Volcano-tectonic Research Articles

Using repeating volcano-tectonic earthquakes to track post-eruptive activity in the conduit system at Redoubt Volcano, Alaska

In the year following the end of the 2009 eruption of Redoubt Volcano, Alaska, four signifi cant swarms of low-frequency, lowmagnitude (M L < 0.1) earthquakes occurred at shallow depths beneath the summit. Because swarms of low-frequency (LF) earthquakes preceded eruptions in 1989 and 2009, the posteruption swarms caused considerable concern and prompted the Alaska Volcano Observatory to raise the monitoring levels on three occasions. None of these swarms led to eruptions, however, and most observers (including us) initially concluded that the swarms had been caused by minor stress adjustments in the new lava dome or in the surrounding summit glaciers. New observations reveal that the shallow LF swarms were accompanied by 2 families of repeating earthquakes at depths between 3 km and 6 km below sea level, where the magma storage region is thought to be. These mid-crustal volcano-tectonic (VT) type earthquakes were identical to earthquakes recorded during the 2009 Redoubt eruption more than 6 months earlier. Focal mechanisms demonstrate that these earthquakes have thrust mechanisms inconsistent with the strike-slip nature of regional faulting. Based on these observations, we conclude that they are generated through processes occurring within the magma storage region. The concurrence of the repeating VT earthquakes with the shallow LF swarms indicates that the shallow LF earthquakes were also magmatically driven. Our results emphasize that even brief episodes of low-amplitude earthquake activity, such as the LF swarms observed at Redoubt following the 2009 eruption, can be indicative of magmatic activity. Perhaps more signifi cant, however, is the demonstration that the conduit system at Redoubt remained active, intact, and capable of transporting heat and fl uids to the surface months after the eruption was considered over.

Repeating volcano-tectonic earthquakes at Mt. Etna volcano (Sicily, Italy) during 1999–2009

Repeating volcano-tectonic (VT) earthquakes, taking place at Mt. Etna during 1999–2009, were detected and analyzed to investigate their behavior. We found 735 families amounting to 2479 VT earthquakes, representing ~38% of all the analyzed VT earthquakes. The number of VT earthquakes making up the families ranges from 2 to 23. Over 70% of the families comprise 2 or 3 VT earthquakes and only 20 families by more than 10 events. The occurrence lifetime is also highly variable ranging from some minutes to ten years. In particular, more than half of the families have a lifetime shorter than 0.5 day and only ~10% longer than 1year. On the basis of these results, most of the detected families were considered “burst-type”, i.e., show swarm-like occurrence, and hence their origin cannot be explained by a temporally constant tectonic loading. Indeed, since the analyzed earthquakes take place in a volcanic area, the rocks are affected not only by tectonic stresses related to the fairly steady regional stress field but also by local stresses, caused by the volcano, such as magma batch intrusions/movements and gravitational loading. We focused on the five groups of families characterized by the longest repeatability over time, namely high number of events and long lifetime, located in the north-eastern, eastern and southern flanks of the volcano. Unlike the first four groups, which similarly to most of the detected families show swarm-like VT occurrences, group “v”, located in the north-eastern sector, exhibits a more “tectonic” behavior with the events making up such a group spread over almost the entire analyzed period. It is clear how both occurrence and slip rates do not remain constant but vary over time, and such changes are time-related to the occurrence of the 2002–2003 eruption. Finally, by FPFIT algorithm a good agreement between directions identified by nodal planes and the earthquake epicentral distribution was generally found.

Source amplitudes of volcano-seismic signals determined by the amplitude source location method as a quantitative measure of event size

The amplitude source location (ASL) method, which uses high-frequency amplitudes under the assumption of isotropic S-wave radiation, has been shown to be useful for locating the sources of various types of volcano-seismic signals. We tested the ASL method by using synthetic seismograms and examined the source amplitudes determined by this method for various types of volcano-seismic signals observed at different volcanoes. Our synthetic tests indicated that, although ASL results are not strongly influenced by velocity structure and noise, they do depend on site amplification factors at individual stations. We first applied the ASL method to volcano-tectonic (VT) earthquakes at Taal volcano, Philippines. Our ASL results for the largest VT earthquake showed that a frequency range of 7–12Hz and a Q value of 50 were appropriate for the source location determination. Using these values, we systematically estimated source locations and amplitudes of VT earthquakes at Taal. We next applied the ASL method to long-period events at Cotopaxi volcano and to explosions at Tungurahua volcano in Ecuador. We proposed a practical approach to minimize the effects of site amplifications among different volcano seismic networks, and compared the source amplitudes of these various volcano-seismic events with their seismic magnitudes. We found a proportional relation between seismic magnitude and the logarithm of the source amplitude. The ASL method can be used to determine source locations of small events for which onset measurements are difficult, and thus can estimate the sizes of events over a wider range of sizes compared with conventional hypocenter determination approaches. Previously, there has been no parameter widely used to quantify the sources of volcano-seismic signals. This study showed that the source amplitude determined by the ASL method may be a useful quantitative measure of volcano-seismic event size.

