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The free core nutation (FCN) is one of the Earth’s rotational eigenmodes, which is caused by the retrograde motion of the liquid core relative to the mantle. The FCN period and Q-factor are determined by the elastic properties of the core/mantle boundary (CMB) and their electromagnetic interaction. In the celestial coordinate system, the FCN period is about 430 days; in the Earth-fixed reference frame this effect manifests itself in the form of the free core resonance (FCR) whose frequency falls in the diurnal tidal band. FCR observation requires highly accurate measurement of the amplitudes and phases of the near-diurnal tidal waves. In particular, the parameter estimates for minor waves K1, P1, Ψ1, and Φ1 are critically important for evaluating the FCR effect, i.e., the period and decay of this resonant mode. The progress in the experimental study of FCR is mainly due to the accumulation of the data from superconducting gravimeters and VLBI; at the same time, also the data of the precision laser strainmeters were used. In this work, the FCR effect is studied based on the long-term strain precision records by two European stations: Baksan, Russia (laser interferometer–strainmeter with a measuring armlength of 75 m [Milyukov et al., 2005; 2007] and Gran Sasso, Italy (two perpendicular laser interferometer–strainmeters, BA and BC, each with a measuring armlength of 90 m [Amoruso and Crescentini, 2009]).

The Free Core Nutation (FCN) is a retrograde mode related to the slight misalignment of the rotation axis of the fluid outer core and the elastic mantle, with a period of about 430 sidereal days in the celestial frame. In the Earth-fixed reference frame, the (complex) frequency of the Free Core Nutation (FCN) is inside the diurnal tidal band and causes a resonant response (Free Core Resonance, FCR) of some diurnal tidal waves to the tide-generating forces.The FCN is usually investigated through its effects on gravity tides and Earth nutations. Here we analyse about 7 years of discontinuous strain records from two 90-m long laser interferometers (strainmeters) operating under the Gran Sasso (Italy) massif and about 4.6 years of discontinuous strain records from two 70-m-long laser interferometers operating the Central Pyrenees (Spain).Starting from the expressions for the vector displacements due to diurnal and semi-diurnal solid tides, we express extension along any azimuthal direction in terms of three complex parameters (related to areal strain and the two shear strain components), which are functions of the latitude-dependent Love and Shida numbers. Those three complex parameters are affected by the FCR through three complex resonance strengths.We find that we can infer 4 model parameters from the inversion of our data, i. e. from the comparison between amplitudes and phases of the measured and theoretical diurnal tides close to the resonance: the FCN period, the FCR quality factor, the imaginary part of one of the three resonance strengths, and the real part of another resonance strength. However, local deformation is distorted with respect to regional deformation because of siting effects. Coupling between local extension (measured by the interferometers) and regional deformation can be described by three coupling coefficients per interferometer, thus introducing 12 additional unknown in the inversions.We minimize misfit between amplitudes and phases of the measured and theoretical tidal strain jointly for all the interferometers by sampling the 4D model parameter space, while optimal coupling coefficients for each interferometer are computed through a simple matrix inversion at each sampled point.Theoretical strain tides is corrected for the effects of the water load oscillations caused by ocean tides. We use FES2014 and TPXO9 ocean models, while the appropriate Earth model for different ocean load areas is chosen basing on the widths of the continental shelves nearby the stations and the inversion misfits.Although we analyse records from two stations only and the amount of data is relatively small, our results for the FCN period and (to some extent) the FCR quality factor are robust and comparable to those obtained from gravity tides and nutations. Moreover, we obtain reliable values of the resonance strengths and robust estimates of the coupling coefficients for all the interferometers.

Earth deformation at the diurnal tidal frequencies includes the resonant tidal-forcing response caused by the Free Core Nutation (FCN), a retrograde mode related to the slight misalignment of the rotation axes of the outer core and mantle. We analyse data from four underground high-sensitivity laser extensometers, whose signal-to-noise ratio in the diurnal tidal band is particularly high, and provide an alternative independent estimate of the FCN complex frequency with respect to more usual techniques (nutation and gravity). Firstly, we differentiate displacements due to diurnal solid tides to obtain extension along any azimuthal direction in terms of three complex parameters (A, S, C) which depend on latitude and frequency. Then, we demonstrate that we can invert the FCN complex frequency and the sensitivity of Im(A) and Re(S) to the resonance from our data. Lastly we obtain the probability distributions of those four parameters. Our results are in full agreement with those from nutation and gravity, as well as with reference IERS (International Earth Rotation and Reference Systems Service) values. Sensitivities of Im(A) and Re(S) to the resonance are estimated here for the first time and are in agreement with values computed using reference Love and Shida numbers from IERS.

