Hz and subdominant frequency at 3.2 Hz. The frequencies observed for discrete signals and ground noise suggest that they are produced by the same repeated mechanism which probably has a volcanic origin. The discrete signals are linked to explosions at the active vents within the craters.

Four pairs of ScP and ScS phases recorded at the station Dumont d'Urville, Adelie Land, at the epicentral distance of about 50°, have been frequency analyzed. The slope of spectral ratio has been utilized to calculate the average value of Qβ in the mantle between the Tasman Sea and the station. With Qα/Qβ = 1.9, we estimate the average value 〈Qβ〉 = 380. This is much higher than the estimation by H. Kanamori of 230 for the mantle beneath southern Arizona and New Mexico.