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A Mw = 6.4 earthquake, possibly related to submarine volcanism, occurred on October 23, 2006, in the vicinity of Monday Seamount, located in the Izu-Bonin-Mariana arc. Despite the size of the earthquake, the precise location of aftershocks is intractable with the routine data processing because of poor station coverage and complex waveforms traversing the oceanic lithosphere. This study overcomes these problems by using the Broadband Ocean Bottom Seismometer (BBOBS) array data with better azimuthal coverage and by developing new methods to detect the P-wave arrival time to locate the aftershocks precisely. Most of the relocated epicenters are located in the rift zone next to the edifice of the seamount. We interpret this pattern as the seismic activity induced by stress perturbation due to the dike intrusion. • We developed a new method to locate epicenters, giving sparsely distributed seismic sites. • We applied the new method to a Mw = 6.4 earthquake and aftershocks that are possibly related to submarine volcanism of Monday Seamount, Izu-Bonin-Mariana arc. • The majority of the newly obtained epicenters are distributed are in the rift zone near the edifice of Monday Seamount. • The aftershock distribution implies dike intrusion to the west of Monday Seamount. [ABSTRACT FROM AUTHOR]

Abstract—The last activation of the Koryakskii volcano in 2008–2009 was accompanied by intense fumarolic and seismic activity. Volcanic activity peaked in March–April 2009 when ash plume rose to a height of 5.5 km and extended laterally over more than 600 km. To understand the dynamics of the volcanic processes and to forecast the further course of the events, it is relevant to establish whether the eruption was associated with a rise of magma to beneath the volcanic edifice or caused by fracturing of the volcano's basement and penetration of groundwater into a high temperature zone. Based on the analysis of the images from the Japanese satellite ALOS-1 using satellite radar interferometry methods, the slope displacements of the Koryakskii volcano during its last activation have been estimated for the first time. The displacements reach 25 cm and cannot be explained by the formation of a layer of volcanic ash deposits or by the slope processes. The most likely cause of the displacements should be recognized to be the intrusion of magmatic material into the volcano edifice with the formation of a fracture with its lower edge at a depth of 0.5 km above sea level, with a size of 1.0 and 2.4 km along the strike and dip, respectively, and with a dip angle from 45° to 60°. Therefore, the processes taking place beneath the volcano can be threatening to the nearby localities and infrastructure and require continuous monitoring. [ABSTRACT FROM AUTHOR]

Rift obliquity is known to be very frequent at divergent plate boundaries and shear components in the opening of fissures have been detected through seismicity in the monitoring of recent rifting events. However, the field structural evidence of obliquity in active rifts for Holocene events is still poorly studied, despite its relevance at both the lithospheric and upper-crust scales. We performed extensive high resolution UAV surveys on Holocene terrains in four areas along the active rift in the Northern Volcanic Zone of Iceland, to map structures and morphotectonic features in detail. We did a kinematic analysis and found an increase of the obliquity from N to S. We further discuss the origins of this obliquity and suggest that the combination of inherited structures reactivation, magmatic intrusions, and topography influence on the propagation of both fractures and magma intrusions can have an impact on the mismatch between rift opening direction and the far-field strain. We conclude that obliquity can be inferred for Holocene rifting events from surface structural data. It is thus fundamental to take into account the existing structural pattern and its (re)activation history in active rift zones to avoid missing out on possible preferential structure reactivation, magma propagation ways, and eruption sites during the monitoring of ongoing rifting events. • Holocene rifting obliquity in Iceland inferred from surface structural field data. • UAV surveys for extensive and high-resolution structural analysis. • Morphotectonic analysis to infer faults and graben reactivation and dike intrusions. [ABSTRACT FROM AUTHOR]

The southern part of the Okinawa Trough forms a narrow back-arc rift basin where evidence for submarine volcanoes and active hydrothermal venting is observed. The region is also known to cause large crustal earthquakes frequently which often accompany a rapid increase in seismicity rate. Although such swarm-like activities are common in active volcanic regions and are considered to be primarily induced by crustal fluid flows, potential interactions between tectonic, magmatic and hydrologic processes have been poorly examined in the southern Okinawa Trough despite these processes happening in the proximity. Here, I report the spatial and statistical characteristics of seismic activity in the southern Okinawa Trough and discuss their relation to other rifting-related phenomena. Most of the earthquakes with magnitude greater than 5 are localized around the rift axes (the Yaeyama Rift and the Yonaguni Rift) where seismic reflection data indicated the presence of solidified magmatic intrusions into the shallow sedimentary layers. I found the areas around the rift axes show low b values of < 0.8 and suggest that stress changes directly induced by dike intrusions beneath the rift axes control the occurrence of swarm activities. On the contrary, regions with high b values (> 1.2) are found around the Ishigaki Knoll and the Hatoma Knoll. These two areas are located between the rift axes and the Ryukyu Islands and correspond to potential submarine volcanoes proposed by seafloor bathymetry and seismic reflection images. This result may constrain the location of the volcanic front in the region. [ABSTRACT FROM AUTHOR]

A half century of monitoring of the Northern Volcanic Zone of Iceland, a branch of the North America—Eurasia plate boundary, shows that the seismicity is very unevenly distributed, both in time and space. The four central volcanoes at the boundary, Þeistareykir, Krafla, Fremrinámar, and Askja, show persistent but very low-level seismicity, spatially coinciding with their high-temperature geothermal systems. On their rift structures, on the other hand, seismicity is almost absent, except during rifting episodes. Krafla went through a rifting episode in 1975–1984 with inflation, interrupted by 20 diking events with extensive rifting, eruptive activity, and intense seismicity along an 80 km long section of the rift. During inflation periods, the seismicity was contained within the caldera of the volcano, reflecting the inflation level of the magma chamber. Diking events were marked by seismicity propagating away from the volcano into the fissure swarms to the south or north of the volcano, accompanied by rapid deflation of the caldera magma chamber. These events lasted from 1 day to 3 months, and the dike length varied between 1 and 60 km. The area around the Askja volcano is the only section of the Northern Volcanic Zone that shows persistent moderate seismicity. The largest events are located between fissure swarms of adjacent volcanic systems. Detailed relative locations of hypocenters reveal a system of vertical strike-slip faults, forming a conjugate system consistent with minimum principal stress in the direction of spreading across the plate boundary. A diking event into the lower crust was identified in the adjacent fissure swarm at Upptyppingar in 2007–2008. Four nests of anomalously deep earthquakes (10–34 km) have been identified in the Askja region, apparently associated with the movements of magma well below the brittle-ductile transition. Several processes have been pointed out as possible causes of earthquakes in the deformation zone around the plate boundary. These include inflation and deflation of central volcanoes, intrusion of propagating dikes, both laterally and vertically, strike-slip faulting on conjugate fault systems between overlapping fissure swarms, migration of magma in the lower, ductile crust, and geothermal heat mining.

