Observations of mount etna seismicity during the 2002–2003 eruption based on deep-sea gravimeter data

This paper concerns observations made by a broadband deep-sea gravimeter installed on the plat-form of the SN-1 multiparameter seafloor observatory. The observatory was deployed at a distance of 25 km from the east coast of Sicily in southern Italy at a depth of 2105 m and was operated in a self-contained mode from October 2002 to February 2003 (134 days). The proximity to Mount Etna and the period of eruptive activity starting in late October 2002 lent added interest to this experiment. The seismic activity of Mount Etna, as recorded by the gravimeter, is characterized by the presence of two signal types, viz., a long-period volcanic tremor of variable amplitude and volcano-tectonic earthquakes. The bulk of energy in the long-period tremor occurs in the spectral interval between 2 and 5 s. The long-period seismic signals due to volcanic and global earthquakes were used to estimate resonant characteristics for Etna’s heterogeneous structures.     

Summit eruptions of Klyuchevskoi Volcano,Kamchatka in the early 21st century, 2003–2013

This paper is concerned with eruptions, seismicity, and deformation on Klyuchevskoi Volcano during the summit eruptions of 2012–2013, with the condition of the central crater during the eruptions, and with the effect that is exerted by the height of the lava in the crater on the start of the eruptions. The recurrence of eruptions in the North Volcanic Cluster (NVC), Kamchatka showed that all the four volcanoes in the cluster (Klyuchevskoi, Tolbachik, Shiveluch, and Bezymyannyi) become active during definite phases that were identified in the 18.6-year lunar cycle. This relationship of the NVC eruptions to the active phases in the 18.6-year lunar cycle, as well as the relationship to the 11-year solar activity, showed that eruptions can be predicted, yielding long-term estimates of activity for the NVC volcanoes. The short-term prediction of volcanic eruptions requires knowledge of seismicity and deformation that occur during the precursory period and during the occurrence of eruptions. Seismic activity during the summit eruptions of 2003–2013 took place in the depth range 20–25 km during repose periods of the volcano and at depths of 0–5 km in the volcanic edifice during the eruption. One notes an almost complete absence of any earthquakes at great depths during the summit eruptions. Volcanic tremor (VT) was recorded from the time that the eruptions began and continued to occur until the end. Geodetic measurements showed that the center of the magma pressure beneath the volcano during the parasitic and summit eruptions of 1979–1989 moved in the 4–17 km depth range, while during the summit eruptions of 2003–2013 the center moved in the 15–20 km range. These changes in the depth of the center of magma pressure may have been related to evacuation from shallow magma chambers.     

Ground tilt variations in the Klyuchevskoi Volcano Area, Kamchatka

We consider the results of ground tilt observations in the area of Klyuchevskoi Volcano, Kamchatka at the Klyuchi and Apakhonchich stations using photoelectric tiltmeters. An anomalous tilt was recorded during a period of increased activity in the summit crater of Klyuchevskoi Volcano in November 1979. In January and February 1980 three more cases of anomalous tilting before the volcano’s active periods were observed. The tilts exhibit some common kinematic features that are quite consistent with the activity periods. Various disturbing influences were estimated as affecting the observations of tilts related to seismic and volcanic events. For the first time in Kamchatka, the parameters of the five principal elastic-tide waves have been determined. Sea tides exerted an indirect effect on the amplitude factors γ(M2) on the E-W component at both sites to decrease them relative to the global value, γ(M2) = 0.7. We investigated the influence of sea tides on the elastic solid earth tide along the Trans-Kamchatka and Kola traverses. The observed and theoretical γ(M2) factors at both traverses situated in different geodynamic areas are in approximate agreement.     

Models of source spectra for tectonic earthquakes in application to some types of volcanic earthquakes: Klyuchevskoi volcano

This study makes use of spectral models for tectonic earthquakes in application to volcano-tectonic (VT) and deep-focus long period volcanic (DL) earthquakes. For these earthquakes we adjusted the parameters of source spectra, and simultaneously, the medium where seismic waves propagate. We derived preliminary results to characterize the models of source spectra for volcanic earthquakes that occur on Klyuchevskoi Volcano. This method can also be used to estimate the attenuation parameters of seismic waves, which are important for the assessment of stress and strain in a volcanic medium.     