Detection of Seismic Signals from Background Noise in the Area of Campi Flegrei: Limits of the Present Seismic Monitoring

Online Material: Table of seismic station location and seismometer type. Seismic monitoring of active volcanoes is necessary for detecting changes in background volcano dynamics to put in evidence possible precursors of an imminent eruption. Unfortunately, there is no clear relationship between the energy of precursory signals and the strength of subsequent volcanic activity. For example, swarms of small and shallow volcano‐tectonic (VT) or long‐period (LP) quakes (for the usual current classification, see Chouet, 2003) or volcanic tremor episodes may be the only seismic precursor of important phreatic activity (see Barberi et al. , 1992; Nakada and Shimizu, 1999). In case of high noise level, these signals may be thus partly or totally masked, making impossible the detection of these precursors. Accurate monitoring procedures should take into account the minimum magnitude detection threshold characteristics of the operating seismic network. In other words, the minimum event energy that can be detected in a given area should be considered as one of the important parameters to measure to test the real capacity of detecting a warning. This is particularly crucial in risky areas, such as Campi Flegrei, which is so densely populated to be considered as one of the most dangerous volcanoes in the world. Campi Flegrei is characterized by very high background seismic noise, and the knowledge of the minimum detectable magnitude is therefore crucial in this area (Petrosino et al. , 2008). For this reason, one of the purposes of the recent research project named UNREST, funded by the Civil Defense of Italy, was to achieve this objective and give useful constraints to the future improvements of the permanent seismic network, which operates in the Campi Flegrei area for volcano monitoring purposes. In the present paper, we will describe the field survey (Fig. 1) and the data analysis of an experiment carried …

Source mechanisms of persistent shallow earthquakes during eruptive and non-eruptive periods between 1981 and 2011 at Mount St. Helens, Washington

Shallow seismicity between 0 and 3-km depth has persisted at Mount St. Helens, Washington (MSH) during both eruptive and non-eruptive periods for at least the past thirty years. In this study we investigate the source mechanisms of shallow volcano-tectonic (VT) earthquakes at MSH by calculating high-quality hypocenter locations and fault plane solutions (FPS) for all VT events recorded during two eruptive periods (1981–1986 and 2004–2008) and two non-eruptive periods (1987–2004 and 2008–2011). FPS show a mixture of normal, reverse, and strike-slip faulting during all periods, with a sharp increase in strike-slip faulting observed in 1987–1997 and an increase in normal faulting in 1998–2004. FPS P-axis orientations show a ~90° rotation with respect to regional σ1 (N23°E) during 1981–1986 and 2004–2008, bimodal orientations (~N-S and ~E-W) during 1987–2004, and bimodal orientations at ~N-E and ~S-W from 2008–2011. We interpret these orientations to likely be due to pressurization accompanying the shallow intrusion and subsequent eruption of magma as domes during 1981–1986 and 2004–2008 and the buildup of pore pressure beneath a seismogenic volume (located at 0–1km) with a smaller component due to the buildup of tectonic forces during 1987–2004 and 2008–2011.

