M. Cocco

The 1997 Umbria‐Marche, Italy, Earthquake Sequence: A first look at the main shocks and aftershocks

A long sequence of earthquakes, six with magnitudes between 5 and 6, struck Central Italy starting on September 26, 1997, causing severe damages and loss of human lives. The seismogenic structure consists of a NW-SE elongated fault zone extending for about 40 km. The focal mechanisms of the largest shocks reveal normal faulting with NE-SW extension perpendicular to the trend of the Apennines, consistently with the Quaternary tectonic setting of the internal sector of the belt and with previous earthquakes in adjacent regions. Preliminary data on the main shocks and aftershocks show that extension in this region of the Apennines is accomplished by normal faults dipping at low angle (∼40°) to the southwest, and confined in the upper ∼8 km of the crust. These normal faults might have reactivated thrust planes of the Pliocene compressional tectonics. The aftershock distribution and the damage patterns also suggest that the three main shocks ruptured distinct 5 to 15 km-long fault segments, adjacent and slightly offset from one another.

Frictional constraints on crustal faulting

We consider how variations in fault frictional properties affect the phenomenology of earthquake faulting. In particular, we propose that lateral variations in fault friction produce the marked heterogeneity of slip observed in large earthquakes. We model these variations using a rate- and state-dependent friction law, where we differentiate velocity-weakening behavior into two fields: the strong seismic field is very velocity weakening and the weak seismic field is slightly velocity weakening. Similarly, we differentiate velocity-strengthening behavior into two fields: the compliant field is slightly velocity strengthening and the viscous field is very velocity strengthening. The strong seismic field comprises the seismic slip concentrations, or asperities. The two “intermediate” fields, weak seismic and compliant, have frictional velocity dependences that are close to velocity neutral: these fields modulate both the tectonic loading and the dynamic rupture process. During the interseismic period, the weak seismic and compliant regions slip aseismically, while the strong seismic regions remain locked, evolving into stress concentrations that fail only in main shocks. The weak seismic areas exhibit most of the interseismic activity and aftershocks but can also creep seismically. This “mixed” frictional behavior can be obtained from a sufficiently heterogeneous distribution of the critical slip distance. The model also provides a mechanism for rupture arrest: dynamic rupture fronts decelerate as they penetrate into unloaded complaint or weak seismic areas, producing broad areas of accelerated afterslip. Aftershocks occur on both the weak seismic and compliant areas around a fault, but most of the stress is diffused through aseismic slip. Rapid afterslip on these peripheral areas can also produce aftershocks within the main shock rupture area by reloading weak fault areas that slipped in the main shock and then healed. We test this frictional model by comparing the seismicity and the coseismic slip for the 1966 Parkfield, 1979 Coyote Lake, and 1984 Morgan Hill earthquakes. The interevent seismicity and aftershocks appear to occur on fault areas outside the regions of significant slip: these regions are interpreted as either weak seismic or compliant, depending on whether or not they manifest interevent seismicity.

Two‐way coupling between Vesuvius eruptions and southern Apennine earthquakes, Italy, by elastic stress transfer

During the past 1000 years, eruptions of Vesuvius have often been accompanied by large earthquakes in the Apennines 50-60 km to the northeast. Statistical investigations had shown that earthquakes often preceded eruptions, typically by less than a decade, but did not provide a physical explanation for the correlation. Here, we explore elastic stress interaction between earthquakes and eruptions under the hypothesis that small stress changes can promote events when the Apennine normal faults and the Vesuvius magma body are close to failure. We show that earthquakes can promote eruptions by compressing the magma body at depth and opening suitably oriented near-surface conduits. Voiding the magma body in turns brings these same normal faults closer to Coulomb failure, promoting earthquakes. Such a coupling is strongest if the magma reservoir is a dike oriented normal to the regional extension axis, parallel to the Apennines, and the near-surface conduits and fissures are oriented normal to the Apennines. This preferred orientation suggests that the eruptions issuing from such fissures should be most closely linked in time to Apennine earthquakes. Large Apennine earthquakes since 1400 are calculated to have transferred more stress to Vesuvius than all but the largest eruptions have transferred to Apennine faults, which may explain why earthquakes more commonly lead than follow eruptions. A two-way coupling may thus link earthquakes and Vesuvius eruptions along a 100-km-long set of faults. We test the statistical significance of the earthquake-eruption correlation in the two-way coupling zone, and find a correlation significant at the 95% confidence level.

