A MW 6.3 earthquake struck on April 6, 2009 the Abruzzi region (central Italy) producing vast damage in the L'Aquila town and surroundings. In this paper we present the location and geometry of the ...fault system as obtained by the analysis of main shock and aftershocks recorded by permanent and temporary networks. The distribution of aftershocks, 712 selected events with ML ≥ 2.3 and 20 with ML ≥ 4.0, defines a complex, 40 km long, NW trending extensional structure. The main shock fault segment extends for 15–18 km and dips at 45° to the SW, between 10 and 2 km depth. The extent of aftershocks coincides with the surface trace of the Paganica fault, a poorly known normal fault that, after the event, has been quoted to accommodate the extension of the area. We observe a migration of seismicity to the north on an echelon fault that can rupture in future large earthquakes.
We retrieve seismic velocity variations within the Earth's crust in the region of L'Aquila (central Italy) by analyzing cross‐correlations of more than two years of continuous seismic records. The ...studied period includes the April 6, 2009, Mw 6.1 L'Aquila earthquake. We observe a decrease of seismic velocities as a result of the earthquake's main shock. After performing the analysis in different frequency bands between 0.1 and 1 Hz, we conclude that the velocity variations are strongest at relatively high frequencies (0.5–1 Hz) suggesting that they are mostly related to the damage in the shallow soft layers resulting from the co‐seismic shaking.
Key Points
Seismic velocities in the Earth crust are reduced after the L'Aquila earthquake
Velocity reduction is caused by co‐seismic shaking of shallow soft layers
Noise correlations provide robust information about tiny crustal variations
Real-time seismology has made significant improvements in recent years, with source parameters now available within a few tens of minutes after an earthquake. It is likely that this time will be ...further reduced, in the near future, by means of increased efficiency in real-time transmission, increasing data coverage and improvement of the methodologies. In this context, together with the development of new ground motion predictive equations (GMPEs) that are able to account for source complexity, the generation of strong ground motion shaking maps in quasi-real time has become ever more feasible after the occurrence of a damaging earthquake. However, GMPEs may not reproduce reliably the ground motion in the near-source region where the finite fault parameters have a strong influence on the shaking.
In this paper we test whether accounting for source-related effects is effective in better characterizing the ground motion. We introduce a modification of the GMPEs within the ShakeMap software package, and subsequently test the accuracy of the newly generated shakemaps in predicting the ground motion. The test is conducted by controlling the performance of ShakeMap as we decrease the amount of the available information. We then update ShakeMap with the GMPE modified with a corrective factor accounting for source effects, in order to better constrain these effects that likely influence the level of (near-source) ground shaking.
We investigate two well-recorded earthquakes from Japan (the 2000 Tottori, M
w 6.6, and the 2008 Iwate-Miyagi, M
w7.0, events) where the instrumental coverage is as dense as needed to ensure an objective appraisal of the results. The results demonstrate that the corrected GMPE can capture only some aspects of the ground shaking in the near-source area, neglecting other multidimensional effects, such as propagation effects and local site amplification.
We present two examples of statistical analysis of seismicity conducted by integrating geological, geophysical and seismological data with the aim to characterize the active stress field and to ...define the spatio-temporal distribution of large earthquakes. Moreover, our data will help to improve the knowledge of the “seismogenic behavior” of the areas and to provide useful information for seismic hazard evaluation.
The earthquakes are described by two non-parametric statistical procedures integrating also tectonic-physical parameters to study the spatio-temporal variability.
The results show that the areas are characterized by: 1) a stress regime with mainly extensional kinematics; 2) tectonic structures mainly oriented with the active stress field (
S
hmin
=
N44°
±
18° in the southern Apennines and
S
hmin
=
N50°
±
17° in the central Apennines); 3) cluster distribution of seismicity and 4) a high probability of earthquake occurrence (
M
>
5.5) in the next 10 years.
The main goal of this work is to provide a probability map for the next moderate to large earthquakes (M ≥ 5.5) in Italy. For this purpose we apply a new nonparametric multivariate model to ...characterize the spatiotemporal distribution of earthquakes. The method is able to account for tectonics/physics parameters that can potentially influence the spatiotemporal variability and tests their relative importance; moreover, it allows straightforward testing of a variety of hypothesis, such as Seismic Gap, Cluster, and Poisson hypothesis. The method has been applied to Italian seismicity of the last four centuries for earthquakes with M ≥ 5.5. Italy has been divided into 61 irregular zones representing areas with homogeneous tectonic regime resulting from active stress data. Besides the magnitude and the time of the earthquakes, the model includes information on the tectonic stress regime, the homogeneity of its orientation, the number of active faults, the dimension of the area and the homogeneity of the topography. The time distribution of the M ≥ 5.5 earthquakes appears clustered in time for a few years after an event, and then the distribution becomes similar to a memoryless Poisson process, leading to a time‐dependent probability map. This map shows that the most likely regions where the next moderate to large earthquakes may occur are Friuli, Umbria‐Marche, and part of Southern Apennines and the Calabrian arc.
