Seismo-volcanic Activity Research Articles

Evolution of shallow volcanic seismicity in the hydrothermal system of La Soufrière de Guadeloupe following the April 2018 Mlv 4.1 earthquake

La Soufrière de Guadeloupe volcano is characterized by seismo-volcanic activity dominantly linked to an active hydrothermal system. This microseismicity is shallow, mainly triggered in swarms and characterized by repeating earthquakes. Four recurrent families of Volcano-Tectonic (VT) repeaters have been identified and the main repeater accounts for 80% of detections. Stacking repeating seismic waveforms produces MASTER events with a high Signal-to-Noise Ratio (SNR) and thanks to the deployment of a temporary seismic nodal array, their absolute locations can be better constrained. Because we observe systematic positive residuals of P-wave arrival times at almost all stations, we investigate potential bias in the velocity model in the shallow part of the dome. By increasing the P-wave velocity and decreasing the VP/VS ratio from 1.8 to 1.69, we reduce these residuals and improve the local velocity model. We use this new velocity model to locate each VT event relatively to is own MASTER hypocenter. This procedure offers a new image of the hydrothermal seismic activity that we locate under the acid lake of the summit Tarissan crater, along a sub-vertical conduit. We also define a linear relationship between the logarithm of the peak amplitude of seismic events and their duration to obtain a pseudo local magnitude. The approach enables to accurately and automatically estimate the magnitude during the event detection. We show that after the occurrence in April 2018 of the largest earthquake (Mlv 4.1) since the last phreatic eruption in 1976–1977, the swarm frequency and amplitude increased significantly, and a new family of VT repeater emerges, accounting for up to 14% of detections. We show a quiescence in the main repeaters activity in the month following the April 2018 earthquake, suggesting a significant stress release within the volcanic system, which is likely related to dynamic stress changes rather than static stress variations caused by the earthquake. Moreover, we detect the emergence of a new family of repeaters 3 months after the event, located ∼100 m above the main repeater family, that could be related to a fluid pressure increase induced by the regional hydrological cycle, superimposed to the internal forcing driven by the hydrothermal system. Finally, using a statistical approach, we highlight seasonal periodicities in the number of events and the released seismic moment at La Soufrière de Guadeloupe, with a dominant peak of seismic activity in October–November and a second, less significant, peak of in April.

The role of the Red Sea rift and the inherited geological structures in the seismo-volcanic activity along the rift flanks

The role of the Red Sea rift on the development and the discrepancy of the seismo-volcanic activity along its flanks is still a debate. Here we tried to resolve this debate by a high-resolution tomographic imaging of the northern Red Sea subsurface structures. For the first time, a large number of arrival time data from the Saudi and Egyptian seismic networks were used. The area comprises the Lunayyir volcanic field in the Saudi Arabia, the Abu-Dabbab seismogenic zone in the Egyptian Red Sea coast, and the Zabargad Shear Zone in the Red Sea. This study revealed clear images of the Red Sea rift-related structures along its flanks. The subsurface extension of the different shear and suture zones existing in the Arabian Nubian shield are well imaged. It is found that the Lunayyir seismo-volcanic activity is possibly controlled by the reactivation of the Yanbu suture zone that is associated with steeply northwestward dipping structure. The suture detachments were observed and identified across the Red Sea as low-V channels. The Egyptian Eastern Desert is found to be highly deformed with crustal-scaled low-V structures which were inherited from the early period of Gondwana collision. The NE-SW strike slip faults along the Red Sea were found to be a part of this deformation that are extended deeply in the crust. The Abu-Dabbab and Marsa-Alam areas were strongly influenced by this deformation with possible magma intrusions. This study provides new insights on the role of the Red Sea in the seismo-volcanic activity along its flanks.