Dikes are the primary mechanisms to transport magma and feed eruptions. Investigations of the surface deformation and seismicity caused by a dike can potentially provide useful information to forecast the expected propagation and associated hazard. On December 24, 2018, a dike intrusion reached the summit of Mt. Etna, feeding an effusive fissure. The intrusion was accompanied by a seismic swarm, with hypocenters beneath the summit craters and eruptive fissure, and by ground deformation. The seismicity continued the following day, with the hypocenters deepening to 3 km b.s.l. due to the propagation of a deeper and thicker dike. This situation generated the fear of feeding a more dangerous eruption in the medium-low flank. Recently it was found an equation that relates the average thickness and dimension of the dike with the expected released mechanical energy and, therefore, to the seismic moment. By using this updated application, it is shown that the observed seismicity could not be accounted for by the first dike. Instead, the cumulative effect of both dikes indicates a total moment from available energy expected that balances the moment recorded by the seismicity. The proposed approach proved very useful in the specific case of Etna volcano eruptions, resulting an effective tool to monitor the state of the intrusion of the magma and, therefore, to predict if a dike has enough energy to continue propagating or to stop.

During the 2018 Kīlauea volcanic eruption, lava erupted from a series of new fissures in the lower East Rift Zone more than 30 km away from the summit through a dike intrusion. Between late May and early August, variations in the effusion rate at the persistent eruptive vent (Fissure 8) were observed following near‐daily summit caldera collapse events. Targeting the ongoing eruptive activity and the subsurface magma movement, we deployed a temporary dense seismic array. The observed time‐lapse changes in seismic velocity associated with the response of the summit collapse in three areas are presented in this study. The results show (1) clear spatially dependent co‐collapse velocity reductions across the newly‐intruded dike structure, (2) a gradual post‐collapse velocity increase near Fissure 8 correlated with the surge of magma supply, and (3) a gradual post‐collapse velocity increase on the summit likely associated with reservoir pressurization and crustal welding. Plain Language Summary: Volcanic eruptions commonly involve failure of rocks surrounding the subsurface magma transport system, such as fracturing and rupturing. During the 2018 Kīlauea volcano eruption, lava mainly erupted from new fissures within the lower East Rift Zone (LERZ) located tens of kilometers away from the magma reservoir beneath the summit. As magma drained from the summit system to feed the LERZ eruption, the summit caldera subsided in an episodic fashion through a series of 62 collapses. Despite this spatial separation, we observe correlation between the summit collapse and the LERZ eruptive activity suggesting the two features are interconnected through a subsurface dike. To better study the eruption dynamics and subsurface magma movement, we deployed a temporary dense seismic array across the Kīlauea volcano system during the steady phase after mid‐June in 2018. In this study, we present the time‐lapse changes in crustal seismic velocity observed across the dense array correlated with the summit collapses. We show that different volcanic components (i.e., summit, dike, and fissure) react to summit magma pressurization differently. Our findings provide new constraints on the time‐evolving mechanical structure of a volcano during a major eruption. Key Points: Temporary seismic array targeting ongoing eruptive activity permits studies on dynamics between preexisting and newly formed structuresWe observe spatially dependent seismic velocity variations associated with the newly intruded dike emplacement in the lower East Rift ZoneMagma pressurization and its transmission through the hydraulically connected dike can be monitored from crustal seismic velocity [ABSTRACT FROM AUTHOR]

We investigate rare seismicity in the neighborhood of the Fentale volcano, main Ethiopian rift that occurred in spring 2015 using data recorded by the Ethiopian Seismic Station Network. Over 1350 earthquakes are located and a seismic swarm is observed to the northeast of Fentale volcano, with the largest earthquake having a magnitude of 4.3 ML. Several VT (volcano-tectonic) and LP (long-period) events are observed in the swarm, indicating that it is induced by magmatic intrusion, consistent with dike-induced deformation in the same area inferred from satellite geodesy. The seismic activity commenced at about 40 km depth and migrated southwestward as it shallowed, towards Fentale volcano, suggesting that the dike feeding system might be influenced by the magma rich Afar Depression further to the northeast rather than the adjacent Fentale volcano. An earlier cluster of seismicity in the same area was recorded by a temporary deployment of seismic stations that ran from November 2001 to January 2003; extensive clusters of less well recorded seismic activity in the region occurred in June 1989, January–February 1981, October–November 2003, and 2004. Such frequent seismicity at the Fentale Volcanic Complex coupled with the adjacent fast-growing Lake Basaka poses an eminent and significant risk to Ethiopia's access to Djibouti port, its most important commercial route. This should be a wakeup call to concerned stakeholders to take precautionary measures by developing risk management and rapid response strategies. • We analyzed the March and April 2015 earthquake swarm that occurred around Tinish Fentale which is found in the northern main Ethiopian rift. • We located more than 1350 earthquakes with maximum magnitude 4.3 ML. Our investigation shows that the activity is caused by dike intrusion. • The cluster of the seismic activity is shown to be closer to Ethiopia's main access road to the port of Djibouti which poses a geohazard risk. [ABSTRACT FROM AUTHOR]

Dike intrusions produce faulting at the surface along with seismic swarms and possible eruptions. Understanding the geometry and kinematics of dike-induced fractures can provide relevant information on what controls magma emplacement and the associated hazards. Here, we focus on the Harrat Lunayyir volcanic field (western Saudi Arabia), where in 2009 a dike intrusion formed a NNW-SSE oriented, ten-kilometer-long and up to one-meter deep graben. This widens from ∼2 km in the SSE to ∼5 km in the NNW, showing a well-defined border normal fault to the west but a diffused fracture zone to the east. We conducted a fixed-wing drone survey to create high resolution (∼3.4 cm) ortho-rectified images and DEMs of the western fault and of a portion of the eastern fracture zone to determine the fracture geometry and kinematics. We then integrated these results with field observations and InSAR data from the 2009 intrusion. Both fault zones contain smaller segments (hundreds of meters long) consisting of normal faults and extension fractures, showing two dominant orientation patterns: NNW-SSE (N330° ± 10°) and NW-SE (N300° ± 10°). The NNW-ESE oriented segments are sub-parallel to the inferred 2009 dike strike (N340°), to pre-historical Harrat Lunayyir eruptive fissures (N330°) and to the overall Red Sea axis (N330°). This suggests that these segments reflect the present-day off-rift stress field close to the Red Sea shoulder. However, the NW-SE oriented segments are oblique to this pattern and exhibit en–echelon structures, suggesting different processes such as: (1) a transfer (or soft-linkage) between dike-parallel fault segments, (2) a topographic control on the fault propagation and (3) a possible reactivation of inherited regional faults. Vertical fault offsets obtained by the drone survey along the western fault vary with the local lithology and these data are not consistent everywhere with the offsets derived from the 2009 InSAR measurements. Field evidence within lava flows also shows the occurrence of previous slipping event(s) on the fault before the 2009 intrusion. Collectively, we suggest that the mismatch between the drone and InSAR datasets is related to the different spatial and temporal resolutions offered by the two techniques.

