By Angela Stallone (PhD candidate in Geophysics at the Istituto Nazionale di Geofisica e Vulcanologia (INGV), Italy) and Giovanni Diaferia (PhD candidate in Geophysics at the University of Roma TRE, Italy)
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A fault under a microscope
When studying earthquakes, the workplace for running experiments can be very different from a well-known chemistry lab. This is the case with Near Fault Observatories (NFOs), which consist of a dense network of sensors installed over a seismically active area. By continuously and closely monitoring the region, these observatories provide high-quality and high-density data that contribute to the understating of earthquake formation.
TABOO
A recent example of a NFO is the Alto Tiberina Near Fault Observatory (TABOO) in central Italy [Chiaraluce et al., 2014]. Here, a group of scientists from the INGV (Istituto Nazionale di Geofisica e Vulcanologia) monitor the Alto Tiberina fault, a 60 km long discontinuity in the northern Apennines. Observed for the first time in the late 90s, it is an extensional fault gently dipping to the northeast.
As is the case with other low angle faults around the globe, its nature is enigmatic and the mechanism of energy release is not totally clear. On one hand, stress can be released abruptly in earthquakes, while on the other hand, slow and continuous aseismic creep can occur. The analysis of historical earthquakes (strong events occurred early in the past and whose evidence is mainly based on written sources) in the Alto Tiberina area shows evidence of three main damaging events in 1352, 1751 and 1781 (red stars in Figure 3). The latter, known as the ‘horrible earthquake of Cagli,’ had an estimated magnitude of Mw 6.4, and caused about 300 casualties. Most of the fatalities occurred in churches between the towns of Cagli and Piobbico, as the quake struck while people were at mass. More recently, two earthquakes struck the area: the Mw 5.2 Gubbio event in 1984 and the Mw 5.1 Gualdo Tadino event in 1998 (blue stars in Figure 3). These earthquakes suggest that the region is very seismically active. Therefore, monitoring the Alto Tiberina Fault is crucial, for it could rupture in damaging earthquakes in the future.
The TABOO installation consists of a dense network of seismometers, GPS receivers, as well as geochemical and electromagnetic stations. Such a multidisciplinary infrastructure provides a close-up look at the Alto Tiberina fault and its seismic activity, which is beneficial for the seismic hazard assessment of the area. As a matter of fact, earthquakes are the result of mechanisms that occur at different scales, ranging from large-scale tectonic loading to site-specific processes. For example, in central Italy, circulation of deep fluids appears to impact seismic activity [Miller et al., 2004; Antonioli et al., 2005; Lombardi et al., 2010].
In five years (2010-2014), the TABOO seismometers recorded around 44.000 earthquakes, mostly ranging from low to very-low magnitude (M < 2). Such a number is the result of continuous recording by the dense network of seismic stations. Besides providing a rich record of seismic events, this modern infrastructure assured extremely high-quality data. For example, earthquake locations were determined with unprecedented precision (Latorre et al. [2016]). Moreover, the minimum recorded magnitude decreased significantly, allowing for the detection of many small earthquakes (the smaller the magnitude, the higher the probability for an event to be missed by a seismometer).
A sharper picture
Like sharpening a blurred image, the abundance of high quality data from the TABOO catalog has returned previously unseen details of the Alto Tiberina fault. The increased number of small events and the better precision of their locations has helped scientists to track the main features of the Alto Tiberina fault system. In fact, the smaller and much more abundant earthquakes contribute to illuminate the subsurface geometry of the tectonic structures. Imaging the main framework of the fault system can therefore help scientists figure out how it could potentially rupture in large, though rare, events.
Regarding the area monitored by TABOO, most of the earthquakes were recorded at low depth, revealing the existence of normal faults auxiliary to the Alto Tiberina fault (Figure 3, profiles on the right). The data also suggests that seismic dislocation along such faults could be facilitated by the circulation of deep fluids (responsible for reducing the mechanical resistance of the rocks involved).
Such high quality data showed that only a small portion (~10%) of seismicity is located along the Alto Tiberina Fault, which is in agreement with stress release accommodated by creep. In general, creep does not totally exclude the possibility of large seismic slip. In this case, taking into account the fault length and rock volumes involved, scientists estimate that the Alto Tiberina fault is capable of rupturing in M=7.2 events.
These results show that the Alto Tiberina Fault system is a highly complex tectonic area where stress release occurs by both seismic slip and creep. These observations are crucial for the correct assessment of the seismic hazard in the northern Apennines, as they provide evidence of the seismogenic potential of the Alto Tiberina fault and its related network of faults.
References
Antonioli, A., Piccinini, D., Chiaraluce, L., & Cocco, M. (2005). Fluid flow and seismicity pattern:
Evidence from the 1997 Umbria-Marche (central Italy) seismic sequence. Geophysical Research
Letters, 32(10).
Chiaraluce, L., Amato, A., Carannante, S., Castelli, V., Cattaneo, M., Cocco, M., … & Marzorati, S.
(2014). The Alto Tiberina Near Fault Observatory (northern Apennines, Italy). Annals of
Geophysics, 57(3).
Lombardi, A. M., Cocco, M., & Marzocchi, W. (2010). On the increase of background seismicity
rate during the 1997–1998 Umbria-Marche, central Italy, sequence: Apparent variation or
fluid-driven triggering?. Bulletin of the Seismological Society of America, 100(3), 1138-1152.
Miller, S. A., Collettini, C., Chiaraluce, L., Cocco, M., Barchi, M., & Kaus, B. J. (2004). Aftershocks driven by a high-pressure CO2 source at depth. Nature, 427(6976), 724-727.
Latorre, D., Mirabella, F., Chiaraluce, L., Trippetta, F., & Lomax, A. (2016). Assessment of earthquake locations in 3‐D deterministic velocity models: A case study from the Altotiberina Near Fault Observatory (Italy). Journal of Geophysical Research: Solid Earth, 121(11), 8113-8135.
Valoroso, L., Chiaraluce, L., Di Stefano, R., & Monachesi, G. (2017). Mixed‐Mode Slip Behavior of the Altotiberina Low‐Angle Normal Fault System (Northern Apennines, Italy) through High‐Resolution Earthquake Locations and Repeating Events. Journal of Geophysical Research: Solid Earth.