Concetta Nostro

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.

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.

Structural constraints on the spatial distribution of aftershocks

[1] Calculations of static stress changes due to large earthquakes have shown that the spatial distribution of aftershocks is predictable to first order, with aftershocks primarily occurring in areas experiencing positive stress changes. Delineation of these areas relies on resolving the stress perturbation onto planes with known orientations; common practice is to use poorly constrained regional stress information to compute optimally oriented failure planes, assuming that they exist everywhere. Here we show that this assumption is not supported by observation but rather that aftershock failure planes are controlled by geological structure. We argue that useful aftershock hazard estimates are better made by replacing information on regional stress with statistical measures of structural orientations.

Seismic anisotropy beneath the Northern Apennines (Italy) and its tectonic implications

We examined shear wave splitting in SKS and S phases from 22 teleseisms at 10 temporary stations on a transect across the Northern Apenninic arc. The array, near 43°N, spans from Corsica Island across the Tyrrhenian region and the Apenninic belt to the Adriatic coast. We applied particle motion, covariance matrix decomposition, and cross correlation methods to estimate the polarization direction of the fast split - shear wave ( ?) and the delay time between split phases (δt). Most of the analyzed shear waves show clear evidence of splitting. The ? in the Adriatic foreland and in the Apennines are approximately parallel to the strike of the mountain belt (NW-SE). The largest δt correspond to the highest elevations, suggesting that anisotropy is related to the compressional tectonics which built the Apennines, and that this tectonic compression involved at least the entire lithosphere. In the Tyrrhenian area we observe ? oriented about E-W, suggesting a reorientation of the mantle fabric due to astenospheric flow, responsible for the E-W post-orogenic extension observed at the surface.

Spherical versus flat models of coseismic and postseismic deformations

We perform an exhaustive study of coseismic and postseismic surface deformations induced by shear dislocations using flat and spherical Earth models. Our aim is to examine the effects of the spherical geometry, the vertical layering, and the self-gravitation on surface displacement field. For a 100 km long fault, spherical and flat models produce comparable coseismic deformations up to a distance of ∼300 km from the epicenter. This distance is sensibly reduced in the postseismic regime and when infinitely long strike-slip faults are considered. The differences between predictions based on flat and spherical models are due both to their global geometry and the effect of the gravity forces. Self-gravitation has a minor role with respect to that of sphericity for surface coseismic deformations, while in the postseismic regime its effects increase considerably. As a case study, we consider the coseismic and postseismic deformations due to the great 1960 Chilean earthquake. The results of the spherical stratified model differ sensibly from those of a flat uniform model. Moreover, within the framework of spherical Earth models, the rheological stratification plays a major role in determining the pattern of the displacement field. We show that the present-day rates of vertical and horizontal deformations are considerably large (∼10 -2 m yr -1 ) for an asthenospheric viscosity ranging from 10 19 to 10 20 Pa s. These rates, which could possibly be detected by geodetic investigations, are found to be also sensitive to the rheological properties of the mantle beneath the asthenosphere.

Normal fault interaction caused by coseismic and postseismic stress changes

We study coseismic and postseismic stress fields caused by a normal faulting earthquake in a self-gravitating, stratified, viscoelastic spherical Earth over distances from a few to hundreds of kilometers. We investigate the contribution of postseismic relaxation on the induced Coulomb stress for extensional tectonic settings accounting for the effects of the Earth stratification. We use a numerical code based on the spherical self-gravitating Earth model developed by Piersanti et al. [1995, 1997]. We study how postseismic relaxation can modify the state of stress at the base of the seismogenic layer where large earthquakes are believed to nucleate. We compare our results with those obtained by means of a three-dimensional dislocation model in an elastic half-space, which does not account for the time-dependent postseismic stress transfer. The viscoelastic relaxation process modifies the coseismic stress changes during time periods from several decades to centuries. The postseismic stress is generally greater than the coseismic stress change. Postseismic relaxation increases the Coulomb stress near the causative faults and tends to reduce the stress shadow areas. The temporal evolution of Coulomb stress reveals that in addition to the viscosity value, the thickness of the elastic layer controls the time at which the relaxation process is completed. A larger thickness of the elastic layer yields a faster relaxation in the first few decades after the seismic event but smaller postseismic stress amplitudes at longer timescales. One of the most significant results of this study is the extreme sensitivity of the timescales of the viscoelastic relaxation to small changes in the thickness and depth of the shallowest viscoelastic layer as well as in variation of the viscosity. Such a result suggests that the interpretation of the time evolution of the postseismic signals only in terms of viscosity values could lead to misleading conclusions.

The role of INGVterremoti blog in information management during the earthquake sequence in central Italy

In this paper, we describe the role the INGVterremoti blog in information management during the first part of the earthquake sequence in central Italy (August 24 to September 30). In the last four years, we have been working on the INGVterremoti blog in order to provide quick updates on the ongoing seismic activity in Italy and in-depth scientific information. These include articles on specific historical earthquakes, seismic hazard, geological interpretations, source models from different type of data, effects at the surface, and so on. We have delivered information in quasi-real-time also about all the recent magnitude M≥4.0 earthquakes in Italy, the strongest events in the Mediterranean and in the world. During the 2016 central Italy, the INGVterremoti blog has continuously released information about seismic sequences with three types of posts: i) updates on the ongoing seismic activity; ii) reports on the activities carried out by the INGV teams in the field and any other working groups; iii) in-depth scientific articles describing some specific analysis and results. All the blog posts have been shared automatically and in real time on the other social media of the INGVterremoti platform, also to counter the bad information and to fight rumors. These include Facebook, Twitter and INGVterremoti App on IOS and Android. As well, both the main INGV home page (http://www.ingv.it) and the INGV earthquake portal ( http://terremoti.ingv.it ) have published the contents of the blog on dedicated pages that were fed automatically. The work done day by day on the INGVterremoti blog has been coordinated with the INGV Press Office that has written several press releases based on the contents of the blog. Since August 24, 53 articles were published on the blog they have had more than 1.9 million views and 1 million visitors. The peak in the number of views, which was more than 800,000 in a single day, was registered on August 24, 2016, following the M 6.0 earthquake.