Bárðarbunga is an active volcano beneath the Vatnajökull icecap in Iceland, where a subglacial dike intrusion occurred in 2014. This area has been studied with interferometric synthetic aperture radar, an important geodetic method that measures crustal deformation; however, ice/snow cover on volcanoes impedes the mapping of crustal deformation because of decorrelation problems. Previous geodetic observations have reported deformation signals at ice‐free regions of a major dike formed in 2014 in the Bárðarbunga volcanic system, but direct observations of the subglacial crustal deformation associated with the dike intrusion have been limited. We applied a pixel tracking approach to various satellite synthetic aperture radar (SAR) data over the northern part of the Vatnajokull icecap and the Holuhraun plain. The pixel tracking data revealed not only crustal deformation fields in the ice‐free region of the magma intrusion, which covers only about 20% of the entire length of dike, but also icecap surface movements over the subglacial part of the dike in the ablation area of the Vatnajökull icecap. Signals above the icecap, suggesting subsidence due to subglacial graben formation, are consistent with the dike propagation path inferred from seismicity during the episode. By subtracting the scaled pre‐diking signals from the co‐diking signals, we corrected for the steady‐state icecap flow signals to derive the subglacial crustal deformations. We showed that the inferred subglacial crustal deformation signals can significantly contribute to the improvement of dike opening/faulting distributions. Applying the pixel tracking to satellite images will enable mapping subglacial crustal deformation in the case of subglacial volcanic activity. Key Points: SAR pixel tracking reveals crustal deformations due to the Bárðarbunga dike intrusion both over ice‐free ground and beneath the icecapSubglacial crustal deformations are inferred by subtracting scaled pre‐diking signals from co‐diking signalsCrustal deformation signals beneath the ice are helpful to improve both dike opening and normal faulting distributions [ABSTRACT FROM AUTHOR]

Seismic swarms close to volcanoes often signal the onset of unrest. Establishing whether magma is the culprit and the unrest can be flagged as magmatic may be challenging. Here we analyze the spatio-temporal pattern of a seismic swarm that occurred in November 2015–February 2016 around Jailolo volcano, a long-dormant and poorly studied volcano located on Halmahera island, North Moluccas, Indonesia. The swarm included four Mw > 5 earthquakes and hundreds of events were felt by the population. We relocate the earthquakes using both the Indonesian Seismic Network and single-station location techniques. We find that the earthquakes cluster in a narrow strip, stretching 5 km E–W and 20 km N–S, migrating southward away from Jailolo volcano at ~10 km/d. We investigate the source mechanisms of the largest earthquakes via full moment tensor inversion. The non-double-couple component is around 50%, such that the earthquakes, besides normal faulting, included a relatively large opening component. After a thorough examination of the possible causes of the Jailolo swarm we conclude that a laterally propagating dike of tens of millions of cubic meters is the triggering mechanism for the seismicity. The swarm marks the first documented magmatic unrest at Jailolo. We find that there is a probability >0.1 that the unrest will last for more than 2 years. This magmatic unrest calls for the classification of Jailolo volcano as active, and for an urgent assessment of the associated volcanic hazard.

Graphical abstract Highlights • Earthquake cross-correlation and relative relocation resolves ∼100 m scale details. • Small-scale complexity of dike propagation pathway and dynamics revealed. • Dike-induced earthquakes linked to propagation of dike tip and opening/inflation. • Seismicity occurs only at dike base where stresses are sufficient to induce failure. • Earthquakes predominantly arise from shear failure of existing crustal weaknesses. Abstract Understanding dikes is vital as they serve both as bodies that build the crust and as conduits that feed eruptions, and must be monitored to evaluate volcanic hazard. During the 2014–15 Bárðarbunga rifting event, Iceland, intense seismicity accompanied the intrusion of a ∼50 km lateral dike which culminated in a 6 month long eruption. We here present relocations of earthquakes induced by the lateral dike intrusion, using cross-correlated, sub-sample relative travel times. The ∼100 m spatial resolution achieved reveals the complexity of the dike propagation pathway and dynamics (jerky, segmented), and allows us to address the precise relationship between the dike and seismicity, with direct implications for hazard monitoring. The spatio-temporal characteristics of the induced seismicity can be directly linked in the first instance to propagation of the tip and opening of the dike, and following this – after dike opening – indicate a relationship with magma pressure changes (i.e. dike inflation/deflation), followed by a general 'post-opening' decay. Seismicity occurs only at the base of the dike, where dike-imposed stresses – combined with the background tectonic stress (from regional extension over >200 yr since last rifting) – are sufficient to induce failure of pre-existing weaknesses in the crust, while the greatest opening is at shallower depths. Emplacement oblique to the spreading ridge resulted in left-lateral shear motion along the distal dike section (studied here), and a prevalence of left-lateral shear failure. Fault plane strikes are predominately independent of the orientation of lineations delineated by the hypocenters, indicating that they are controlled by the underlying host rock fabric. This high-resolution study provides unprecedented opportunity for comparison with both geodetic and field (frozen dike) observations, and development and consolidation of analytical and analogue models, with implications for rifting processes and real-time monitoring of magma intrusion. [ABSTRACT FROM AUTHOR]

Abstract In this investigation, we use a recent constitutive framework and remeshing technique for tetrahedra to analyze the pressure-driven crack propagation of limestone intruded by basaltic magma. Limestone is represented by an elasto-plastic capped Drucker-Prager model with an hypoelastic term in order to account for inelastic effects from plastic signature. Kinematic hardening is considered for limestone, whereas magma is modeled by means of a compressible Bingham fluid. Classical limit surfaces of the capped model are used to initiate the crack events in the limestone. Propagation is performed by a local remeshing technique with mesh smoothing for uncracked elements. Arnold's MINI element is used to avoid locking in the quasi-incompressible case. Verification (for limestone) examples and a crack propagation example in 3D are performed. A mesh convergence study is performed, exhibiting very promising results. Highlights • Coupled basaltic magma/limestone interaction with fracture. • Dike intrusion by remeshing. • Use of MINI element. • Capped Drucker-Prager - Bingham fluid implementations in finite strains. • 3D Fracture verification test. [ABSTRACT FROM AUTHOR]