Sudden alteration in the seismic behavior of the Domuyo volcanic complex in the southern volcanic backarc zone, Argentina

The Domuyo volcanic complex (DVC) and its geothermal field in the retroarc zone of the southern Central Andes of Argentina present reduced seismicity according to different catalogs (USGS and INPRES). However, in 2015/2016, a local project was carried out in the area to describe its seismo-volcanic activity, registering a large number of volcano-tectonic (VT) events (538 VT). Considering there is scarce information on these events, this study focuses on analyzing the Domuyo Volcanic Complex (DVC) to assess its seismicity. Therefore, we installed a local seismological network in the study area and compared results with data registered by other authors. Four seismological networks were used, to obtain a more precise location of the seismic events and calculate the focal mechanisms of earthquakes with magnitudes greater than 2. For the first record of crustal seismicity detected by INPRES the September 10th, 2016 with a Ml 3.3, we calculated the focal mechanism with two possible solutions: a thrust solution with a strike component and a favored normal solution with a strike component. Additionally, we relocated the largest event in the Domuyo region on March 27th, 2019, with a magnitude of 4.4 (NEIC – USGS) and focal mechanism with a normal solution and a small strike component, obtaining a shallower depth of 3.9 km instead of 10 km. The new seismological data used in this paper, correspond to September 10th 2016, and two different time periods, the first comprising continuous data from March to April 2019, when the largest registered earthquake occurred in the Domuyo region, and the second from December 2019 to January 2021. At these periods, registered seismicity had magnitudes Ml between 1.9 and 2.8, and focal depths between 1.8 and 5.2 km. Four of these events count with focal mechanisms with extensional and limited strike-slip components that are tentatively linked to the known neotectonic structures affecting the western slope of the DVC. This seismic sequence agrees with previous proposals in which degasification from a magmatic body at shallow depths constitutes the trigger factor.

Array analysis of seismo-volcanic activity with distributed acoustic sensing

SUMMARY Continuous seismic monitoring of volcanoes is challenging due to harsh environments and associated hazards. However, the investigation of volcanic phenomena is essential for eruption forecasting. In seismo-volcanic applications, distributed acoustic sensing (DAS) offers new possibilities for long-duration surveys. We analyse DAS strain rate signals generated by volcanic explosions and tremor at Stromboli volcano (Italy) recorded along 1 km of dedicated fibre-optic cable. We validate DAS recordings with colocated nodal seismometers. Converting node measurements to strain rate, we observe a perfect match in phase between DAS and node waveforms. However, DAS amplitudes appear to be around 2.7 times smaller than those of node records, which we explain as due to the inefficient ground-to-fibre strain transfer in the loose cable. We invert time delays between strain rate waveforms and confirm that the DAS enables us to retrieve a dominant and persistent seismic source in the proximity of active craters. This stable source location is confirmed by node array analyses. Despite an observed high noise level of strain rate signals outside a range of 2–15 Hz, our results demonstrate the potential of this new technology in monitoring volcanic areas.

The composition of gas emissions at Petite Terre (Mayotte, Comoros): inference on magmatic fingerprints

The Comoros archipelago is an active geodynamic region located in the Mozambique Channel between East continental Africa and Madagascar. The archipelago results from intra-plate volcanism, the most recent eruptions having occurred on the youngest island of Grande Comore and on the oldest one of Mayotte. Since 2018, the eastern submarine flank of Mayotte has been the site of one of the largest recent eruptive events on Earth in terms of erupted lava volume. On land, the most recent volcanic activity occurred in Holocene on the eastern side of Mayotte, corresponding to the small Petite Terre Island, where two main and persistent gas seep areas are present (Airport Beach, namely BAS, and Dziani Dzaha intracrateric lake). The large submarine eruption at the feet of Mayotte (50 km offshore; 3.5 km b.s.l.) is associated with deep (mantle level) seismic activity closer to the coast (5–15 km offshore) possibly corresponding to a single and large magmatic plumbing system. Our study aims at characterizing the chemical and isotopic composition of gas seeps on land and assesses their potential link with the magmatic plumbing system feeding the Mayotte volcanic ridge and the recent submarine activity. Data from bubbling gases collected between 2018 and 2021 are discussed and compared with older datasets acquired between 2005 and 2016 from different research teams. The relation between 3 He/ 4 He and δ 13 C-CO 2 shows a clear magmatic origin for Mayotte bubbling gases, while the variable proportions and isotopic signature of CH 4 is related to the occurrence of both biogenic and abiogenic sources of methane. Our new dataset points to a time-decreasing influence of the recent seismo-volcanic activity at Mayotte on the composition of hydrothermal fluids on land, whose equilibrium temperature steadily decreases since 2018. The increased knowledge on the gas-geochemistry at Mayotte makes the results of this work of potential support for volcanic and environmental monitoring programs.

