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Time-frequency representation of earthquake accelerograms and inelastic structural response records using the adaptive chirplet decomposition and empirical mode decomposition
In this paper, the adaptive chirplet decomposition combined with the Wigner-Ville transform and the empirical mode decomposition combined with the Hilbert transform are employed to process various non-stationary signals (strong ground motions and structural responses). The efficacy of these two adaptive techniques for capturing the temporal evolution of the frequency content of specific seismic signals is assessed. In this respect, two near-field and two far-field seismic accelerograms are analyzed. Further, a similar analysis is performed for records pertaining to the response of a 20-story steel frame benchmark building excited by one of the four accelerograms scaled by appropriate factors to simulate undamaged and severely damaged conditions for the structure. It is shown that the derived joint time–frequency representations of the response time histories capture quite effectively the influence of non-linearity on the variation of the effective natural frequencies of a structural system during the evolution of a seismic event; in this context, tracing the mean instantaneous frequency of records of critical structural responses is adopted.
The study suggests, overall, that the aforementioned techniques are quite viable tools for detecting and monitoring damage to constructed facilities exposed to seismic excitations
Possibility between earthquake and explosion seismogram differentiation by discrete stochastic non-Markov processes and local Hurst exponent analysis
The basic purpose of the paper is to draw the attention of researchers to new
possibilities of differentiation of similar signals having different nature.
One of examples of such kind of signals is presented by seismograms containing
recordings of earthquakes (EQ's) and technogenic explosions (TE's). We propose
here a discrete stochastic model for possible solution of a problem of strong
EQ's forecasting and differentiation of TE's from the weak EQ's. Theoretical
analysis is performed by two independent methods: with the use of statistical
theory of discrete non-Markov stochastic processes (Phys. Rev. E62,6178 (2000))
and the local Hurst exponent. Time recordings of seismic signals of the first
four dynamic orthogonal collective variables, six various plane of phase
portrait of four dimensional phase space of orthogonal variables and the local
Hurst exponent have been calculated for the dynamic analysis of the earth
states. The approaches, permitting to obtain an algorithm of strong EQ's
forecasting and to differentiate TE's from weak EQ's, have been developed.Comment: REVTEX +12 ps and jpg figures. Accepted for publication in Phys. Rev.
E, December 200
Modulated Oscillations in Three Dimensions
The analysis of the fully three-dimensional and time-varying polarization
characteristics of a modulated trivariate, or three-component, oscillation is
addressed. The use of the analytic operator enables the instantaneous
three-dimensional polarization state of any square-integrable trivariate signal
to be uniquely defined. Straightforward expressions are given which permit the
ellipse parameters to be recovered from data. The notions of instantaneous
frequency and instantaneous bandwidth, generalized to the trivariate case, are
related to variations in the ellipse properties. Rates of change of the ellipse
parameters are found to be intimately linked to the first few moments of the
signal's spectrum, averaged over the three signal components. In particular,
the trivariate instantaneous bandwidth---a measure of the instantaneous
departure of the signal from a single pure sinusoidal oscillation---is found to
contain five contributions: three essentially two-dimensional effects due to
the motion of the ellipse within a fixed plane, and two effects due to the
motion of the plane containing the ellipse. The resulting analysis method is an
informative means of describing nonstationary trivariate signals, as is
illustrated with an application to a seismic record.Comment: IEEE Transactions on Signal Processing, 201
Noise Corruption of Empirical Mode Decomposition and Its Effect on Instantaneous Frequency
Huang's Empirical Mode Decomposition (EMD) is an algorithm for analyzing
nonstationary data that provides a localized time-frequency representation by
decomposing the data into adaptively defined modes. EMD can be used to estimate
a signal's instantaneous frequency (IF) but suffers from poor performance in
the presence of noise. To produce a meaningful IF, each mode of the
decomposition must be nearly monochromatic, a condition that is not guaranteed
by the algorithm and fails to be met when the signal is corrupted by noise. In
this work, the extraction of modes containing both signal and noise is
identified as the cause of poor IF estimation. The specific mechanism by which
such "transition" modes are extracted is detailed and builds on the observation
of Flandrin and Goncalves that EMD acts in a filter bank manner when analyzing
pure noise. The mechanism is shown to be dependent on spectral leak between
modes and the phase of the underlying signal. These ideas are developed through
the use of simple signals and are tested on a synthetic seismic waveform.Comment: 28 pages, 19 figures. High quality color figures available on Daniel
Kaslovsky's website: http://amath.colorado.edu/student/kaslovsk
Seismic Tremor Reveals Spatial Organization and Temporal Changes of Subglacial Water System
©2019. American Geophysical Union. All Rights Reserved.Subglacial water flow impacts glacier dynamics and shapes the subglacial environment. However, due to the challenges of observing glacier beds, the spatial organization of subglacial water systems and the time scales of conduit evolution and migration are largely unknown. To address these questions, we analyze 1.5‐ to 10‐Hz seismic tremor that we associate with subglacial water flow,
that is, glaciohydraulic tremor, at Taku Glacier, Alaska, throughout the 2016 melt season. We use frequency‐dependent polarization analysis to estimate glaciohydraulic tremor propagation direction (related to the subglacial conduit location) and a degree day melt model to monitor variations in melt‐water
input. We suggest that conduit formation requires sustained water input and that multiconduit flow paths can be distinguished from single‐conduit flow paths. Theoretical analysis supports our seismic interpretations that subglacial discharge likely flows through a single‐conduit in regions of steep hydraulic potential gradients but may be distributed among multiple conduits in regions with shallower potential gradients. Seismic tremor in regions with multiple conduits evolves through abrupt jumps between stable configurations that last 3–7 days, while tremor produced by single‐conduit flow remains more stationary.
