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

Luigi Vadacca

doi:10.3390/geosciences10040144

Abstract

Geological and geophysical evidence suggests that the Altotiberina low-angle (dip angle of 15–20 ° ) normal fault is active in the Umbria–Marche sector of the Northern Apennine thrust belt (Italy). The fault plane is 70 km long and 40 km wide, larger and hence potentially more destructive than the faults that generated the last major earthquakes in Italy. However, the seismic potential associated with the Altotiberina fault is strongly debated. In fact, the mechanical behavior of this fault is complex, characterized by locked fault patches with a potentially seismic behavior surrounded by aseismic creeping areas. No historical moderate (5 ≤ Mw ≤ 5.9) nor strong (6 ≤ Mw ≤ 6.9)-magnitude earthquakes are unambiguously associated with the Altotiberina fault; however, microseismicity is scattered below 5 km within the fault zone. Here we provide mechanical evidence for the potential activation of the Altotiberina fault in moderate-magnitude earthquakes due to stress transfer from creeping fault areas to locked fault patches. The tectonic extension in the Umbria–Marche crustal sector of the Northern Apennines is simulated by a geomechanical numerical model that includes slip events along the Altotiberina and its main seismic antithetic fault, the Gubbio fault. The seismic cycles on the fault planes are simulated by assuming rate-and-state friction. The spatial variation of the frictional parameters is obtained by combining the interseismic coupling degree of the Altotiberina fault with friction laboratory measurements on samples from the Zuccale low- angle normal fault located in the Elba island (Italy), considered an older exhumed analogue of Altotiberina fault. This work contributes a better estimate of the seismic potential associated with the Altotiberina fault and, more generally, to low-angle normal faults with mixed-mode slip behavior.

Highlights

The Altotiberina fault (ATF) is located at the Tuscany–Umbria–Marche regional boundary within the Northern Apennines (Figure 1), a NE- verging thrust-fold belt undergoing NE-trending extension at a rate of about 3 mm/yr [1]
At 10,000 years slip along the ATF plane begins to propagate in the central part, while the entire Gubbio fault (GF) plane has reached failure
It should be noted that GF slips completely only after 10,000 years, even though, with an average dip angle of 40◦, GF is classified as a well-oriented normal fault [20,22]

Summary

Introduction

The Altotiberina fault (ATF) is located at the Tuscany–Umbria–Marche regional boundary within the Northern Apennines (Figure 1), a NE- verging thrust-fold belt undergoing NE-trending extension at a rate of about 3 mm/yr [1]. The microseismicity nucleating on the ATF is not able to explain the amount of deformation associated with the short- and long-term slip rate inferred by geological [7] and geodetic studies [8,9], suggesting a prevalent aseismic deformation In support of this hypothesis, talc minerals, characterized by a very low friction coefficient (0.05 < μS < 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].

Model Geometry and Material Properties

Rate- and State-Dependent Friction Law

Modeling the Mixed-Mode Fault Slip Behavior

Initial Stress Field Conditions

Fault Slip Condition

Reactivation of Misoriented and Potentially Seismic Fault Patches

Stick-Slip Versus Creeping Deformation

Discussion and Conclusions