While low‐angle normal faults have been recognized worldwide from geological studies, whether these structures are active or capable of generating big earthquakes is still debated. We provide new ...constraints on the role and modes of the Altotiberina fault (ATF) in accommodating extension in the Northern Apennines. We model GPS velocities to study block kinematics, faults slip rates and interseismic coupling of the ATF, which is active and accounts, with its antithetic fault, for a large part of the observed chain normal 3 mm/yr tectonic extension. A wide portion of the ATF creeps at the long‐term slip rate (1.7 ± 0.3 mm/yr), but the shallow locked portions are compatible with M > 6.5 earthquakes. We suggest that positive stress accumulation due to ATF creep is most likely released by more favorable oriented splay faults, whose rupture may propagate downdip along low‐angle normal fault surface and reduce the probability of occurrence of a seismic rupture of the shallower locked portion.
Key Points
First geodetic image of heterogeneous interseismic coupling on a continental LANF
The locked portion of the Alto Tiberina LANF can potentially be the locus of M > 6 earthquakes
Stress accumulated by long‐term creep on the ATF can be released by failure of hanging wall splay faults
A critical point in the analysis of ground displacement time series, as those recorded by space geodetic techniques, is the development of data-driven methods that allow the different sources of ...deformation to be discerned and characterized in the space and time domains. Multivariate statistic includes several approaches that can be considered as a part of data-driven methods. A widely used technique is the principal component analysis (PCA), which allows us to reduce the dimensionality of the data space while maintaining most of the variance of the dataset explained. However, PCA does not perform well in finding the solution to the so-called blind source separation (BSS) problem, i.e., in recovering and separating the original sources that generate the observed data. This is mainly due to the fact that PCA minimizes the misfit calculated using an
L
2
norm (
χ
2
)
, looking for a new Euclidean space where the projected data are uncorrelated. The independent component analysis (ICA) is a popular technique adopted to approach the BSS problem. However, the independence condition is not easy to impose, and it is often necessary to introduce some approximations. To work around this problem, we test the use of a modified variational Bayesian ICA (vbICA) method to recover the multiple sources of ground deformation even in the presence of missing data. The vbICA method models the probability density function (pdf) of each source signal using a mix of Gaussian distributions, allowing for more flexibility in the description of the pdf of the sources with respect to standard ICA, and giving a more reliable estimate of them. Here we present its application to synthetic global positioning system (GPS) position time series, generated by simulating deformation near an active fault, including inter-seismic, co-seismic, and post-seismic signals, plus seasonal signals and noise, and an additional time-dependent volcanic source. We evaluate the ability of the PCA and ICA decomposition techniques in explaining the data and in recovering the original (known) sources. Using the same number of components, we find that the vbICA method fits the data almost as well as a PCA method, since the
χ
2
increase is less than 10 % the value calculated using a PCA decomposition. Unlike PCA, the vbICA algorithm is found to correctly separate the sources if the correlation of the dataset is low (
<
0.67) and the geodetic network is sufficiently dense (ten continuous GPS stations within a box of side equal to two times the locking depth of a fault where an earthquake of
M
w
>
6
occurred). We also provide a cookbook for the use of the vbICA algorithm in analyses of position time series for tectonic and non-tectonic applications.
Purpose
Many questions concerning Turner syndrome (TS) remain unresolved, such as the long-term complications and, therefore, the optimal care setting for adults. The primary aim of this long-term ...cohort study was to estimate the incidence of comorbid conditions along the life course.
Methods
A total of 160 Italian patients with TS diagnosed from 1967 to 2010 were regularly and structurally monitored from the diagnosis to December 2019 at the University Hospital of Bologna using a structured multidisciplinary monitoring protocol.
Results
The study cohort was followed up for a median of 27 years (IQR 12–42). Autoimmune diseases were the comorbid condition with the highest incidence (61.2%), followed by osteoporosis and hypertension (23.8%), type 2 diabetes (16.2%) and tumours (15.1%). Median age of onset ranged from 22 years for autoimmune diseases to 39 years for type 2 diabetes. Malignant tumours were the most prominent type of neoplasm, with a cumulative incidence of 11.9%. Papillary thyroid carcinoma was the most common form of cancer, followed by skin cancer and cancer of the central nervous system. Only one major cardiovascular event (acute aortic dissection) was observed during follow-up. No cases of ischaemic heart disease, heart failure, stroke or death were recorded.
Conclusions
This cohort study confirms the need for continuous, structured and multidisciplinary lifelong monitoring of TS, thus ensuring the early diagnosis of important comorbid conditions, including cancer, and their appropriate and timely treatment. In addition, these data highlight the need for the increased surveillance of specific types of cancer in TS, including thyroid carcinoma.
Changes in water level are commonly reported in regions struck by a seismic event. The sign and amplitude of such changes depend on the relative position of measuring points with respect to the ...hypocenter, and on the poroelastic properties of the rock. We apply a porous media flow model (TOUGH2) to describe groundwater flow and water‐level changes associated with the first ML5.9 mainshock of the 2012 seismic sequence in Emilia (Italy). We represent the earthquake as an instantaneous pressure step, whose amplitude was inferred from the properties of the seismic source inverted from geodetic data. The results are consistent with the evolution recorded in both deep and shallow water wells in the area and suggest that our description of the seismic event is suitable to capture both timing and magnitude of water‐level changes. We draw some conclusions about the influence of material heterogeneity on the pore pressure evolution, and we show that to reproduce the observed maximum amplitude it is necessary to take into account compaction in the shallow layer.
