ALBERT

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.