Statistical analyses and characteristics of volcanic tremor on Stromboli Volcano (Italy)

 A study of volcanic tremor on Stromboli is carried out on the basis of data recorded daily between 1993 and 1995 by a permanent seismic station (STR) located 1.8 km away from the active craters. We also consider the signal of a second station (TF1), which operated for a shorter time span. Changes in the spectral tremor characteristics can be related to modifications in volcanic activity, particularly to lava effusions and explosive sequences. Statistical analyses were carried out on a set of spectra calculated daily from seismic signals where explosion quakes were present or excluded. Principal component analysis and cluster analysis were applied to identify different classes of spectra. Three clusters of spectra are associated with two different states of volcanic activity. One cluster corresponds to a state of low to moderate activity, whereas the two other clusters are present during phases with a high magma column as inferred from the occurrence of lava fountains or effusions. We therefore conclude that variations in volcanic activity at Stromboli are usually linked to changes in the spectral characteristics of volcanic tremor. Site effects are evident when comparing the spectra calculated from signals synchronously recorded at STR and TF1. However, some major spectral peaks at both stations may reflect source properties. Statistical considerations and polarization analysis are in favor of a prevailing presence of P-waves in the tremor signal along with a position of the source northwest of the craters and at shallow depth. Received: 15 December 1996 / Accepted: 31 March 1998     

The February–July 2007 eruption of the summit crater of Klyuchevskoi volcano, Kamchatka

Observations of the summit eruption of Klyuchevskoi volcano in the period from February 15, 2007 to July 9, 2007 are considered. This typical (for this volcano) summit eruption was explosive-effusive in character. The ejectamenta volume is estimated at 0.025 km³. Calculation of active phases of the volcano was carried out in accordance with V.A. Shirokov’s technique. The identified active phases agree well with the eruptive periods. The 2007 summit eruption corresponds to an active phase (May 2006 to May 2009) favorable for the volcano’s eruption. Geodetic observations carried out since 1979 along a radial profile have revealed uplifts and subsidences of the northeastern slope of the volcano. The maximum displacement of 23 cm was recorded in 2007 on the site closest to the volcano crater at a distance of 11 km from the summit crater center. In the course of two previous summit eruptions (2003–2004 and 2005) insignificant uplifts and subsidences of the slope were also noted, although the general ascent of the slope remained. This indicated possible repeated eruptions in the nearest future. Changes in the seismicity before, during and after the eruption are also discussed.     

Geochemical evidence of the renewal of volcanic activity inferred from CO<Subscript>2</Subscript> soil and SO<Subscript>2</Subscript> plume fluxes: the 2007 Stromboli eruption (Italy)

On 27 February 2007, a new eruption occurred on Stromboli which lasted until 2 April. It was characterized by effusive activity on the Sciara del Fuoco and by a paroxysmal event (15 March). This crisis represented an opportunity for us to refine the model that had been developed previously (2002–2003 eruption) and to improve our understanding of the relationship between the magmatic dynamics of the volcano and the geochemical variations in the fluids. In particular, the evaluation of the dynamic equilibrium between the volatiles (CO₂ and SO₂) released from the magma and the corresponding fluids discharged from the summit area allowed us to evaluate the level of criticality of the volcanic activity. One of the major accomplishments of this study is a 4-year database of summit soil CO₂ flux on the basis of which we define the thresholds (low–medium–high) for this parameter that are empirically based on the natural volcanological evolution of Stromboli. The SO₂ fluxes of the degassing plume and the CO₂ fluxes emitted from the soil at Pizzo Sopra la Fossa are also presented. It is noteworthy that geochemical signals of volcanic unrest have been clearly identified before, during and after the effusive activity. These signals were found almost simultaneously in the degassing plume (SO₂ flux) and in soil degassing (CO₂ flux) at the summit, although the two degassing processes are shown to be clearly different. The interpretation of the results will be useful for future volcanic surveillance at Stromboli.     

