H. Tkalcic | The Australian National University (original) (raw)
Papers by H. Tkalcic
AGU Fall Meeting Abstracts, Dec 1, 2016
AGU Fall Meeting Abstracts, Dec 1, 2012
AGU Fall Meeting Abstracts, Dec 1, 2017
Over the last decade, a rolling array of seismometers has been sequentially deployed throughout s... more Over the last decade, a rolling array of seismometers has been sequentially deployed throughout southeast Australia to record passive seismic activity. To date, nearly 350 separate sites have been occupied with nominal station spacings of 40-50 km on the mainland and 15-20 km in Tasmania. Deployment periods for each of the eight arrays installed so far have varied between 4-10
AGU Fall Meeting Abstracts, Dec 1, 2020
Journal of Geophysical Research: Solid Earth, 2015
The origin of the microseismic wavefield is associated with deep ocean and coastal regions where,... more The origin of the microseismic wavefield is associated with deep ocean and coastal regions where, under certain conditions, ocean waves can excite seismic waves that propagate as surface and body waves. Given that the characteristics of seismic signals generally vary with frequency, here we explore the frequency-and azimuth-dependent properties of microseisms recorded at a medium aperture (25 km) array in Australia. We examine the frequency-dependent properties of the wavefield, and its temporal variation, over two decades (1991-2012), with a focus on relatively high-frequency microseisms (0.325-0.725 Hz) recorded at the Warramunga Array, which has good slowness resolution capabilities in this frequency range. The analysis is carried out using the incoherently averaged signal Capon beamforming, which gives robust estimates of slowness and back azimuth and is able to resolve multiple wave arrivals within a single time window. For surface waves, we find that fundamental mode Rayleigh waves (R g) dominate for lower frequencies (<0.55 Hz) while higher frequencies (>0.55 Hz) show a transition to higher mode surface waves (L g). For body waves, source locations are identified in deep ocean regions for lower frequencies and in shallow waters for higher frequencies. We further examine the association between surface wave arrivals and a WAVEWATCH III ocean wave hindcast. Correlations with the ocean wave hindcast show that secondary microseisms in the lower-frequency band are generated mainly by ocean swell, while higher-frequency bands are generated by the wind sea, i.e., local wind conditions. A number of studies have focused on locating the source regions of SM Rayleigh wave energy with seismic arrays [
Physics of the Earth and Planetary Interiors, 2006
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2012
Seismologists construct images of the Earth's interior structure using observations, derived ... more Seismologists construct images of the Earth's interior structure using observations, derived from seismograms, collected at the surface. A common approach to such inverse problems is to build a single ‘best’ Earth model, in some sense. This is despite the fact that the observations by themselves often do not require, or even allow, a single best-fit Earth model to exist. Interpretation of optimal models can be fraught with difficulties, particularly when formal uncertainty estimates become heavily dependent on the regularization imposed. Similar issues occur across the physical sciences with model construction in ill-posed problems. An alternative approach is to embrace the non-uniqueness directly and employ an inference process based on parameter space sampling. Instead of seeking a best model within an optimization framework, one seeks an ensemble of solutions and derives properties of that ensemble for inspection. While this idea has itself been employed for more than 30 year...
The SEAL3 experiment represents one of the twelve array deployments that so far comprise the larg... more The SEAL3 experiment represents one of the twelve array deployments that so far comprise the large and ambitious WOMBAT project, which aims to cover a significant portion of the Australian continent with a rolling array of seismometers. SEAL3 consists of 55 three-component short-period instruments located in central and southern New South Wales (NSW) with an average station spacing of approximately
ABSTRACT PKP waveforms are the main source of information regarding lowermost mantle and core sei... more ABSTRACT PKP waveforms are the main source of information regarding lowermost mantle and core seismological structures. An extension of our previously developed Simulated Annealing algorithm to analyze body waves is presented. It allows us to resolve the interference between the different PKP phases and their corresponding depth phases (pPKP and sPKP), which makes possible to process shallow earthquakes previously discarded, and thus to improve the sampling of the deep Earth. The algorithm is applied to various PKP waveform data sets in order to retrieve differential travel times and amplitude ratios of the core phases, and to demonstrate its efficiency. We present a first order analysis of differential travel times, amplitude ratios and waveforms of this new data base of PKP core phases in order to constrain the inner core seismic anisotropy, attenuation and deep structure. The introduction of shallow event data confirms the hemispherical pattern of the inner core anisotropy and the increase of the inner core quality factor with depth. An anti-correlation is found at all epicentral distances between differential travel time residuals and amplitude ratios. This anti-correlation can only be explained by a high attenuation of fast rays inside the inner core, produced by inner core fabric. Amplitude ratios and waveforms of core phases are also analyzed tin order to detect inner core discontinuities below the inner core boundary.
We report the first observational evidence that the complex rotational dynamics of the Earth'... more We report the first observational evidence that the complex rotational dynamics of the Earth's inner core appear to be in close relationship with the geomagnetic field. We infer from a newly observed collection of earthquake doublets that the Earth's inner core" shuffles", exhibiting both prograde and retrograde rotation in the reference frame of the mantle. Evidence for a complex pattern in the rotation of the inner core characterized by episodes of both prograde and retrograde motion is presented. A key feature of the new ...