We analyze the coseismic stress perturbation during the 17 June 2000 south Iceland seismic sequence; the main shock (Ms 6.6) was followed by three large events within a few tens of seconds (8, 26, ...and 30 s) located within 80 km. The aim of this paper is to investigate short‐term fault interaction and instantaneous triggering. This happens when a fault perturbed by a stress change fails before the end of the transient stress perturbation. We compute the shear, normal, and Coulomb stress changes as functions of time in a stratified elastic half‐space by using discrete wave number and reflectivity methods. We calculate dynamic stresses caused by the main shock at the hypocenters of these three subsequent events. Our numerical results show that the onset of the last two events is slightly delayed with respect to the arrival time of the second positive peak of Coulomb stress variation, while the first event occurred after the first positive stress peak. We have also analyzed the response of a spring slider system representing a fault governed by a rate‐ and state‐dependent friction law, perturbed by shear and normal stress variations caused by the main shock. The fault response to the computed stress perturbations is always clock advanced. We have found suitable constitutive parameters of the modeled fault that allow the instantaneous dynamic triggering of these three earthquakes. If the initial sliding velocity is comparable with the tectonic loading velocity, we obtained failure times close to the observed origin times for low values of the initial effective normal stress.
In this study we aim to understand the effect of static and dynamic stress changes in promoting earthquake failures on secondary faults. Toward this goal we solve the equation of motion of a ...spring‐slider dynamic system including inertia and using rate‐ and state‐dependent constitutive laws. We separately investigate the dynamic response of this fault analog system to a sudden stress change represented either as a stress step or as a stress pulse, which are used to model permanent (static) and transient (dynamic) stress perturbations. The induced earthquake failure does not occur immediately at the application of the coseismic stress change, but it is delayed in time: we define this time interval as the triggering delay. For a given stress perturbation, we analyze the dependence of triggering delays on different system conditions and constitutive parameters. Our results clearly show that the effects of static and dynamic stress changes are quite different. While a static stress change is able to advance as well as to delay an induced instability depending on its sign, a dynamic stress pulse is only able to promote a nearly instantaneous failure, provided its amplitude is positive and large enough with respect to the direct effect of friction. In other words, dynamic stress changes can only cause nearly instantaneous failures, without any relevant triggering delay. These results should be considered in interpreting the seismicity rate changes caused by large earthquakes at least as long as seismic events are interpreted as sliding instabilities obeying rate‐ and state‐dependent friction laws.
Summary
In this paper we model the crack growth in an elastic medium constituted by two welded half-spaces with different rigidities. We implement a 2D Boundary Element Method (BEM) computing shear ...and normal tractions acting on the crack and the slip accommodating stress drop from an arbitrary initial configuration to a final frictional configuration. The direction of crack growth follows the criterion of maximum energy release (strain and gravitational energy) provided that it overcomes the surface fracture energy and the work dissipated by friction. The energetic criterion leads to estimates of the dip angle of seismic faults depending on the amplitude of the initial stress and it includes the classical Anderson's results as a particular case. Moreover, in presence of a sharp rigidity contrast, the direction of crack growth is strongly deflected. The model simulates non-planar, complex, fault geometries, as in the case of detachment and listric faults and it explains the increase of dip angles for both normal and reverse faults, when they enter soft sedimentary layers.
Analyzing the displacement time series from continuous GPS (cGPS) with an Independent Component Analysis, we detect a transient deformation signal that correlates both in space and time with a ...seismic swarm activity (maximum Mw=3.69 ± 0.09) occurred in the hanging wall of the Altotiberina normal fault (Northern Apennines, Italy) in 2013–2014. The geodetic transient lasted ∼6 months and produced a NW‐SE trending extension of ∼5.3 mm, consistent with the regional tectonic regime. The seismicity and the geodetic signal are consistent with slip on two splay faults in the Altotiberina fault (ATF) hanging wall. Comparing the seismic moment associated with the geodetic transient and the seismic events, we observe that seismicity accounts for only a fraction of the measured geodetic deformation. The combined seismic and aseismic slip decreased the Coulomb stress on the locked shallow portion of the ATF, while the transition region to the creeping section has been loaded.
Plain Language Summary
Among the open problems in Solid Earth Science the comprehension of earthquakes is one of the most intriguing. In order to understand how earthquakes behave it is of fundamental importance to study the whole spectrum of sizes and magnitudes. Small earthquake study is prevented because of the small deformation signals, usually covered by noise. In this work we present the study of a small seismic swarm activity in the northern Apennines (Italy), and we demonstrate that it produced observable deformation at the surface recorded by a dense geodetic network of continuous GPS. Comparing the total deformation recorded from the geodetic network and the one associated with the seismic events, we are able to determine the relative importance of seismic and aseismic motion for small earthquakes.The results indicate that aseismic deformation is not negligible, and seismic crises like the one documented must be taken into account in order to better evaluate the hazard posed by a fault.
Key Points
Independent Component Analysis of GPS time series detects small tectonic deformation transient in slow strain‐rates region
Seismic and geodetic data reveal large aseismic contribution for a swarm‐like activity in a normal fault environment
The transient slip decreased Coulomb stress on the locked portion of an active low‐angle normal fault
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