Real-time characterization of large earthquakes using the predominant period derived from 1Hz GPS data

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

Atomic clocks as a tool to monitor vertical surface motion

Atomic clock technology is advancing rapidly, now reaching stabilities of $\Delta f/f \sim 10^{-18}$, which corresponds to resolving $1$ cm in equivalent geoid height over an integration timescale of about 7 hours. At this level of performance, ground-based atomic clock networks emerge as a tool for monitoring a variety of geophysical processes by directly measuring changes in the gravitational potential. Vertical changes of the clock's position due to magmatic, volcanic, post-seismic or tidal deformations can result in measurable variations in the clock tick rate. As an example, we discuss the geopotential change arising due to an inflating point source (Mogi model), and apply it to the Etna volcano. Its effect on an observer on the Earth's surface can be divided into two different terms: one purely due to uplift and one due to the redistribution of matter. Thus, with the centimetre-level precision of current clocks it is already possible to monitor volcanoes. The matter redistribution term is estimated to be 2-3 orders of magnitude smaller than the uplift term, and should be resolvable when clocks improve their stability to the sub-millimetre level. Additionally, clocks can be compared over distances of thousands of kilometres on a short-term basis (e.g. hourly). These clock networks will improve our ability to monitor periodic effects with long-wavelength like the solid Earth tide.Comment: 11 pages, 5 figures, accepted as express letter in the Geophysical Journal Internationa

Consistency of PPP GPS and strong-motion records: case study of Mw9.0 Tohoku-Oki 2011 earthquake

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. Several studies have shown the consistency of the two data sets within at certain frequency (e.g., 0.03<f<0.2Hz). The compatibility of Precise Point Positioning (PPP) GPS and strong-motion data was assessed by comparing their respective displacement waveforms for several frequency bands (f<0.3Hz). For this purpose, there are used GPS and strong-motion records of the Mw9.0 Tohoku 2011 earthquake at 23 very close spaced sites and conclude that the agreement between the two datasets depends on the frequency of the excitation, the direction of the excitation signal and the distance from the excitation source

Lithosphere strain rate and stress field orientations near the Alpine arc in Switzerland

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)

Analysis of 1-Hz GPS data for the estimation of long-period surface motion of Tohoku-Oki Mw9.0 2011 earthquake

The GPS displacement time series are today mostly used for the estimation of the transient (period <2sec) and coseismic (static) displacement of an earthquake, while the estimation of the velocity and acceleration is based only seismic networks. In this study, we examine whether the GPS records can be used to capture the spectral characteristics of the long-period signal and their consistency with the corresponding seismic motion sensors, for the displacement, velocity and acceleration. For this purpose, we use the 1-Hz GPS network records of the Tohoku-Oki 2011 earthquake processed in Precise Point Positioning (PPP). The derived GPS and strong-motion waveforms were analysed resulting into displacement, velocity and acceleration for periods ranging from 3 to 100s. The derived GPS and strong-motion time series were compared and it was found that these are of similar pattern and amplitude. However, there is a non-constant phase shift between the corresponding GPS and strong-motion time series, resulting in significant difference between the time series in the time domain. On the contrary in the frequency domain, the GPS and strong-motion time series are consistent for periods larger than 3-4s. Finally the GPS and the strong-motion records were compared in the time-frequency domain based on wavelet analysis, revealing that both GPS and seismic records express consistently the variation of the long-period of the seismic signal. Thus, it is proved that the GPS records can be used for the estimation of the long-period ground motion and contribute to the reliable estimation of the corresponding characteristics (displacement, velocity, acceleration)

Analysis of 1-Hz GPS data for the estimation of long-period surface motion of Tohoku-Oki Mw9.0 2011 earthquake

The GPS displacement time series are today mostly used for the estimation of the transient (period <2sec) and coseismic (static) displacement of an earthquake, while the estimation of the velocity and acceleration is based only seismic networks. In this study, we examine whether the GPS records can be used to capture the spectral characteristics of the long-period signal and their consistency with the corresponding seismic motion sensors, for the displacement, velocity and acceleration. For this purpose, we use the 1-Hz GPS network records of the Tohoku-Oki 2011 earthquake processed in Precise Point Positioning (PPP). The derived GPS and strong-motion waveforms were analysed resulting into displacement, velocity and acceleration for periods ranging from 3 to 100s. The derived GPS and strong-motion time series were compared and it was found that these are of similar pattern and amplitude. However, there is a non-constant phase shift between the corresponding GPS and strong-motion time series, resulting in significant difference between the time series in the time domain. On the contrary in the frequency domain, the GPS and strong-motion time series are consistent for periods larger than 3-4s. Finally the GPS and the strong-motion records were compared in the time-frequency domain based on wavelet analysis, revealing that both GPS and seismic records express consistently the variation of the long-period of the seismic signal. Thus, it is proved that the GPS records can be used for the estimation of the long-period ground motion and contribute to the reliable estimation of the corresponding characteristics (displacement, velocity, acceleration)