Trends in the aggregated rate of pre-eruptive volcano-tectonic seismicity at Kilauea volcano, Hawaii

Accelerating rates of volcano-tectonic (VT) earthquakes are commonly observed during volcanic unrest. Understanding the repeatability of their behaviour is essential to evaluating their potential to forecast eruptions. Quantitative eruption forecasts have focused on changes in precursors over intervals of weeks or less. Previous studies at basaltic volcanoes in frequent eruption, such as Kilauea in Hawaii and Piton de La Fournaise on Reunion, suggest that VT earthquake rates tend to follow a power-law acceleration with time about 2 weeks before eruption, but that this trend is often obscured by random fluctuations (or noise) in VT earthquake rate. These previous studies used a stacking procedure, in which precursory sequences for several eruptions are combined to enhance the signal from an underlying acceleration in VT earthquake rate. Such analyses assume a common precursory trend for all eruptions. This assumption is tested here for the 57 eruptions and intrusions recorded at Kilauea between 1959 and 1984. Applying rigorous criteria for selecting data (e.g. maximum depth; restricting magnitudes to be greater than the completeness magnitude, 2.1), we find a much less pronounced increase in the aggregate rate of earthquakes than previously reported. The stacked trend is also strongly controlled by the behaviour of one particular pre-eruptive sequence. In contrast, a robust signal emerges among stacked VT earthquake rates for a subset of the eruptions and intrusions. The results are consistent with two different precursory styles at Kilauea: (1) a small proportion of eruptions and intrusions that are preceded by accelerating rates of VT earthquakes over intervals of weeks to months and (2) a much larger number of eruptions that show no consistent increase until a few hours beforehand. The results also confirm the importance of testing precursory trends against data that have been filtered according to simple constraints on the spatial distribution and completeness magnitude of the VT earthquakes.

Seismic observations of Redoubt Volcano, Alaska — 1989–2010 and a conceptual model of the Redoubt magmatic system

Seismic activity at Redoubt Volcano, Alaska, has been closely monitored since 1989 by a network of five to ten seismometers within 22km of the volcano's summit. Major eruptions occurred in 1989–1990 and 2009 and were characterized by large volcanic explosions, episodes of lava dome growth and failure, pyroclastic flows, and lahars.Seismic features of the 1989–1990 eruption were 1) weak precursory tremor and a short, 23-hour-long, intense swarm of repetitive shallow long-period (LP) events centered 1.4km below the crater floor, 2) shallow volcano-tectonic (VT) and hybrid earthquakes that separated early episodes of dome growth, 3) 13 additional swarms of LP events at shallow depths precursory to many of the 25 explosions that occurred over the more than 128day duration of eruptive activity, and 4) a persistent cluster of VT earthquakes at 6 to 9km depth.In contrast the 2009 eruption was preceded by a pronounced increase in deep-LP (DLP) events at lower crustal depths (25 to 38km) that began in mid-December 2008, two months of discontinuous shallow volcanic tremor that started on January 23, 2009, a strong phreatic explosion on March 15, and a 58-hour-long swarm of repetitive shallow LP events. The 2009 eruption consisted of at least 23 major explosions between March 23 and April 5, again accompanied by shallow VT earthquakes, several episodes of shallow repetitive LP events and dome growth continuing until mid July. Increased VT earthquakes at 4 to 9km depth began slowly in early April, possibly defining a mid-crustal magma source zone.Magmatic processes associated with the 2009 eruption seismically activated the same portions of the Redoubt magmatic system as the 1989–1990 eruption, although the time scales and intensity vary considerably among the two eruptions. The occurrence of precursory DLP events suggests that the 2009 eruption may have involved the rise of magma from lower crustal depths. Based on the evolution of seismicity during the 1989–1990 and 2009 eruptions the Redoubt magmatic system is envisioned to consist of a shallow system of cracks extending 1 to 2km below the crater floor, a magma storage or source region at roughly 3 to 9km depth, and a diffuse magma source region at 25 to 38km depth. Close tracking of seismic activity allowed the Alaska Volcano Observatory to successfully issue warnings prior to many of the hazardous explosive events that occurred in 2009.

Interaction between regional and magma-induced stresses and their impact on volcano-tectonic seismicity

Recent seismological observations have reported volcano-tectonic (VT) earthquakes with fault-plane solutions exhibiting a change of ~90° in their pressure axes relative to the regional stress field. Interestingly, they are recorded mainly during periods preceding eruptive activity and coexisting with those VTs showing a regional trend. This study explains the occurrence of such trends in VT seismicity and discusses the possible patterns of earthquake locations related to the interaction of regional and magma-induced stresses caused by pressurization or depressurization of magmatic sources. Our analysis shows that in the presence of a dominant regional stress field, faulting will occur on faults whose associated slip direction is close to or in agreement with the background regional stress. Failure on faults with an opposite slip direction is unlikely to occur. As magma pressure starts counter-acting the regional stresses, the likelihood of faults to slip in either a regional or opposite sense of slip relative to regional maximum compression increases, allowing the co-existence of possible failure with both slip tendencies, however the spatial distribution of possible faulting differs. As the pressure is progressively increased, the stress patterns gradually approach those corresponding to the absence of a regional stress field. The presented modeling results have implications for volcanic monitoring routines aiming to detect changes in stress patterns. They will ultimately help to improve the correct interpretation of volcano-tectonic seismicity.

