Christoph Sens-Schönfelder

Passive image interferometry and seasonal variations of seismic velocities at Merapi Volcano, Indonesia

[1] We propose passive image interferometry as a technique for seismology that allows to continuously monitor small temporal changes of seismic velocities in the subsurface. The technique is independent of sources in the classical sense and requires just one or two permanent seismic stations. We retrieve the Green’s functions that we use for interferometry from ambient seismic noise. Applying passive image interferometry to data from Merapi volcano we show that velocity variations can be measured with an accuracy of 0.1% with a temporal resolution of a single day. At Mt. Merapi the velocity variations show a strong seasonal influence and we present a depth dependent hydrological model that describes our observations solely based on precipitation. Citation: Sens

Comprehensive observation and modeling of earthquake and temperature‐related seismic velocity changes in northern Chile with passive image interferometry

We report on earthquake and temperature-related velocity changes in high-frequency autocorrelations of ambient noise data from seismic stations of the Integrated Plate Boundary Observatory Chile project in northern Chile. Daily autocorrelation functions are analyzed over a period of 5 years with passive image interferometry. A short-term velocity drop recovering after several days to weeks is observed for the Mw 7.7 Tocopilla earthquake at most stations. At the two stations PB05 and PATCX, we observe a long-term velocity decrease recovering over the course of around 2 years. While station PB05 is located in the rupture area of the Tocopilla earthquake, this is not the case for station PATCX. Station PATCX is situated in an area influenced by salt sediment in the vicinity of Salar Grande and presents a superior sensitivity to ground acceleration and periodic surface-induced changes. Due to this high sensitivity, we observe a velocity response of several regional earthquakes at PATCX, and we can show for the first time a linear relationship between the amplitude of velocity drops and peak ground acceleration for data from a single station. This relationship does not hold true when comparing different stations due to the different sensitivity of the station environments. Furthermore, we observe periodic annual velocity changes at PATCX. Analyzing data at a temporal resolution below 1 day, we are able to identify changes with a period of 24 h, too. The characteristics of the seismic velocity with annual and daily periods indicate an atmospheric origin of the velocity changes that we confirm with a model based on thermally induced stress. This comprehensive model explains the lag time dependence of the temperature-related seismic velocity changes involving the distribution of temperature fluctuations, the relationship between temperature, stress and velocity change, plus autocorrelation sensitivity kernels.

Monitoring stress changes in a concrete bridge with coda wave interferometry

Coda wave interferometry is a recent analysis method now widely used in seismology. It uses the increased sensitivity of multiply scattered elastic waves with long travel-times for monitoring weak changes in a medium. While its application for structural monitoring has been shown to work under laboratory conditions, the usability on a real structure with known material changes had yet to be proven. This article presents experiments on a concrete bridge during construction. The results show that small velocity perturbations induced by a changing stress state in the structure can be determined even under adverse conditions. Theoretical estimations based on the stress calculations by the structural engineers are in good agreement with the measured velocity variations.

Seismic interferometry and stationary phase at caustics

Waves that propagate between two receivers can be extracted by cross correlating noise recorded at these receivers if the noise is generated by sources on a closed surface surrounding the receivers. This concept is called seismic interferometry. Of all these noise sources, those for whom the travel time difference for propagation to the two receivers is in the stationary phase zone, give the dominant contribution. In this paper we analyze the stationary phase properties when one receiver is at a caustic for waves leaving the other receiver. A simple model for a waveguide and a general treatment of caustics show that, at a caustic, the curvature of the travel time difference at the stationary phase zone vanishes. As a result the stationary phase region is considerably wider at a caustic than at other points, and it is more likely that noise sources are present in the stationary phase region. This explains why it is easier to extract waves from seismic interferometry when the receivers are at caustics such as antipodal receivers on the Earth.

Seismic shear waves as Foucault pendulum

Earths rotation causes splitting of normal modes. Wave fronts and rays are, however, not affected by Earths rotation, as we show theoretically and with observations made with USArray. We derive that the Coriolis force causes a small transverse component for P waves and a small longitudinal component for S waves. More importantly, Earths rotation leads to a slow rotation of the transverse polarization of S waves; during the propagation of S waves the particle motion behaves just like a Foucault pendulum. The polarization plane of shear waves counteracts Earths rotation and rotates clockwise in the Northern Hemisphere. The rotation rate is independent of the wave frequency and is purely geometric, like the Berry phase. Using the polarization of ScS and ScS2 waves, we show that the Foucault-like rotation of the S wave polarization can be observed. This can affect the determination of source mechanisms and the interpretation of observed SKS splitting.

Elastic-wave propagation and the Coriolis force

Seismic waves and Foucault pendula share intriguing similarities in the way they respond to Earth’s rotation.

Monitoring a volcano with passive image interferometry

Ambient seismic noise has been used successfully as a source of information for structural investigations. Ballistic surface as well as body waves were reconstructed by correlation of noise and used in tomographic studies. In these cases it is of course assumed that the medium under study is stationary, i.e., that the reconstructed Green’s function does not change with time. In this contribution we show that medium changes can well be monitored by means of subtle changes in the Green’s function. Using an interferometric approach applied to the coda part of the Green’s function, we detect temporal changes of delay times on Merapi volcano (Indonesia). The changes of delay time depend on lapse time, which indicates that the velocity changes inside the volcano are spatially heterogeneous. We present a hydrological model that can explain the temporal changes of delay time as well as its lapse time dependence changes of the ground water level induced by precipitation. From this analysis we conclude that (a) sei...