Swarm Of Volcano-tectonic Earthquakes Research Articles

A statistical analysis of volcano-tectonic earthquake swarms in the off Nicobar region of the Andaman Sea

A statistical analysis of volcano-tectonic earthquake swarms in the off Nicobar region of the Andaman Sea

Features of the Kizimen Volcano area seismicity prior to and during the 2010–2013 eruption

After 82 years of dormancy, Kizimen Volcano in Kamchatka (Russia) produced its first eruption that was supported by seismological observations. The 2010–2013 eruption lasted about three years and was preceded by high rates of seismicity lasting more than one and a half years. In this study we conduct a time-space seismicity analysis and consider the dynamics of the b-value (the slope of the cumulative distribution function of magnitudes). It is shown that there was a constant increase in stress. Which ended with an earthquake with MLmax = 5.2 followed by an eruption. The eruption was characterized by explosive phases and the squeezing out of a very viscous lava flow in 2011–2012. A change from effusive activity to explosive following swarms of volcano-tectonic earthquakes was observed throughout the entire period of the eruption. This confirms the poor mixing of magma in the chamber under the Kizimen Volcano.

On the origin of recent seismic unrest episodes at Deception Island volcano, Antarctica

Deception Island (South Shetland Islands, Antarctica) is an active volcano characterized by a moderate level of seismic activity, dominated by long-period seismicity related to hydrothermal processes in a shallow aquifer. Nevertheless, in the last few decades the volcano has undergone at least three episodes of seismic unrest, in 1992, 1999, and 2015. During these episodes, the pattern of seismicity changed, and swarms of volcano-tectonic earthquakes with hundreds of events in time spans of a few months were detected. These episodes are interpreted as consequences of magmatic intrusions. However, the seismic series display significant differences that lead us to think that the processes initiating the series are not exactly the same in all cases. The 1999 series comprised mostly small-magnitude earthquakes, produced regularly during 1.5 months, and located at shallow depths (<4 km) within the caldera, mostly along a WSW-ENE trend that parallels the Bransfield rift. No precursory seismic activity was reported, and a few months after the series onset the seismicity was back to normal levels. The 2015 series included earthquakes with larger magnitudes, occurring during 5 months in temporal clusters separated by aseismic periods. They were located at deeper levels (<10 km) with epicenters distributed all around Deception Island, at distances up to 30 km. Additionally, distal (~35 km) VT seismicity was reported SE of Livingston Island months before the 2015 series onset, and the seismicity at Deception Island remained anomalously high during a few years. Taking into account the limited data available for the 1992 unrest, we conclude that the 1992 and 1999 series were produced by shallow, short-lived, small-volume (~4·104 m3) intrusions that affected the shallowmost part of the volcanic edifice. On the contrary, the 2015 series was consequence of a deep, long-lasting intrusion that involved a larger volume of ~5·106 m3 (in the range of a VEI 2 eruption) and modified the stress field of the whole volcanic edifice.

Campi Flegrei, Vesuvius and Ischia Seismicity in the Context of the Neapolitan Volcanic Area

Studying seismicity in a volcanic environment provides important information on the state of activity of volcanoes. The seismicity of the Neapolitan volcanoes, Campi Flegrei, Vesuvius, and Ischia, shows distinctive characteristics for each volcano, covering a wide range of patterns and types. In this study we relocated some significant volcano-tectonic earthquake swarms that occurred in Campi Flegrei and Vesuvius. Moreover, we compared the earthquake occurrence evolution, the magnitude and the seismic energy release of the three volcanoes. Also, we considered the results of seismic analysis in the light of geochemical and ground deformation data that contribute to defining the state of activity of volcanoes. In Campi Flegrei, which is experiencing a long term unrest, we identified a seismogenic structure at shallow depth in Pisciarelli zone that has been activated repeatedly. The increasing seismicity accompanies an escalation of the hydrothermal activity and a ground uplift phase. At Vesuvius a very shallow seismicity is recorded, which in recent years has shown an increase in terms of the number of events per year. Earthquakes are usually located right beneath the crater axis. They are concentrated in a volume affected by the hydrothermal system. Finally, Ischia generally shows a low level of seismicity, however, in Casamicciola area events with a moderate magnitude can occur and these are potentially capable of causing severe damage to the town and population, due to their small hypocentral depth (typically &amp;lt; 2.5 km). After the seismic crisis of August 21, 2017 (mainshock magnitude M = 4), the seismicity returned to a low level in terms of occurrence rate and magnitude of earthquakes. The seismicity of these three different volcanic areas shows some common aspects that highlight a relevant role of hydrothermal processes in the seismogenesis of volcanic areas. However, while the main swarms in Campi Flegrei and most of the Vesuvian earthquakes are distributed along conduit-like structures, the seismicity of Ischia is mainly located along faults. Furthermore, the temporal evolution of seismicity in Neapolitan volcanic area suggests a concomitant increase in the occurrence of earthquakes both in Campi Flegrei and Vesuvius in recent years.