The abstract is available here: https://uscholar.univie.ac.at/o:1054129

Two laser strainmeters are being operated in the Canfranc Underground Laboratory (LSC, Central Pyrenees, Spain) at about 350 m depth. One of the two laser strainmeters (GAL16, striking 76°) is located about 670 m from the Spanish entrance of a decommissioned train tunnel, along the side wall of one of the bypasses connecting a recent highway tunnel to the train tunnel. The other strainmeter (LAB780, striking -32°) is located about 780 m from the Spanish entrance of the train tunnel, inside two narrow side halls parallel to and built at the same time as the train tunnel. Their mechanical and optical setups derive from a previous installation at Gran Sasso, Italy, with some changes and improvements. Here we show the main instrument features in the frequency range of 100–0.001 mHz. At frequencies lower than 4 mHz, strain noise compares well with the best laser strainmeters made till now, while at higher frequencies strain noise is higher than at Kamioka, Japan, probably because of frequency instabilities of the laser source. Environmental (air temperature and pressure) effects on measured strain are quite small; thus, signal-to-noise ratio in the tidal bands is unusually high. In particular, diurnal Ψ1 and Φ1 tides clearly emerge from noise even using a 2-year-long strain record, giving the opportunity to improve previous determinations of the Free Core Nutation parameters from strain data as soon as more data are acquired. The features of the LSC strainmeters allow investigating the Earth in a very broad frequency range, with a signal-to-noise ratio as good as or better than that of the best laser strainmeters in the world.

Caldera unrest is often attributed to magma intrusion into a sill. In several cases, like Fernandina and Sierra Negra, Kilauea south caldera, and Campi Flegrei, the sill is km-sized and km-deep. A few questions related to sill emplacement at calderas seem still unanswered: how do sills form and spread, why can magma propagate for kilometers without solidifying, and why do ground deformation data rarely, if ever, detect sill propagation.When considering isoviscous incompressible magma intruding at a constant rate into a homogeneous half-space under non-isothermal conditions and forming a circular sill, mathematical modeling includes: a fluid-dynamic equation (relying on lubrication theory), a fracture propagation criterion, an elasticity equation (link between fluid overpressure and sill opening), and a heat-transfer magma-solidification equation. As already known, a small lag must exist between the fluid (magma) and fracture fronts, because of the large pressure gradients required to drive a viscous liquid into a narrow opening.We show that the free-surface effects on the elasticity equation are negligible, provided that depth-to-radius is smaller than one, as at the above-mentioned calderas; thus, spreading occurs like in an infinite medium. Taking advantage of published studies on hydraulic fracture propagation, first we consider isothermal spreading, as governing equations admit approximate analytical solutions for sill radius, sill opening, fluid overpressure and lag size. Next we consider non-isothermal spreading of an isoviscous incompressible single-component magma, which is initially at its solidification temperature.We show that if the sill is at least a couple of kilometers deep and the product of viscosity and injection rate is sufficiently small, then the lag between the magma and fracture fronts is much smaller than the sill radius during most of the propagation process; as a consequence, propagation velocity is practically unaffected by the lag, except for the initial phase. Because of the way solidified magma thickness and sill opening grow with distance from the tip in the near-tip region, zero-lag non-isothermal spreading would stop after travelling unrealistically short distances, unless magma intrudes rocks that are as hot as the solidification temperature or has unrealistic overpressures. Thus, we consider how the lag might affect the sill maximum size, by preventing solidification at the tip. We compute non-isothermal propagation velocity and the solidified magma thickness by adapting the approach originally developed by Dontsov (2016) for the zero-lag propagation of penny-shaped hydraulic fractures with leak-off; then we relate the lag size to the propagation velocity using the isothermal solutions.We find that the lag plays a fundamental role in postponing the sill arrest by magma solidification, because heat exchange between the magma and the hosting rock is effective only behind the lag, where the magma has some finite thickness and sill opening grows with distance from the tip faster than thickness of solidified magma.As for ground deformation, we show that its pattern does not change appreciably over time if the final sill radius is smaller than 2 to 3 km: this explains why it is usually attributed to the inflation of a stationary source.

This work presents and discusses the tidal analysis of about 2.5 years of strain data recorded by two 70 m-long high-resolution laser strainmeters, which are operating in the Canfranc underground laboratory (Central Pyrenees, Spain). Spectra show clear tidal peaks whose frequencies range from the diurnal band to at least 8 cycles per day; strain amplitudes (relative change in length for each strainmeter) range from few 10−12 to 10−8. The reliability of observations and corrections for local distortions are testified by the excellent agreement between measurements and computations for all the diurnal and semidiurnal tides included in the TPXO8 and FES2012 tidal atlases, with the exception of L2. Observed higher-frequency strain tides are mostly attributed to loading by nonlinear shallow-water constituents in the Bay of Biscay, more than 120 km from the measurement site. The signals are quantitatively compared with computations using TPXO8 (MN4, M4, and MS4) and FES2012 (M3, N4, MN4, M4, MS4, and M6). Computations fully agree with M4 observations for one strainmeter, overestimate M4 by about 30% for the other strainmeter and M6 by about a factor of two, and underestimate the other tides. This work shows that data from high-sensitivity strainmeters installed in high-quality sites may provide valuable additional information for studying the nonlinear tidal dynamics and energetics of coastal waters and minor ocean tides, at spatial resolutions of tens to thousands square kilometers, depending on the strainmeter location with respect to the coastline.