Results of seismic monitoring surveys of Deception Island volcano, Antarctica, from 1999–2011

AbstractDeception Island volcano (South Shetland Islands, Antarctica) has been monitored in summer surveys since 1994. We analyse the seismicity recorded from 1999–2011 with a local network and seismic arrays. It includes long-period (LP) events, volcanic tremor episodes and volcano-tectonic (VT) earthquakes. Long-period events are conspicuous, ranging from 58 (2007–08) to 2868 events (2003–04). The highest number of LP events in one day is 243 on 2 February 2001, and there are several discrete periods of intense LP activity. These variations may be related to alterations in the shallow hydrothermal system of Deception Island. The number of VT earthquakes recorded during the surveys range from 4 (2008–09) to 125 (2007–08). In some periods VT distributions are temporally and spatially homogeneous, with a generally low level of seismicity. In other periods we observe a peak of VT activity lasting a few days, concentrated in a particular area. These two patterns may respond to different processes, involving regional stresses and local tectonic destabilization induced by volcanic activity. Overall, this study indicates that over the period 1999–2011 the volcano presented a moderate level of seismicity, and suggests that there has been no significant reactivation of the volcano since the 1999 seismic crisis.

Source Separation on Seismic Data: Application in a Geophysical Setting

This article gives a brief description of the degenerate unmixing estimation technique (DUET) and applies it in a geophysical setting. Source separation has not been fully addressed by geophysicists and is a crucial first step to locating simultaneous sources, which in turn helps with understanding the dynamics of the sources and their source mechanisms. DUET is applied to synthetic seismic signals. The source separation method works success fully to separate two contemporary explosive sources, and two simultaneous oblique tensile cracks in a three-dimensional (3-D) structural model of Mount Etna. The method has also been applied to field recordings on Mount Etna since 2008. The method separates long-period (LP) events from tremor, LP events from volcano tectonic (VT) events, and different sources of tremor from each other.

Multiple causes for non-eruptive seismic swarms at Mt. Martin, Katmai Volcanic Cluster, Alaska (2004–2008)

In January 2006, the Alaska Volcano Observatory recorded a non-eruptive swarm of over 778 volcano-tectonic (VT) earthquakes in the Katmai Volcanic Cluster (KVC), Alaska. The swarm earthquakes have estimated hypocentral depths of 0–3km below sea level (BSL) immediately below the summit of Mt. Martin volcano. In an effort to determine the cause of this swarm, we calculated 134 double-couple fault plane solutions (FPS) for a pre-swarm period (Jan 2004–Dec 2005), the swarm period (January 2006), and a post-swarm period (Feb 2006–Dec 2008), and examined temporal changes in FPS orientations using directional statistical analysis. Statistical tests of uniformity indicate that FPS orientations were heterogeneous during the pre-swarm period, and homogeneous during the 2006 swarm. 3D tests on P- and T-axis orientation data also indicate that FPS homogeneity may have persisted during the post-swarm period. Best-fit axial Von Mises distributions indicate a strongly preferred NNE–SSW P-axis azimuth for swarm FPS, and a NW–SE mean P-axis azimuth for the pre- and post-swarm period FPS. The NW–SE background P-axis azimuth is approximately parallel to regional maximum compressive stress along the northern segment of the Aleutian arc. Thus, we find evidence for a temporary ~90° reorientation of FPS P-axes during the 2006 swarm, and suggest that the change in FPS orientation may be indicative of intrusion of a shallow, small-volume magma- or fluid-filled dike beneath Mt. Martin in early 2006. In contrast, FPS for VT earthquakes comprising minor swarms in 2007 and 2008 indicate only normal faulting, suggesting a fundamentally different causative process from the major 2006 swarm. Our results demonstrate that the VT earthquake swarms which are a common occurrence in the KVC may be caused by several processes, including magma intrusion, tectonic activity, and hydrothermal fluid circulation; and highlight the need for increased monitoring and analysis during future periods of geophysical unrest.