Temporal change of the magma plumbing system at Galeras volcano, Colombia, revealed by repeated seismic tomography in 2009–2018

We performed repeated three-dimensional seismic tomography in 2009–2018 for the area influenced by the intersection of the Silvia–Pijao (Buesaco) and Cauca–Almaguer fault systems beneath Galeras volcano, Colombia. We analyzed two periods, 2009–2012 and 2013–2018, which were characterized by explosive eruptive activity and relative quiescence at Galeras, respectively. We used P- and S-wave arrival times of 2848 (2009–2012) and 2289 (2013–2018) earthquakes that occurred beneath the volcano. P- and S-wave velocity (Vp and Vs, respectively) anomalies and their changes between the two periods were interpreted to be associated with magma movement in the volcanic plumbing system, which was controlled by the intersection of the faults. Two possible pathways of magma ascent and accumulation were identified in the fault systems. One, located SW of the active crater, consisted of areas with high Vp/Vs values (>1.9) at depths of −2 to 4 km below sea level, and the other located NE of the active crater with high and low Vp/Vs values. The low-Vp/Vs (<1.7) area at depths of 0 to 2 km may represent vesiculated magma that was partly erupted during explosive eruptions in 2009–2010. These anomalies, found in 2009–2012, displayed clear changes in 2013–2018. Large surface deformation observed between January 2013 and February 2014 may be associated with magma movement from the SW pathway. The largest volcano-tectonic (VT) earthquake, with a magnitude of 5.1, ever recorded at Galeras occurred on 12 June 2018 and was followed by a swarm of VT earthquakes. This VT activity may have been caused by magma intrusion along the NE pathway. On the basis of our tomographic images and geodetic, petrological, and geochemical results, we propose a model of the plumbing system beneath Galeras during 2009–2018.

Disruption of Long-Term Effusive-Explosive Activity at Santiaguito, Guatemala

A comprehensive understanding of the dynamic evolution of active volcanic systems requires extensive sets of geophysical and geological data to fully constrain and understand shifts in eruptive behavior. The Santiaguito dome complex, Guatemala, is a remarkable example of an open-vent volcanic system where continuous eruptive activity has historically been characterized by cycles of effusion and frequent, small to moderate, gas-and-ash explosions. During 2015-2016 the volcano experienced a rapid intensification of activity including large vulcanian explosions, frequently accompanied by pyroclastic density currents. Here we present a chronology of the eruptive activity at Santiaguito from November 2014 - May 2017, compiled from field observations (visual and thermal) and activity reports. We also present seismic and acoustic infrasound data collected during the same period, the longest and largest dataset collected at Santiaguito to date. Three major phases of eruptive activity took place during the study period. The first phase was consistent with the long-term eruptive behavior reported at Santiaguito by previous studies: lava effusion simultaneous with small (\textless1 km plume height), regular (25-200 minute intervals), gas-and-ash explosions. The second phase from July 2015 to September 2016 was defined by large (\textless5-7 km plume height) vulcanian explosions at irregular intervals and often accompanied by pyroclastic density currents. The third phase was marked by a return to effusive activity in October 2016 interspersed by small, gas-rich explosions. Over 6000 explosive events were recorded by seismic and infrasound during the study period and clearly delineate the three phases of activity at the volcano. Furthermore, we present the first documented geophysical evidence of explosion blast waves and volcano-tectonic earthquake swarms at Santiaguito. An important implication of observations is that negative trends in explosion rates at silicic lava dome eruptions cannot be used alone as an indicator of future weaker activity and reduced risk. The dataset introduced here will serve as the foundation for future studies of the long-lived effusive eruption and rapid transitions to explosive volcanic activity at Santiaguito dome complex.