In March 2010 two borehole strainmeters and three Michelson tiltmeters within the Campi Flegrei volcanic system, Italy, registered an abrupt deformation signal that was followed 20 min later by seismic slip on a pair of onshore normal faults. We demonstrate that the observed strain changes were caused by a small but rapid volume decrease in a previously identified offshore ellipsoidal magma source or part of it. Although the total deflation was below the detectability of interferometric synthetic aperture radar and GPS, deflation observed rates were briefly 2 orders of magnitude more rapid than decadal inflation rates. We conclude that this high dilatational contraction rate was responsible for triggering seismicity and that this process may be responsible for the normal faulting often observed in the Campi Flegrei region. Our study quantifies the crucial role played by a transient, minor reduction in dilatational stress, in triggering slip on a fault near critical failure. Our subsurface measurements of strain and tilt registered anomalous deformation three sigma above background noise levels 17 min before the onset of microseismicity suggesting strain measurements have potential utility as an early warning system for the city of Naples.

The volume collects papers submitted to Pure and Applied Geophysics following a call on the topic “Geodynamics and Earth Tides”. Partly, the authors had participated in the 18th Geodynamics and Earth Tides Symposium held inTrieste, Italy, in June 2016. TheEarth tides constitute the leading thread through the book, since instrumentation sensitive enough to observe them, also records a broad spectrum of signals generated by Earth dynamic processes.

The Gran Sasso massif is a carbonate fractured aquifer with a spring discharge of more than 18 m3 s−1. The water table has been partially drained by two motorway tunnels and an underground laboratory (UL), located into the core aquifer. Karst features have limited role below the water table, where groundwater flow is mainly regulated by the fracture network. Two paired laser extensometers (BA and BC) recorded ground deformation in the UL. Changes in deformation correlate with the seasonal recharge/discharge cycle of groundwater flow and its long-term changes. Hydrostatic conditions prevail during the recharge phases because of the low permeability of local fractures, favoring compression, and hydraulic gradient increase above the UL. Fast groundwater flow through the high-permeability fault outcropping in the UL can enhance local dilatation for short periods. Spring discharge during exhaustion periods is fed by the low-permeability fracture network, fostering hydrodynamic conditions by hydraulic gradient decrease, diminishing compression and consequently favoring dilatation. Independent support to this conceptual model comes from local tests and a numerical model which highlights the hydromechanical strain effects induced by the hydrological cycle on the jointed rock mass along BA and the role of the hydraulic gradient on the rock mass deformation.

From 2006 to spring 2013, Campi Flegrei (CF) caldera, Italy, was mostly uplifting at an increasing rate, particularly high from 2011. We show that the 2011–2013 accelerated uplift and 1980–2010 inflation and deflation phases can be explained by a two-source conceptual model similar to that proposed by Amoruso et al. (2014) (reference model). However, pressurization of the sole thin quasi-horizontal ∼4000 m deep source, responsible for large-scale 1980–2010 deformation, can explain the whole 2011–2013 deformation, while activity of the shallower Solfatara hydrothermal source, responsible for residual 1980–2010 deformation, appears constant. These results suggest a predominantly magmatic unrest in 2011–2013. Near-real-time comparison of observations and reference model predictions can provide additional information for short-term eruption forecasting at CF; a similar approach could be followed also in other volcanic environments.

We analyze 1980–2010 ground displacements, to discern similarities or differences between Campi Flegrei (CF) inflations and deflations and highlight possible anomalies in particular areas. We show that the deformation pattern can be decomposed into two stationary (constant over time, except for a mere scaling factor) parts; both of them are satisfied by simple deformation sources. A quasi-horizontal elongated crack (oriented NW to SE, and embedded in an elastic layered half-space at a depth of about 3600 m) satisfies large-scale deformation. All source parameters but potency (volume change) are constant over time. Residual deformation is confined to the area of the Solfatara fumarolic field and satisfied by a small spheroid located at about 1900 m in depth. Again, all source parameters but potency are constant over time. The histories of the two sources are somewhat similar but not equal, supporting the existence of a genuine local deformation source at Solfatara against the emergence of a mere distortion of large-scale deformation. Although reality is probably much more complex, our simple model explains 1980–2010 CF deformation within ground-displacement data errors and is consistent with Solfatara geochemical conceptual models, fumarolic geochemical data, and seismic attenuation imaging of CF. The observation that the CF deformation pattern can be decomposed into two stationary parts is hardly compatible with several recent works which proposed multiple sources with different features acting in different periods, fluid injections implying ample changes of large-scale deformation pattern over time, complex spatio-temporal patterns of distributions of volumetric sources.

