SUMMARY
The computation of the Love numbers (LNs) for a spherically symmetric self-gravitating viscoelastic Earth is a classical problem in global geodynamics. Here we revisit the problem of the ...numerical evaluation of loading and tidal LNs in the static limit for an incompressible planetary body, adopting a Laplace inversion scheme based upon the Post-Widder formula as an alternative to the traditional viscoelastic normal modes method. We also consider, within the same framework, complex-valued, frequency-dependent LNs that describe the response to a periodic forcing, which are paramount in the study of the tidal deformation of planets. Furthermore, we numerically obtain the time-derivatives of LNs, suitable for modelling geodetic signals in response to surface loads variations. A number of examples are shown, in which time and frequency-dependent LNs are evaluated for the Earth and planets adopting realistic rheological profiles. The numerical solution scheme is implemented in ALMA3 (the plAnetary Love nuMbers cAlculator, version 3), an upgraded open-source Fortran 90 program that computes the LNs for radially layered planetary bodies with a wide range of rheologies, including transient laws like Andrade or Burgers.
We propose a procedure for uncertainty quantification in Probabilistic Tsunami Hazard Analysis (PTHA), with a special emphasis on the uncertainty related to statistical modelling of the earthquake ...source in Seismic PTHA (SPTHA), and on the separate treatment of subduction and crustal earthquakes (treated as background seismicity). An event tree approach and ensemble modelling are used in spite of more classical approaches, such as the hazard integral and the logic tree. This procedure consists of four steps: (1) exploration of aleatory uncertainty through an event tree, with alternative implementations for exploring epistemic uncertainty; (2) numerical computation of tsunami generation and propagation up to a given offshore isobath; (3) (optional) site-specific quantification of inundation; (4) simultaneous quantification of aleatory and epistemic uncertainty through ensemble modelling. The proposed procedure is general and independent of the kind of tsunami source considered; however, we implement step 1, the event tree, specifically for SPTHA, focusing on seismic source uncertainty. To exemplify the procedure, we develop a case study considering seismic sources in the Ionian Sea (central-eastern Mediterranean Sea), using the coasts of Southern Italy as a target zone. The results show that an efficient and complete quantification of all the uncertainties is feasible even when treating a large number of potential sources and a large set of alternative model formulations. We also find that (i) treating separately subduction and background (crustal) earthquakes allows for optimal use of available information and for avoiding significant biases; (ii) both subduction interface and crustal faults contribute to the SPTHA, with different proportions that depend on source-target position and tsunami intensity; (iii) the proposed framework allows sensitivity and deaggregation analyses, demonstrating the applicability of the method for operational assessments.
On April 6, 2009, 01:32:39 GMT, the city of L'Aquila was struck by a Mw 6.3 earthquake that killed 307 people, causing severe destruction and ground cracks in a wide area around the epicenter. Four ...days before the main shock we augmented the existing permanent GPS network with five GPS stations of the Central Apennine Geodetic Network (CaGeoNet) bordering the L'Aquila basin. The maximum horizontal and vertical coseismic surface displacements detected at these stations was 10.39 ± 0.45 cm and −15.64 ± 1.55 cm, respectively. Fixing the strike direction according to focal mechanism estimates, we estimated the source geometry with a non linear inversion of the geodetic data. Our best fitting fault model is a 13 × 15.7 km2 rectangular fault, SW‐dipping at 55.3 ± 1.8°, consistent with the position of observed surface ruptures. The estimated slip (495 ± 29 mm) corresponds to a 6.3 moment magnitude, in excellent agreement with seismological data.
The 2016–2017 seismic sequence, in central Italy, was caused by the Mt. Vettore‐Mt. Bove active fault system, which generated three mainshocks, the largest one of Mw 6.6, on 30 October 2016. On 18 ...January 2017, four Mw 5–5.5 seismic events nucleated on the Campotosto active fault. The fault is considered as potentially responsible for M ~6.6 seismic events and a seismic gap. The structure also nucleated a Mw 5.4 aftershock during the 2009 L'Aquila seismic sequence. We here combined new geological/geomorphological field data, 1945 aerial photographs interpretation, geodetic (Differential Interferometric Synthetic Aperture Radar and Global Positioning System) data, and local historical seismicity analysis to determine how much of the Campotosto seismogenic source ruptured during the 2009 and 2016–2017 seismic sequences. The obtained residual seismic moment of the Campotosto seismic gap is in the range Mw 6.4–6.6 earthquake. Moreover, the review of the regional tectonic setting and evolution allowed us a brand new and comprehensive reading of the seismotectonics for the region, in terms of both seismic potential and segmentation of the seismogenic sources, which results strictly connected to the long‐term activity of inherited lithospheric discontinuities, among which the major Ancona‐Anzio fault. We here first highlight that such regional structures strongly influenced the structural setting and evolution of the surrounding regions, in a progressive and continuous process of tectonic inversions, which implies that structural heritage must be understood and considered when dealing with seismotectonics of a given tectonically active region.