An Overview of the Dynamics of the Volcanic Paroxysmal Explosive Activity, and Related Seismicity, at Andesitic and Dacitic Volcanoes (1960–2010)

Understanding volcanic paroxysmal explosive activity requires the knowledge of many associated processes. An overview of the dynamics of paroxysmal explosive eruptions (PEEs) at andesitic and dacitic volcanoes occurring between 1960 and 2010 is presented here. This overview is based mainly on a description of the pre-eruptive and eruptive events, as well as on the related seismic measurements. The selected eruptions are grouped according to their Volcanic Explosivity Index (VEI). A first group includes three eruptions of VEI 5-6 (Mount St. Helens, 1980; El Chichon, 1982, and Pinatubo, 1991) and a second group includes three eruptions of VEI 3 (Usu volcano, 1977; Soufriere Hills Volcano (SHV), 1996, and Volcan de Colima, 2005). The PEEs of the first group have similarity in their developments that allows to propose a 5-stage scheme of their dynamics process. Between these stages are: long (more than 120 years) period of quiescence (stage 1), preliminary volcano-tectonic (VT) earthquake swarm (stage 2), period of phreatic explosions (stage 3) and then, PEE appearance (stage 4). It was shown also that the PEEs of this group during their Plinian stage “triggered” the earthquake sequences beneath the volcanic structures with the maximum magnitude of earthquakes proportional to the volume of ejecta of PEEs (stage 5). Three discussed PEEs of the second group with lower VEI developed in more individual styles, not keeping within any general scheme. Among these, one PEE (SHV) may be considered as partly following in development to the PEEs of the first group, having stages 1, 3 and 4. The PEEs of Usu volcano and of Volcan de Colima had no preliminary long-term stages of quiescence. The PEE at Usu volcano came just at the end of the preceding short swarm of VT earthquakes. At Volcan de Colima, no preceding swarm of VT occurred. This absence of any regularity in development of lower VEI eruptions may refer, among other reasons, to different conditions of opening of the magmatic conduit during these eruptions.

Complex surface deformation of Akutan volcano, Alaska revealed from InSAR time series

Akutan volcano is one of the most active volcanoes in the Aleutian arc. An intense swarm of volcano-tectonic earthquakes occurred across the island in 1996. Surface deformation after the 1996 earthquake sequence has been studied using Interferometric Synthetic Aperture Radar (InSAR), yet it is hard to determine the detailed temporal behavior and spatial extent of the deformation due to decorrelation and the sparse temporal sampling of SAR data. Atmospheric delay anomalies over Akutan volcano are also strong, bringing additional technical challenges. Here we present a time series InSAR analysis from 2003 to 2016 to reveal the surface deformation in more detail. Four tracks of Envisat data acquired from 2003 to 2010 and one track of TerraSAR-X data acquired from 2010 to 2016 are processed to produce high-resolution surface deformation, with a focus on studying two transient episodes of inflation in 2008 and 2014. For the TerraSAR-X data, the atmospheric delay is estimated and removed using the common-master stacking method. These derived deformation maps show a consistently uplifting area on the northeastern flank of the volcano. From the TerraSAR-X data, we quantify the velocity of the subsidence inside the caldera to be as high as 10mm/year, and identify another subsidence area near the ground cracks created during the 1996 swarm.