长白山天池火山2002~2005年火山扰动机制研究

地震、形变、火山气体地球化学等观测结果表明2002~2005年长白山天池火山经历了1次扰动事件。长白山站地震台（CBS台）记录到了扰动事件前后连续稳定的宽频带地震观测资料。前人的观测研究结果认为长白山天池火山扰动期间的火山地震类型主要为构造型火山地震，伴随少量的谐频型地震。本文通过匹配滤波技术，对1999~2007年扰动事件前后CBS台单台三分量地震观测数据进行模板扫描，获得3763个清晰的火山地震事件，其中谐频（HS）事件125个，构造（VT）事件3618个，并发现长周期（LP）事件20个。进而将火山扰动期间火山地震事件分为3种类型：构造型事件、长周期事件和谐频型事件，并提出2002~2005年长白山天池火山扰动机制模型：深源地震-火山能量传递模型，即汪清深源地震能量释放和传递，引发长白山火山区岩石圈应力状态波动。地幔岩浆房受应力干扰后，岩浆通道打开，少量岩浆侵入地壳岩浆房。岩浆混合脱气导致地壳岩浆房升压，引起顶部岩石微破裂，产生构造型火山地震，气体和流体填充这些裂隙，从而产生LP和HS型火山地震事件。     

Estimation of shear-wave attenuation characteristics for the Klyuchevskoi volcanic edifice

This paper reports a study of intrinsic attenuation and scattering in the Klyuchevskoi volcanic edifice. The data set consisted of small volcano-tectonic earthquakes occurring as deep as 30 km beneath Klyuchevskoi Volcano and recorded by radio telemetry seismograph stations installed on the edifice and near it. The digital seismograms of small volcano-tectonic earthquakes were processed by the multiple lapse time window analysis (MLTWA) method currently in use in seismology. The method uses experimental normalized integrals of 3D seismic energy density determined from several time windows applied to earthquake records that have been put through bandpass filters. The parameters that characterize the intrinsic attenuation and scattering of seismic energy of small volcano-tectonic earthquakes in Klyuchevskoi Volcano were estimated by adjusting the coefficients to ensure the best fit between experimental and theoretical integrals. An analytical solution to the seismic energy transfer equation was used to calculate the theoretical integrals of 3D seismic energy. Reliable estimates of the parameters that characterize the intrinsic attenuation and scattering in the material of the Klyuchevskoi volcanic edifice have been obtained. Our estimates of the Q for the edifice are below those derived by other workers for the Kamchatka lithosphere.     

Magmatic Systems and the Conditions for Hydrothermal Circulation at Depth in the Klyuchevskoi Volcanic Cluster as Inferred from Observations of Local Seismicity and Thermo-Hydrodynamic Simulation

An analysis of local seismicity within the Klyuchevskoi Volcanic Cluster and Shiveluch Volcano for the period 2000–2017 revealed a sequence of plane-oriented earthquake clusters that are interpreted here as the emplacement of dikes and sills (magmatic fracking). The geometry of magma bodies reflects the geomechanical conditions in volcanic plumbing systems and at the bases of the volcanoes. Magmatic fracking within active magmatic plumbing systems results in the formation of permeable reservoirs whose vertical extent can reach 35 km (Klyuchevskoi) and can be as wide as 15 km across (Shiveluch), depending on the geomechanical condition of the host rocks. These reservoirs will be the arena of subsequent hydrothermal circulation, producing geothermal and ore fields, as well as hydrocarbon fields. TOUGH2-EOS1sc simulation tools were used to estimate the conditions for the formation of hydrothermal reservoirs at temperatures below 1200°С and pressures below 1000 bars.     