A Kinematic Source-Time Function Compatible with Earthquake Dynamics

We propose a new source-time function, to be used in kinematic modeling of ground-motion time histories, which is consistent with dynamic propagation of earthquake ruptures and makes feasible the dynamic interpretation of kinematic slip models. This function is derived from a source-time function first proposed by Yoffe (1951), which yields a traction evolution showing a slip-weakening behavior. In order to remove its singularity, we apply a convolution with a triangular function and obtain a regularized source-time function called the regularized Yoffe function. We propose a parameterization of this slip-velocity time function through the final slip, its duration, and the duration of the positive slip acceleration ( Tacc ). Using this analytical function, we examined the relation between kinematic parameters, such as peak slip velocity and slip duration, and dynamic parameters, such as slip-weakening distance and breakdown-stress drop. The obtained scaling relations are consistent with those proposed by Ohnaka and Yamashita (1989) from laboratory experiments. This shows that the proposed source-time function suitably represents dynamic rupture propagation with finite slip-weakening distances.

Redistribution of dynamic stress during coseismic ruptures: Evidence for fault interaction and earthquake triggering

We investigate the spatiotemporal evolution of dynamic stress outside a rupturing extended fault. The dynamic stress variations caused by a coseismic rupture in a half space are computed by using the discrete wavenumber and reflectivity methods. After a transient phase, the stress time history evolves to the final static stress value. We compare the static stress changes resulting from this model with those computed from a static dislocation model. We have applied this method to study the interactions between the first two normal faults which ruptured during the 1980 (MS 6.9) Irpinia earthquake. These two subevents are separated in time by nearly 20 s, while the third (and last) subevent occurred 40 s after the rupture onset. We compute the dynamic stress changes caused by the rupture of the first subevent. Our modeling results show that the dynamic stress peak on the second subevent fault plane is reached between 7 s and 8 s after the rupture initiation on the main fault. On the average the static stress level on the second subevent (20 s) fault plane is reached nearly after 14 s. The dynamic rupture did not jump from a rupturing segment to the adjacent one immediately, but the triggering of the 20 s subevent is delayed by roughly 10 s with respect to the instant of occurrence of the dynamic stress peak induced by the 0 s event. The dynamic stress pulse propagates along the strike direction of the second subevent fault plane at an average velocity of nearly 3.4 km/s. The delayed triggering of the second subevent can be interpreted in terms of the frictional properties of the faults. In particular, rate- and state-dependent frictional law can explain a delayed instability after a sudden change in stress. Using the estimated values of the subevent triggering delay and the shear stress change, we attempt to constrain the parameter Aσ on the 20 s fault. The values here inferred agree well with those resulting from previous studies.

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.

Frictional response induced by time-dependent fluctuations of the normal loading

We study the effect of time-variable normal stress perturbations on a creeping fault which satisfies a velocity-weakening rate- and state-dependent friction law and is slipping at constant speed. We use the spring-block model and include the effect of inertia. To account for the variable normal stress, we use the description introduced by Linker and Dieterich [1992], which links normal stress fluctuations to changes of the state variable. We consider periodic perturbations of the normal stress in time (as caused, for instance, by tides) and compare the behavior for two commonly used friction laws (the “slip” and the “ageing” laws). Their mechanical response is shown to be significantly different for normal stress fluctuations. It could be used to probe these two laws during laboratory friction experiments. We show that there is a resonance phenomenon, involving strong amplification of the shear and velocity response of the interface, when the spring stiffness is modestly above its critical value (or when, at a given stiffness, the normal stress is modestly below its critical value). We show that such an amplification is also observed when periodic fluctuations of the shear loading are considered, making the resonance phenomenon a general feature of the response of a near-critical creeping surface to periodic fluctuations of the external loading. Analytical solutions are based on a linear expansion for low amplitude of normal or shear stress variations and are in very good agreement with numerical solutions. A method to find the evolution of friction in the case of an arbitrary perturbation of the normal stress is also presented. The results show that a creeping fault may be destabilized and enter a stick-slip regime owing to small normal stress oscillations. This may also account for a mechanism for the generation of “creep bursts.” However, these phenomena require very specific parameter ranges to excite the resonance, which may not be met very generally in nature. This study illustrates the importance of the normal stress fluctuations on stable sliding and suggests further friction laboratory experiments.