Abstract: Spreading processes at the axes of fast spreading ridges are mainly controlled by magmatic activity, whereas tectonic activity dominates further away from the axis. High‐resolution near‐bottom bathymetry data, photographs, videos, and human observations from submersible surveys are used to develop a detailed tectonic analysis of the 16°N segment of the East Pacific Rise (EPR). These data are used to evaluate how a highly magmatic segment, close to a hot spot, affects the nucleation and evolution of faulting patterns and impacts the evaluation of tectonic strain within 2 km of the spreading axis. Our study shows that (1) the growth of tectonic features differs in response to dike intrusion and tectonic extension, (2) the initiation of brittle extension is strongly controlled by the location of the axial magma lens and the development of layer 2A, and (3) the high magmatic budget and the off‐axis magma lens in the west part of the plateau do not significantly impact the initiation of brittle extension along the central portion of the 16°N segment. Within the axial summit region, more than 2% of plate separation at 16°N on the EPR is accommodated by brittle extension, as is observed at other EPR segments. The interaction of the Mathematician hot spot with this EPR segment has no significant influence on the initiation of the tectonic deformation, but it does reduce the development of the brittle deformation. [ABSTRACT FROM AUTHOR]

Fissure eruptions are commonly linked to magma dikes at depth and are associated with elastic and inelastic surface deformation. Elastic deformation is well described by subsidence occurring above the dike plane and uplift and lateral widening occurring perpendicular to the dike plane. Inelastic deformation is associated with the formation of a graben, which is bordered by graben parallel faults that might express as sets of fractures at the surface. Additionally, secondary structures, such as push-ups, bends and step overs, yield information about the deforming domain. However, once these structures are formed during fissure eruptions, they are rarely preserved in nature, due to the effects of rapid erosion, sediment coverage or overprinting by other faulting events. Therefore, simple normal fault displacements are commonly assumed at dikes. At the 2014/2015 Holuhraun eruption sites (Iceland), increasing evidence suggests that developing fractures exhibited variations in their displacement modes. In an attempt to investigate these variations, a fieldwork mapping project combining Terrestrial Laser Scanning (TLS) and Unmanned Aerial Vehicle (UAV)-based aerophoto analysis was undertaken. Using these data, we generated local high-resolution Digital Elevation Models (DEMs) and a structural map that facilitated the identification of kinematic indicators and the assessment of the observed structures. We identified 315 fracture segments from these satellite data. We measured the strike directions of single segments, including the amount of opening and opening angles, which indicate that many of the measured fractures show transtensional dislocations. Of these, ~81% exhibit a significant left-lateral component and only ~17% exhibit a right-lateral component. Here, we demonstrate that the local complexities in these fracture traces and geometries are closely related to variations in their transtensional opening directions. Moreover, we identified local changes in fracture azimuths and offsets close to eruption sites, which we speculate are associated with geometric changes in the magma feeder itself. The results highlight that the opening of fractures associated with an erupting fissure may record transtensional modes with both, left-lateral and right-lateral components. These results further highlight the value of using UAV-based high-resolution data to contribute to the integrity of the observations of the structural complexities produced by local geologic events.

Harrat Lunayyir is one of the smaller lava fields in western Saudi Arabia that is of current interest due to a dike intrusion episode in 2009, an ongoing swarm of earthquakes and the possible hazard that pose. In addition to seismology, other geophysical data have been used to study the structure of the area, and the available aeromagnetic information is shown and discussed here. The reduced to the pole (RTP) magnetic grid and its enhancements have been used to define some of the main anomalies, and these have been correlated with the known geology, including the Red Sea coastal dike system. There are numerous linear features that are possibly related to the dikes or faulting, but within the area of the harrat, details due to the underlying structure are largely obscured by the magnetic surface lavas. Northeasterly trends in the magnetic data may indicate old zones of weakness that intersect one of the main coastal dikes at the location of the recent seismic activity and surface fissure, suggesting that this is a point of weakness resulting in the volcanism and seismicity that also appears to be largely limited to two of the NE trends. The association of the recent seismicity with a known geological and aeromagnetic feature is important in determining the seismic hazard for the region, especially if the location of future activity can be used to reduce overall uncertainty in the analysis by identifying potential fault sources. Here, the seismicity appears to lie on one of the NNW-trending coastal dikes that have been reactivated recently along a section between two NE-trending older faults. [ABSTRACT FROM AUTHOR]

We present observations of ground deformation at Sakurajima in August 2015 and model the deformation using a combination of GNSS, tilt and strain data in order to interpret a rapid deformation event on August 15, 2015. The pattern of horizontal displacement during the period from August 14 to 16, 2015, shows a WNW-ESE extension, which suggests the opening of a dike. Using a genetic algorithm, we obtained the position, dip, strike length, width and opening of a dislocation source based on the combined data. A nearly vertical dike with a NNE-SSW strike was found at a depth of 1.0 km below sea level beneath the Showa crater. The length and width are 2.3 and 0.6 km, respectively, and a dike opening of 1.97 m yields a volume increase of 2.7 × 10 m. 887 volcano-tectonic (VT) earthquakes beside the dike suggest that the rapid opening of the dike caused an accumulation of strain in the surrounding rocks, and the VT earthquakes were generated to release this strain. Half of the total amount of deformation was concentrated between 10:27 and 11:54 on August 15. It is estimated that the magma intrusion rate was 1 × 10 m/h during this period. This is 200 times larger than the magma intrusion rate prior to one of the biggest eruptions at the summit crater of Minami-dake on July 24, 2012, and 2200 times larger than the average magma intrusion rate during the period from October 2011 to March 2012. The previous Mogi-type ground deformation is considered to be a process of magma accumulation in preexisting spherical reservoirs. Conversely, the August 2015 event was a dike intrusion and occurred in a different location to the preexisting reservoirs. The direction of the opening of the dike coincides with the T-axes and direction of faults creating a graben structure. [ABSTRACT FROM AUTHOR]