Inferences on the source of long-period seismicity at Campi Flegrei from polarization analysis and reconstruction of the asymptotic dynamics

In the present paper, we analyse long-period (LP) events and seismic noise recorded at Campi Flegrei during the uplift episode of 2004–2006. The results of a detailed polarization analysis indicate that the large number of LPs detected during the seismic crisis were accompanied by a sustained activity consisting of very low-energy volcanic signals. These weak volcanic signals, which are usually absent in ambient noise recorded in the studied area, and LP events have similar properties, namely they are radially polarized towards the same LP source. The large dataset analysed allows us to study the statistics of the polarization, leading to the extraction of the average properties of the volcanic signals. An estimate of the complexity of the system can be provided by the degrees of freedom necessary to describe the asymptotic dynamics in a reconstructed phase space. This analysis shows that both LPs and low-energy signals can be described by a low-dimensional dynamical system, while ambient noise unrelated to volcanic activity is higher dimensional. Based on these observations, we interpret the phenomena observed during the seismic crisis in the framework of the theory of self-oscillating systems, in which LPs and low-energy signals represent self-oscillations generated by a persistent hydrothermal source.     

Major-ion bulk deposition around an active volcano (Mt. Etna, Italy)

Bulk atmospheric deposition of major cations (Na, K, Ca, Mg) and anions (Cl, F, SO₄) were measured at 15 sites around an active volcano, Mount Etna, from 2001 to 2003. Their composition indicates several natural sources, among which deposition of plume-derived volcanogenic gas compounds is prevalent for F, Cl and S. Plume-derived acidic compounds are also responsible for the prevailing acidic composition of the samples collected on the summit of the volcano (pH in the 2.45–5.57 range). Cation species have complex origin, including deposition of plume volcanogenic ash and aerosols and soil-dust wind re-suspension of either volcanic or carbonate sedimentary rocks. Variation of the deposition rates during the March 2001–March 2003 period, coupled with previous measurements from 1997 to 2000 (Appl Geochem 16:985–1000, 2001), were compared with the variation of SO₂ flux, volcanic activity and rainfall. The deposition rate was mainly controlled by rainfall. Commonly, about 0.1–0.9% of HF, HCl and SO₂ emitted by the summit crater's plume were deposited around the volcano. We estimate that ∼2 Gg of volcanogenic sulphur were deposited over the Etnean area during the 2002–2003 flank eruption, at an average rate of ∼24 Mg day⁻¹ which is two orders of magnitude higher than that typical of quiescent degassing phases.     

Analysis of seismic patterns observed at Nevado del Ruiz volcano, Colombia during August – September 1985

The seismic activity of the Nevado del Ruiz volcano was monitored during August–September 1985 using a three-component portable seismograph station placed on the upper part of the volcano. The objective was to investigate the frequency content of the seismic signals and the possible sources of the volcanic tremor. The seismicity showed a wide spectrum of signals, especially at the beginning of September. Some relevant patterns from the collected records, which have been analyzed by spectrum analysis, are presented. For the purpose of analysis, the records have been divided into several categories such as long-period events, tremor, cyclic tremor episodes, and strong seismic activity on September 8, 1985.The origin of the seismic signals must be considered in relation to the dynamical and acoustical properties of fluids and the shape and dimensions of the volcano's conduits.The main results of the present experiment and analysis show that the sources of the seismic signals are within the volcanic edifice. The signal characteristics indicate that the sources lie in fluid-phase interactions rather than in brittle fracturing of solid components.     

Enhancement of long period components of recorded and synthetic ground motions using InSAR

Tall buildings and flexible structures require a better characterization of long period ground motion spectra than the one provided by current seismic building codes. Motivated by that, a methodology is proposed and tested to improve recorded and synthetic ground motions which are consistent with the observed co-seismic displacement field obtained from interferometric synthetic aperture radar (InSAR) analysis of image data for the Tocopilla 2007 earthquake (M_w=7.7) in Northern Chile. A methodology is proposed to correct the observed motions such that, after double integration, they are coherent with the local value of the residual displacement. Synthetic records are generated by using a stochastic finite-fault model coupled with a long period pulse to capture the long period fling effect.It is observed that the proposed co-seismic correction yields records with more accurate long-period spectral components as compared with regular correction schemes such as acausal filtering. These signals provide an estimate for the velocity and displacement spectra, which are essential for tall-building design. Furthermore, hints are provided as to the shape of long-period spectra for seismic zones prone to large co-seismic displacements such as the Nazca-South American zone.     