Changes in continental water storage generate vertical surface deformation, induce crustal stress perturbations, and modulate seismicity rates. However, the degree to which regional changes in ...terrestrial water content influence crustal stresses and the occurrence of earthquakes remains an open problem. We show how changes in groundwater storage, computed for a ∼1,000 km2 basin, focus deformation in a narrow zone, causing large horizontal, nonseasonal displacements. We present results from a karstic mountain range located at the edge of the Adria‐Eurasia plate boundary system in Northern Italy, where shortening is accommodated across an active fold‐and‐thrust belt. The presence of geological structures with high permeabilities and of deeply rooted hydrologically active fractures focus groundwater fluxes and pressure changes, generating transient surface horizontal displacements up to 5 mm and perturbations of crustal stress up to 25 kPa at seismogenic depths. The background seismicity rates appear correlated, without evident temporal delay, with groundwater storage changes in the hydrological basin. With no evidence of pore pressure propagation from the hydrologically active fractures, seismicity modulation is likely affected by direct stress changes on faults planes.
Plain Language Summary
The natural water cycle, by changing how water is stored on the continents, can cause nonnegligible deformation at the Earth's surface. Redistribution of water masses has long been known to alter the state of stress in the crust and potentially modulate seismicity rates. However, the degree to which regional changes in groundwater storage influence crustal stresses and the occurrence of earthquakes at fault scales remains an open problem. We study a karst area located in a tectonically active region of Northern Italy, where plate convergence is accommodated across a complex system of faults and folds. We use GPS, hydrological, meteorological and seismological observations, integrated by hydrological and mechanical models, to show that there is a direct elastic connection between changes in groundwater storage, crustal deformation, and seismicity rates. We show that hydrologically active fractures and seismically active fractures might be disjoint, and that pore pressure propagation is not required to generate stress changes at seismogenic depths. Indeed, the convergence of water from upstream catchment toward permeable fractures connected to the surface can generate large pressure changes on the wall of these fractures, causing horizontal displacements and perturbations of the crustal stress that modulate background seismicity rates.
Key Points
Regional groundwater storage changes in heterogeneous media modulate horizontal transient deformation
Water pressure changes in shallow permeable fractures cause large elastic stress changes at seismogenic depth
Background seismicity rates are correlated with groundwater storage changes
SUMMARY
A new non‐parametric multivariate model is provided to characterize the spatio‐temporal distribution of large earthquakes. The method presents several advantages compared to other more ...traditional approaches. In particular, it allows straightforward testing of a variety of hypotheses, such as any kind of time dependence (i.e. seismic gap, cluster, and Poisson hypotheses). Moreover, it may account for tectonics/physics parameters that can potentially influence the spatio‐temporal variability, and tests their relative importance. The method has been applied to the Italian seismicity of the last four centuries. The results show that large earthquakes in Italy tend to cluster; the instantaneous probability of occurrence in each area is higher immediately after an event and decreases until it reaches, in few years, a constant value representing the average rate of occurrence for that zone. The results also indicate that the clustering is independent of the magnitude of the earthquakes. Finally, a map of the probability of occurrence for the next large earthquakes in Italy is provided.
The Istituto Nazionale di Geofisica e Vulcanologia (INGV) is an Italian research institution, with focus on Earth Sciences. INGV runs the Italian National Seismic Network (Rete Sismica Nazionale, ...RSN) and other networks at national scale for monitoring earthquakes and tsunami as a part of the National Civil Protection System coordinated by the Italian Department of Civil Protection (Dipartimento di Protezione Civile, DPC). RSN is composed of about 400 stations, mainly broadband, installed in the Country and in the surrounding regions; about 110 stations feature also co-located strong motion instruments, and about 180 have GPS receivers and belong to the National GPS network (Rete Integrata Nazionale GPS, RING). The data acquisition system was designed to accomplish, in near-real-time, automatic earthquake detection, hypocenter and magnitude determination, moment tensors, shake maps and other products of interest for DPC. Database archiving of all parametric results are closely linked to the existing procedures of the INGV seismic monitoring environment and surveillance procedures. INGV is one of the primary nodes of ORFEUS (Observatories & Research Facilities for European Seismology) EIDA (European Integrated Data Archive) for the archiving and distribution of continuous, quality checked seismic data. The strong motion network data are archived and distributed both in EIDA and in event based archives; GPS data, from the RING network are also archived, analyzed and distributed at INGV. Overall, the Italian earthquake surveillance service provides, in quasi real-time, hypocenter parameters to the DPC. These are then revised routinely by the analysts of the Italian Seismic Bulletin (Bollettino Sismico Italiano, BSI). The results are published on the web, these are available to both the scientific community and the general public. The INGV surveillance includes a pre-operational tsunami alert service since INGV is one of the Tsunami Service providers of the North-eastern Atlantic and Mediterranean Tsunami warning System (NEAMTWS).
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