Episodes of nonvolcanic tremor and accompanying slow slip recently have been observed in the subduction zones of Japan and Cascadia. In Cascadia, such episodes typically last a few weeks and differ from "normal" earthquakes in their source location and moment-duration scaling. The three most recent episodes in the Puget Sound/southern Vancouver Island portion of the Cascadia subduction zone were exceptionally well recorded. In each episode, we saw clear pulsing of tremor activity with periods of 12.4 and 24 to 25 hours, the same as the principal lunar and lunisolar tides. This indicates that the small stresses associated with the solid-earth and ocean tides influence the genesis of tremor much more effectively than they do the genesis of normal earthquakes. Because the lithostatic stresses are 10(5) times larger than those associated with the tides, we argue that tremor occurs on very weak faults.

Nonvolcanic tremor is difficult to locate because it does not produce impulsive phases identifiable across a seismic network. An alternative approach to identifying specific phases is to measure the lag between the S and P waves. We cross-correlate vertical and horizontal seismograms to reveal signals common to both, but with the horizontal delayed with respect to the vertical. This lagged correlation represents the time interval between vertical compressional waves and horizontal shear waves. Measurements of this interval, combined with location techniques, resolve the depth of tremor sources within +/-2 kilometers. For recent Cascadia tremor, the sources locate near or on the subducting slab interface. Strong correlations and steady S-P time differences imply that tremor consists of radiation from repeating sources.

This study summarizes the results of structural, geochemical and seismological surveys carried out at Nisyros volcano (Aegean Sea, Greece) during 1999-2001. Field mapping and mesostructural measurements at the summit caldera (Lakki plain) indicate that faults follow two main strikes: NE-SW and N-S. The N-S striking fault depicts extensional features accommodating the left-lateral component of motion of the NE-SW-striking main faults. The NE-SW preferred strike of the Lakki faults and of the mineral-filled veins as well as the distribution and NE-SW elongation of the hydrothermal craters indicate that tectonics plays a major role in controlling the fluid pathway in the Nisyros caldera. The same NE-SW trend is depicted by CO 2 anomalies revealed through detailed soil CO 2 flux surveys, thus indicating a structural control on the pattern of the hydrothermal degassing. Degassing processes account for a thermal energy release of about 43 MW, most of which occurs at Lofos dome, an area that was affected by hydrothermal eruptions in historical times. The seismic study was conducted in June 2001, using a deployment specifically aimed at detecting signals of magmatic-hydrothermal origin. Our instruments recorded local and regional earthquakes, a few local longperiod events (LP), and bursts of monochromatic tremor. Local earthquake activity is concentrated beneath the caldera, at depths generally shallower than 6 km. Planewave decomposition of tremor signal indicates a shallow (<200 m) source located in the eastern part of the caldera. Conversely, LP events depict a source located beneath the central part of the caldera, in the area of Lofos dome, at depths in the 1-2-km range. In agreement with geochemical and structural measurements, these data suggest that both the deeper and shallower part of the hydrothermal system are subjected to instability in the fluid flow regimes, probably consequent to transient pressurization of the reservoir. These instabilities may be related to input of hot fluids from the deeper magmatic system, as suggested by the variations in geochemical parameters observed after the 1997-1999 unrest episode. The significance of seismological and geochemical indicators as precursors of hydrothermal explosive activity at Nisyros is discussed.

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In this article, we analyze the seismic signals produced by two landslides that occurred at the Stromboli volcano on 30 December 2002, recorded by both broadband and short-period seismic stations located in the 2.5-22-km distance range from the source. For both landslides, the characteristics of the low-frequency seismograms indicate a complex time history in the release of seismic energy. The first landslide occurred over the submerged part of the northwest sector of the volcano and had associated a large-amplitude, low-frequency pulse representative of the abrupt detachment of a large mass. Lower amplitude phases in the following 3 minutes possibly indicate minor detachment events. The highest amplitude, lowfrequency signals are well described by a single-force source model. The second mass-failure episode is also characterized by a complex source and can be interpreted as a multiple event, with a less abrupt onset and at least four detachments occurring during 4-5 minutes and producing low-frequency signals. Synthetic seismograms generated by a shallow single force located in the submerged area of Sciara del Fuoco and directed upslope, fit well the first low-frequency seismic pulse recorded at Stromboli and Panarea by three-component stations. From this simulation, we estimated the force exerted by the first mass failure. The estimate of the volume through two different procedures, gives values in the range of 1.0-1.5 million m 3 and about 14 million m 3 , respectively. The landslides, which involved both the submarine and the subaerial northwest flank of the volcano, produced a tsunami that struck the coast of Stromboli Island and in a few minutes reached the other islands of the Aeolian Archipelago. Three broadband seismic stations installed on land about 100 m from the coastline at Panarea Island, located 20 km southwest of Stromboli, recorded very long period seismic signals produced by the tsunami waves. Analysis of these signals gives invaluable information on the spectral content and propagation properties of tsunami waves and on their interaction with the ground at a short distance from the coast. Synthetic tsunami waves, obtained by a landslide source model and taking into account the bathymetry of the sea surrounding Stromboli and Panarea Islands, fit the observed phenomena and the experimental data very well.

Abstract. The seismovolcanic signals associated with the volcanic activity of Deception Island (Antarctica), recorded during three Antarctic summers (1994-1995, 1995-1996 and 1996-1997), are analyzed using a dense small-aperture (500 m) seismic array. The visual and spectral classification of the seismic events shows the existence of long-period and hybrid isolated seismic events, and of low-frequency, quasi-monochromatic and spasmodic continuous tremors. All spectra have the highest amplitudes in the frequency band between 1 and 4 Hz, while hybrids and spasmodic tremors have also significant amplitudes in the high-frequency band (4-10 Hz). The array analysis indicates that almost all the well-correlated low-frequency signals share similar array parameters (slowness and back azimuth) and have the same source area, close to the array site. The polarization analysis shows that phases at high-frequency are mostly composed of P waves, and those phases dominated by low frequencies can be interpreted as surface waves. No clear shear waves are evidenced. From the energy evaluation, we have found that the reduced displacement values for surface and body waves are confined in a narrow interval. Volcanotectonic seismicity is located close to the array, at a depth shallower than 1 km. The wave-field properties of the seismovolcanic signals allow us to assume a unique source model, a shallow resonating fluid-filled crack system at a depth of some hundreds of meters. All of the seismic activity is interpreted as the response of a reasonably stable stationary geothermal process. The differences observed in the back azimuth between low and high frequencies are a near-field effect. A few episodes of the degassification process in an open conduit were observed and modeled with a simple organ pipe.

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Preliminary analysis of deep tremor recorded during July, 2004, in the Cascadia Subduction zone shows that small aperture arrays can resolve the slowness and back azimuth of seismic waves with a useful resolution. Data were collected by three dense arrays of short‐period seismometers specifically deployed in the Puget Sound area under an US‐Italy‐Canada cooperative effort. Slowness analyses at the three arrays indicate that the 2–4 Hz tremor wave‐field is composed of waves propagating with apparent velocities higher than 4 km/s. Combining this with polarisation analysis show these waves to be transverse (SH) waves. However, P‐waves, though smaller in amplitude, can be detected by different slowness values obtained for the radial and transverse components. The intersection of wave vectors determined by the back azimuth and slowness values measured at the three arrays provides a preliminary estimate of source location for a sample of the recorded deep tremor.