Faults Geometry and the Role of Fluids in the 2016–2017 Central Italy Seismic Sequence

Abstract

The 2016–2017 Central Italy seismic sequence ruptured overlapping normal faults of the Apennines mountain chain, in nine earthquakes with magnitude Mw > 5 within a few months. Here we investigate the structure of the fault system using an extensive aftershock data set, from joint permanent and temporary seismic networks, and 3‐D Vp and Vp/Vs velocity models. We show that mainshocks nucleated on gently west dipping planes that we interpret as inverted steep ramps inherited from the late Pliocene compression. The two large shocks, the 24 August, Mw = 6.0 Amatrice and the 30 October, Mw = 6.5 Norcia occurred on distinct faults reactivated by high pore pressure at the footwall, as indicated by positive Vp/Vs anomalies. The lateral extent of the overpressurized volume includes the fault patch of the Norcia earthquake. The irregular geometry of normal faults together with the reactivated ramps leads to the kinematic complexity observed during the coseismic ruptures and the spatial distribution of aftershocks.

Shear Wave Splitting Evidence and Relations With Stress Field and Major Faults From the “Amatrice‐Visso‐Norcia Seismic Sequence”

“…The imbricate thrust zone causes the overlaying of turbiditic succession with consequent: sediment consolidation, pore closure, and dewatering. The presence of accreted sediment could be related to the low Vp / Vs value (1.80) up to 4‐km depth, as proposed by Chiarabba et al (). At deeper layers, high Vp / Vs values (up to 1.95) are found, probably related to the Upper Triassic ‐Miocene carbonatic Umbria‐Marche succession.…”

“…The Central Apennine has been affected by multiphased contractional and extensional deformation started in the Early Tertiary. A postorogenic Quaternary extension of the fold‐and‐thrust belt is still ongoing and striking SW‐NE as documented by instrumental seismicity (Chiarabba et al, ; Chiaraluce et al, ; Michele et al, ), stress data (Mariucci & Montone, ), geodetic measurements (Cheloni et al, ), and geological studies (Bonini et al, ; Lavecchia et al, ; Walters et al, ).…”

Rupture Kinematics and Structural‐Rheological Control of the 2016 Mw6.1 Amatrice (Central Italy) Earthquake From Joint Inversion of Seismic and Geodetic Data

“…This plane intersects our fault in correspondence of the splitting zone identified by the rupture evolution (Figure 2e). Comparing the imaged slip distribution with the Vp values along the fault plane (Figure 5c), we observe that Vp' values increase moving from the high slip zone to north crossing the identified plane (dashed line in Figure 5c), highlighting the presence of a bulge/up-warp geometry compatible with a thrust-and-fold structure (Chiarabba et al, 2018). Moreover, we observe that the zone of transition with enhanced Poisson ratio values (see blue mapped color in Figure 5d) coincides with the fault area where the along-strike rupture propagation has been impeded and split.…”

“…This feature seems to be in agreement with the presence of a structural and stress barrier controlling the entire seismic sequence evolution, as proposed by previous studies (Chiaraluce et al, 2017;Mildon et al, 2017;Pizzi et al, 2017). In order to characterize the geometry of this barrier, we compare our fault model with the aftershock hypocenters relocated by Chiaraluce et al (2017), the mapped fault traces Villani et al, 2018), and Vp and Poisson ratio tomographic model obtained by Chiarabba et al (2018). The aftershocks alignment identifies a 190°striking and 32°dipping plane emerging in correspondence to the STF (Figures 5a and 5b and S7).…”

“…In order to fit both geodetic and seismological observations, we build up a fault plane striking N164°E (consistent with the strike angle well constrained by geodetic data, Cheloni et al, 2017, magenta planes in Figure S3) and dipping 50°to the SW. The adopted hypocenter, located at 13.238°E, 42.704°N, at a depth of 6.3 km (red star in Figure S3) is in full agreement with the hypocenter relocated by Chiarabba et al (2018; blue star in Figure S3).…”