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In this study we test whether principal components of the strain rate and stress tensors align within Switzerland. We find that 1) Helvetic Nappes line (HNL) is the relevant tectonic boundary to define different domains of crustal stress/surface strain rates orientations and 2) orientations of T- axes (of moment tensor solutions) and long-term asthenosphere cumulative finite strain (from SKS shear wave splitting) are consistent at the scale of the Alpine arc in Switzerland. At a more local scale, we find that seismic activity and surface deformation are in agreement but in three regions (Basel, Swiss Jura and Ticino); possibly because of the low levels of deformation and/or seismicity. In the Basel area, deep seismicity exists while surface deformation is absent. In the Ticino and the Swiss Jura, where seismic activity is close to absent, surface deformation is detected at a level of ~2 10E−8/yr (~6.3 10E−16/s)., Scientific Reports, 8, ISSN:2045-2322

Earthquake early warning (EEW) systems’ performance is driven by the trade-off between the need for a rapid alert and the accuracy of each solution. A challenge for many EEW systems has been the magnitude saturation for large events (Mw>7) and the resulting underestimation of seismic moment magnitude. In this study, we test the performance of high-rate (1Hz) GPS, based on seven seismic events, to evaluate whether long-period ground motions can be measured well enough to infer reliably earthquake predominant periods. We show that high-rate GPS data allow the computation of a GPS-based predominant period (τg) to estimate lower bounds for the magnitude of earthquakes and distinguish between large (MW>7) and great (MW>8) events and thus extend the capability of EEW systems for larger events. It is also identified the impact of the different value of the smoothing factor α on the τg results and how the sampling rate and the computation process differentiates τg from the commonly used τp.

Direct observations of vertical movements of the earth's surface are now possible with space-based GPS networks, and have applications to resources, hazards and tectonics. Here we present data on vertical movements of the Earth's surface in New Zealand, computed from the processing of GPS data collected between 2000 and 2015 by 189 permanent GPS stations. We map the geographical variation in vertical rates and show how these variations are explicable within a tectonic framework of subduction, volcanic activity and slow slip earthquakes. Subsidence of >3 mm/yr is observed along southeastern North Island and is interpreted to be due to the locked segment of the Hikurangi subduction zone. Uplift of 1–3 mm/yr further north along the margin of the eastern North Island is interpreted as being due to the plate interface being unlocked and underplating of sediment on the subduction thrust. The Volcanic Plateau of the central North Island is being uplifted at about 1 mm/yr, which can be explained by basaltic melts being injected in the active mantle-wedge at a rate of ∼6 mm/yr. Within the Central Volcanic Region there is a 250 km2 area that subsided between 2005 and 2012 at a rate of up to 14 mm/yr. Time series from the stations located within and near the zone of subsidence show a strong link between subsidence, adjacent uplift and local earthquake swarms.

GPS and strong-motion sensors are broadly used for the monitoring of structural health and Earth surface motions, focusing on response of structures, earthquake characterization and rupture modeling. Most studies have shown differences between the two systems at very long periods (e.g. >100sec). The aim of this study is the assessment of the compatibility of GPS and strong-motion records by comparing the consistency in the frequency domain and by comparing their respective displacement waveforms for several frequency bands. For this purpose, GPS and strong-motion records of 23 collocated sites of the Mw9.0 Tohoku 2011 earthquake were used to show that the consistency between the two datasets depends on the frequency of the excitation, the direction of the excitation signal and the distance from the excitation source., Smart Structures and Systems, 2015

Monitoring surface deformation in real-time help at planning and protecting infrastructures and populations, manage sensitive production (i.e. SEVESO-type) and mitigate long-term consequences of modifications implemented. We present RT-SHAKE, an algorithm developed to detect ground motions associated with landslides, sub-surface collapses, subsidences, earthquakes or rock falls. RT-SHAKE detects first transient changes in individual GPS time series before investigating for spatial correlation(s) of observations made at neighbouring GPS sites and eventually issue a motion warning. In order to assess our algorithm on fast (seconds to minute), large (from 1cm to meters) and spatially consistent surface motions, we use the 1Hz GEONET GNSS network data of the Tohoku-Oki MW9.0 2011 as a test scenario. We show the delay of detection of seismic wave arrival by GPS records is of ~10 seconds with respect to an identical analysis based on strong-motion data and this time delay depends on the level of the time-variable noise. Nevertheless, based on the analysis of the GPS network noise level and ground motion stochastic model, we show that RT-SHAKE can narrow the range of earthquake magnitude, by setting a lower threshold of detected earthquakes to MW6.5-7, if associated with a real-time automatic earthquake location system., Geophysical Journal International, 214 (2), ISSN:0956-540X, ISSN:1365-246X