Early Carboniferous minor intrusions of the Clyde Plateau: new data from the Cumbrae islands

ABSTRACT:The Cumbrae islands are unmatched for study of minor intrusions associated with the Early Carboniferous Clyde Lava Plateau. The transitional to subalkaline rock suite is present in a linear swarm of c. 250 ENE to NE dykes – 82 ‘basaltic’ (basalt to hawaiite) and 18 ‘trachytic’ (benmoreite to felsite), with a crustal dilation of 13·3. This swarm provides a unique window on the shallow crustal feeder system of the lava plateau. A ‘trachytic’ secondary swarm, focused on the plateau volcano of Misty Law in the Renfrewshire Hills, is analogous to a similar swarm in the Campsie Fells linked to the Meikle Bin volcano, and a ‘basaltic’ secondary swarm, unusually dense, with crustal dilation of 24·7, may record projection of the Dumbarton–Fintry Volcano–Tectonic lineament from the Campsie and Kilpatrick hills into the Firth of Clyde. Accompanied by bosses, plugs, vents and sheeted structures, the swarms are inferred to overlie fracture systems in the Caledonian basement of the Midland Valley. Internal relationships among extrusive and intrusive rocks of the Early Carboniferous suite and external relationships to dyke suites recording regional tectono-magmatic events of Late Carboniferous and Palaeogene age allow formulation of a new relative chronology for magmatic activity in the western off-shore Midland Valley.

Precursors to dyke-fed eruptions at basaltic volcanoes: insights from patterns of volcano-tectonic seismicity at Kilauea volcano, Hawaii

To investigate the physical controls on volcano-tectonic (VT) precursors to eruptions and intrusions at basaltic volcanoes, we have analyzed the spatial and temporal patterns of VT earthquakes associated with 34 eruptions and 23 dyke intrusions that occurred between 1960 and 1983 at Kilauea, in Hawaii. Eighteen of the 57 magmatic events were preceded by an acceleration of the mean rate of VT earthquakes located close to the main shallow magma reservoir. Using a maximum-likelihood technique and the Bayesian Information Criterion for model preference, we demonstrate that an exponential acceleration is preferred over a power-law acceleration for all sequences. These sequences evolve over time-scales of weeks to months and are consistent with theoretical models for the approach to volcanic eruptions based on the growth of a population of fractures in response to an excess magma pressure. Among the remaining 40 magmatic events, we found a significant correlation between swarms of VT earthquakes located in the mobile south-flank of Kilauea and eruptions and intrusions. The behaviour of these swarms suggests that at least some of the magmatic events are triggered by transient episodes of elevated rates of aseismic flank movement, which could explain why many eruptions and intrusions are not preceded by longer-term precursory signals. In none of the 57 cases could a precursory sequence be used to distinguish between the approach to an eruption or an intrusion, so that, even when a precursory sequence is recognized, there remains an empirical chance of about 40% (24 intrusions from 57 magmatic events) of issuing a false alarm for an imminent eruption.

Mechanism of the 1996–97 non-eruptive volcano-tectonic earthquake swarm at Iliamna Volcano, Alaska

A significant number of volcano-tectonic (VT) earthquake swarms, some of which are accompanied by ground deformation and/or volcanic gas emissions, do not culminate in an eruption. These swarms are often thought to represent stalled intrusions of magma into the mid- or shallow-level crust. Real-time assessment of the likelihood that a VT swarm will culminate in an eruption is one of the key challenges of volcano monitoring, and retrospective analysis of non-eruptive swarms provides an important framework for future assessments. Here we explore models for a non-eruptive VT earthquake swarm located beneath Iliamna Volcano, Alaska, in May 1996–June 1997 through calculation and inversion of fault-plane solutions for swarm and background periods, and through Coulomb stress modeling of faulting types and hypocenter locations observed during the swarm. Through a comparison of models of deep and shallow intrusions to swarm observations, we aim to test the hypothesis that the 1996–97 swarm represented a shallow intrusion, or “failed” eruption. Observations of the 1996–97 swarm are found to be consistent with several scenarios including both shallow and deep intrusion, most likely involving a relatively small volume of intruded magma and/or a low degree of magma pressurization corresponding to a relatively low likelihood of eruption.