Many calderas show repeated unrest over centuries. Though probably induced by magma, this unique behaviour is not understood and its dynamics remains elusive. To better understand these restless calderas, we interpret deformation data and build thermal models of Campi Flegrei caldera, Italy. Campi Flegrei experienced at least 4 major unrest episodes in the last decades. Our results indicate that the inflation and deflation of magmatic sources at the same location explain most deformation, at least since the build-up of the last 1538 AD eruption. However, such a repeated magma emplacement requires a persistently hot crust. Our thermal models show that this repeated emplacement was assisted by the thermal anomaly created by magma that was intruded at shallow depth ~3 ka before the last eruption. This may explain the persistence of the magmatic sources promoting the restless behaviour of the Campi Flegrei caldera; moreover, it explains the crystallization, re-melting and mixing among compositionally distinct magmas recorded in young volcanic rocks. Our model of thermally-assisted unrest may have a wider applicability, possibly explaining also the dynamics of other restless calderas.

Coda wave attenuation imaging is able to detect fluid/melt accumulation and ancient magmatic bodies in volcanoes. Here we use recently developed space-weighting sensitivity functions to invert for the spatial distributions of multifrequency coda wave attenuation ( Qc−1), measured during the largest monitored unrest at Campi Flegrei caldera (1983–1984). High-attenuation anomalies are spatially correlated with the regions of highest structural complexities and cross faulting. They characterize deep fluid circulation in and around the aseismic roots of the 1534 A.D. Mount Nuovo eruption and fluid accumulation in the areas of highest hydrothermal hazard. Just offshore Pozzuoli, and at the highest frequency (wavelengths of ∼150 m), the main cause of ground deformation and seismicity during the unrest is an aseismic low-attenuation circular anomaly, similar in shape and nature to those produced by ancient magmatic reservoirs and active sills at other volcanoes.

SUMMARY We analyse eight years of discontinuous strain records from two crossed 90-m long laser interferometers (extensometers) operating in the Gran Sasso underground observatory (Central Italy) to investigate the FCN parameters. At first, inverting realistic synthetic tidal parameters (amplitude and phase of eight diurnal tidal components) we show that the resolving power of strain tides is comparable to that of gravity tides if tidal parameters are inverted minimizing the misfit (as usually done). Both resolving powers improve if data are inverted minimizing the misfit, and this improvement is particularly notable for gravity tides. The analyses of strain records have been performed after correcting measured strain for local distortion of the regional strain field and ocean loading. Data are inverted minimizing the misfit. Parameter uncertainty assessment is performed both inverting synthetic tidal parameter sets, obtained from the real one by adding a random noise, and estimating the Bayesian marginal probabilities from an ensemble of parameter space samples. Even if we analyse data from a single station, our results for the period of the Free Core Nutation (about 429 sidereal days) are robust and comparable to those from gravity tides, obtained from the joint inversion of data from several stations. The quality factor is not well constrained because of the large uncertainty on the Ψ1 phase; however our results are consistent with recently published values (≈20 000). The agreement between observations and predictions looks better than in any previous work that makes use of strain tides. Our inversions provide also an independent estimate of Re(h1− 3l1), which is the dominating term in the resonance strength expression, in agreement with IERS Conventions (2003).

Two laser interferometers for geophysical researches are operating under the Gran Sasso massif, Central Italy, since several years. Their electro-optical set-up has been recently modified with respect to the original version to reduce costs and improve reliability. Output of each interferometer now consists of two quadrature signals, whose Lissajous figure is inverted for changes in length of the 90-m-long baselines. These top-rank instruments give high-quality strain data, but siting effects produce coupling among the different strain components, so that measured strain is not equal to the large-scale Earth strain. We estimate local effects using reference tidal strains and a matrix representation of the relationship between instrument and remote strains. The three coupling coefficient of each interferometer (obtained by best-fitting) are able to reduce differences between observed and predicted tides down to negligible levels.

A very large uplift (about 1.8 m) occurred in the period 1982–1984 at Campi Flegrei caldera, Italy, without culminating in an eruption. A still-standing controversy accompanies the interpretation of deformation and gravity changes recorded during the unrest, which were interpreted to result from the sub-surface magmatic reservoir by some authors and from the hydrothermal system or hybrid sources by others. Here for the first time we take into account crustal layering while inverting leveling, EDM, and gravity data using uniformly-pressurized sources, namely small vertical spheroids and finite horizontal penny-shaped sources. The weight of EDM data in the misfit function is chosen from a trade-off curve in order to balance the compromise between fitting the leveling and the EDM data well. Models using a homogeneous medium cannot give a good simultaneous fit to leveling and EDM deformation data of the 1982–1984 unrest, whereas incorporating a layered structure (determined from seismically derived estimates of the P wave speed for the crust, and not adjusted to improve the fit in any of the inversions) allows a significantly better fit. Also, layering affects the sub-surface mass redistribution effects on gravity changes, and we show that the retrieved intrusion density is in full agreement with densities for highly evolved magmas expected at the Campi Flegrei caldera for depths of 3 to 4 km, ruling out hydrothermal fluids as the primary cause of the 1982–1984 unrest. The source of the 1982–1984 CF unrest was probably a shallow (about 3-km deep) penny-shaped magma intrusion fed by a deeper magma chamber; source overpressure was few MPa.