Fast wavefield decomposition of volcano-tectonic earthquakes into polarized P and S waves by Independent Component Analysis

In the present work a new approach for the analysis of polarization of seismic signals is proposed. The method is based on Independent Component Analysis and allows the identification and separation of the basic sources, which are naturally polarized into the vertical and horizontal planes. The results from the case study of a swarm of volcano-tectonic earthquakes occurred at Campi Flegrei in October 2015 are impressive: a clear separation of the P- and S-wave seismic phases in the time domain is obtained. In addition, the efficiency of the method in retrieving the polarization parameters is demonstrated by the comparison with other standard techniques. The presented approach provides wavefield decomposition and polarization analysis in a single step, thus avoiding a priori cumbersome filtering procedures and segmentation of the signals. It is useful for discriminating and analysing different seismic phases and can be applied to a variety of volcanic and tectonic signals, therefore it can strongly support all the studies on propagation and source mechanism. Moreover, due to its fastness and robustness this stand-alone tool can be routinely used in the volcano monitoring practice.

Convolutive independent component analysis for processing massive datasets: a case study at Campi Flegrei (Italy)

A novel procedure is proposed to analyse continuous seismic signal on hourly scales to have a prompt discrimination among the different sources. Specifically, this approach is applied to a massive dataset recorded at Campi Flegrei caldera during the year 2006 when a swarm of volcano-tectonic earthquakes occurred. The convolutive independent component analysis is adopted to obtain a clear separation among meteo-marine microseism, anthropogenic noise, hydrothermal tremor in the absence of volcano-tectonic activity, whereas in non-stationary conditions a contribution connected to the corner frequency of the earthquakes emerges. A coarse-grained variable to be monitored continuously is introduced, i.e. the frequency associated with the maximum amplitude of the power spectral density of the deconvolutive independent components. That parameter is sensitive to the variation in the frequency bands of interest (e.g. that corresponding to the corner frequencies of volcano-tectonic events) and can be used as marker of the insurgence of seismic activity.

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.

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.

The origin of volcano-tectonic earthquake swarms

Research Article| June 01, 2006 The origin of volcano-tectonic earthquake swarms Diana C. Roman; Diana C. Roman 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar Katharine V. Cashman Katharine V. Cashman 2Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Diana C. Roman 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Katharine V. Cashman 2Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA Publisher: Geological Society of America Received: 21 Sep 2005 Revision Received: 07 Jan 2006 Accepted: 20 Jan 2006 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 The Geological Society of America, Inc. Geology (2006) 34 (6): 457–460. https://doi.org/10.1130/G22269.1 Article history Received: 21 Sep 2005 Revision Received: 07 Jan 2006 Accepted: 20 Jan 2006 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Diana C. Roman, Katharine V. Cashman; The origin of volcano-tectonic earthquake swarms. Geology 2006;; 34 (6): 457–460. doi: https://doi.org/10.1130/G22269.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Volcano-tectonic (VT) seismicity is commonly recorded prior to and during eruptions. VT seismicity may reflect stresses induced by dike propagation, as indicated by propagating hypocenters and fault-plane solutions (FPS) reflecting regional stresses, or stresses induced by dike inflation, indicated by randomly distributed hypocenters and FPS with pressure axes rotated ∼90° to regional maximum compression. Although numerical models of dike-induced stresses indicate that both regimes should occur during dike emplacement, this has not yet been observed, according to published seismic data. Instead, ∼90° rotated FPS are observed in some cases, while propagating hypocenters mark dike formation in other cases. We suggest that differences in the seismic expression of upper crustal magma migration may result from differences in the regional tectonic setting and in the nature of magma–wall-rock interactions. Ultimately, it may be possible to use information contained in VT seismicity to forecast changes in a volcano's behavior by establishing the characteristic stress field response for a given volcano, or through a deeper understanding of the complex relationships between VT seismicity, local crustal stresses, and the physical mechanisms of magma migration. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Spectral characteristics of volcano-tectonic earthquake swarms in Nevado del Ruiz Volcano, Colombia

