Along strike variation in fault creep on the active Alto Tiberina low angle normal fault inferred from GPS geodesy
Abstract
The Alto Tiberina fault (ATF) in central Italy is a low angle normal fault (LANF) dipping ~20° to the east-northeast. The fault is inferred from surface geology, deep boreholes, seismic reflection lines, abundant microseismicity, and crustal motion data. Balanced cross sections show that the fault plays a major role in accommodating regional extension in central Italy, having accommodated up to 10 km of extension over the past 3 Ma. However, no large earthquakes have been attributed to the ATF. Instead, large earthquakes in the area occur on high angle west dipping normal faults that cut the ATF hanging wall. Several lines of evidence, including fine grained foliations composed of velocity strengthening phyllosilicate minerals in exhumed fault rocks, high fault fluid over-pressures observed in footwall boreholes (~85% lithostatic pressure at 3.7-4.8 km depth), persistent microseismicity coincident with the ATF fault plane, and pattern of geodetically observed crustal motions suggest that the ATF accommodates slip primarily by aseismic creep up to shallow (~4 km) depth in the crust. Previous studies using a simple fault model consisting of an edge dislocation buried in and elastic halfspace supported the shallow creeping hypothesis. But newer realizations of the crustal motion field, imaged with more precision and higher spatial resolution than previously reported, are not adequately explained by this 1-D creeping-fault model. Moreover, significant variations in the occurrence of large hanging wall earthquakes are observed along the strike of the ATF and may be indicative of along-strike variation in ATF fault mechanics. To test whether the along-strike variation in earthquake occurrence is accompanied by similar variation in the rate of fault creep on the ATF, we analyzed crustal motion data derived from more than a decade of continuous GPS measurements in central Italy. We used the TDEFNODE software to parameterize the ATF using the available high-resolution constraints on fault geometry provided by seismic reflection data and seismicity. Our modeling suggests that north of latitude ~43.3°N the ATF creeps at nearly the full fault slip rate of ~2 mm/yr between 3 and 6 km depth. However, south of ~43.3°N the fault appears to be locked to a depth of 9 to 12 km. This apparent locked zone correlates closely with the locations of recent and pre-instrumental large magnitude hanging wall earthquakes, including the 1979 Mw 5.8 Norcia earthquake, 1984 Mw 5.6 Gubbio earthquake, and a sequence of three events Mw 5.6-6.0 near Colfiorito in 1997. In contrast, there are no earthquakes larger than Mw4.0 recorded in the hanging wall overlying the inferred creeping segment of the ATF.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T51H..02B
- Keywords:
-
- 8163 TECTONOPHYSICS Rheology and friction of fault zones;
- 7209 SEISMOLOGY Earthquake dynamics;
- 1242 GEODESY AND GRAVITY Seismic cycle related deformations