Insights into the relationship between the Red Sea rift-related structures and the seismo-volcanic activity in Harrat Lunayyir, Saudi Arabia: A seismic tomography study

Harrat Lunayyir is one of the youngest Cenozoic volcanic fields in western Saudi Arabia, which in 2009 experienced a seismic swarm of about 34,000 events with a maximum magnitude of M5.4. Many studies debated about the relationship of a depicted shallow-level magma intrusion with the Red Sea rifting and the channelized northward flow from the Afar mantle plume. In this study, a detailed seismic tomography inversion for the P- and S-wave velocities, and the Poisson’s ratio structures beneath Harrat Lunayyir and the adjacent Red Sea offshore area is presented. The tomographic images showed evidence of magma intrusions beneath the Lunayyir Swarm Area (LSA) characterized by low P- and S-wave velocity and high Poisson’s ratio anomalies down to 21 km depth. These anomalies are in juxtaposition with some high-velocity anomalies attributed to remnants of solidified magma intrusions relevant to previous episodes of volcanicity. Beneath the Red Sea, the lithospheric mantle structure is clearly observed as a high P-wave velocity high-Poisson’s ratio anomaly at a depth of 20–40 km. Traces of high-Vp anomalies with anomalous Poisson’s ratio are observed in the area between the Red Sea and the LSA suggesting that a channel of hot lithospheric mantle material may link the Red Sea rift structure with the LSA. We suggest that the Red Sea drifting and its related processes in the Zabargad Shear Zone might have significantly contributed to the early formation of the Lunayyir volcanic field and could possibly have in an impact on any magmatic activity in the future.

Offshore Mayotte volcanic plumbing revealed by local passive tomography

A new submarine volcano has been discovered offshore Mayotte, a part of the Comoros volcanic archipelago located between Africa and Madagascar. The edifice arose from the sea-floor following a seismo-volcanic crisis that started in May 2018. This seismo-volcanic activity highlights very deep magma reservoirs and dykes in the East Mayotte volcanic system. Since the crisis, the region has experienced >2000 earthquakes with magnitude ≥3.5 and activity continues today (August 17, 2021). The earthquakes are unusually deep and distributed into two swarms: one 5–15 km east of Petite-Terre at 25–55 km depth and a second 25 km away at 30–50 km depth. Significant subsidence of Mayotte to the East has been assigned to the drainage of a deep magma chamber, inferred to be located 30 km from the coast. However, at present, the earthquake locations and geodetic observations have not been sufficient to image entirely the structure of the volcanic plumbing system. In this study, we construct Vp, Vs, dVp, dVs and Vp/Vs 3D velocity models to assess the deeper structure of the young volcano plumbing system, offshore and East of Mayotte. Using >3000 earthquakes from an ongoing monitoring effort, and a 1D velocity model determined onboard, we jointly inverted for velocity structures, earthquake locations, origin times, and station corrections using LOTOS software. The calculated 3D velocity models highlight a complex volcanic system down to 40 km depth. Specifically, we image 3 interpreted reservoirs, more or less consolidated/old. The main reservoir is located at about 30 km depth and deeper, making it one of the deepest magmatic chamber imaged. The reservoirs are connected by several old crystallized conduits, whose existence could have been influenced by the presence of an old fracture zone, globally oriented N130°, due to a regional strike-slip motion of the lithosphere. Moreover, gas-saturated rock may be present below the currently degassing Horse Shoe structure. We were unable to image connections between the new volcanic edifice and reservoirs or conduits due to a lack of resolution in that part of the study area.

