Ambient Earth noise: A survey of the Global Seismographic Network

Abstract

[1] It has been a decade since the last comprehensive model of ambient Earth noise was published (Peterson, 1993). Since then, observations of ambient Earth noise from the Incorporated Research Institutions for Seismology (IRIS) Global Seismographic Network (GSN) of widely distributed, similarly equipped, and well-calibrated stations have become available. The broad geographic sampling of this large data set and the ease of access to waveform data provided by the IRIS Data Management System facilitate analysis of global noise samples. We have analyzed data from the 118 GSN stations operating during the year July 2001 through June 2002. On the basis of over 738,000 hourly spectral estimates computed from these stations' data, we have developed a robust noise model that exhibits significant differences from previous models both in the normal mode and body wave bands. Our analysis technique has the advantage that we do not need to search for quiet periods but can include all data where the instruments are operating correctly.

Transient gravity perturbations induced by earthquake rupture

“…Torsion-bar sensitivities have surpassed 10 −7 Hz −1/2 at 0.1 Hz (Shoda et al 2014). Naturally, work on detector designs needs to be accompanied by careful selection of instrument sites, which can have a big impact on ambient seismic or infrasound fields (Berger et al 2004;Brown et al 2014), and also on the associated gravity noise.…”

“…This is also illustrated in the figures in Section 3. Measurements of such weak gravity transients are masked by a foreground of seismic noise (Berger et al 2004;Brown et al 2014), which is of the order of 100 nm s −2 between 0.1 Hz and 1 Hz. A novel class of instruments, for example seismically isolated gravity gradiometers (also referred to as gravity strainmeters (Harms et al 2013)), is required to detect these early transients.…”

Clear evidence for the sign-reversal of the pressure admittance to gravity near 3mHz

“…Of course, because seismic waves and noise of instrumental and other origin (e. g. Forbriger, 2007) will provide some energy at these frequencies thus the best one can expect is to see a minimum. Peterson (1993) and Berger et al (2004) show in their studies of broad-band seismic noise at many stations of the Global Seismic Network for many time windows that the lower envelope of vertical acceleration noise power spectral densities (PSD) has a minimum near 3 mHz not present in the horizontal components. This minimum in the vertical PSD and the difference between vertical and horizontal noise at these frequencies is nicely explained by the contributions from the atmosphere as described in detail by Zürn and Wielandt (2007) and .…”

Ambient infrasound noise

“…In addition, we define preliminary infrasound noise models for the infrasound network. Noise models for seismic networks [Peterson, 1993;McNamara and Buland, 2004;Berger et al, 2004] have been valuable for evaluating station performance and for helping to estimate network detection capability.…”