Seismo-acoustic signals associated with degassing explosions recorded at Shishaldin Volcano,Alaska, 2003–2004

In summer 2003, a Chaparral Model 2 microphone was deployed at Shishaldin Volcano, Aleutian Islands, Alaska. The pressure sensor was co-located with a short-period seismometer on the volcano’s north flank at a distance of 6.62 km from the active summit vent. The seismo-acoustic data exhibit a correlation between impulsive acoustic signals (1–2 Pa) and long-period (LP, 1–2 Hz) earthquakes. Since it last erupted in 1999, Shishaldin has been characterized by sustained seismicity consisting of many hundreds to two thousand LP events per day. The activity is accompanied by up to ∼200 m high discrete gas puffs exiting the small summit vent, but no significant eruptive activity has been confirmed. The acoustic waveforms possess similarity throughout the data set (July 2003–November 2004) indicating a repetitive source mechanism. The simplicity of the acoustic waveforms, the impulsive onsets with relatively short (∼10–20 s) gradually decaying codas and the waveform similarities suggest that the acoustic pulses are generated at the fluid–air interface within an open-vent system. SO₂ measurements have revealed a low SO₂ flux, suggesting a hydrothermal system with magmatic gases leaking through. This hypothesis is supported by the steady-state nature of Shishaldin’s volcanic system since 1999. Time delays between the seismic LP and infrasound onsets were acquired from a representative day of seismo-acoustic data. A simple model was used to estimate source depths. The short seismo-acoustic delay times have revealed that the seismic and acoustic sources are co-located at a depth of 240±200 m below the crater rim. This shallow depth is confirmed by resonance of the upper portion of the open conduit, which produces standing waves with f=0.3 Hz in the acoustic waveform codas. The infrasound data has allowed us to relate Shishaldin’s LP earthquakes to degassing explosions, created by gas volume ruptures from a fluid–air interface.     

Source location of long period seismicity at Volcàn de Colima,México

This paper presents an analysis of seismicity associated with the volcanic activity of Volcàn de Colima (México) and recorded in the period November 2005–April 2006 during a field survey by the Istituto Nazionale di Geofisica e Vulcanologia (INGV)–Osservatorio Vesuviano, the Observatorio Vulcanologico de Colima of Colima University and the Instituto Andaluz de Geofisica, University of Granada. Three different types of volcanic earthquakes have been identified on the basis of their spectral properties: Type A (0.3–1 Hz), Type B (1–5 Hz) and Type C (3–4 Hz). Results of polarization analysis applied to Type A events show a predominance of radial motion, indicating that the wavefield comprises compressional waves (P) and shear waves polarized in the vertical plane (SV), while the signal always begins with a negative polarity. Type A, B and C earthquakes have been located using both a flat layered model and a 3D model including topography. Hypocentre distributions indicate that the source of Type A signals is very shallow and confined to a small volume lying about 1 km below the crater. In contrast, the source of Type B and C events is significantly deeper, with most hypocentres located in a volume of about 1 km³ centred at 2.5–3 km depth. A cluster analysis based on the cross-correlation among the waveforms of different events recorded at the same station was applied to Type A earthquakes. Only two clusters, which include only a small percentage of events were found, indicating that earthquake families were uncommon during the period of our survey.     

Eruption-induced modifications to volcanic seismicity at Ruapehu,New Zealand,and its implications for eruption forecasting

Broadband seismic data collected on Ruapehu volcano, New Zealand, in 1994 and 1998 show that the 1995-1996 eruptions of Ruapehu resulted in a significant change in the frequency content of tremor and volcanic earthquakes at the volcano. The pre-eruption volcanic seismicity was characterized by several independent dominant frequencies, with a 2 Hz spectral peak dominating the strongest tremor and volcanic earthquakes and higher frequencies forming the background signal. The post-eruption volcanic seismicity was dominated by a 0.8-1.4 Hz spectral peak not seen before the eruptions. The 2 Hz and higher frequency signals remained, but were subordinate to the 0.8-1.4 Hz energy. That the dominant frequencies of volcanic tremor and volcanic earthquakes were identical during the individual time periods prior to and following the 1995-1996 eruptions suggests that during each of these time periods the volcanic tremor and earthquakes were generated by the same source process. The overall change in the frequency content, which occurred during the 1995-1996 eruptions and remains as of the time of the writing of this paper, most likely resulted from changes in the volcanic plumbing system and has significant implications for forecasting and real-time assessment of future eruptive activity at Ruapehu.     