We show that high-rate GPS can have a vital role to play in near real-time monitoring of potentially destructive earthquakes. We do this by investigating the potential of GPS in recording strong ground motions from earthquakes in Switzerland and Japan. The study uses finite-fault stochastic ground motion simulation based on Fourier amplitude spectra and duration models previously developed for both countries, allowing comparisons in terms of both Fourier and time domain characteristics (here the Peak Ground Velocity, PGV). We find that earthquakes of magnitude Mw>5.8 can be expected to be recorded by GPS in real-time at 10 km distance, i.e. their Fourier spectrum exceeds the noise of the instruments enough to be used in strong motion seismology. Post-processing of GPS time series lowers the noise and can improve the minimum observable magnitude by 0.1-0.2. As GPS receivers can record at higher rates (> 10 sps), we investigate which sampling rate is sufficient to optimally record earthquake signals and conclude that a minimum sampling rate of 5 sps is recommended. This is driven by recording events at short distances (below 10 km for magnitude 6 events and below 30 km for magnitude 7 events). Furthermore, the Maximum Ground Velocity derived from GPS is compared to the actual PGV for synthetic signals from the stochastic simulations and the 2008 Mw=6.9 Iwate earthquake. The proposed model, confirmed by synthetic and empirical data, shows that a reliable estimate of PGV for events of about magnitude 7 and greater can be basically retrieved by GPS in real-time and could be included for instance in ShakeMaps for aiding post-event disaster management.

We decipher the strain history of the upper mantle in California through the comparison of the long-term finite strain field in the mantle and the surface strain-rate field, respectively inferred from fast polarization directions of seismic phases (SKS and SKKS), and Global Positioning System (GPS) surface velocity fields. We show that mantle strain and surface strain-rate fields are consistent in the vicinity of San Andreas Fault (SAF) in California. Such an agreement suggests that the lithosphere and strong asthenosphere have been deformed coherently and steadily since >1 Ma. We find that the crustal stress field rotates (up to 40° of rotation across a 50 km distance from 50° relative to the strike of the SAF, in the near-field of SAF) from San Francisco to the Central Valley. Both observations suggest that the SAF extends to depth, likely through the entire lithosphere. From Central Valley towards the Basin and Range, the orientations of GPS strain-rates, shear wave splitting measurements and seismic stress fields diverge indicating reduced coupling or/and shallow crustal extension and/or presence of frozen anisotropy.

Reduced near-infrared reflectance observed in September 1973 in Skylab images of the western flank of Mt. Etna has been interpreted as an eruption precursor of the January 1974 eruption. Until now, it has been unclear when this signal started, whether it was sustained and which process(es) could have caused it. By analyzing tree-ring width time-series, we show that the reduced near-infrared precursory signal cannot be linked to a reduction in annual tree growth in the area. However, comparing the tree-ring width time-series with both remote sensing observations and volcano-seismic activity enables us to discuss the starting date of the pre-eruptive period of the 1974 eruption., Scientific Reports, 7, ISSN:2045-2322

© 2015 IEEE. We evaluate the potential of troposphere models derived from ground meteorological data (pressure, temperature, and relative humidity) and Global Positioning System (GPS) data to improve InSAR measurements and models derived from them. We test this approach on an ERS-2/Envisat data set collected during a transient surface deformation episode that occurred from January to July 2005 in the San Gabriel Valley, southern California, USA. We find that the interferometric phase change observed over the corresponding period cannot be solely attributed to hydrological uplift associated with rising groundwater levels but also includes a significant contribution from differential tropospheric delay due to differing quantities of water vapor in the troposphere on the two SAR observation dates. We show that, if the tropospheric phase contribution is mistakenly interpreted as the range change associated with changes in groundwater storage, both the surface displacement and the groundwater storage coefficient may be overestimated by up to 30%. This method could be applied in real time where meteorological measurements are available near one or more GPS permanent site(s).

Earth and Planetary Science Letters, 345–348, ISSN:0012-821X, ISSN:1385-013X

We show that geodetic data from the Bay Area Regional Deformation (BARD) network indicate asymmetric motion across the San Andreas fault in the San Francisco Bay Area (SFBA), resulting from a strong contrast in rigidity across the fault, as determined previously from seismological data. Assuming asymmetric motion across the fault, we determine the location and size of the maximum strain rate in the region. We find that, compared to the determination using a symmetric model of deformation, it is shifted eastward and its value increases from ∼0.4 μstrain/yr to ∼0.65 μstrain/yr. Such strain rate amplitudes are consistent with previous geodetic slip rate estimates. We confirm that the geological units located east of SAF are entrained by the motion of the Pacific Plate and that the San Andreas fault (SAF) is the real rheological limit between the Pacific and North-American Plates. The asymmetry of rheology constrained in this study implies the strain rate maximum in SFBA is likely located between SAF and the Hayward fault system. This also has implications for hazards in the northern SFBA, in particular on the Rodgers creek fault.