Response of Mount Etna to dynamic stresses from distant earthquakes

Influences of distant earthquakes on volcanic systems by dynamic stress transfer are well documented. We analyzed seismic signals and volcanic activity at Mount Etna during two periods, January 2006 and May 2008, that clearly showed variations coincident with distant earthquakes. In the first period, characterized by mild volcano activity, the effect of the dynamic stress transfer, caused by an earthquake in Greece (M = 6.8), was twofold: (1) banded tremor activity changed its features and almost disappeared; (2) a swarm of volcano‐tectonic (VT) earthquakes took place. The changes of the banded tremor were likely due to variations in rock permeability, caused by fluid flows driven by dynamic strain. The VT earthquake swarm probably developed as a secondary process, promoted by the dynamically triggered activation of magmatic fluids. The second period, May 2008, showed an intense explosive activity. During this interval, the dynamic stress transfer, associated with the arrival of the seismic waves of the Sichuan earthquake (M = 7.9), affected the character of the seismo‐volcanic signals and on the following day triggered an eruption. In particular, we observed changes in volcanic tremor and increases of both occurrence rate and energy of long period events. In this case, we suggest that dynamic stress transfer caused nucleation of new bubbles in volatile‐rich magma bodies with consequent buildup of pressure, highlighted by the increase of long period activity, followed by the occurrence of an eruption. We conclude that stresses from distant earthquakes are capable of modifying the state of the volcano.

Effects of the 1989 fracture system in the dynamics of the upper SE flank of Etna revealed by volcanic tremor data: The missing link?

Fracture reactivation is a widespread process in nature even though evidence of magma‐induced reactivation is less documented. Here we provide evidence of the reactivation of a fracture system on the upper flank of the Mt. Etna volcano and consider its possible implications in understanding the recent volcanic and tectonic activity. A NNW–SSE trending fracture, partly accompanied by magma emplacement in the form of a laterally propagating dike, formed in 1989 on the upper SE flank of Etna. Lava effusions in 1991–1993, 2001, and 2006 were associated with volcano‐tectonic (VT) seismicity and ground deformations on the upper part of the volcano, which document the seismogenetic involvement of the 1989 fractures, although without eruptive phenomena along the discontinuity. In addition to the aforementioned episodes of VT seismicity, differences in the characteristics of the background seismic radiation (volcanic tremor) were measured at stations close to these fractures during the eruptive activity on 24 November 2006, for which more detailed volcanological and seismological time histories are available. Moving on from these findings, we analyze volcanic tremor data recorded close to the summit and along the S flank of the volcano to highlight the interactions between seismic radiation and the 1989 fracture system. Centroid location of volcanic tremor and wave field characteristics at stations of the permanent local seismic network of Etna highlight the guidance role played by the 1989 fractures during the eruptive activity on 24 November 2006. In addition, the collected data shed light on hitherto unknown structural features, which appear to connect the volcano summit to the lower SE slope and also play an important role in controlling the instability of the E flank. More generally, this study shows how (1) using an integrated approach, it is possible to link apparently different features to a common structure, showing uniform and distinct dynamics relevant at the volcano scale, and (2) fracture reactivation can also occur by means of magma intrusion, playing an important role in the transfer of magma within a volcanic edifice.

Spatio-temporal evolution of volcano seismicity: A laboratory study

We report a laboratory and microstructural study of a suite of deformation experiments in which basalt from Mount Etna volcano is deformed and fractured at an effective confining pressure representative of conditions under a volcanic edifice (40 MPa). Particular attention was paid to the formation of a fracture and damage zone with which to stimulate coupled hydro-mechanical interactions that create the various types of seismicity recorded on volcanic edifices, and which usually precede eruption. Location of AE events through time shows the formation of a fault plane during which waveforms exhibit the typical high frequency characteristics of volcano-tectonic (VT) earthquakes. We found that these VT earthquakes were particularly pronounced when generated using dry samples, compared to samples saturated with a pore fluid (water). VT events generated during deformation of water saturated sample are characterised by a distinctive high frequency onset and a longer, low frequency coda exhibiting properties often seen in the field as hybrid events. We present evidence that hybrid events are, in fact, the common type of volcanic seismic event with either VT or low frequency (LF) events representing end members, and whose proportion depend on pore fluid being present in the rock type being deformed, as well as how close the rock is to failure. We find a notable trend of reducing instances of hybrid events leading up to the failure stage in our experiments, suggesting that during this stage, the pore fluid present in the rock moves sufficiently quickly to provide a resonance, seen as a LF coda. Our data supports recent modeling and field studies that postulate that hybrid events generated in volcanic areas are likely to be generated through the interaction of hydrothermal fluids moving through a combination of pre-existing microcrack networks and larger faults, such as those we observe in forensic (post-test) examination.