A dike propagated ~ 48 km from the Bárðarbunga central volcano into one of its fissure swarms in August and early September 2014. The dike intrusion was accompanied by caldera subsidence in the ice-covered Bárðarbunga central volcano, as well as a propagating earthquake swarm, graben subsidence, and fissure eruptions in the fissure swarm. Most of the dike was emplaced in the glacier-covered part of the fissure swarm, but a 10 km long section at the distal (northern) end of the dike is ice-free. New ground fractures were observed in the ice-free part during the dike propagation and prior to the outbreak of a major lava-producing eruption. The fractures delineate two graben structures in direct continuation of each other. Here we show the extent and depth of the grabens and maps of fractures and eruptive fissures that were formed or reactivated north of the Vatnajökull glacier during the dike intrusion. The active graben subsidence was identified 3 days after seismicity reached the ice-free area. Then fractures delineated a 700–1000 m wide and 5 km long graben visible from the northern edge of the Vatnajökull glacier. Farther north a narrower, 250–450 m wide graben was seen. A short-lived minor eruption occurred between these grabens on the 29th of August 2014, 2 days after the grabens were identified. The northern graben was extended southwards during or before the eruption, and included the eruptive fissure. On the 31st of August 2014, a much larger eruption began on the same eruptive fissure. The fissure quickly obtained its maximum length of 2 km. This eruption lasted 6 months. A minor eruption, lasting 2 days or less, occurred on the southern graben on the 5th of September 2014. The first two eruptions took place on the same eruptive fissure that formed the pre-existing Holuhraun lava, presumably in the 18th century. Measurements in January 2015 show that the southern graben was 3.5–5.5 m deep, but the northern graben had been covered by lava. From the depth and the opening of the graben, we estimate that the dip of the boundary faults of the graben is ~ 75°. The narrow grabens observed before the eruptions indicated that magma was at shallow depth, likely much less than 300 m, 2 days before it opened to the surface. Such observations can serve as a tool to estimate likelihood of eruptions during dike intrusions. [ABSTRACT FROM AUTHOR]

In July 2007, an earthquake swarm initiated in northern Tanzania near Lake Natron and lasted for about two months. Mt. Oldoinyo Lengai, located to the southwest of the swarm, began to erupt effusively about a month prior to the swarm, and increased its eruption intensity on September when the swarm almost ceased. Several previous studies have already reported the crustal deformation signals associated with the swarm using Interferometric Synthetic Aperture Radar (InSAR). However, nearly all the published data are based on the C-band ENVISAT/ASAR images acquired only from the descending path. We use the L-band ALOS/PALSAR images acquired from both ascending and descending paths, which allow us to examine the deformation signals in more detail. In addition to the InSAR data, we employ the offset-tracking technique to detect the signals along the azimuth direction. Using InSAR and offset-tracking data, we obtain the full 3D displacement fields associated with the episode. Besides the horizontal extension and subsidence signals due to the dike intrusion as already reported, the inferred full 3D displacements further indicate that the subsiding zone was horizontally moving by ~ 48 cm toward SSW. To explain the displacements, we performed fault source modeling, assuming an elastic half space. The fault slip distribution indicates that the contribution of the strike–slip component is about 20% of total moment release. Because almost all the focal mechanisms of earthquakes during the 2007 event indicate nearly pure normal faulting, aseismic strike–slip must have been responsible for the horizontal movement of the subsiding zone. The strike–slip at the shallowest depths suggests the presence of transtensive stress, which seems to be reasonable to generate the relay zones that are widely observed in the East African Rift. We also confirmed that the stress changes due to the dike intrusion were consistent with the inferred fault slip distributions. [ABSTRACT FROM AUTHOR]

Although extensive studies have been conducted on coals influenced by igneous intrusion, the changes of concentration and composition of polycyclic aromatic hydrocarbons (PAHs) in coals responding to this kind of geological rapid heating are largely unknown. In the present study, thirteen coal samples, including unaltered coal, thermally altered coal and a dike-coal contact sample were collected along a transect approaching an igneous dike intrusion from the No. 3 coal seam of the Zhuji coal mine in Huainan coalfield, China. These samples represent a broad range of coal ranks ranging from high volatile A bituminous to anthracite. Vitrinite reflectance (%Ro) profiles indicate a progressive increase in %Ro value from ambient levels of 0.87–1.02% to 5.16% and include an alteration occurring out to ∼3.6 m from the dike. Bulk chemical composition from proximate and ultimate analyses shows the typical trend with distance from the dike. The total amount of extractable PAHs (ΣPAH 35 ), US-EPA priority pollutant PAHs (ΣPAH 16 ) and carcinogenic PAHs (ΣPAH 8 ) range from 3.4–81.1 μg/g, 2.5–28.7 μg/g and 0.7–16.9 μg/g, respectively. PAH concentrations vary regularly with coal rank, reaching a maximum in low volatile bituminous coal (1.62 %Ro), located 3.5 m from the dike. This rank is higher than reported for coals not affected by contact metamorphism. Contact metamorphism has a dramatic effect on the coalification process and results in a different relationship between the PAH maximum concentration and coal rank. The igneous dike intrusion also has a significant effect on PAH distributions, where the proportions of 4–6 ring PAHs decrease toward the coal-dike contact and PAHs are dominated by those with 2 and 3 rings. The thermally altered coals show a decrease in alkylation compared with that of the unaltered coals, and the dealkylation process is apparently accompanied by the cross linking reaction of PAHs to form a more compact structure of organic macromolecule. [ABSTRACT FROM AUTHOR]

Dike intrusion is a fundamental process during upper oceanic crustal accretion at fast- to superfast-spreading ridges. Based on the distribution of magma along fast-spreading centers inferred from marine geophysical data, models predict systematic steep flow at magmatically robust segment centers and shallow magma flow toward distal segment ends. Anisotropy of magnetic susceptibility (AMS) fabrics from 48 fully-oriented block samples of dikes from upper oceanic crust exposed at Hess Deep Rift and Pito Deep Rift reveal a wide range of magma flow directions that are not consistent with such simple magma supply models. The AMS is interpreted to arise from distribution anisotropy of titanomagnetite crystals based on weak shape-preferred orientation of opaque oxide and plagioclase crystals generally parallel to AMS maximum eigenvectors. Most dike samples show normal AMS fabrics with maximum eigenvector directions ranging from subvertical to subhorizontal. The distributions of inferred magma flow lineations from maximum eigenvectors show no preferred flow pattern, even after structural correction. We use a Kolmogorov–Smirnov test (KS-test) to show that the distribution of bootstrapped flow lineation rakes from Pito Deep are not statistically distinct from Hess Deep, and neither are distinguishable from Oman and Troodos Ophiolite AMS data. Magma flow directions in sheeted dikes from these two seafloor escarpments also do not correlate with available geochemistry in any systematic way as previously predicted. These results indicate distinct compositional sources feed melt that is injected into dikes at fast- to superfast-spreading ridges with no preference for subhorizontal or subvertical magma flow. Collectively, results imply ephemeral melt lenses at different along-axis locations within the continuous axial magma chamber and either direct injection or intermingling of melt from other deeper ridge-centered or off-axis sources. [ABSTRACT FROM AUTHOR]

