Mixed‐Mode Slip Behavior of the Altotiberina Low‐Angle Normal Fault System (Northern Apennines, Italy) through High‐Resolution Earthquake Locations and Repeating Events

Abstract

We generated a 4.5‐year‐long (2010–2014) high‐resolution earthquake catalogue, composed of ~37,000 events with ML < 3.9 and MC = 0.5 completeness magnitude, to report on the seismic activity of the Altotiberina (ATF) low‐angle normal fault system and to shed light on the mechanical behavior and seismic potential of this fault, which is capable of generating a M7 event. Seismicity defines the geometry of the fault system composed of the low‐angle (15°–20°) ATF, extending for ~50 km along strike and between 4 and 16 km at depth showing an ~1.5 km thick fault zone made of multiple subparallel slipping planes, and a complex network of synthetic/antithetic higher‐angle segments located in the ATF hanging wall (HW) that can be traced along strike for up to 35 km. Ninety percent of the recorded seismicity occurs along the high‐angle HW faults during a series of minor, sometimes long‐lasting (months) seismic sequences with multiple MW3+ mainshocks. Remaining earthquakes (ML < 2.4) are released instead along the low‐angle ATF at a constant rate of ~2.2 events per day. Within the ATF‐related seismicity, we found 97 clusters of repeating earthquakes (RE), mostly consisting of doublets occurring during short interevent time (hours). RE are located within the geodetically recognized creeping portions of the ATF, around the main locked asperity. The rate of occurrence of RE seems quite synchronous with the ATF‐HW seismic release, suggesting that creeping may guide the strain partitioning in the ATF system. The seismic moment released by the ATF seismicity accounts for 30% of the geodetic one, implying aseismic deformation. The ATF‐seismicity pattern is thus consistent with a mixed‐mode (seismic and aseismic) slip behavior.

Repeating earthquakes in western Corinth Gulf (Greece): implications for aseismic slip near locked faults

“…Rubin et al 1999; and are interpreted as the boundary of the seismogenic zone where the locked part of the faults lay above them. Another example is the case of Alto Tiberina fault, where microseismicity defines a low-angle normal fault (Chiaraluce et al 2007;Valoroso et al 2017), lower parts of which are creeping aseismically whereas the shallow locked portions are compatible with an M > 6.5 earthquake (Anderlini et al 2016). In the case of western Corinth Gulf, the shallow north-dipping zone was interpreted as a detachment zone Figure 15.…”

The Altotiberina Low-Angle Normal Fault (Italy) Can Fail in Moderate-Magnitude Earthquakes as a Result of Stress Transfer from Stable Creeping Fault Area

“…These structures, characterized by higher dip angles compared to the ATF, have generated small-moderate-magnitude earthquakes; the largest one of M w = 5.1 occurred in 1984 on the Gubbio fault plane [4]. According to the works of [5,6] only microseismic events (<2.3 M L ) have been located along the 500-1000 m thick fault zone cross-cutting the upper crust from 4-5 km down to 14-16 km depth and coinciding with the geometry and location of the ATF (Figure 1). The seismicity nucleating along the ATF is characterized by a nearly constant rate of earthquake production r = 7.30 × 10 −4 earthquakes day −1 km 2 , corresponding to about three events per day with M L < 2.3.…”

“…In support of this hypothesis, talc minerals, characterized by a very low friction coefficient (0.05 < µ S < 0.23; [10]) with a velocity-strengthening slip behavior (e.g., creeping), have been observed to form interconnected foliated networks within the Zuccale fault core, a low-angle normal fault located in the Elba island (Italy) and considered the (older) exhumed analogue of the ATF [11][12][13]. Only recently these hypotheses have been confirmed [6,14]. Indeed, to investigate spatial variations of the frictional behavior along the ATF surface, Anderlini et al [14] mapped the coefficient of interseismic coupling (the ratio of the long-term seismic slip rate to the tectonic slip rate) by inverting GPS data.…”

“…They found that about half of the ATF surface below 5 km of depth is characterized by creep, producing a long-term slip rate of 1.7 ± 0.3 mm/yr, while the remaining portion of the same fault is locked and it may be capable of generating M6.5+ earthquakes. In addition, thanks to a 5-year-long (2010-2014) high-resolution earthquake catalogue of about 40,000 events [6,15] observed a striking positive correlation between the creeping regions and the microseismic activity (<2.3 M L ), characterized by clusters of repeating earthquakes, whereas locked portions (asperities) are noticeably less productive or almost silent. The seismic moment released by the ATF seismicity accounts for 30% of the geodetic one [6], implying aseismic deformation.…”

“…In addition, thanks to a 5-year-long (2010-2014) high-resolution earthquake catalogue of about 40,000 events [6,15] observed a striking positive correlation between the creeping regions and the microseismic activity (<2.3 M L ), characterized by clusters of repeating earthquakes, whereas locked portions (asperities) are noticeably less productive or almost silent. The seismic moment released by the ATF seismicity accounts for 30% of the geodetic one [6], implying aseismic deformation. Finally, Independent Component Analysis of GPS time series revealed a large aseismic contribution for swarm-like activity that occurred in the hanging wall of the ATF in 2013-2014 [16].…”