Strain data recorded by two crossed laser extensometers operating in the Gran Sasso underground observatory recorded seismic free oscillations excited by the 2004 December 26 Sumatra–Andaman earthquake. The main source of instrumental noise can be mitigated by differencing data from the crossed strainmeter arms, so that the resulting differential strain data set offers an unprecedented resolution of the seismic toroidal free oscillations with periods T > 1000 s.We reconstruct the time evolution of selected free-oscillations for comparison with synthetic seismograms that include normal-mode coupling effects from Coriolis force, attenuation and ellipticity. Envelopes estimated for the Gran Sasso differential data set for free oscillations with period T < 1000 s (frequencies f > 1 mHz) are approximated adequately by a composite Centroid–Moment–Tensor (CMT) source with five subevents and an aggregate Mw = 9.3 moment-magnitude. Envelopes for several toroidal free oscillations with T > 1000 s are predicted less well. The amplitude of the rarely observed mode 0T 2 is overpredicted at Gran Sasso by roughly a factor of two, and other modes are underpredicted. The amplitude discrepancy for 0T 2 is confirmed at selected exceptionally low-noise seismic stations. Hypothetical explanations include a slow-slip component of the seismic moment release, errors in the composite-CMT source model, unmodelled coupling effects to Earth’s secular modes and feedback from the Sumatra–Andaman tsunami on Indian Ocean coastlines. Of these hypotheses, either an extended-duration strain release or tsunami feedback seem most plausible. The viability of the tsunami-feedback mechansim deserves further investigation.

SUMMARY Even if proper treatment of error statistics is potentially essential for the reliability of experimental data inversion, a critical evaluation of its effects on levelling data inversion is still lacking. In this paper, we consider the complete covariance matrix for levelling measurements, obtained by combining the covariance matrix due to measurement errors and the covariance matrix due to non-measurement errors, under the simple hypothesis of uncorrelated nonmeasurement errors on bench mark vertical displacements. The complete covariance matrix is reduced to diagonal form by means of a rotation matrix; the same rotation transforms the data to independent form. The eigenvalues of the complete covariance matrix give the uncertainties of the transformed independent data. This procedure can be used also with non-normal distributions of errors, in which case misfit functions other than χ 2 (e.g. the mean absolute deviation) are minimized. Here we focus on two test cases (the 1989 Loma Prieta earthquake and the 1908 Messina earthquake) inverting both real data and synthetics. The inversion of synthetic data sets does not evidence any systematic dependence of retrieved parameter values on the covariance matrix. Most retrieved fault parameter values are close to what used in the forward model, whatever covariance matrix is used. As a consequence, large discrepancies among results obtained using covariance matrices including different combinations of measurement and non-measurement errors when inverting measured and synthetic data sets would possibly indicate the need for further investigations. While measurement errors can be a priori evaluated, it is difficult to estimate non-measurement errors. Our synthetic tests using a uniform-slip rectangular fault in a homogeneous elastic half-space show that, if measurement errors have been correctly evaluated, average non-measurement errors can be estimated by choosing their weight inside the covariance matrix so that the ratio between the total square residual for the χ 2 best-fitting model and the number of degrees of freedom is about one. This result holds even if non-measurement errors are different from bench mark to bench mark and/or not normally distributed. In case of more complex models (e.g. one distributed-slip fault) it is difficult to estimate the actual number of degrees of freedom. Our synthetic tests show that this difficulty can be surmounted by using the Akaike Information Criterion, which actually decreases when turning from a uniform-slip inverse model to a distributed-slip inverse model only if the correct amount of non-measurement errors is included in the covariance matrix.

SUMMARY The destructive 1980 Campania-Lucania earthquake was a complex event, consisting of at least three subevents. While the main features of the first (0 s) and of the last (40 s) subevents are widely accepted, the second subevent (20 s) is still controversial. Fault geometries are here determined from levelling data, using a global minimization technique. Even if uplifts of southernmost benchmarks (the most sensitive to the 20 s subevent) are small, new proper statistical analysis allows us to draw a few robust conclusions. Our results agree with previously accepted models as regards the 0 and 40 s subevents. As regards the 20 s subevent, previous models always assumed a northeast-dipping fault, while levelling data show that it could have been equivalently caused by a rupture dipping either southwest or northeast. The statistical significance of the two models is about the same, and strike and dip of the two faults are consistent with the two focal planes previously obtained from teleseismic data. Aftershock distribution seems to favour the southwest-dipping fault model.