Changes in Diffuse Degassing From the Summit Crater of Teide Volcano (Tenerife, Canary Islands) Prior to the 2016 Tenerife Long‐Period Seismic Swarm

AbstractGround surface diffuse degassing and heat flow surveys were conducted at the summit crater of Teide volcano (Tenerife) on a roughly monthly basis from 2013 to 2017 to evaluate their degree of temporal variation and compare with seismovolcanic activity in and around Tenerife Island. We report that significant changes in survey physicochemical and chemical parameters through time were observed. Several geochemical signals record the occurrence of magmatic fluid injections in the hydrothermal system of Tenerife: during June‐August of 2016 there were increases in diffuse H2S emission, observed relatively higher values of diffuse H2S/CO2 He/CO2, and CO2/CH4 degassing ratios and changes in the carbon isotopic ratio of diffuse CO2 degassing. Convective mixing within a magma reservoir triggered by an injection of fresh magma is confirmed as the most plausible scenario to explain the source of fluid injection into the hydrothermal system of Teide.

Volcanic and volcano-tectonic activity forecasting: a review on seismic approaches

Forecasting volcanic activity is a difficult problem that is being addressed worldwide from different perspectives. Significant advances have been made after the introduction of non-linear dynamical systems theory and the use of power-law distributions of different geophysical parameters in the Earth Sciences. In particular, frequency-magnitude power-law statistics evidences the scale-invariance and self-organization of seismicity, and brittle fracture models show that under certain conditions, a precursory causal evolution characterized by an accelerating strain rate culminates in a catastrophic failure of the system under stress. The precursory organization of the seismicity and the distinct characteristics of the seismic events have allowed the development of forecasting tools. In this work we present some examples of forecasting methods based on seismic observations at different volcanoes in the world, and how results and experiences has been used to improve both hardware and software tools developed for short-term forecasting of volcanic and seismo-volcanic activity.

Multi-parametric approach to the analysis of soil radon gas for its validation as geoindicator in two sites of the Phlegrean Fields caldera (Italy)

Radon is considered one of the short- and immediate-term earthquake precursors by International Association of Seismology and Physics of the Earth’s Interior. For this reason, the accurate and reliable measurement of its specific activity in soil gas is mandatory. Since its presence is influenced by many environmental factors, in order to eliminate this influence on the radon activity measurement a multi-parametric and multi-site analysis is necessary. Following this approach, a continuous radon monitoring was carried out in two sites of the Phlegrean Fields caldera (Campania, Italy) using the RaMonA system, which also allows for the measurement of temperature, relative humidity and pressure. In particular, the dependence of measured radon specific activity on the meteorological conditions was studied using the multiple linear regression method. The analysis was implemented also by testing data averages on different timescales. In this paper, the results of a preliminary analysis performed on data collected throughout 2012 are presented; the outcomes obtained allow to emphasize site-specific behavior and to discriminate apparent radon “anomalies” of climatic origin from those tied to the phenomena occurring in the earth’s crust. The method employed showed its usefulness by removing the effects of meteorological conditions and thus to better identify the possible radon anomalies caused by seismo-volcanic activity. The peculiarities of the different sites and some correlations with earthquakes are discussed.

Development and recent activity of the San Andrés landslide on El Hierro, Canary Islands, Spain

Extremely voluminous landslides with a long run-out (also known as megalandslides) on oceanic volcanic islands are infrequent denudational processes on such islands. At the same time, they represent a major geological hazard that must be looked into to avoid negative consequences for the inhabitants of these islands. Their occurrence can be related to periods of intense seismo-volcanic activity, similar to that which occurred on El Hierro Island over 2011–2012. Landslides on volcanic islands are studied using onshore and offshore geological, geophysical and geomorphological records, considering their unique triggering conditions (e.g. lava intrusions, eruptive vents, magma chamber collapses). Previous work has pointed out similarities between specific cases of landslides on volcanic islands and deep-seated gravitational slope deformations (DSGSDs) which are typical in high mountain settings. Nevertheless, the methodological approaches and concepts used to investigate DSGSDs are not commonly applied on volcanic islands studies, even though their use may provide new information about the development stage, recent movements and future hazards. Therefore, this approach for studying the San Andrés landslide (SAL) on El Hierro (Canary Islands) has been developed applying a detailed morphological field mapping, an interpretation of digital elevation models, structural measurements, kinematic testing, and a precise movement monitoring system. The acquired information revealed a strong structural influence on the landslide morphology and the presence of sets of weakened planes acting as the sliding surfaces of the SAL or secondary landslides within its body. The presence of secondary landslides, deep erosive gullies, coastal cliffs and high on-shore relative relief also suggests a high susceptibility to future landslide movement. Direct monitoring on the landslide scarps and the slip plane, performed between February 2013 and July 2014, using an automated optical-mechanical crack gauge with a precision of up to 10−2mm, detected creep movement in the order of 1mmyr−1 with a persistent sinistral component as well as episodic horizontal and a subtle vertical movement. This monitoring will continue in the future in order to verify the initial observations, which instead of long-term movement patterns, could represent a landslide response to the period of an intense seismo-volcanic activity during 2011–12.

