Patterns of Volcanic Activity of Piton de la Fournaise (Réunion Island, Indian Ocean). A Synthesis Based on Monitoring Data Between 1980 and July 1985, and on Historic Records Since 1930
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ABSTRACT The integrated analysis of geological, seismological and field observations with lineament data derived from satellite images allows the identification of a possible seismogenic fault zone for an earthquake which occurred near Etne in southwestern Norway, on 29 February 1989. The hypocentre of the earthquake was located at the mid‐crust at a depth of 13.8±0.9 km which is typical of small intraplate earthquakes. The Etne earthquake occurred as a result of normal faulting with a dextral strike‐slip component on a NW–SE trending fault. Available geological and lineament data indicate correlation of the inferred seismogenic fault with the NW–SE trending Etne fault zone. An aeromagnetic anomaly related to the Etne fault zone forms a regional feature intersecting both Precambrian basement and allochthonous Caledonian rocks. Based on these associations the occurrence of the Etne event is ascribed to the reactivation of a zone of weakness along the Etne fault zone. Slope‐instabilities developed in the superficial deposits during the Etne event demonstrate the existence of potentially hazardous secondary‐effects of such earthquakes even in low seismicity areas such as southwestern Norway.
Lineament
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Magma and related hydrothermal fluid movement, and their interaction with solid rock, in active volcanic regions, generate a wide variety of seismic waves whose characterization can mitigate the risk of a potential eruption. Located in the western region of the Trans-Mexican Volcanic Belt, Ceboruco Volcano, whose last eruptive period was 1870–1875, is considered to be one of the most hazardous volcanoes in Mexico. We have conducted a detailed study of the seismicity in the surroundings of Ceboruco's volcanic edifice to assess the current state of this volcano. A dense temporary seismic network with 25 seismic stations in an area of 16 km × 16 km was deployed between November 2016 and July 2017, as part of the P-24 project of the CeMIEGeo consortium; this effort has allowed the detection of 81 earthquakes concentrated beneath the crater with depths between 4 and 8 km. In this study, We observe that the recorded seismicity occurs in swarms, and we specifically identify four sequences that we characterize in detail via the first focal mechanisms available for this volcano. Our results suggest a change in the local seismicity distribution compared to earlier observations, which reported seismic activity near the volcano edifice associated with fluid migration along zones of weakness related to the extensional stresses of the Tepic-Zacoalco rift. The changes in seismic patterns and obtained focal mechanisms are consistent with observed fluid effects at many geothermal sites worldwide, but also could suggest resumption of activity at this currently dormant volcano.
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Stress relaxation
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Abstract Nabro volcano, situated to the east of the Afar Rift Zone, erupted on 12 June 2011. Eruptions at such off‐rift volcanoes are infrequent, and consequently, the plumbing systems are poorly understood. We present posteruption Synthetic Aperture Radar (SAR) images from the TerraSAR‐X satellite and posteruption continuous seismic activity from a local seismic array. Interferometric analysis of SAR data, reveals a circular, 12 km wide, signal subsiding at ∼200 mm/yr. We inverted for the best fit Mogi source finding a 4 ± 1 × 10 7 m 3 /yr volume decrease at 7 ± 1 km depth. Between 31 August and 7 October 2011, we located 658 and relocated 456 earthquakes with local magnitudes between −0.4 and 4.5. Seismicity beneath the SE edge of Nabro at 11 km depth is likely associated with high strain rates from deep magma flow into the modeled reservoir. This suggests that magma is supplied through a narrow conduit and then stored at ∼7 km depth. We interpret seismicity at 4–6 km depth as brittle fracturing above the inferred magma reservoir. Focal mechanisms delineate a thrust fault striking NE‐SW and dipping 45° to the SE across the caldera floor. We propose that the crustal response is to slip on this fault which crosscuts the caldera rather than to deform on ring faults. The NE‐SW fault plane is not associated with measurable surface deformation, indicating that it does not contribute much to the caldera deformation. We show that subsidence of the caldera is controlled by magma chamber processes rather than fault slip.
Caldera
Magma chamber
Rift zone
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Abstract The high seismic productivity of volcanic areas provides the chance to investigate the local stress conditions with great resolution, by analysing the slope of the frequency-magnitude distribution of earthquakes, namely the b- value. Here we investigated the seismicity of Mt. Etna between 2005 and 2019, focusing on one of the largest known episodes of unrest in December 2018, when most of the intruding magma aborted, rather oddly, its ascent inside the volcano. We found a possible stress concentration zone along magma pathways, which may have inhibited the occurrence of a larger eruption. If the origin of such hypothetical loaded region is related to tectonic forces, one must consider the possibility that geodynamic processes can locally result in such rapid crustal strain as to perturb the release of magma. Strong b- value time-variations occurred a few days before the unrest event, suggesting new possibilities for investigating the volcano state and impending eruptions.
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Abstract We constructed a catalog of deep low‐frequency (DLF) earthquakes beneath 52 volcanic regions in Japan to investigate their seismicity based on three analyses: relocation, classification, and detection. Relocation and classification analyses were based on waveform correlation, and detection analysis was conducted using the matched filter technique. We detected a total of 105,327 DLF earthquakes and found that DLF earthquakes in many regions are spatially clustered with a spatial spread of only 1–2 km with approximately 5 km intervals between the lower limit of crustal earthquakes and Moho discontinuity. Based on temporal seismicity patterns, DLF earthquake groups in each region can be classified into episodic and non‐episodic types. Episodic groups consist of seismic swarms and quiescence. In some episodic groups, DLF earthquakes have constant recurrence interevent times or increasing interevent times as a function of time. Swarms of DLF earthquakes sometimes are associated with volcanic activities at the surface, which are crustal deformations of Meakan volcano, 2007 and 2014 eruptions of Ontake volcano, and the 2015 magma intrusion of Sakurajima volcano. The spatiotemporal characteristics of DLF earthquake groups may be linked to the movement of magmatic fluids. The discrete vertical separation of DLF earthquake groups may reflect small‐scale heterogeneities, such as injected magma. Periodic activity patterns may be caused by volcanic mechanisms, such as intermittent magma flow. The variety of DLF earthquake patterns may suggest that multiple mechanisms, including magma cooling and intermittent magma flow in complex magma supply systems, may trigger DLF earthquakes rather than a single mechanism.
Discontinuity (linguistics)
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Seafloor Spreading
Seismometer
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