SUMMARY We develop a methodology to derive fast and reliable peak ground velocity (PGV) ShakeMaps from kinematic finite-source models of earthquake rupture inferred from geodetic static displacements. The temporal variations in slip on the fault are based on the simple assumption that larger slip takes longer time to accumulate. Assuming constant rupture and slip velocities, slip is distributed in time to produce a variable rise-time model. Sensitivity tests on finite-source models of the 1994 Mw 6.7 Northridge, California earthquake show that distribution of peak ground velocities derived using 3.0 km s −1 rupture velocity and 76.8 cm s −1 slip velocity matches, to a large extent, observed PGV, suggesting that rapid assessment of strong ground motions derived from geodetic data can aid emergency response, particularly in areas with sparse seismic station coverage.

International audience; Relying on densely spaced GPS data collected between 1996 and 2007 and on InSAR data spanning the 1992–2001 time period, we explore the effects of lateral rheological heterogeneities across the San Andreas Fault near Point Reyes on the interseismic velocity field. We evaluate heterogeneous elastic dislocation models, including an asymmetric model with contrasting rigidities on either side of the San Andreas Fault and models with a narrow compliant fault zone that lead to high near-fault strain rates. We show that we cannot resolve a compliant zone from interseismic surface displacement measurements unless independent constraints on the locking depth exist. We infer an 18 ± 1 mm yr− 1 slip rate on the San Andreas Fault with a 10 ± 2 km locking depth and a 40% higher rigidity to the SW of the fault. At Bodega Bay, GPS and EDM data suggest a compliant zone on the SAF with a 50-to-60% rigidity decrease, but 25 km further south we find no evidence for a weak fault zone. This suggests substantial variability in fault zone properties along the San Andreas Fault.

We thank (Cayol and Battaglia, 2009) (hereafter referred to as CB2009) for commenting on our work (Houlie and Montagner, 2007). We start our response by reviewing our model and then discuss the potential of discrimination of tilt and displacement at a VBB seismometer from seismic records. There appears to be a misunderstanding of the approach and the interpretation of our results presented in Houlie and Montagner (2007). We do not question the discovery by Battaglia et al. (2000) of transient deformation before volcanic eruptions, as well as the existence of 2 types of signals, but the interpretation is not necessarily as simple as proposed in their paper. We note that the three components in the seismic record of displacement might have different explanations. From a theoretical point of view, any perturbation of displacement can be expressed as follows: δuiðtÞ = ∑j ∂uiðtÞ xj dxj. It means that δui(t) can be expressed by 9 independent terms. Since σzj=0 at the surface of the Earth, there remain 6 degrees of freedom. Usually, the symmetric part (deformation eij) related to the stress tensor (in the elastic case) is separated from the anti-symmetric part that is related to rotation or tilt. Consequently, distinguishing displacement and tilt is a real scientific challenge. Very long period (period TN100 s) transients are interpreted in terms of tilts (Battaglia et al., 2000), but we explore the possibility that a large part of seismic record is related to translational displacement in addition of or rather than tilt (rotation). Following this simple idea, we draw the consequences of this hypothesis and propose a model which differs from previous approaches (Battaglia et al., 2000; Battaglia and Bachelery, 2003; Battaglia et al., 2005). There are two extreme models for interpreting the data: the first one is only based on tilt (associated with the dislocation model (Okada, 1992)) and a second one is based on ground displacement generated by a spherical source. CB2009 claim that the whole signal can be explained by tilt, but we claim that a significant part of the signal is related to ground displacement and that actual data cannot yet discriminate between these two models. It is likely that both effects are present, and the next challenge will be to determine where the right loci of the true solution are between these 2 extrememodels. The intention of our paper (Houlie andMontagner, 2007) was to draw attention to this fundamental issue, which has important consequences for the understanding of volcano eruption dynamics. We suggest that the actual data are not sufficient to provide a definitive answer to this issue. Our work shows that an interpretation of the currently available seismic record in terms of translational displacements is perfectly valid and that further work will be needed to discriminate between tilt and displacement models and combinations of the two.