Advances in seismic monitoring at Deception Island volcano (Antarctica) since the International Polar Year

<p>Deception Island is an active volcano located in the south Shetland Islands, Antarctica. It constitutes a natural laboratory to test geophysical instruments in extreme conditions, since they have to endure not only the Antarctic climate but also the volcanic environment. Deception is one of the most visited places in Antarctica, both by scientists and tourists, which emphasize the importance of volcano monitoring. Seismic monitoring has been going on since 1986 during austral summer surveys. The recorded data include volcano-tectonic earthquakes, long-period events and volcanic tremor, among others. The level of seismicity ranges from quiet periods to seismic crises (e.g. 1992-1993, 1999). Our group has been involved in volcano monitoring at Deception Island since 1994. Based on this experience, in recent years we have made the most of the opportunities of the International Polar Year 2007-2008 to introduce advances in seismic monitoring along four lines: (1) the improvement of the seismic network installed for seismic monitoring during the summer surveys; (2) the development and improvement of seismic arrays for the detection and characterization of seismo-volcanic signals; (3) the design of automated event recognition tools, to simplify the process of data interpretation; and (4) the deployment of permanent seismic stations. These advances help us to obtain more data of better quality, and therefore to improve our interpretation of the seismo-volcanic activity at Deception Island, which is a crucial step in terms of hazards assessment.</p>

Evidence of persistent seismo-volcanic activity at Marsili seamount

<p>The Marsili submarine volcano is the largest European volcano, and it can be considered as the key to our understanding of the dynamics of the spreading and back-arc lithosphere formation in the Tyrrhenian sector [Marani et al. 2004, and references therein]. Despite its size, it is very difficult to monitor due to its geographical position [D'Alessandro et al. 2011], and it still remains little known. In 2006, the Centro Nazionale Terremoti (National Earthquake Centre) of the Istituto Nazionale di Geofisica e Vulcanologia (INGV) deployed a broadband ocean-bottom seismometer with hydrophone (OBS/H) [Mangano et al. 2011] on the flat top of Marsili volcano, at a depth of ca. 790 m. In only nine days, the instrument recorded ca. 800 seismo-volcanic events [D'Alessandro et al. 2009]. This revealed the intense seismo-volcanic activity of Marsili volcano for the first time. […]</p><p> </p>

Forecasting seismo-volcanic activity by using the dynamical behavior of volcanic earthquake rates

We present a novel approach for short-term forecasting of volcano seismic activity. Volcanic earthquakes can be seen as a response mechanism of the earth crust to stresses induced by magma injection. From this point of view the temporal evolution of seismicity can be represented as a diffusion process which compensates pressure differences. By means of this dynamical approach we are able to estimate the system behavior in the near future which in turn allows us to forecast the evolution of the earthquake rate for the next time span from actual and past observations. For this purpose we model the earthquake rate as a random walk process embedded in a moving and deforming potential function. The center of the potential function is given by a moving average of the random walk's trace. We successfully apply this procedure to estimate the next day seismicity at Soufrière Hills volcano, Montserrat, over a time period of six years. When comparing the dynamical approach to the well known method of material failure forecast we find much better predictions of the critical stages of volcanic activity using the new approach.