In a volcanic area, the orientation and composition of dikes record the development of the magmatic system that feeds intrusive and eruptive activity. At Spanish Peaks, Colorado, curved dike trajectories issuing from a single focal area have been attributed to horizontal propagation from a pressurized central reservoir in a deviatoric tectonic stress field. These dikes, however, are nowhere in contact with the central intrusion, are younger than it by about 1 My and are not filled with the same magma. They were emplaced at shallow depths ( ≈ 1 km ), where the local stress field is very sensitive to surface loads. Here, we show that their trajectories can be set by the load of a volcanic edifice in a tectonic stress field. The orientation and distribution of the Spanish Peaks dikes have changed in the course of two million years as magmas were evolving chemically. Early dikes that were parallel to each another and filled with primitive melts document ascent in the regional tectonic stress field. They were replaced by curved dikes carrying evolved melts, which record the influence of a sizable volcanic edifice. Beneath this edifice, the induced compression prevented dense primitive magmas from erupting in the focal area and diverted intermediate magmas sideways. The growth of this large volcanic cone was probably responsible for the formation of a magma reservoir. The mechanisms that have shaped the Spanish Peaks dike swarm may control the spatial distribution and migration of eruptive centers in many active volcanic areas. [ABSTRACT FROM AUTHOR]

Abstract: Coal maturation due to igneous intrusions offers the ideal opportunity to study the chemical-structural evolution of rapidly heated coal. Variations in the chemical structures of kerogens from high volatile bituminous coal near two igneous dike intrusions in the Illinois Basin were characterized in detail by advanced solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. With decreasing distance to dike contacts and increasing thermal maturity (vitrinite reflectance R o from 0.62% to 5.03%), the new insights into the chemical structural changes in coal kerogens include (i) the C(CH2)C groups removed faster than CCH3 groups, suggesting that the major cleavage of aliphatics should not occur at aryl rings, (ii) a progressive decrease of the relative abundance of protonated aromatic carbons, possibly due to replacement of aromatic hydrogens by aryl groups via cross-linking, and (iii) an increase in the aromatic cluster size, in addition to the well established changes of the elimination of aliphatics and aromatic C-O groups and an increase in aromaticity. For the first time the growth of aromatic cluster sizes with increasing maturity was quantitatively estimated. Another novel finding is that coal kerogen (kerogen #22) directly at the contact with the large dike retained considerable aliphatic components with olefinic and COO groups. These aliphatic components could have been entrapped and failed to diffuse out rapidly enough because intense and very fast heating rendered this kerogen partially fluidized. Our advanced solid-state NMR techniques provided deep insights into the structural changes in kerogen from bituminous coal in response to dike intrusions which cannot be achievable using routine 13C cross polarization/magic angle spinning technique. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

major seismic swarm occurred near Parícutin volcano between the end of May and early July 2006. More than 700 earthquakes with magnitude (M) exceeding 2.4 were located. Parícutin, located in the Michoacán-Guanajuato volcanic field in western Mexico, is well known as the site of the 1943 eruption in which a new 400 m cinder cone was constructed in what had been farmland. The 2006 swarm exhibits all of the characteristics typically associated with swarms of volcanic origins. The earthquake rate showed the typical ramp up and ramp down over the course of several days. Magnitudes were evenly distributed in time with a notably high b-value of 2.45. The earthquake locations cluster around a northeast-striking trend extending approximately 6 km. Over the first two weeks, hypocenters migrated steadily a few hundred meters per day, rising from 9 to 5 km depth and moving northeast about 5 km. On approximately June 7, the ascent of hypocenters stalled. For the next three weeks, hypocenters held their depth while migrating laterally back to the southwest. Focal mechanisms during the first part of the swarm reflected the increased stress caused by dike inflation. Following June 7, the stress orientation changed and became more consistent with the inflation of horizontal sill-like structures. Though only limited information is available from the seismic swarm preceding the 1943 eruption, several features, including the swarm duration and magnitude relationships, were comparable to those of the 2006 episode. The strong indicators of a magmatic origin to the 2006 swarm suggest that at this location there are few, if any, traditional seismic discriminants that could be used to distinguish which seismic swarms and dike emplacement events might culminate in eruption. [ABSTRACT FROM AUTHOR]

Abstract: Many volcanic edifices are subject to flank failure, usually produced by a combination of events, rather than any single process. From a dynamic point of view, the cause of collapse can be divided into factors that contribute to an increase in shear stress, and factors that contribute to the reduction in the friction coefficient μ of a potential basal failure plane. We study the potential for flank failure at Mount Etna considering a schematic section of the eastern flank, approximated by a wedge-like block. For such geometry, we perform a (steady state) limit equilibrium analysis: the resolution of the forces parallel to the possible basal failure plane allows us to determine the total force acting on the potentially unstable wedge. An estimate of the relative strength of these forces suggests that, in first approximation, the stability is controlled primarily by the balance between block weight, lithostatic load and magmatic forces. Any other force (sea load, hydrostatic uplift, and the uplift due to mechanical and thermal pore-fluid pressure) may be considered of second order. To study the model sensitivity, we let the inferred slope α of the basal surface failure vary between −10° and 10°, and consider three possible scenarios: no magma loading, magmastatic load, and magmastatic load with magma overpressure. We use error propagation to include in our analysis the uncertainties in the estimates of the mechanics and geometrical parameters controlling the block equilibrium. When there is no magma loading, the ratio between destabilizing and stabilizing forces is usually smaller than the coefficient of friction of the basal failure plane. In the absence of an initiating mechanism, and with the nominal values of the coefficient of friction μ =0.7±0.1 proposed, the representative wedge will remain stable or continue to move at constant speed. In presence of magmastatic forces, the influence of the lateral restraint decreases. If we consider the magmastatic load only, the block will remain stable (or continue to move at constant speed), unless the transient mechanical and thermal pressurization significantly decrease the friction coefficient, increasing the instability of the flank wedge for α >5° (seaward dipping decollement). When the magma overpressure contribution is included in the equilibrium analysis, the ratio between destabilizing and stabilizing forces is of the same order or larger than the coefficient of friction of the basal failure plane, and the block will become unstable (or accelerate), especially in the case of the reduction in friction coefficient. Finally, our work suggests that the major challenge in studying flank instability at Mount Etna is not the lack of an appropriate physical model, but the limited knowledge of the mechanical and geometrical parameters describing the block equilibrium. [Copyright &y& Elsevier]

Abstract: An active seismic survey with unprecedented density of seismometers found a high velocity zone to the west of the summit of Asama Volcano, Japan. The high velocity zone coincides with an area of magma-filled crack (dike) intrusion associated with the 2004 eruptions inferred from precise earthquake relocations and ground deformation modeling. It also coincides with an area of high resistivity surrounded by low resistivity, indicating that the solidification of magma due to repeating intrusions is responsible for the high velocity. This thus endorses the magma pathway previously speculated by seismic and geodetic observations. These findings demonstrate that dense seismic exploration combined with geophysical monitoring is an effective way to understand the dynamics of volcanic eruptions. [Copyright &y& Elsevier]

