Six days after the start of the seismic sequence in Central Italy, the analysis of the seismological, geological and geodetic data collected by the monitoring networks and by the teams of researchers and technicians on the ground has made it possible to reconstruct a first detailed picture of what is happened. In this article we report a summary of the work carried out so far by INGV and collected in the "FIRST SUMMARY REPORT ON THE AMATRICE ML 6.0 EARTHQUAKE OF AUGUST 24, 2016 (CENTRAL ITALY)".
The area affected by the earthquake of August 24th is located within a well-known seismic belt, characterized by high danger and affected in the ancient and recent seismic history by other strong earthquakes, as described in an article of August 24th and as indicated in the figure below.
Sequences of the last decades in the central Apennine sector affected by the Amatrice sequence that began with the earthquake of August 24, 2016 (red star and gray area). The blue symbols identify the 1979 Valnerina (Norcia) sequence; the orange symbols identify the Gubbio seismic sequence of 1984; the red symbols are the 1997 Colfiorito sequence (also known as the Umbria-Marche earthquakes); the dark blue symbols the Gualdo Tadino sequence from 1998; the green symbols are the sequence of the 2009 Eagle; finally, the black symbols in the northwest identify the seismicity of the upper Tiber valley and the Pietralunga sequence of 2010. The magnitudes of the respective main shocks are shown in the figure together with the focal mechanisms of the shocks of major magnitude and the magnitude of completeness ( Mc) of the seismic catalogs used for the figure. Rectangles indicate the approximate locations of major historic earthquakes in the area.
Some initial considerations can be made on the fault responsible for the earthquake of 24 August. By comparing the data recorded by the National Seismic Network (supplemented by temporary instruments since the first hours after the event), by satellite data (LINK), both GPS and SAR), from the soil analyses, a very coherent picture of the process that generated the earthquake is obtained. Further investigations and elaborations, still in progress, will help to better clarify what happened and will be able to provide indications on what could happen in the future.
The first observations of co-seismic ground displacement obtained with the Japanese satellite ALOS 2 show an elongated ground subsidence in a NNW-SSE area, with maximum values of about 20 cm in two areas to the north and south (INGV processing)
The structure responsible for the seismic event (seismogenic fault) is oriented in a north-north-west – south-south-east direction and extends for 25-30 km between the centers of Norcia, to the north, and that of Amatrice to the south. The plan extension of the area affected by aftershocks is over 300 km2, and the crustal volume involved extends from the surface to a depth of about 10 km. The fault activated by the earthquake has a complex geometry, with a main plane dipping from east to west, which moved with a bilateral rupture direction, from the Accumoli area towards the two ends of the fault. This did not move in the same way along its entire extension, but inside it two areas were identified where the most important cosismic movement took place, of the order of 1 meter (blue areas in the figure below).
Preliminary model of displacement distribution on the fault from continuous ALOS2, Sentinel 1 and GPS data. In red the epicenter of the main event, the black dots are the replicas preliminarily relocated with the NonLinLoc procedure.
The sudden displacement of the fault, at 3:36 on August 24, lasted less than ten seconds but the shaking perceived by the population in the epicentral area was much greater, on the order of a minute or two. On the surface, the displacements detected by geologists confirm this geometry of the fault, but the movements detected so far on the escarpment (in the area of Monte Vettore and Vettoretto) are at most 15-20 centimetres. This difference between depth and surface displacement is often observed for these types of earthquakes and faults (termed extensional or normal).
Co-seismic fracture at the base of the fault plane on Monte Vettoretto.
The analysis of satellite data has allowed us to verify that the sector west of the fault was lowered during the earthquake of August 24th. On the surface, this translated into a 10-20 cm subsidence of an elongated zone parallel to the direction of the fault (NNO-SSE). The long-term effect of these deformations (ie after tens of thousands of years and hundreds of earthquakes) is the creation of the intramontane valleys (the lowered parts) which characterize the landscape of the Apennines.
These reported here are just some of the information found in the "FIRST SUMMARY REPORT ON THE AMATRICE ML 6.0 EARTHQUAKE ON AUGUST 24, 2016 (CENTRAL ITALY) ".