Seismicity and active tectonics at Coloumbo Reef (Aegean Sea, Greece): Monitoring an active volcano at Santorini Volcanic Center using a temporary seismic network

The volcanic center of Santorini Island is the most active volcano of the southern Aegean volcanic arc. Α dense seismic array consisting of fourteen portable broadband seismological stations has been deployed in order to monitor and study the seismo-volcanic activity at the broader area of the Santorini volcanic center between March 2003 and September 2003. Additional recordings from a neighbouring larger scale temporary network (CYCNET) were also used for the relocation of more than 240 earthquakes recorded by both arrays. A double-difference relocation technique was used, in order to obtain optimal focal parameters for the best-constrained earthquakes. The results indicate that the seismic activity of the Santorini volcanic center is strongly associated with the tectonic regime of the broader Southern Aegean Sea area as well as with the volcanic processes. The main cluster of the epicenters is located at the Coloumbo Reef, a submarine volcano of the volcanic system of Santorini Islands. A smaller cluster of events is located near the Anydros Islet, aligned in a NE–SW direction, running almost along the main tectonic feature of the area under study, the Santorini–Amorgos Fault Zone. In contrast, the main Santorini Island caldera is characterized by the almost complete absence of seismicity. This contrast is in very good agreement with recent volcanological and marine studies, with the Coloumbo volcanic center showing an intense high-temperature hydrothermal activity, in comparison to the corresponding low-level activity of the Santorini caldera. The high-resolution hypocentral relocations present a clear view of the volcanic submarine structure at the Coloumbo Reef, showing that the main seismic activity is located within a very narrow vertical column, mainly at depths between 6 and 9 km. The focal mechanisms of the best-located events show that the cluster at the Coloumbo Reef is associated with the “Kameni–Coloumbo Fracture Zone”, which corresponds to the western termination of the major ENE–WSW Santorini–Amorgos Fault Zone. Stress–tensor inversion of the available fault plane solutions from Coloumbo Reef, as well as existing neotectonic fault information from NE Santorini (Coloumbo peninsula), suggests that the NE Santorini–Coloumbo faults belong to a single rupture system, with a ~ 30° rotation of the local stress field with respect to the NNW–SSE regional extension field of the southern Aegean Sea. The observed change of the fault plane solutions shows that local conditions at the Coloumbo submarine volcano area control the observed faulting pattern.

Temporal relationship between the 2000 Miyake–Kozu seismovolcanic activity and the 2000 Tokai slow-slip event

We investigated the temporal relationship between the two events, namely, the seismovolcanic activity near the Miyakejima and Kozushima islands and the slow-slip event along the plate boundary in the Tokai district. The islands are located on the east of Tokai, and the Tokai slow-slip event was discovered immediately after the large crustal deformation caused by the volcanic activity in the Miyakejima–Kozushima regions ceased. However, the order of occurrence of these events is still controversial and its recognition will help us to understand the tectonic processes of the central part of Japan, where many volcanic and seismic activities occur. For this purpose, we applied the statistical approach (Kawamura, M., Yamaoka, K., 2006. Spatiotemporal characteristics of the displacement field revealed with principal component analysis and the mode-rotation technique, Tectonophys., 419, 55–73), which consists of principal component analysis (PCA) and a mode rotation procedure, to the displacement field provided by the nationwide GPS network (GEONET) in order to obtain the characteristic structures of spatiotemporal crustal deformation caused by the above two events. We divided the time period of analysis into two sections (namely, June 26, 1999 to June 25, 2000 and June 26, 2000 to June 25, 2002) by the day when the magma intrusion occurred beneath the Miyakejima volcano on June 26, 2000. The spatial and temporal modes for the first time period did not indicate any significant spatiotemporal patterns corresponding to the two events. This indicates the absence of episodic crustal deformations during this time period. On the contrary, the modes for the latter time period included the changes caused by these events. The two major modes included the spatiotemporal structures of the first and latter half periods of the Miyake–Kozu seismovolcanic activity. The characteristic pattern of the crustal deformation corresponding to the Tokai slow-slip event was found in the fourth mode, which was prominent after the beginning of the Miyake–Kozu seismovolcanic activity. From these results, we conclude that the crustal deformation caused by the Tokai slow-slip event did not start before the Miyake–Kozu seismovolcanic activity.