The Canfranc underground laboratory (LSC), excavated under the Central Pyrenees, is mainly devoted to the study of phenomena which needs “cosmic silence.” It also hosts a geodynamical facility, named Geodyn, which holds an accelerometer, a broadband seismometer, and two high-resolution laser strainmeters. During the routine processing of the seismic data, we detected an unusual spectral signature in the 2–10 Hz frequency band, which does not correspond to the typical sources of seismic noise and which can also be recognized in the strain records. After checking against meteorological and hydrological data, we can relate those signals to variations in the discharge by the Aragon River, an Alpine-style river in the southern Pyrenees, located about 400 m from the LSC Geodyn facility. Four main episodes have been identified since early 2011, each lasting 1–2 to 6–8 days. Additionally, a limited number of shorter episodes have also been detected. Three types of river-generated seismic events have been identified, related respectively to moderate rainfall, snowmelt, and flooding events associated to severe storms. Each of those types has distinctive characteristics which allow monitoring the hydrological events from the analysis of seismic and deformation data. A few previous studies have already described the seismic noise close to rivers with larger discharge or in small-scale experimental settings, and we are showing here that the so-called “fluvial seismology” can be useful to study the hydrological evolution of Alpine style streams and may have a potential interest for the civil authorities in charge of the management of hydrological basins.

A new set of measurements of the absorption cross-section of oxygen in the wavelength range 208–240 nm is reported. These new data, obtained using a 90 m long cell, are about 15% lower than the most recently published.

Summary In the carbonate fractured aquifer of the Gran Sasso (Central Italy), numerical simulations have been applied to validate a conceptual model of the recharge/discharge cycle at regional scale. To identify the effective aquifer features that mostly affect the spring discharges, simple parsimonious models, based on few key parameters whose effective values are well-constrained by previous hydrological/geological studies, are used. Input data are represented by infiltration, calculated considering rainfall, snowmelt and runoff in endorheic basins, and effective permeability distribution, estimated by geological, tectonic and hydrogeological settings of the aquifer. Discharges of tapped springs and tunnel drainages for the 1999–2009 period have been simulated, after estimating the recharge area of each monitored spring by the hydrogeological setting and checking them by mean isotope elevation values. Discharge monitoring has been compared with the simulations, obtaining a good agreement both at seasonal and yearly time basis. The simulations will also be useful for previewing discharge in drought period and evaluating protection zones of tapped springs in the territory of a National Park.

Strain data recorded by two laser interferometer–strainmeters operating in the Baksan (Russia) and Gran Sasso (Italy) underground observatories, and the length-of-day (LOD) data describing the variable rate of the Earth's rotation are used to study the relation between the deformation processes in the lithosphere and the global geodynamics of the Earth over short time intervals. The methods applied are based on analysis of the coherence of the studied processes, and correlation analysis. A significant (90%) correlation is revealed between the local deformation fields at two remote observation stations, which proves the existence of a global (at least on the scale of the Eurasian plate) component in the Earth's deformation field that manifests itself at characteristic time intervals of up to 1–2 months. At the same level of significance, the correlation between the local deformation fields and variations in the rate of the Earth's rotation has also been identified. The found correlations in the tidal low-frequency range are caused by the direct impact of the long-period tidal loading ( M f and M tm waves) on the lithosphere and the length-of-the-day (LOD). On the contrary, the significant correlation in the non-tidal range is probably linked to irregular perturbations of the continental character, which create a coherent interference in the studied processes. The global mechanism that causes this coherent noise requires further study.

[1] Magmatic unrest can be successfully monitored and studied from modeling of the induced surface deformation; one limiting factor however is the small number of available magmatic source models. Here we have obtained expressions (quadrupole approximation) for displacements and stresses from the inflation of any pressurized triaxial ellipsoid in an infinite elastic medium. The expressions can be evaluated by combining the effects of seven suitable point sources and are approximately valid also for a heterogeneous half-space. Till now, the only available (approximate or exact) expressions for finite expansion sources referred to spheres, prolate spheroids, and horizontal circular cracks embedded in a homogeneous half-space. Our approach allows to model also oblate spheroids and non-axisymmetric sources, whose effects were previously estimable only in the far field through a moment tensor representation. We also show that when the deformation source is a vertically flattened ellipsoid, the inversion of superficial displacements using a general moment tensor may lead to wrong physical models of the deformation source itself. This may be the case for the Campi Flegrei caldera, as well as for other calderas.