Spectral analyses for volcano-tectonic earthquakes were carried out at Nevado del Ruiz Volcano (NRV) for the period 1985–1996 for several earthquake swarms around the volcano, named North, East, West, South and Crater swarm zones. Important spectral peaks for each earthquake swarm zone were found by counting the number of spectra that had the same spectral peaks. Each swarm zone showed some characteristic peaks, which could help to differentiate between them; however, the most important peaks were similar for all the zones. These results suggest that the earthquake swarms at NRV were influenced directly by the source (activity of the volcano) and could also be influenced by the site effect. Some temporal changes were observed in spectral parameters such as a change in the frequency contents in almost all the swarm zones, and the frequency of the P-waves in the West earthquake swarm zone. Before the eruptions on November 13, 1985 and September 1, 1989, P-waves showed low frequencies (1–2Hz) at the West earthquake swarm. After the eruptions, the frequencies of P increased (2–4Hz). This fact showed that changes (decreasing of frequencies) in the spectra of P-waves at the West earthquake swarm could help in the monitoring of volcanic activity at NRV. This swarm zone seems to be related directly with the most important volcanic crises that have occurred. This suggests that the West swarm zone should be monitored in more detail in the future.

Temporal and spectral characteristics of seismicity observed at Popocatepetl volcano, central Mexico

Popocatepetl volcano entered an eruptive phase from December 21, 1994 to March 30, 1995, which was characterized by ash and fumarolic emissions. During this eruptive episode, the observed seismicity consisted of volcano-tectonic (VT) events, long-period (LP) events and sustained tremor. Before the initial eruption on December 21, VT seismicity exhibited no increase in number until a swarm of VT earthquakes was observed at 01:31hours local time. Visual observations of the eruption occurred at dawn the next morning. LP activity increased from an average of 7 events a day in October 1994 to 22 events per day in December 1994. At the onset of the eruption, LP activity peaked at 49 events per day. LP activity declined until mid-January 1995 when no events were observed. Tremor was first observed about one day after the initial eruption and averaged 10h per episode. By late February 1995, tremor episodes became more intermittent, lasting less than 5min, and the number of LP events returned to pre-eruption levels (7 events per day). Using a spectral ratio technique, low-frequency oceanic microseismic noise with a predominant peak around 7s was removed from the broadband seismic signal of tremor and LP events. Stacks of corrected tremor episodes and LP events show that both tremor and LP events contain similar frequency features with major peaks around 1.4Hz. Frequency analyses of LP events and tremor suggest a shallow extended source with similar radiation pattern characteristics. The distribution of VT events (between 2.5 and 10km) also points to a shallow source of the tremor and LP events located in the first 2500m beneath the crater. Under the assumption that the frequency characteristics of the signals are representative of an oscillator we used a fluid-filled-crack model to infer the length of the resonator.

Soufrière Hills Eruption, Montserrat, 1995–1997: Volcanic earthquake locations and fault plane solutions

A total of 9242 seismic events, recorded since the start of the eruption on Montserrat in July 1995, have been uniformly relocated with station travel‐time corrections. Early seismicity was generally diffuse under southern Montserrat, and mostly restricted to depths less than 7 km. However, a NE‐SW alignment of epicentres beneath the NE flank of the volcano emerged in one swarm of volcano‐tectonic earthquakes (VTs) and later nests of VT hypocentres developed beneath the volcano and at a separated location, under St. George's Hill. The overall spatial distribution of hypocentres suggests a minimum depth of about 5 km for any substantial magma body. Activity associated with the opening of a conduit to the surface became increasingly shallow, with foci concentrated below the crater and, after dome building started in Fall 1995, VTs diminished and repetitive swarms of ‘hybrid’ seismic events became predominant. By late‐1996, as magma effusion rates escalated, most seismic events were originating within a volume about 2 km diameter which extended up to the surface from only about 3 km depth ‐ the diminution of shear failure earthquakes suggests the pathway for magma discharge had become effectively unconstricted. Individual and composite fault plane solutions have been determined for a few larger earthquakes. We postulate that localised extensional stress conditions near the linear VT activity, due to interaction with stresses in the overriding lithospheric plate, may encourage normal fault growth and promote sector weaknesses in the volcano.