M4.1 earthquake rattles northern Italy
This part of the Southern Alps has previously hosted M6+ earthquakes
Citation: Hubbard, J. and Bradley, K., 2024. M4.1 earthquake rattles northern Italy. Earthquake Insights, https://doi.org/10.62481/d7ba5970
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A magnitude 4.1 earthquake struck northern Italy on March 27, 2024 at 22:19 local time. The earthquake was widely felt in northeastern Italy, and shaking was also reported in neighboring countries, including Austria, Slovenia, Croatia, and Germany. As far as we are aware, no damage has been reported. Shaking in the earthquake has been precisely mapped using dozens of seismometers by the INGV, which indicate maximum intensities of V-VI (moderate to strong). Visit the INGV’s post for this event to see more details.
Although the earthquake was not large, it was among the largest in recent years. Note that on the map and timeline below, the event is shown as a M4.5 - a variety of magnitudes have been reported, from M4.1 to M4.5. In this post, we prefer M4.1, to match the moment tensor reported by the INGV. However, the map below uses the default EMSC magnitude instead.
The recent earthquake occurred at the eastern edge of the Southern Alps. Here, Italy is moving northward toward central Europe at about 2 mm/yr. This is far slower than, for instance, the shortening rate at the front of the Himalaya (13-18 mm/yr). However, while the current rate of convergence is low, it is important to remember that earthquake magnitude does not depend on deformation rate - a slow-moving fault system can produce large earthquakes. The slip rate controls how often earthquakes should occur, but does not limit the magnitude of the largest events.
The largest regional earthquake in recent history was the 1976 Friuli earthquake - a M6.5 earthquake that killed nearly one thousand people and was followed by a number of large aftershocks. This event occurred due to slip on one of the many thrusts that dip gently beneath the mountain range.
Because there are many thrusts stacked on top of each other, and the Friuli earthquake did not rupture to the surface, it is difficult to know exactly which fault slipped in the mainshock - although geologists have tried. In fact, it appears likely that a number of faults were activated in the ensuing aftershock sequence.
Based on its location, the recent M4.1 occurred on a fault within this area. Unlike the 1976 mainshock, which ruptured a gently north-dipping fault, this earthquake was a was oblique, and occurred on a steeply dipping plane. Its focal mechanism indicates that slip occurred on a 50-60° dipping plane, dipping either to the south, or to the northwest. Given the complexity of the geology here it is perhaps not surprising to see diverse types of earthquakes.
Why do these faults exist? The Alps are basically a geologist’s playground - exposing a wide range of deformation that accumulated over 100 million years or more. While today it appears as a single mountain range, geologists divide it into parts, to identify rocks sourced from collisions of different microplates. Mixed in with those rocks are ophiolites - remnants of the ocean basins that once existed between those microplates.
Today, the Alps are a bi-vergent structure - in other words, there are thrusts along the north side of the mountain range, dipping south, and a second set of thrusts on the south side of the mountain range, dipping north. The lower part of the European Plate, which once subducted southward, has broken off. Because plate movements are mostly driven by sinking oceanic crust, this break-off caused the forces driving collision to fade, although not entirely. Some scientists suggest that now it is the Adriatic Plate that is sinking northward - i.e. that the subduction polarity reversed.
Other cross-sections simply show collision today, with the crust of northern Italy wedging its way into the Alps. The deep structure of the Alps has been a topic of great interest (and argument) for more than a century, and despite major technological and conceptual advances there is clearly still much to be learned here.
Today’s earthquake in the Southern Alps was small, but history tells us that earthquakes here can reach much larger magnitudes. That is not limited to the 1976 earthquake: M6+ events have been identified along strike in 1695, 1873, and 1936. A number of faults that could generate earthquakes above M6 have been identified along the range front. It is not possible to predict when the next large earthquake will occur here, but it is likely that when it does, it will be quite damaging.
In some cases, large earthquakes in Italy have been preceded by foreshocks. This includes the 1976 Friuli earthquake, as well as some more recent events like the 2009 M6.3 L’Aquila earthquake. While these events can act as warnings, the challenge is identifying which small earthquakes might herald larger events. Furthermore, the time between a foreshock and a mainshock is not well defined; for the 1976 event, there was only one minute between the two earthquakes, while at L’Aquila, the events were separated by six days. These variations mean that it is very difficult to use small events as effective warnings. Could the recent M4.1 earthquake be a foreshock of a larger event? It is possible, but statistically unlikely; only time will tell.
And although Italy has advanced monitoring systems and good modern building codes, it also has a large number of older buildings that remain vulnerable to shaking, liquefaction, and slope failures. In 2023, it was announced that the latest iteration of the national seismic hazard map of Italy had been rejected by a panel of referees, due to disagreements about the most appropriate ways to estimate seismic hazard. As always, the best way to prepare for large earthquakes continues to include construction of earthquake-resistant buildings, and retrofitting of existing vulnerable structures.
References:
D'Agostino, N., Cheloni, D., Mantenuto, S., Selvaggi, G., Michelini, A. and Zuliani, D., 2005. Strain accumulation in the southern Alps (NE Italy) and deformation at the northeastern boundary of Adria observed by CGPS measurements. Geophysical Research Letters, 32(19). https://doi.org/10.1029/2005GL024266
Galadini, F., Poli, M.E. and Zanferrari, A., 2005. Seismogenic sources potentially responsible for earthquakes with M≥ 6 in the eastern Southern Alps (Thiene-Udine sector, NE Italy). Geophysical Journal International, 161(3), pp.739-762. https://doi.org/10.1111/j.1365-246X.2005.02571.x
Granado, P., 2017. Inversion Tectonics in the Alpine Foreland, Eastern Alps (Austria). Ph.D. Thesis, University of Barcelona. http://hdl.handle.net/2445/116804
INGV, 2024. Mw 4.1 (Ml 4.5) seismic event, in the province of Udine, 27 March 2024. https://ingvterremoti.com/2024/03/27/evento-sismico-mw-4-1-ml-4-5-in-provincia-di-udine-27-marzo-2024/
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Picozzi, M., Spallarossa, D., Iaccarino, A.G. and Bindi, D., 2022. Temporal evolution of radiated energy to seismic moment scaling during the preparatory phase of the Mw 6.1, 2009 L’Aquila earthquake (Italy). Geophysical Research Letters, 49(8), p.e2021GL097382. https://doi.org/10.1029/2021GL097382
Sabelli, C., 2023. Italy’s new seismic hazard map is back to square one. Nature Italy, https://doi.org/10.1038/d43978-023-00072-1.
Thornton, J.M., Mariethoz, G. and Brunner, P., 2018. A 3D geological model of a structurally complex Alpine region as a basis for interdisciplinary research. Scientific data, 5(1), pp.1-20. https://doi.org/10.1038/sdata.2018.238
Zelenin, E., Bachmanov, D., Garipova, S., Trifonov, V. and Kozhurin, A., 2022. The Active Faults of Eurasia Database (AFEAD): the ontology and design behind the continental-scale dataset. Earth System Science Data, 14(10), pp.4489-4503. https://doi.org/10.5194/essd-14-4489-2022