Seismicity at White Island volcano, New Zealand: a revised classification and inferences about source mechanism

The classification of earthquakes at White Island volcano, New Zealand, has been revised to address problems in existing classification schemes, to better reflect new data and to try to focus more on source processes. Seismicity generated by the direct involvement of magmatic or hydrothermal fluids are referred to as volcanic, and that generated by fault movement in response to stresses caused by those fluids, regional stresses, thermal effects and so on are referred to as volcano-tectonic. Spasmodic bursts form a separate category, as we have insufficient information to classify them as volcanic or volcano-tectonic. Volcanic seismicity is divided into short-duration, long-period volcanic earthquakes, long-duration volcanic earthquakes, and harmonic- and non-harmonic volcanic tremor, while volcano-tectonic seismicity is divided into shallow and deep volcano-tectonic earthquakes. Harmonic volcanic tremor is related to sub-surface intrusive processes, while non-harmonic volcanic tremor originates close to active craters at shallow depth, and usually occurs during eruptive activity. Short-duration, long-period volcanic earthquakes come from a single source close to the active craters, but originate deeper than non-harmonic volcanic tremor, and are not related to eruptive activity. Long-duration volcanic earthquakes often accompany larger discrete eruptions. The waveform of these events consists of an initial low-frequency part from a deep source, and a later cigar-shaped part of mixed frequencies from a shallow crater source.     

Monitoring Seismo-volcanic and Infrasonic Signals at Volcanoes: Mt. Etna Case Study

Volcanoes generate a broad range of seismo-volcanic and infrasonic signals, whose features and variations are often closely related to volcanic activity. The study of these signals is hence very useful in the monitoring and investigation of volcano dynamics. The analysis of seismo-volcanic and infrasonic signals requires specifically developed techniques due to their unique characteristics, which are generally quite distinct compared with tectonic and volcano-tectonic earthquakes. In this work, we describe analysis methods used to detect and locate seismo-volcanic and infrasonic signals at Mt. Etna. Volcanic tremor sources are located using a method based on spatial seismic amplitude distribution, assuming propagation in a homogeneous medium. The tremor source is found by calculating the goodness of the linear regression fit (R ²) of the log-linearized equation of the seismic amplitude decay with distance. The location method for long-period events is based on the joint computation of semblance and R ² values, and the location method of very long-period events is based on the application of radial semblance. Infrasonic events and tremor are located by semblance–brightness- and semblance-based methods, respectively. The techniques described here can also be applied to other volcanoes and do not require particular network geometries (such as arrays) but rather simple sparse networks. Using the source locations of all the considered signals, we were able to reconstruct the shallow plumbing system (above sea level) during 2011.     

The active volcanoes of Kamchatka and Paramushir Island, North Kurils in 2007

Eight strong eruptions of four Kamchatka volcanoes (Bezymyannyi, Klyuchevskoi, Shiveluch, and Karymskii) and Chikurachki Volcano on Paramushir Island, North Kurils took place in 2007. In addition, an explosive event occurred on Mutnovskii Volcano and increased fumarole activity was recorded on Avacha and Gorelyi volcanoes in Kamchatka and Ebeko Volcano on Paramushir Island, North Kurils. Thanks to close cooperation with colleagues involved in the Kamchatkan Volcanic Eruption Response Team (KVERT) project from the Elizovo Airport Meteorological Center and volcanic ash advisory centers in Tokyo, Anchorage, and Washington (Tokyo VAAC, Anchorage VAAC, and Washington VAAC), all necessary precautions were taken for flight safety near Kamchatka.