Abstract: We examine the relationship between dike emplacement and normal fault displacement along the south-western margin of Okataina Volcanic Centre, New Zealand. At this location dikes (including a basaltic dike up to 5 m wide at the ground surface intruded during the 1886 AD Tarawera eruption) formed over the last 22 kyr in the Tarawera linear vent zone within a few kilometres of the Paeroa Fault zone. Displacement analysis and mapping of fault scarps indicate that fault and dike extension are complementary over time periods of tens of thousands of years. Extension that is primarily accommodated by faulting outside the volcanic complex is achieved by dike intrusion and faulting within it. Historical observations following the 1886 AD eruption and previously published active fault trenching collectively indicate, however, that dike intrusion generally does not induce slip within the Paeroa Fault zone close to the lateral dike tip. We suggest that displacement in the Paeroa Fault zone outside the Okataina Volcanic Centre is generally not triggered by eruptions and that this fault is principally tectonic in origin. [Copyright &y& Elsevier]

A 2D finite elements study was carried out to analyse the effects caused by dike intrusion inside a heterogeneous medium and with a realistic topography of Mt. Etna volcano. Firstly, the method (dimension domain, elements type) was calibrated using plane strain models in elastic half-spaces; the results were compared with those obtained from analytical dislocation models. Then the effects caused both by the topographic variations and the presence of multi-layered medium on the surface, were studied. In particular, an application was then considered to Mt. Etna by taking into account the real topography and the stratification deduced from seismic tomography. In these conditions, the effects expected by the dike, employed to model the 2001 eruption under simple elastic half-space medium conditions, were computed, showing that topography is extremely important, at least in the near field.

Key tectonic events at the southern rim of the South Fiji Basin-Three Kings Ridge-Norfolk Basin region of the Southwest Pacific are revealed by analysis of the extremely complex structural sequence in a coastal section of Mt Camel Terrane units (Houhora Complex). The Cretaceous to early Tertiary clastic sedimentary and igneous lithologies of the terrane were deposited in an oblique-slip extensional back-arc basin. All units were deformed at relatively shallow levels (zeolite facies). Pervasive shearing to broken formation, and at least two phases of early folding, occurred during Tertiary accretion tectonics in a nascent subduction zone along the margin of the New Zealand micro-continent. Accretionary processes were followed by pervasive sinistral shearing and associated folding and movement on low-angle faults during overthrusting of the Northland Allochthon. Deformation was postdated by intrusion of dikes of intermediate composition, associated with establishment of the subduction-related Miocene Northland volcanic arc. [ABSTRACT FROM AUTHOR]

Abstract: An earthquake swarm struck the North Tanzania Divergence, East African Rift over a 2 month period between July and September 2007. It produced approximately 70 M>4 earthquakes (peak magnitude Mw 5.9), and extensive surface deformation, concurrent with eruptions at the nearby Oldoinyo Lengai volcano. The spatial and temporal evolution of the entire deformation event was resolved by Interferometric Synthetic Aperture Radar (InSAR) observations, owing to a particularly favorable acquisition programming of the Envisat and ALOS satellites, and was verified by detailed ground observations. Elastic modeling based on the InSAR measurements clearly distinguishes between normal faulting, which dominated during the first week of the event, and intermittent episodes of dike propagation, oblique dike opening and dike-induced faulting during the following month. A gradual decline in the intensity of deformation occurred over the final weeks. Our observations and modeling suggest that the sequence of events was initiated by pressurization of a deep-seated magma chamber below Oldoinyo Lengai which opened the way to lateral dike injection, and dike-induced faulting and seismicity. As dike intrusion terminated, silicate magma ascended the volcano conduit, reacted with the carbonatitic magma, and set off a major episode of explosive ash eruptions producing mixed silicate-carbonatitic ejecta. The rise of the silicate magma within the volcano conduit is attributed to bubble growth and buoyancy increase in the magma chamber either due to a temporary pressure drop after the termination of the diking event, or due to the dynamic effects of seismic wave passage from the earthquake swarm. Similar temporal associations between earthquake swarms and major explosive ash eruptions were observed at Oldoinyo Lengai over the past half century. [Copyright &y& Elsevier]

Abstract: Weak long-lived ground deformation associated with the volcanic unrest of Mt. Iwate, northeastern Japan in 1998–2002 is detected from borehole strainmeter, tiltmeter and global positioning system (GPS) data. This deformation is relatively weak (0.01 μ/day strain, 0.01 μrad/day tilt, and 0.01 cm/day displacement), but persists for a period of 5 years. To detect such weak long-term deformation, a new decomposition method based on Bayesian theory is developed and applied to the recorded data. This method is effective for extracting secular drift from the strain and tilt data, and seasonal changes from the GPS data. From a selection of possible models, a composite source model consisting of a dike series and a Mogi source is identified as the best fit to the observed data. The deformation is divided into 8 phases based on the characteristic variations in tilt and strain. Based on this model, the deformation sequence is considered to have initiated with the intrusion of a dike at intermediated depth below the summit region between February and April 1998, with short periods of shallow dikes intrusion toward the west in March and April. Shallow dike intrusion resumed in September, reaching a final volume of 5.9×106 m3. The Mogi source is inferred to have been located 3 km beneath the western flank of the volcano, slowly inflating to a maximum volume 1.2×107 m3 by the end of 2000, then deflating by 49% to 2002. This ground deformation was accompanied by seismic migration, from the summit region in the vicinity of the intruding dike to the west following the dike extension. This dike-correlating seismic activity ceased simultaneously with deflation of the Mogi source, suggesting that the Mogi source was a hydrothermal reservoir that was breached by the intruding dike. The Bayesian decomposition method developed in this study makes it possible to clarify the detailed characteristics of dike intrusions and the long-lived activity of the Mogi source utilizing the high sensitivity of strain and tiltmeters coupled with the stability of GPS. [Copyright &y& Elsevier]

Abstract: Dike emplacement in volcanic rift zones is often associated with the injection of “blade-like” dikes, which propagate long distances parallel to the rift, but frequently remain trapped at depth and erupt only near the tip of the dike. Over geologic time, this style of dike injection implies that a greater percentage of extension is accommodated by magma accretion at depth than near the surface. In this study, we investigate the evolution of faulting, topography and stress state in volcanic rift zones using a kinematic model for dike injection in an extending 2-D elastic-viscoplastic layer. We show that the intrusion of blade-like dikes focuses deformation at the rift axis, leading to the formation of an axial rift valley. However, flexure associated with the development of the rift topography generates compression at the base of the plate. If the magnitude of these deviatoric compressive stresses exceeds the deviatoric tensile stress associated with far-field extension, further dike injection will be inhibited. In general, this transition from tensile to compressive deviatoric stresses occurs when the rate of accretion in the lower crust is greater than 50–60% of the far-field extension rate. These results indicate that, over geologic timescales, the injection of blade-like dikes is a self-limiting process in which dike-generated faulting and topography result in an efficient feedback mechanism that controls the time-averaged distribution of magma accretion within the crust. [Copyright &y& Elsevier]

