ALBERT

Description: How fast, and how foreseeable, is magma ascent is one of the most compelling and unanswered issues of volcanology. The velocity of the magma upwelling depends on the local conditions of the volcanic conduit and rheology of the magma. During magma emplacement in the shallow crust, transient variations of physical properties underneath active volcanoes are expected and in a few cases observed. The predictability of such changes strongly depends on how fast this process is, compared to our ability to handle geophysical data and consistently resolve transient anomalies in the physical properties of the medium. Mount Etna (Italy) is a perfect natural laboratory to investigate such issues, due to the almost continuous magmatic activity and the high quality of seismologic and geodetic data. Here we show, for the first time, that seismic attenuation of local earthquakes strongly increases due to the emplacement of magma within the crust, forecasting an incipient eruption at Mount Etna.

Description: On 23 October 2011, an M w  7.1 earthquake struck the Van Lake region of eastern Turkey; causing vast damage in the cities of Van and Erciş. The mainshock was followed by a large number of aftershocks, which define a 60–70 km long and 30–35 km wide northeast–southwest-trending structure, in agreement with the source rupture models derived for the main event. In this paper, we take advantage of this large data set to examine the spatial and temporal properties of the Van earthquake aftershock activity. We derive the spatial distribution of b -value of the Gutenberg–Richter law, as well as complementary seismicity parameters, along the surface projection of the fault plane. Recent studies have been published on the same issue, presenting controversial and sometimes opposite results. With respect to previous studies, we rely on a possibly higher quality catalog of relocated earthquakes. Furthermore, we adopt a more conservative approach, excluding from the analysis the first few days of data, until the M c reaches a stable completeness threshold; finally, we conduct statistical tests in order to check the significance of the spatial and temporal variation of b -value across the fault plane. Calculations are made for the complete catalog and for two independent aftershock subcatalogs, after which a stable magnitude of completeness M c is reached. For each catalog, we correlate the observed b -value patterns with slip distribution models of the mainshock obtained through the inversion of seismological and geodetic data. Overall, the b -values vary from 0.9 to 1.5 along the Van rupture fault zone. The higher b -values (〉1.1) are observed around the epicenter of the mainshock characterizing the higher coseismic slip area on the fault projection. Low b -values are concentrated at the peripheral portion of the fault, away from high-slip patches. Moreover, the b -value distribution over the fault plane undergoes significant variation throughout the aftershock sequence.

Description: The 2016 central Italy seismic sequence consists so far of a series of moderate-to-large earthquakes activating within a few months along a 60-km-long and Apenninic-trending normal-fault system. Regrettably, the high vulnerability of the local infrastructure and the shallowness of the largest events (depth around 8 km) resulted in 299 casualties and more than 20,000 homeless, with great difficulties in the disaster management. The sequence evolved around its largest event ( M w  6.5, 30 October) that occurred right in the middle of a fault system already activated two months before with a first M w  6.0 mainshock (on 24 August) located to the south near the town of Amatrice. Then, another M w  5.9 mainshock occurred just four days before the largest mainshock (26 October) at the northernmost extent of the sequence, near the town of Visso. We analyze the space–time evolution of the first four months of seismic activity through the relocation of ~26,000 earthquakes and the kinematic source models of the three mainshocks. All the main events nucleated at the base of a southwest-dipping normal-fault system segmented by the presence of crosscutting compressional structures. The presence of these inherited faults, separating diverse geological domains, appears to modulate evolution of the sequence interfering with coseismic slip distribution and fault segments interaction. Several secondary antithetic and synthetic faults are located at a shallow depth (〈4 km), both in the hanging wall and footwall. The whole normal fault system, confined within the first 8 km of the upper crust, is bounded below by a shallow east-dipping and 2–3-km-thick layer in which small events plus a series of large extensional aftershocks ( M w  4) occur, possibly decoupling the upper and lower crusts.

Description: 〈span〉〈div〉Abstract〈/div〉The M〈sub〉w〈/sub〉 6.1 (6 April 2009) L’Aquila (Italy) earthquake occurred in one of the most seismically active areas of central Italy and was preceded by a three-month-long foreshock period. Thanks to recordings by a regional permanent network, we derive for the first time P- and S-wave velocity tomographic models of a major fault prone to an imminent main shock. Close to the M〈sub〉w〈/sub〉 6.1 hypocenter, we observe high Vp (〉6.8 km/s) and high Vp/Vs (〉1.9) consistent with thick dolomitic volumes filled with fluids sealed by impermeable anhydritic layers. Significant changes in velocities defined by time-lapse imaging during the foreshock period suggest rapid fluid migration through the locked fault zone. The complex positive feedback between fluid pressure buildup and hydrofracturing of the dolomitic reservoir, testified by foreshock production, eventually provoked the catastrophic coseismic breaching of the fault seal. Our results show that foreshock time-lapse tomography provides clues on the preparatory phase of a large normal-faulting earthquake.〈/span〉

Description: 〈span〉The M〈sub〉w〈/sub〉 6.1 (6 April 2009) L’Aquila (Italy) earthquake occurred in one of the most seismically active areas of central Italy and was preceded by a three-month-long foreshock period. Thanks to recordings by a regional permanent network, we derive for the first time P- and S-wave velocity tomographic models of a major fault prone to an imminent main shock. Close to the M〈sub〉w〈/sub〉 6.1 hypocenter, we observe high Vp (〉6.8 km/s) and high Vp/Vs (〉1.9) consistent with thick dolomitic volumes filled with fluids sealed by impermeable anhydritic layers. Significant changes in velocities defined by time-lapse imaging during the foreshock period suggest rapid fluid migration through the locked fault zone. The complex positive feedback between fluid pressure buildup and hydrofracturing of the dolomitic reservoir, testified by foreshock production, eventually provoked the catastrophic coseismic breaching of the fault seal. Our results show that foreshock time-lapse tomography provides clues on the preparatory phase of a large normal-faulting earthquake.〈/span〉