Ground-based optical atomic clocks as a tool to monitor vertical surface motion

According to general relativity, a clock experiencing a shift in the gravitational potential ΔU will measure a frequency change given by Δf/f≈ΔU/c2. The best clocks are optical clocks. After about 7 hr of integration they reach stabilities of Δf/f∼10−18 and can be used to detect changes in the gravitational potential that correspond to vertical displacements of the centimetre level. At this level of performance, ground-based atomic clock networks emerge as a tool that is complementary to existing technology for monitoring a wide range of geophysical processes by directly measuring changes in the gravitational potential. Vertical changes of the clock's position due to magmatic, post-seismic or tidal deformations can result in measurable variations in the clock tick rate. We illustrate the geopotential change arising due to an inflating magma chamber using the Mogi model and apply it to the Etna volcano. Its effect on an observer on the Earth's surface can be divided into two different terms: one purely due to uplift (free-air gradient) and one due to the redistribution of matter. Thus, with the centimetre-level precision of current clocks it is already possible to monitor volcanoes. The matter redistribution term is estimated to be 3 orders of magnitude smaller than the uplift term. Additionally, clocks can be compared over distances of thousands of kilometres over short periods of time, which improves our ability to monitor periodic effects with long wavelength like the solid Earth tid

Long-period surface motion of the multi-patch Mw9.0 Tohoku-Oki earthquake

We show that it is possible to capture the oscillatory ground motion induced by the Tohoku-Oki event for periods ranging from 3 to 100s using Precise Point Positioning (PPP). We find that the ground motions of the sedimentary basins of Japan were large (respectively > 0.15m/s and >0.15m/s2 for velocity and acceleration) even for periods larger than 3s. We compare geodetic observables with a Ground Motion Prediction Equation (GMPE) designed for Japan seismicity and find that the Spectral Acceleration (SA) is well estimated for periods larger than 3s and distances ranging from 100 to 500km. At last, through the analysis of the displacement attenuation plots, we show that the 2011 Tohoku-Oki event is likely composed of multiple rupture patches as suggested before by time-reversal inversions of seismic data

Tree-ring stable isotopes and radiocarbon reveal pre- and post-eruption effects of volcanic processes on trees on Mt. Etna (Sicily, Italy)

Early detection of volcanic eruptions is of major importance for protecting human life. Ground deformation and changes in seismicity, geochemistry, petrology, and gravimetry are used to assess volcanic activity before eruptions. Studies on Mt. Etna (Italy) have demonstrated that vegetation can be affected by pre-eruptive activity before the onset of eruptions. During two consecutive years before Mt. Etna's 2002/2003 flank eruption, enhanced vegetation index (NDVI) values were detected along a distinct line which later developed into an eruptive fissure. However, the mechanisms by which volcanic activity can lead to changes in pre-eruption tree growth processes are still not well understood. We analysed ${\delta}^{13}$C, ${\delta}^{18}$O and $^{14}$C in the rings of the survived trees growing near to the line where the pre-eruptive increase in NDVI was observed in order to evaluate whether the uptake of water vapour or fossil volcanic CO2 could have contributed to the enhanced NDVI. We found a dramatic decrease in ${\delta}^{18}$O in tree rings formed before 2002/2003 in trees close to the eruption fissure, suggesting uptake of volcanic water by trees during pre-eruptive magma degassing. Moist conditions caused by outgassing of ascending magma may also have led to an observed reduction in tree-ring ${\delta}^{13}$C following the eruption. Furthermore, only ambiguous evidence for tree uptake of degassed CO2 was found. Our results suggest that additional soil water condensed from degassed water vapour may have promoted photosynthesis, explaining local increases in NDVI before the 2002/2003 Mt. Etna flank eruption. Tree-ring oxygen stable isotopes might be used as indicators of past volcanic eruptions

New approach to detect seismic surface waves in 1Hz-sampled GPS time series

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)