Abstract: A time-dependent model of magma intrusion is presented for the Miyake–Kozu Island area in central Japan based on global positioning system (GPS) measurements at 28 sites recorded between June 27 and August 27, 2000. A model derived from a precise hypocenter distribution map indicates the presence of three dikes between Miyake and Kozu Islands. Other dike intrusion models, including a dike with aseismic creep and a dike associated with a deep deflation source are also discussed. The optimal parameters for each model are estimated using a genetic algorithm (GA) approach. Using Akaike''s information criteria (AIC), the three-dike model is shown to provide the best solution for the observed deformation. Volume changes in spherical inflation and deflation sources, as well as three dikes, are calculated for seven discretized periods after GA optimization of the dike geometry. The optimization suggests a concentration of dike expansion near Miyake Island in the period from June 27 to July 1 associated with large deflation at a depth of about 7 km below Miyake volcano, indicating magma supply from depth below Miyake Island. In the period from July 9 to August 10, a huge dike intrusion near Kozu Island is inferred, accompanied by expansion of the lower parts of a central dike, suggesting magma supply from depth in the region between Miyake and Kozu Islands. [Copyright &y& Elsevier]

Abstract: The high-frequency GPS analysis at Miyakejima volcano captured rapid deformation following the early stage of Miyakejima 2000 activity. Given a specified dike and spherical source geometry, we solve the opening distribution on each sub-dike that minimizes a linear combination of the norm of the weighted data residual and roughness of the opening distribution. The time dependence of dike intrusion and spherical source deflation shows that magma migrated laterally from the southern region to the west coast. Although the depth of the maximum opening does not change with time, a very shallow intrusion can be seen at the eastern part of the dike. This may explain why cracks and graben structures developed on the west coast. Most of the magma migrated northwest, with only a fraction coming to the surface in an eruption below sea level. [Copyright &y& Elsevier]

The model for the 2000 dike intrusion event between Kozushima and Miyakejima volcano, Japan, was reinvestigated. After the sudden earthquake swarm in Miyakejima volcano, a dike intrusion of large volume was detected by the nationwide GPS network (Geonet). The displacements detected with GPS stations over an area with a radius of about 200 km shows a distribution that is consistent with the dike source being located near Miyakejima volcano. The dike was intruded northwestwards between Miyakejima and the neighboring Kozushima volcano. We searched for the parameters in the models that reproduce the regional displacements due to dike intrusion between Miyakejima and Kozushiima islands. We tested three models, (1) the model with a single dike, (2) the model with a dike and a point dislocation source which represents a creep dislocation source and (3) the model with a dike and a deflation source which represents a magma reservoir. Though all three models can match the horizontal displacements near the source area, model 1 fails to reproduce the regional displacements in the central part of Japan. Both models 2 and 3 can reproduce the regional displacement for horizontal components. Model 3 produces slightly better results than model 2 for vertical components. The balance in the volume budget for models 2 and 3 is also consistent with the observations. These results show that we cannot distinguish between the two models using only GPS observation. As there is no direct evidence for such a large creep or ductile source (corresponds to M7 or more) as proposed in model 2 and the active seismic region migrated back and forth within the linear swarm region, the model with a dike and a deep magma source is preferable. For the deflation point source, we obtained a deflation volume of 1.5 km3 at the depth of 20 km below the dike. An additional ~0.95 km3 of volume loss through caldera collapse and edifice deflation took place at Miyakejima. We conclude that the magma that intruded the dike came in part from below Miyakejima and in part from below the sea floor between Miyakejima and Kozushima, perhaps from reservoirs at the Moho. [ABSTRACT FROM AUTHOR]

From June 26, 2000, an earthquake swarm started in Miyakejima, about 50 km south off Honshu, central Japan. Eruptions of Miyakejima and five large earthquakes with magnitudes 6.0 and above occurred for the following 2 months together with a large number of smaller earthquakes over 100,000. In this study, we focused on spatio-temporal crustal deformation observed at GPS stations in this area. In order to explain the seismicity and the crustal deformation, we considered a dike intrusion model placed between Miyakejima and Kozushima. Then, we attempted to obtain spatio-temporal distributions, the amount of the dike intrusion, deflation beneath Miyakejima, and fault slips of the seven large events by geodetic data inversion. For this purpose, we divided the time series of GPS data from June 12 to August 27 into 10 periods, which are related to significant events. In order to find location, depth, strike and dip of the dike plane and a depth of deflation beneath Miyakejima, we used the Monte Carlo method and tested 10,000 models for each period. As a result, we estimated that the total amount of the dike intrusion and the deflation beneath Miyakejima reached about 1.1×109 and 5.4×108 m3, respectively. The maximum amount of the dike intrusion and the deflation beneath Miyakejima reached over 3.5×108 m3 during the period from July 20 to July 28, 2000, and over 1.7×108 m3 during the period from June 15 to June 28, 2000, respectively. Temporal change of the amount of the dike intrusion corresponds well to that of the deflation beneath Miyakejima until the middle of July. However, since large amount of the dike intrusion from July 20 to 28 did not correspond to that of the deflation beneath Miyakejima, we deduced that the magma source changed from Miyakejima to Kozushima and the magma might come from sub-crustal magma pockets from the middle of July. If we assume the open crack associated with the dike intrusion and the deflation beneath Miyakejima are filled with magma with a density of 2500 kg/m3, the mass would be about 2.75×109 and 1.35×109 tons, respectively. We deduced that at least 1.4×109 tons of magma came from sub-crustal magma pockets. [Copyright &y& Elsevier]

Hydrodynamic mingling of magma and liquefied sediments is generally accepted to represent the key process in the formation of some peperites. Experimental studies on simulant liquids and calculations based on recent empirical findings in the field of polymer research were undertaken to investigate the effectiveness of this process. These studies show that for a wide range of shear rates a laminar flow behaviour of the system magma–liquefied sediment can be expected, i.e. turbulent mingling is not a realistic scenario. Formation of peperitic fabrics with grain sizes on a cm scale can hydrodynamically be explained under realistic intrusion velocities. In addition to the hydrodynamics, cooling processes must also be considered during peperite formation. Calculations of the cooling history of the juvenile magmatic component under realistic cooling conditions demonstrate a significant limitation of the hydrodynamic mingling time. The consequence for peperite formation under normal intrusion conditions is that domains of magmatic melt and domains of liquefied sediment smaller than 10 cm are not probable. However, as smaller domain sizes (peperitic grain sizes) exist in nature, we conclude that either those peperites represent the products of explosive events or that hydrodynamic mingling was accompanied by additional fragmentation processes. [Copyright &y& Elsevier]