Forecasting eruptions after long repose intervals from accelerating rates of rock fracture: The June 1991 eruption of Mount Pinatubo, Philippines

Mount Pinatubo, Philippines erupted on 7th June 1991, after 500 years of repose and two months of detected seismic unrest. Accelerating volcano-tectonic (VT) earthquake rates in the final days before this eruption have previously been used to develop eruption forecasting models based on extension and coalescence of fractures in the volcanic edifice forming a new pathway for magma ascent to the surface. Here, the precursory acceleration in VT earthquake rates is re-analysed with these forecasting models to test their accuracy and sensitivity to how the VT earthquake data are gathered. Binning the data into fixed VT number intervals rather than fixed time intervals gives more precise forecasts, but gives a ‘forecast’ 24 hours before the eruption began. This is interpreted as the forecast time indicating when the new magma conduit formed. The continued high rate of VT seismicity in the final day before the magma reached the surface is interpreted as widening of the conduit and friction between the magma and country rock during magma ascent.

Emergence of Lava Dome from the Crater Lake of Kelud Volcano, East Java

DOI: 10.17014/ijog.v4i4.83Kelud Volcano (+1731 m) in East Java is one of the most active and dangerous volcanoes in Indonesia. A large lake occupies the summit crater. Historical eruptions generally only lasted for a very short time, mostly no longer than a few hours. The outburst is usually accompanied by pyroclastic flows. On August 2007, the activity of the volcano was initiated by the increase of the temperature of lake water and the change of the colour from typical green to yellow. Activities of the volcano are discussed following the swarms of volcano-tectonic (VT) earthquakes on September 10th, September 26th to 29th, and October 24th to November 2nd. On September 26th to 29th, hypocentral distribution of those VT shifted from 5 km deep to just beneath the crater. The highest number of VT earthquakes occurred on November 1st attaining 50 events, then followed by a swarm of B-type events, where the number reached 1437 events in a day. The volcanic activity peaked on November 3rd when seismic records became saturated, which then was preceded by a sharp increase of lake temperature and a sudden deflation of radial tilt. It suggests that the lava extrusion forming a lava dome was taking place.

Temporal change of characteristics of shallow volcano-tectonic earthquakes associated with increase in volcanic activity at Kuchinoerabujima volcano, Japan

Shallow volcano-tectonic (VT) earthquakes recorded at the Kuchinoerabujima island volcano in southwest Japan are analyzed in order to clarify the role of hydrothermal activity in the development of volcanic seismicity. From analysis of shallow VT earthquakes in 2006, two specific episodes of elevated seismicity are observed in April and November 2006. The VT earthquakes have hypocenters at depths of 0–0.4 km beneath the summit crater, and normal fault focal mechanisms with WNW–ESE extension consistent with the tensional stress field indicated by the alignment of craters and fissures. Although the hypocenters and focal mechanisms are found to be largely invariant during these episodes, the corner frequencies of the VT earthquakes underwent a pronounced increase and decrease accompanying the changes in seismicity rates. The corner frequencies increased to 20–25 Hz approximately one month prior to the onset of elevated seismicity, and then decreased to 10–15 Hz in the period of peak seismicity. The rupture length also decreased at the onset of seismicity, thereafter increasing as the seismicity continued. The peak seismicity in terms of the daily number of VT events was accompanied by inflation around the crater, suggestive of a pressure increase in the volcanic system. It is inferred that the increase in shallow VT seismicity and rupture length is related to the development of a fractured zone. The pressure increase in the volcanic system is attributed to the intrusion of hydrothermal fluids, which is supported by an observed increase in fumarolic temperature and activity. The preceding monochromatic events are thus considered to be generated by the effect of fluid-filled cracks. The shortening of rupture length is then inferred to be related to the closing of non-fluid-filled cracks in the fracture zone under the increasing pressure field, leading to a transition from monochromatic events to low-frequency and shallow VT seismicity.