Abstract. The configuration of the Pacific plate subducted beneath the Kamchatka peninsula and the stress distribution in the Kamchatka subduction zone (KSZ) were studied using the catalog of the Kamchatka regional seismic network, focal mechanism solutions estimated from P wave first motions, the formal inversion of long-period waveforms, and centroid moment tensor solutions. To the south of-55øN, the slab shows an approximately constant dip angle of -55 ø. To the north of-55øN, the dip of the slab becomes shallower reaching -35 ø. The maximum depth of seismicity, Din, varies from -500 km depth near 50øN to -300 km depth at -55øN. The volcanic front is almost linear along the main part of the KSZ whereas it is sharply shifted landward to the north of-55øN. The variation of Dm is apparently consistent with the standard empirical relation Dm --f (rp), where rp is the thermal parameter of the subducted slab.To the north of-55øN, the slab is offset toward the northwest, and it is sharply deformed in a narrow contorted zone which is -30 km wide (-56øN, -161 øE). To the north of this contortion, Dm decreases to -100 km. The landward shift of the northern part of the slab is reflected by a sharp deviation of the volcanic front to the northwest which follows the -90-160 km isodepth range of the subducted slab. The observed value of Dm in the northern segment significantly diverges from the global relation Dm --f (rp). We interpret this as an effective decrease of the thermal thickness of the subducted lithosphere.
The origin of the volcanic tremor is still under debate. Many theories have been proposed in the last years, but none has yet been completely accepted. In 1993, highly sensitive pressure sensors (2.175 Pa/Volt) used to monitor the explosive activity at Stromboli have revealed unexpected correlation between small spike‐shaped pressure signals (1–2 Pa) and volcanic tremor. These pressure pulses repeat regularly in time with a recurrent period of ca. 1 s. Video camera images allowed us to correlate the pressure pulses with small gas bursts occurring at one of the active vents. The striking correlation (0.971) between infrasonic and seismic energy fluctuations is particularly meaningful in the frequency domain. Infrasonic and seismic signal share the same spectral content (3 Hz) for every station within a range of 700 m around the craters. Correlations in time and frequency domain remained unaltered during the 1994 field experiments. Moreover, during 1994, the increased degassing activity has been followed by an increase in pressure release (7–8 Pa) and by a shift towards higher frequencies (8 Hz) both in the infrasonic and seismic records. Infrasonic waves and volcanic tremor show similar energy fluctuations and frequency contents, appearing therefore to be produced by the same dynamical process. On this basis, we claim that volcanic tremor at Stromboli originates by continuous outbursting of small gas bubbles in the upper part of the magmatic column.
Abstract. Volcanic tremor at Stromboli (Aeolian islands, Italy) is correlated to small infrasonic transients [Ripepe et al., 1996] which repeat almost rythmically intime in a range between 0.8 and 1.2 s. We demonstrate that infrasonic transients are associated to small gas bubble (-•0.5 m) burstings which produces no transients in the seismic signal. Tremor ground displacement attenuates with the inverse of the distance from the craters indicating that the source is shallow. Short-term energy release shows that infrasonic and seismic signals are linked to the same dynamical process, while at the long-term scale it is evident that the two signals are controlled by two distinctive mechanisms. We suggest that the possible physical model acts in two steps' first, gas coalescence and, then, gas bursting. In our model, the seismic signal is related to the coalescence of a gas bubble from a layer of small bubbles, while the infrasonic signal is linked to the bursting of the bubble when it reaches the magma surface. Gas bubbles could form by free coalescence in magma or could be forced to coalesce by a structural barrier. We calculate that forced coalescence induces in magma a pressure change (-.•104 Pa) 2 orders of magnitude higher than free coalescence, and it explains best the tremor ground displacement (10 -• m) recorded at Stromboli. Moreover, forced coalescence evidences the role of a structural barrier, such as a dike, in volcanic tremor source dynamics. In this gas dynamic process, the delay time of 1-2 s between infrasonic pulses could reflect the gas nucleation interval of basaltic magma [Thomas et al., 1993;Manga, 1996]. We propose that the source function for the shallow volcanic tremor at Stromboli could be the viscoelastic reaction of the magma to the pressure decrease induced by gas bubble growth rate under constant depressurization. The spectrum of our source function is controlled by the time duration of the pressure pulse, which represents the viscoelastic relaxation time of the magma and gas bubble growth rate. The predicted asymptotic decay of the frequency contents fits t;he spectral behavior of the vocanic tremor ground displacement recorded at Stromboli. We show that the same spectral behavior can be found in ground displacement spectra of volcanic tremor recorded on different volcanoes.
We analyse daily cross-correlation computed from continuous records by permanent stations operating in vicinity of the Klyuchevskoy group of volcanoes (Kamchatka). Seismic waves generated by volcanic tremors are clearly seen on the cross-correlations between some pairs of stations as strong signals at frequencies between 0.2 and 2 Hz and with traveltimes typically shorter than those corresponding to interstation propagation. First, we develop a 2-D sourcescanning algorithm based on summation of the envelops of cross-correlations to detect seismic tremors and to determine locations from which the strong seismic energy is continuously emitted. In an alternative approach, we explore the distinctive character of the cross-correlation waveforms corresponding to tremors emitted by different volcanoes and develop a phasematching method for detecting volcanic tremors. Application of these methods allows us to detect and to distinguish tremors generated by the Klyuchevskoy and the Tolbachik, volcanoes and to monitor evolution of their intensity in time.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.