The Mw = 6·3 L'Aquila earthquake on 6 April 2009 produced a mainshock that caused significant changes in the hydrogeology of the Gran Sasso carbonate fractured aquifer: (i) the sudden disappearance at the time of the mainshock of some springs located exactly along the surface trace of the Paganica normal fault (PF); (ii) an immediate increase in the discharge of the Gran Sasso highway tunnel drainages and of other springs and (iii) a progressive increase of the water table elevation at the boundary of the Gran Sasso aquifer during the following months. Using the data collected since the 1990s that include aftershock monitoring as well as data regarding spring discharge, water table elevations, turbidity and rainfall events, a conceptual model of the earthquake's consequences on the Gran Sasso aquifer is proposed herein. In this model that excludes the contribution of seasonal recharge, the short-term hydrologic effects registered immediately after the mainshock are determined to have been caused by a pore pressure increase related to aquifer deformation. Mid-term effects observed in the months following the mainshock suggest that there was a change in groundwater hydrodynamics. Supplementary groundwater that flows towards aquifer boundaries and springs in discharge areas reflects a possible increase in hydraulic conductivity in the recharge area, nearby the earthquake fault zone. This increase can be attributed to fracture clearing and/or dilatancy. Simulations by numerical modelling, related to pore pressure and permeability changes with time, show results in accordance with observed field data, supporting the conceptual model and confirming the processes that influenced the answer of the Gran Sasso aquifer to the L'Aquila earthquake. Copyright © 2010 John Wiley & Sons, Ltd.

This study performs a detailed reconstruction of the rupture mechanism of the 1980 Campania–Lucania (southern Italy) earthquake. This is achieved by relocation of the main event through computation of fault-plane solutions of the aftershocks, P-wave velocity inversion, and analysis of post-seismic ground deformation, which provide an overall picture of the faulting mechanism. All of these data are in favor of a complex rupture mechanism, as already identified by many studies, which consists of three separate events. The present study defines a graben-like rupture, with the first event rupturing a (>20-km-long) segment of a large, high-angle, NE-dipping, SE-NW-striking, normal fault. The two successive ruptures occurred separately, the first along the southern segment, and the second along the northern segment, of a complementary SW-dipping, normal fault. This mechanism is well evidenced by the revised location of the hypocenter of the main event, and the location of the aftershocks and their fault-plane solutions, as well as by the underlying three-dimensional P-wave velocity structure. The model proposed by Amoruso et al. [2005a] that was based on the inversion of co-seismic vertical displacement data is confirmed by the present analyses, as it satisfies all of the available experimental observations, and better constrains the location and fault-plane solutions of the aftershocks, the velocity discontinuities, and the rupture observations at the surface. This conclusion is also supported by analyses of the post-seismic data.

Simple systems and procedures based on personal computer hardware to analyze the fringe pattern in a Michelson interferometer, retrieve information on the irregularities of the combined phase front and use it to obtain a correct interferogram are presented. In one system, where one of the mirrors can be moved piezoelectrically, good sensitivity (lambda/30) is obtained while using an economical array of 1-bit image sensing elements. In another system, based on a TV camera and a 6-bit frame grabber, images of the two separate laser beams and of the fringe patterns are stored; by adequate numerical analysis, higher sensitivity (lambda/100) is achieved.

[1] Two laser strainmeters that operate at 1400-m depth, about 20 km NE of the epicenter of the 2009/04/06 magnitude-6.3 L'Aquila, Italy, earthquake, have produced records of strain since 1995. During the two years before the event, no anomalous signal larger than a few tens of nanostrains is visible, limiting the volume of the possible earthquake preparation zone to less than 100 km3; moreover, earth tidal response is stable within 0.5% in amplitude and 0.5° in phase. Thus, reality of large-scale precursory phenomena seems unlikely. During the last few days, there is some evidence of dilatancy of satured rock over the earthquake causative fault, maybe related to the foreshocks. Seconds before the event, strain is stable at the 10−12 level and prerupture nucleation slip in the hypocentral region is constrained to have a moment less than 2 × 1012 Nm, i. e. 0.00005% of the main shock seismic moment.

The recent paper “The 28 December 1908 Messina Straits Earthquake ( Mw 7.1): A Great Earthquake throughout a Century of Seismology” by Pino et al. (2009) provides a review of the observations, early studies, and main accomplishments in the investigation of the source of the 1908 Messina Straits earthquake. Figure 9 in Pino et al. (2009) is the main reason for this comment. The authors assert that the figure shows the slip distribution along-strike of the fault resulting from various studies, including Amoruso et al. (2002). In fact, they compare the seismic moment per unit fault length (shown as an equivalent along-strike slip distribution under the assumption of uniform along-dip slip distribution on a 20-km wide fault) obtained by Pino et al. (2000) from seismic data with the maximum slip along each section of the distributed-slip fault resulting from different geodetic models. Here we show that Figure 9 in Pino et al. (2009) is inconsistent and misleading, because incomparable quantities are compared. Before we address this point, we will discuss the reference in Table 2 of Pino et al. (2009) to the size (100 × 30 km2) of the variable-slip fault of Amoruso et al. (2002), clarifying its real meaning through a short summary of our computation procedure. We also report the main results in Amoruso et al. (2006), omitted in Pino et al. (2009). In Amoruso et al. (2002) we performed a joint inversion of P -wave first-motion polarities and geodetic data (leveling lines along the Calabrian and Sicilian sides of the Straits) under the assumption that uplifts are uncorrelated experimental data. The static baseline corrections for leveling lines were additional free parameters of the model. Fault slip was at first assumed uniform, then independent in a small set of coplanar subfaults sharing the same rake angle (see Figure …