Monitoring the reawakening of Canary Islands' Teide Volcano

Following more than 30 years of seismic and volcanic quiescence, the Canary Islands region located off the northwestern coast of Africa started to show signs of seismovolcanic activity at the end of 2003 (Figure 1). In spring 2004, there was a significant increase in the number of seismic events (a mixture of volcano‐tectonic events and regional earthquakes with pure volcanic events such as tremors and long‐period signals) located inland on Tenerife Island.The increase ofactivity in 2004 coincided with an increase of fumarolic activity at the Teide volcano on Tenerife Island, an increase in the emission of carbon dioxide in the northwestern part of the island, and changes in the gravimetric field on the northern flank of the volcano. After several seismic events had been felt by the population, the first alert level was declared by the civil protection division of the local government.This apparent reawakening of Teide, which last erupted in 1909, provides an opportunity to study from the initial stages the reactivation of this volcanic area and its related phenomena.

A36 傾斜及び歪みの連続観測で明らかになった伊東沖における地震・火山活動の特徴

A36 傾斜及び歪みの連続観測で明らかになった伊東沖における地震・火山活動の特徴

The recent seismo-volcanic activity at Deception Island volcano

Abstract This paper reviews the recent seismic studies carried out at Deception Island, South Shetland Islands, Antarctica, which was monitored by the Argentinean and Spanish Antarctic Programs since 1986. Several types of seismic network have been deployed temporarily during each Antarctic summer. These networks have consisted of a variety of instruments, including radio-telemetered stations, autonomous digital seismic stations, broadband seismometers, and seismic arrays. We have identified two main types of seismic signals generated by the volcano, namely pure seismo-volcanic signals, such as volcanic tremor and long-period (LP) events, and volcano-tectonic (VT) earthquakes. Their temporal distributions are far from homogeneous. Volcanic tremors and LP events usually occur in seismic swarms lasting from a few hours to some days. The number of LP events in these swarms is highly variable, from a background level of less than 30/day to a peak activity of about 100 events/h. The occurrence of VT earthquakes is even more irregular. Most VT earthquakes at Deception Island have been recorded during two intense seismic crises, in 1992 and 1999, respectively. Some of these VT earthquakes were large enough to be felt by researchers working on the island. Analyses of both types of seismic events have allowed us to derive source locations, establish seismic source models, analyze seismic attenuation, calculate the energy and stress drop of the seismic sources, and relate the occurrence of seismicity to the volcanic activity. Pure seismo-volcanic signals are modelled as the consequence of hydrothermal interactions between a shallow aquifer and deeper hot materials, resulting in the resonance of fluid-filled fractures. VT earthquakes constitute the brittle response to changes in the distribution of stress in the volcanic edifice. The two VT seismic series are probably related to uplift episodes due to deep injections of magma that did not reach the surface. This evidence, however, indicates the high potential for future volcanic eruptions at Deception Island.

A new strategy to measure radon in an active volcanic island (Tenerife, Canary Islands)

A promising tool in forecasting seismo-volcanic activity is the analysis of temporal and spatial variations of soil gases, in particular radon (222Rn). However, the modulation of the radon signal induced by environmental factors could mask information directly related to volcanic activity and, hence, radon sensors should be placed in locations where the number and effect of environmental parameters are minimised. There is a vast network of galleries, underground tunnels for deep-water prospecting, in the volcanic island of Tenerife, Canary Islands. They provide very stable conditions where radon monitoring could be carried out. In this sense, radon data were collected for 2 years at two different galleries in an attempt to assess the potential of these locations as observatories of the gas dynamics within the volcanic edifice of Tenerife. It turned out that natural ventilation, driven by temperature gradients between the air inside and outside the gallery, strongly disturbs the radon signal. We recommend to place detectors where the thermal gradient is minimised, namely far away from the entrance of the gallery or where internal temperatures are much higher than external air temperatures. In any case, we believe galleries are good places to monitor radon emissions in the island of Tenerife.