We also find that polarized glaciohydraulic tremor wave types are potentially linked to the distance from source to station and that multiple peak frequencies propagate from a similar direction. Tremor appears undetectable at distances beyond 2–6 km from the source. This new understanding of the spatial organization and temporal development of subglacial conduits informs our understanding of dynamism within the subglacial hydrologic system.Raw seismic data described in this paper are available through the Incorporated Research Institutions for Seismology Data Management Center (http://ds.iris.edu/mda/ZQ? timewindow=2015‐2016; Amundson et al., 2015). The raw weather data used in this paper can be found through the Arctic Data Center (https://doi.org/ 10.18739/A2H98ZC7V; Bartholomaus & Walter, 2018). Python code developed to carry out the analyses presented here is available at https://github.com/ voremargot/Seismic‐Tremor‐Reveals‐ Spatial‐Organization‐and‐Temporal‐ Changes‐of Subglacial‐Water‐System and https://github.com/ tbartholomaus/med_spec. This study was made possible with support from the University of Texas Institute for Geophysics and the University of Idaho. We thank Ginny Catania for the loan of weather stations. J. P. W.'s and J. M. A.'s contributions to this work were supported by the U.S. National Science Foundation (OPP‐1337548 and OPP‐ 1303895). T. C. B. thanks Dylan Mikesell for an early conversation, which inspired the analysis presented here.Ye
Seismic Tremor Reveals Spatial Organization and Temporal Changes of Subglacial Water System
©2019. American Geophysical Union. All Rights Reserved.Subglacial water flow impacts glacier dynamics and shapes the subglacial environment. However, due to the challenges of observing glacier beds, the spatial organization of subglacial water systems and the time scales of conduit evolution and migration are largely unknown. To address these questions, we analyze 1.5‐ to 10‐Hz seismic tremor that we associate with subglacial water flow,
that is, glaciohydraulic tremor, at Taku Glacier, Alaska, throughout the 2016 melt season. We use frequency‐dependent polarization analysis to estimate glaciohydraulic tremor propagation direction (related to the subglacial conduit location) and a degree day melt model to monitor variations in melt‐water
input. We suggest that conduit formation requires sustained water input and that multiconduit flow paths can be distinguished from single‐conduit flow paths. Theoretical analysis supports our seismic interpretations that subglacial discharge likely flows through a single‐conduit in regions of steep hydraulic potential gradients but may be distributed among multiple conduits in regions with shallower potential gradients. Seismic tremor in regions with multiple conduits evolves through abrupt jumps between stable configurations that last 3–7 days, while tremor produced by single‐conduit flow remains more stationary.
We also find that polarized glaciohydraulic tremor wave types are potentially linked to the distance from source to station and that multiple peak frequencies propagate from a similar direction. Tremor appears undetectable at distances beyond 2–6 km from the source. This new understanding of the spatial organization and temporal development of subglacial conduits informs our understanding of dynamism within the subglacial hydrologic system.Raw seismic data described in this paper are available through the Incorporated Research Institutions for Seismology Data Management Center (http://ds.iris.edu/mda/ZQ? timewindow=2015‐2016; Amundson et al., 2015). The raw weather data used in this paper can be found through the Arctic Data Center (https://doi.org/ 10.18739/A2H98ZC7V; Bartholomaus & Walter, 2018). Python code developed to carry out the analyses presented here is available at https://github.com/ voremargot/Seismic‐Tremor‐Reveals‐ Spatial‐Organization‐and‐Temporal‐ Changes‐of Subglacial‐Water‐System and https://github.com/ tbartholomaus/med_spec. This study was made possible with support from the University of Texas Institute for Geophysics and the University of Idaho. We thank Ginny Catania for the loan of weather stations. J. P. W.'s and J. M. A.'s contributions to this work were supported by the U.S. National Science Foundation (OPP‐1337548 and OPP‐ 1303895). T. C. B. thanks Dylan Mikesell for an early conversation, which inspired the analysis presented here.Ye
Low Frequency Tilt Seismology with a Precision Ground Rotation Sensor
We describe measurements of the rotational component of teleseismic surface
waves using an inertial high-precision ground-rotation-sensor installed at the
LIGO Hanford Observatory (LHO). The sensor has a noise floor of 0.4 nrad at 50 mHz and a translational coupling of less than 1 rad/m
enabling translation-free measurement of small rotations. We present
observations of the rotational motion from Rayleigh waves of six teleseismic
events from varied locations and with magnitudes ranging from M6.7 to M7.9.
These events were used to estimate phase dispersion curves which shows
agreement with a similar analysis done with an array of three STS-2
seismometers also located at LHO
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