Key Points
Geological/geomorphological information is coupled with seismological and geodetic data related to the 2009 and 2016–2017 central Italy seismic sequences
Coseismic dislocation model, based on GPS and InSAR data related to the 18 January 2017 earthquakes on the Campotosto fault, is presented
The residual seismogenic potential of the Campotosto fault is on the order of 6.4–6.5 moment magnitude earthquake
Space‐time clustering is the most striking departure of large earthquakes occurrence process from randomness. These clusters are usually described ex‐post by a physics‐based model in which ...earthquakes are triggered by Coulomb stress changes induced by other surrounding earthquakes. Notwithstanding the popularity of this kind of modeling, its ex‐ante skill in terms of earthquake predictability gain is still unknown. Here we show that even in synthetic systems that are rooted on the physics of fault interaction using the Coulomb stress changes, such a kind of modeling often does not increase significantly earthquake predictability. Earthquake predictability of a fault may increase only when the Coulomb stress change induced by a nearby earthquake is much larger than the stress changes caused by earthquakes on other faults and by the intrinsic variability of the earthquake occurrence process.
Key PointsOften, fault interaction models do not improve earthquake forecasting skillWe define a simple parameter that indicates when fault interaction mattersWe find some requirements to get a time synchronization of faults
We adopted a multidisciplinary approach to investigate the seismotectonic scenario of the 30 October 2016, Mw 6.5, Norcia earthquake, the largest shock of the 2016–2017 central Italy earthquake ...sequence. First, we used seismological and geodetic data to infer the dip of the main slip patch of the seismogenic fault that turned out to be rather low‐angle (~37°). To evaluate whether this is an acceptable dip for the main seismogenic source, we modeled earthquake deformation using single‐ and multiple‐fault models deduced from aftershock pattern analyses. These models show that the coseismic deformation generated by the Norcia earthquake is coherent with slip along a rather shallow‐dipping plane. To understand the geological significance of this solution, we reconstructed the subsurface architecture of the epicentral area. As the available data are not robust enough to converge on a single fault model, we built three different models encompassing all major geological evidence and the associated uncertainties, including the tectonic style and the location of major décollement levels. In all models the structures derived from the contractional phase play a significant role: from controlling segmentation to partially reusing inherited faults, to fully reactivating in extension a regional thrust, geometrically compatible with the source of the Norcia earthquake. Based on our conclusions, some additional seismogenic sources falling in the eastern, external portions of the Apennines may coincide with inherited structures. This may be a common occurrence in this region of the chain, where the inception of extension is as recent as Middle‐Upper Pleistocene.
Key Points
We used a multidisciplinary, multistage approach to investigate the seismotectonic setting of the 2016, Mw 6.5, Norcia earthquake
We tested different models against geological, seismological, and geodetic data
We find that the 30 October 2016, Mw 6.5, Norcia earthquake was generated by the extensional reactivation of a major thrust fault
The present work reports the analysis of a possible relationship due to stress transfer between the two earthquakes that hit the province of Van, Eastern Turkey, on October 23, 2011 (Mw = 7.2) and on ...November 9, 2011 (Mw = 5.6). The surface displacement field of the mainshock has been obtained through a combined data set made up of differential interferograms from COSMO-SkyMed and ENVISAT satellites, integrated with continuous GPS recordings from the Turkish TUSAGA-AKTIF network. This allowed us to retrieve the geometry and the slip distribution of the seismic source and to compute the Coulomb Failure Function (CFF) variation on the aftershock plane, in order to assess a possible causal relationship between the two events. Our results show that the November 9 earthquake could have been triggered by the October 23 shock, with transferred stress values largely exceeding 1 bar.
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