[1] Two laser strainmeters that operate at 1400-m depth, about 20 km NE of the epicenter of the 6 April 2009 magnitude-6.3 L'Aquila, Italy, earthquake, have produced a clear record of postseismic strain. Here we show the results from the analysis of the data related to the first few days after the event. Strain after about 1.5 days is fully consistent with afterslip on a stationary region of the earthquake causative fault. The preceding few-hour-long transient (whose seismic moment history is quasi-exponential) is fully consistent with unilateral diffusive slip propagation toward the shallower part of the same fault. The propagation path ends where later afterslip probably occurred. Slip propagation similar to heat diffusion has been suggested to explain the observed scaling law between amplitude and duration of slow earthquakes; here we give the first observational evidence of the role and details of slow rupture propagation.

[1] We present exact expressions for the volume change of a pressurized ellipsoidal cavity in an infinite homogeneous elastic medium. The expressions can be used as approximate solutions also for a homogeneous half-space. We show that previously published widely used expressions are correct for spheres but underestimate the ratio of the volume change to the product of pressure and volume in any other case. We discuss the capability to infer the shape of a single ellipsoidal cavity from far-field deformation measurements. Our results indicate that source axis ratios may often be hard to estimate, whereas it may be easier to infer the volume change of the source. We also consider the case of a source region consisting of interconnected pressurized ellipsoidal cavities, neglecting mutually induced stress. If all the cavities share the same shape and orientation, the source is seen in the far field as a single ellipsoidal cavity and it is possible to compute the total volume change from surface displacements. The shape of the apparent single ellipsoid is the same as the shape of the constituting cavities and not of the source region. In any other case a single ellipsoidal cavity might even be unable to give the same surface displacements as the ensemble of cavities. Since sudden volume change of a cavity can generate seismic waves, we discuss the decomposition of the related moment tensor into isotropic, double-couple, and compensated linear-vector dipole force systems in case of magma exchange between two ellipsoidal cavities, giving relations for the moment tensor components.

A new laser strainmeter, inspired by those used in the first gravitational waves detectors, is presented. A wide‐band feedback circuit controls the optical length of the reference arm of an unequal‐arm Michelson interferometer by means of a Pockels cell. No modulation is added to the Pockels cell supply, since laser power fluctuations are proved to be of negligible effect if some care is taken in balancing the two outputs of the interferometer. A reset circuit follows variations in arm length greater than λ, obscuring the servo for 50 μs. As a consequence, an analog signal combined with a digital signal allow to detect any change in the arm length greater than 10−3 λ. The frequency band extends from continuous signals to 100 kHz.

The 1997 September-October Umbria-Marche sequence has been extensively studied in the past by analyzing coseismic displacement data (GPS, leveling, SAR). Here we focus on synthetic data representative of the main event of the 1997 Umbria-Marche sequence and investigate the effects of a crustal layering proper to the Colfiorito area on surface displacements and inferred source features when inverting coseismic geodetic data without taking into account layering. We compare bootstrapping and NA-Bayes as tools for parameter uncertainty assessment and show how the Akaike Information Criterion can be used to select the model which is most likely to be correct. Since SAR images offer the most complete coverage of the study area, we use synthetic line-ofsight displacement data.

This paper reviews the main observations on slow earthquakes and low frequency tremor made along the Apennines, the main seismic active zone of Italy. These observations have been made using a geodetic interferometer system operating since 1994 in the underground tunnel of Gran Sasso, central Italy, and an underground seismic array (UNDERSEIS) operating since 2002 in the same environment. The observations made in recent years indicate that both phenomena are quite rare and apparently uncorrelated. Slow earthquakes, mainly recorded in 1997 and occasionally later, have probably been caused by the activity of a shallow fault system located near the interferometers. Until now only one tremor episode characterized by low frequency content and duration of several hours has been detected in January 2004, without any correlations with the occurrence of slow or regular earthquakes. The signal to noise ratio of this event is very low, but the results of our detailed analysis show that its frequency contents and wave field characteristics are compatible with a low frequency non volcanic tremor.

[1] We study the effects of crustal layering on ground displacements and gravity changes due to a spheroidal expanding source. Soft superficial layers affect the deformation pattern (giving an apparent shallower source if layering is not taken into account), the ratio of horizontal to vertical displacements (biasing the source shape), and the subsurface mass redistribution effects on gravity changes. Retrieved intrusion density is biased toward very low values if the source is modelled as a penny-shaped crack (point and finite) in a homogeneous half-space and the resulting density misestimate can lead to a possibly incorrect assessment of volcanic hazard. As an application, we consider the 1982–1984 Campi Flegrei unrest. We show that the large discrepancy between densities of spherical and penny-shaped sources, given in the literature and obtained inverting ground displacement and gravity data in a homogeneous half-space, is at least partially a consequence of the homogeneous half-space assumption.