Static stress changes and fault interaction during the 1997 Umbria-Marche earthquake sequence

We study the static stress changes caused by moderatemagnitude earthquakes that occurred in Umbria-Marcheduring a seismic sequence which started on September3, 1997, with a ML 4.7 foreshock and consisted ofeight earthquakes whose magnitudes range between 5.0and 6.0. The earthquakes occurred on normal faultsstriking in the Apennine direction and dipping at lowangles towards the SW. The goal is to verify if stresschanges induced by each mainshock can explain theoccurrence of subsequent events. Our results show thatthe foreshock slightly increased the Coulomb stress onthe first mainshock fault plane. The distribution ofseismicity that followed the foreshock is clustered inthe area of Coulomb stress increase comprised betweenthe two faults which ruptured in opposite directionsduring the two largest shocks of September 26. Thelocations and the geometry of the three largestearthquakes agree well with the pattern of Coulombstress changes suggesting elastic interaction betweenthese faults. However, we were not able to model thewhole sequence of ML ≥ 5.0 events in terms ofCoulomb stress changes. The difficulties are due tothe similarity of fault plane solutions for eventslocated very close to each other and in the hangingwall of the mainshock rupture planes. Our results showthat normal stress changes agree better with thespatial pattern of the whole sequence of moderatemagnitude events. If previous ruptures unclamp thefault planes of subsequent earthquakes, fluid flow canplay a dominant role in promoting earthquakes duringthe seismic sequence.

Slip heterogeneity and directivity of the ML 6.0, 2016, Amatrice earthquake estimated with rapid finite-fault inversion

On 24 August 2016 a magnitude ML 6.0 occurred in the Central Apennines (Italy) between Amatrice and Norcia causing nearly 300 fatalities. The main shock ruptured a NNW-SSE striking, WSW dipping normal fault. We invert waveforms from 26 three-component strong motion accelerometers, filtered between 0.02 and 0.5 Hz, within 45 km from the fault. The inferred slip distribution is heterogeneous and characterized by two shallow slip patches updip and NW from the hypocenter, respectively. The rupture history shows bilateral propagation and a relatively high rupture velocity (3.1 km/s). The imaged rupture history produced evident directivity effects both N-NW and SE of the hypocenter, explaining near-source peak ground motions. Fault dimensions and peak slip values are large for a moderate-magnitude earthquake. The retrieved rupture model fits the recorded ground velocities up to 1 Hz, corroborating the effects of rupture directivity and slip heterogeneity on ground shaking and damage pattern.

Spatio-temporal distribution of seismic activity during the Umbria-Marche crisis, 1997

We present the spatio-temporal distribution of more than 2000 earthquakesthat occurred during the Umbria-Marche seismic crisis, between September 26and November 3, 1997. This distribution was obtained from recordings of atemporary network that was installed after the occurrence of the first two largest shocks (Mw =, 5.7, Mw = 6.0) of September 26. This network wascomposed of 27 digital 3-components stations densely distributed in theepicentral area. The aftershock distribution covers a region of about 40 km long and about2 km wide along the NW-SE central Apennines chain. The activity is shallow,mostly located at less than 9 km depth. We distinguished three main zonesof different seismic activity from NW to SE. The central zone, that containsthe hypocenter of four earthquakes of magnitude larger than 5, was the moreactive and the more complex one. Sections at depth identify 40–50°dipping structures that agree well with the moment tensor focalmechanisms results. The clustering and the migration of seismicity from NW to SE and the generalfeatures are imaged by aftershock distribution both horizontally and at depth.