Broadband seismic data enable us to test whether Ultra Long Period (ULP) signals can be used to investigate magma chamber pressure state and to monitor volcanic eruptions. By systematically investigating seismic signals at GEOSCOPE station RER, we find ULP signals associated with volcanic events, during the last 2 decades. We tentatively propose to interpret these signals as events highlighting the activity of the upper magma feeding system of the Piton de la Fournaise volcano and as being related to the deep magma chamber. Some of these unusual signals were previously detected and usually interpreted in term of tilt [Battaglia, J., Aki, K., Montagner, J.-.J.-P. Tilt signals derived from a GEOSCOPE VBB station on the Piton de la Fournaise volcano, Geophys. Res. Lett. 27 (5) (2000) 605–608.]. They are detected using STS1 seismometers sensitive in the bandwidth 10 −3 to 10 −2 Hz. This alternative model enables to quantify each event in terms of a pressure drop in the upper magma reservoir of the volcano. Two kinds of seismic waveforms can be associated with two types of fracturing events. The first one is related to the fast rise of a magma batch coming from the deep magma chamber and the second is the result of a slow rise in pressure inside a shallow reservoir. In both cases, we are able to detect these events on very long period seismic data. The interpretation of these signals supports the idea of a single magma source at the sea level. Thus, we propose to classify all the eruptions occurring during the last two decades into two groups, corresponding to the two primary pressure states of the magma chamber. © 2007 Elsevier B.V. All rights reserved.

Geophysical Journal International, 202 (3), ISSN:0956-540X, ISSN:1365-246X

International audience; The Azores archipelago is located near the triple junction between the Nubian (NU), North America (NA), and Eurasia (EU) plates. It is characterized by a relatively strong seismicity and active volcanism. The best estimate of the current plate velocities in the area over geologic timescales comes from the NUVEL1-NNR model (DeMets et al.) . At the geodetic timescale, plate motion models (Altamimi et al., Sella et al.) including GPS, DORIS, SLR and VLBI data currently do not yet include data from the Azores. In the framework of a research project focused on the seismotectonics and volcanotectonics of São Miguel island, we have analysed GPS data collected there in 1993, 1994, 1997, 2000 and 2002. Our analysis determines both the velocity in the REVEL (Sella et al.) reference frame of São Miguel and its internal deformation. The comparison of the 2000 and 2002 coordinates indicates that São Miguel accommodates moderate WNW-ESE extension at a rate slower than 5 mm yr-1.

Flank-fissure eruptions involve lateral injection and propagation of magma in a volcanic edifice along pre-existing fractures in the direction of the rift zones where magma intrusion and lava flow production are concentrated over time. Thus, the identification and mapping of active fractures and faults is a fundamental aspect of studies of active volcanic systems. However, gradual dyke wedge emplacement at depth in well-fractured zones on volcano flanks and in volcanic rift zones does not necessarily trigger large amplitude deformation signals susceptible to be recorded months or even years before the actual eruption. Here we show that active and potentially eruptive areas can be detected up to 2 yrs before the arrival to the surface of the final eruptive dyke and venting of lava flows by processing satellite images applying a normalized difference vegetation index (NDVI) algorithm. A posteriori analysis of satellite images indeed reveals that the surficial effects of dyke wedge injection and ascent on plant growth were apparent for Mt. Etna from 2000 to 2002 and for Mt. Nyiragongo in 2001, thus months to years before they erupted.

Geophysical Journal International, 199, ISSN:0956-540X, ISSN:1365-246X

We compute a series of finite-source parameter inversions of the fault rupture of the 2004 Parkfield earthquake based on 1 Hz GPS records only. We confirm that some of the co-seismic slip at shallow depth (, Scientific Reports, 4, ISSN:2045-2322

Tectonophysics, 599, ISSN:0040-1951, ISSN:1879-3266

On Mt. Etna (Italy), an enhanced Normalized Difference in Vegetation Index (NDVI) signature was detected in the summers of 2001 and 2002 along a distinct line where, in November 2002, a flank eruption subsequently occurred. These observations suggest that pre-eruptive volcanic activity may have enhanced photosynthesis along the future eruptive fissure. If a direct relation between NDVI and future volcanic eruptions could be established, it would provide a straightforward and low-cost method for early detection of upcoming eruptions. However, it is unclear if, or to what extent, the observed enhancement of NDVI can be attributed to volcanic activity prior to the subsequent eruption. We consequently aimed at determining whether an increase in ambient temperature or additional water availability owing to the rise of magma and degassing of water vapour prior to the eruption could have increased photosynthesis of Mt. Etna's trees. Using dendro-climatic analyses we quantified the sensitivity of tree ring widths to temperature and precipitation at high elevation stands on Mt. Etna. Our findings suggest that tree growth at high elevation on Mt. Etna is weakly influenced by climate, and that neither an increase in water availability nor an increase in temperature induced by pre-eruptive activity is a plausible mechanism for enhanced photosynthesis before the 2002/2003 flank eruption. Our findings thus imply that other, yet unknown, factors must be sought as causes of the pre-eruption enhancement of NDVI on Mt. Etna. ISSN:1932-6203

