Lauro Chiaraluce

Complex Normal Faulting in the Apennines Thrust-and-Fold Belt: The 1997 Seismic sequence in Central Italy

A long sequence of moderate-magnitude earthquakes (5 M 6) struck central Italy in September and October 1997. At the end of the sequence a year later, the seismogenic area extends for about 60 km along the Apennines. The analysis of historical seismicity suggests that this seismic sequence filled a 700-year gap in this portion of the chain. Other historical sequences in the same area are characterized by prolonged seismic release on adjacent fault segments, probably due to the in- volvement of shallow and complex structures inherited by the compressive tectonics. The distribution of seismicity and the fault-plane solutions show that the extension in this region is accomplished by normal faults dipping at relatively low angles (40) to the southwest. The focal mechanisms of the largest shocks reveal normal faulting with extension perpendicular to the Apenninic chain (northeast-southwest), consistently with the Quaternary tectonics of the internal sector of the northern Apen- nine belt and with previous earthquakes in adjacent regions. Three mainshocks oc- curred on distinct 5- to 10-km-long fault segments, adjacent and slightly offset be- tween each other. High-quality aftershock locations show that seismicity is confined within the sedimentary Mesozoic cover in the upper 8 km of the crust and that most of the aftershocks are shallower than the largest shocks, which nucleated at 6-km depth. Faults evidenced by aftershock locations have a planar geometry and show increased complexity toward the surface. Most of the aftershock focal mechanisms are dominated by normal faulting. Several strike-slip events occurred at shallow depths, reactivating portions of pre-existing thrust planes that segment the normal fault system. The spatiotemporal evolution of seismicity shows a peculiar migration of hypocenters along the strike of the main faults with multiple ruptures and the activation of fault segments before the occurrence of the main rupture episodes.

On the Relationship between Mw and ML for Small Earthquakes

Abstract Estimating the moment magnitudes ( M w ) of a small earthquake is a challenging task. One viable option to measure its size is to calculate its local magnitude ( M L ) and convert it to the physically based M w . Unfortunately, to correctly perform such a conversion is not easy; moreover, even though many studies demonstrate that the equivalence between M L and M w is incorrect for small events, these two parameters are sometimes thought to be strictly equivalent, regardless of the earthquake’s size. Using random vibration theory, we show that, below M w ∼4, the M L of a small earthquake is proportional to the logarithm of its seismic moment, and the following relationship holds: We test our findings on a high‐quality data set in the Upper Tiber Valley (northern Apennines, Italy), composed of events in the range of 0≤ M L ≤3.8, for which we compute accurate estimates of M L and M w . Online Material: Details of the processing procedure, figures of the empirical regional attenuation functional, and source terms of 1191 events from the Alto Tiberina fault (ATF) data set and earthquake catalog.

On the Relationship betweenMwandMLfor Small Earthquakes

Abstract Estimating the moment magnitudes ( M w ) of a small earthquake is a challenging task. One viable option to measure its size is to calculate its local magnitude ( M L ) and convert it to the physically based M w . Unfortunately, to correctly perform such a conversion is not easy; moreover, even though many studies demonstrate that the equivalence between M L and M w is incorrect for small events, these two parameters are sometimes thought to be strictly equivalent, regardless of the earthquake’s size. Using random vibration theory, we show that, below M w ∼4, the M L of a small earthquake is proportional to the logarithm of its seismic moment, and the following relationship holds: We test our findings on a high‐quality data set in the Upper Tiber Valley (northern Apennines, Italy), composed of events in the range of 0≤ M L ≤3.8, for which we compute accurate estimates of M L and M w . Online Material: Details of the processing procedure, figures of the empirical regional attenuation functional, and source terms of 1191 events from the Alto Tiberina fault (ATF) data set and earthquake catalog.

On the Relationship between Mw and ML for Small EarthquakesShort Note

Abstract Estimating the moment magnitudes ( M w ) of a small earthquake is a challenging task. One viable option to measure its size is to calculate its local magnitude ( M L ) and convert it to the physically based M w . Unfortunately, to correctly perform such a conversion is not easy; moreover, even though many studies demonstrate that the equivalence between M L and M w is incorrect for small events, these two parameters are sometimes thought to be strictly equivalent, regardless of the earthquake’s size. Using random vibration theory, we show that, below M w ∼4, the M L of a small earthquake is proportional to the logarithm of its seismic moment, and the following relationship holds: We test our findings on a high‐quality data set in the Upper Tiber Valley (northern Apennines, Italy), composed of events in the range of 0≤ M L ≤3.8, for which we compute accurate estimates of M L and M w . Online Material: Details of the processing procedure, figures of the empirical regional attenuation functional, and source terms of 1191 events from the Alto Tiberina fault (ATF) data set and earthquake catalog.