Recently, co-seismic seismic source characterization based on GPS measurements has been completed in near- and far-field with remarkable results. However, the accuracy of the ground displacement measurement inferred from GPS phase residuals is still depending of the distribution of satellites in the sky. We test here a method, based on the double difference (DD) computations of Line of Sight (LOS), that allows detecting 3D co-seismic ground shaking. The DD method is a quasi-analytically free of most of intrinsic errors affecting GPS measurements. The seismic waves presented in this study produced DD amplitudes 4 and 7 times stronger than the background noise. The method is benchmarked using the GEONET GPS stations recording the Hokkaido Earthquake (2003 September 25th, Mw = 8.3)., Scientific Reports, 1, ISSN:2045-2322

[1] In the paper ‘‘Large scale ground deformation of Etna observed by GPS between 1994 and 2001’’ by N. Houlie et al. (Geophysical Research Letters, 33, L02309, doi:10.1029/2005GL024414, 2006) the correct volume rate of the Mogi source estimated from the GPS data of Mt. Etna from 1994 to 2001 is 27 10 m/yr instead of 60 10m/yr as reported in paragraph 14, page 3. GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L20309, doi:10.1029/2006GL026892, 2006

International audience; [1] We have processed thirty Global Positioning System (GPS) campaigns carried out at Etna from 1994 to early 2001 between the last two main flank eruptions of the Mt. Etna (Sicily, Italy). This rest period allowed us to investigate the deep magma plumbing system of the Mt. Etna. The temporal dynamics of twenty-three points observed three times or more were analyzed. All the time series show a first-order linear trend during the five years period. It suggests that the volcano was continuously deformed by the action of a deep source while a discrete activity of the volcano was observed at the summit. We have interpreted the residual deformation field as the result of an major eastward motion of the eastern flank of the volcano.

[1] We processed 79 days of GPS data collected at four stations on Miyakejima volcano (Japan) before and during its eruption in 2000. This period includes a strong seismic swarm (27 June to 8 July) followed by five eruptions (8 and 14–15 July, 10, 18, and 29 August). Surface motion is strong during the seismic swarm, then remains almost steady during eruptions. However, during each eruption, we found anomalous values of the LC (ionosphere free) double differences, especially on 18 August, with a peak of 0.6 cycles (i.e., an apparent 290 mm increase of the satellite-station range). We cannot explain these anomalies by temporary ground displacements during the events. Instead, we propose that they are due to path delay effects in the hot volcanic plume. By adapting a seismic tomography algorithm, the refractivity anomaly and the inferred temperature in the plume is mapped in time and space.

Eruptions can produce not only flows of incandescent material along the slopes of a volcano but also ash plumes in the troposphere [Sparks et al., 1997] that can threaten aircraft flying in the vicinity [Fisher et al., 1997]. To protect aircraft, passengers, and crews, the International Civil Aviation Organization and the World Meteorological Organization created eight Volcanic Ash Advisory Centers (VAAC, http://www.ssd.noaa.gov/VAAC/vaac.html) around the globe with the goal of tracking volcanic plumes and releasing eruption alerts to airports, pilots, and companies. Currently, the VAAC monitoring system is based mostly on the monitoring systems of any local volcano observatories and on real-time monitoring of data acquired by meteorological satellites.

We present a comparison of long‐period (30 T M w 8.3) with both very broadband seismograms and synthetic waveforms. We show that GPS provides valuable waveform data between 40 and 160 s periods, especially in the near field of the earthquake where seismograms are saturated. There are no long‐period broadband seismic data available in the near field of large earthquakes (∼100 km for earthquakes M w >7 and ∼500 km for earthquakes M w >8). The comparison with synthetic seismic displacement waveforms shows that the GPS waveforms are performing well at periods of 40–80 s at epicentral distances between 150 and 600 km and at periods of 80–160 s at epicentral distances between 150 and 400 km. At longer periods and larger epicentral distances, the amplitudes do not always exceed the noise level (∼2–3 mm), but we still observe at selected GPS sites that the phase and the amplitude of the surface waves are of good quality, up to ∼1200 km. We conclude that GPS waveforms in the near field of a large event can be used for seismological applications. In this article, we recover the focal mechanism of the Tokachi‐Oki event by inverting the GPS data recorded within 300 km of the epicenter.