Geophysical Journal International, 2010
We present a significant addition to the data set of traveltimes of seismic PKP waves that sample... more We present a significant addition to the data set of traveltimes of seismic PKP waves that sample the Earth's lowermost mantle and core along the Earth's rotation axis. Recorded at permanent Global Seismic Network (GSN) and temporary SSCUA deployment broadband seismographic stations in Antarctica, the new data improve the previously poor and biased coverage that underlies the seismic constraints on recent models of inner core structure and anisotropy. On the one hand, new differential PKP traveltime measurements improve the sampling of predominantly the eastern inner core hemisphere. PKPab-df and PKPbc-df differential traveltime residuals, with respect to the spherically symmetric model ak135, are consistently smaller than two seconds along the north-south paths sampled. Axially symmetric models of inner core seismic anisotropy with fast axis parallel to the Earth's rotation axis require a weak anisotropy of (0.7 ± 0.1) per cent to be consistent with our PKPbc-df observations. PKPbc-df residuals from the quasi-eastern hemisphere indicate (0.4 ± 0.1) per cent anisotropy. If only PKPbc-df observations from the top 200 km of this hemisphere are considered, this is reduced to (0.1 ± 0.2) per cent, consistent with an isotropic layer. On the other hand, new absolute PKP traveltime measurements add to the sampling of both hemispheres of the inner core, but it is difficult to use them with more confidence to assess structure of the core since they are affected by crustal and mantle structure and source uncertainties. The newly collected data set also increases constraints on D" structure beneath the South Pole. In contrast to previous inferences based on data from northern stations, we find no evidence of a velocity heterogeneity in the outer core near the inner core boundary associated with the cylinder tangent to the inner core in the southern hemisphere.
Bulletin of the Seismological Society of America, 2009
A volcanic earthquake with M w 5:6 occurred beneath the Bárdarbunga caldera in Iceland on 29 Sept... more A volcanic earthquake with M w 5:6 occurred beneath the Bárdarbunga caldera in Iceland on 29 September 1996. This earthquake is one of a decade-long sequence of M 5 events at Bárdarbunga with non-double-couple mechanisms in the Global Centroid Moment Tensor catalog. Fortunately, it was recorded well by the regional-scale Iceland Hotspot Project seismic experiment. We investigated the event with a complete moment tensor inversion method using regional long-period seismic waveforms and a composite structural model. The moment tensor inversion using data from stations of the Iceland Hotspot Project yields a non-double-couple solution with a 67% vertically oriented compensated linear vector dipole component, a 32% doublecouple component, and a statistically insignificant (2%) volumetric (isotropic) contraction. This indicates the absence of a net volumetric component, which is puzzling in the case of a large volcanic earthquake that apparently is not explained by shear slip on a planar fault. A possible volcanic mechanism that can produce an earthquake without a volumetric component involves two offset sources with similar but opposite volume changes. We show that although such a model cannot be ruled out, the circumstances under which it could happen are rare.
AGU Fall Meeting Abstracts, Dec 1, 2016
AGU Fall Meeting Abstracts, Dec 1, 2012
AGU Fall Meeting Abstracts, Dec 1, 2017
Over the last decade, a rolling array of seismometers has been sequentially deployed throughout s... more Over the last decade, a rolling array of seismometers has been sequentially deployed throughout southeast Australia to record passive seismic activity. To date, nearly 350 separate sites have been occupied with nominal station spacings of 40-50 km on the mainland and 15-20 km in Tasmania. Deployment periods for each of the eight arrays installed so far have varied between 4-10
AGU Fall Meeting Abstracts, Dec 1, 2020
Journal of Geophysical Research: Solid Earth, 2015
The origin of the microseismic wavefield is associated with deep ocean and coastal regions where,... more The origin of the microseismic wavefield is associated with deep ocean and coastal regions where, under certain conditions, ocean waves can excite seismic waves that propagate as surface and body waves. Given that the characteristics of seismic signals generally vary with frequency, here we explore the frequency-and azimuth-dependent properties of microseisms recorded at a medium aperture (25 km) array in Australia. We examine the frequency-dependent properties of the wavefield, and its temporal variation, over two decades (1991-2012), with a focus on relatively high-frequency microseisms (0.325-0.725 Hz) recorded at the Warramunga Array, which has good slowness resolution capabilities in this frequency range. The analysis is carried out using the incoherently averaged signal Capon beamforming, which gives robust estimates of slowness and back azimuth and is able to resolve multiple wave arrivals within a single time window. For surface waves, we find that fundamental mode Rayleigh waves (R g) dominate for lower frequencies (<0.55 Hz) while higher frequencies (>0.55 Hz) show a transition to higher mode surface waves (L g). For body waves, source locations are identified in deep ocean regions for lower frequencies and in shallow waters for higher frequencies. We further examine the association between surface wave arrivals and a WAVEWATCH III ocean wave hindcast. Correlations with the ocean wave hindcast show that secondary microseisms in the lower-frequency band are generated mainly by ocean swell, while higher-frequency bands are generated by the wind sea, i.e., local wind conditions. A number of studies have focused on locating the source regions of SM Rayleigh wave energy with seismic arrays [
Physics of the Earth and Planetary Interiors, 2006
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2012
Seismologists construct images of the Earth's interior structure using observations, derived ... more Seismologists construct images of the Earth's interior structure using observations, derived from seismograms, collected at the surface. A common approach to such inverse problems is to build a single ‘best’ Earth model, in some sense. This is despite the fact that the observations by themselves often do not require, or even allow, a single best-fit Earth model to exist. Interpretation of optimal models can be fraught with difficulties, particularly when formal uncertainty estimates become heavily dependent on the regularization imposed. Similar issues occur across the physical sciences with model construction in ill-posed problems. An alternative approach is to embrace the non-uniqueness directly and employ an inference process based on parameter space sampling. Instead of seeking a best model within an optimization framework, one seeks an ensemble of solutions and derives properties of that ensemble for inspection. While this idea has itself been employed for more than 30 year...
The SEAL3 experiment represents one of the twelve array deployments that so far comprise the larg... more The SEAL3 experiment represents one of the twelve array deployments that so far comprise the large and ambitious WOMBAT project, which aims to cover a significant portion of the Australian continent with a rolling array of seismometers. SEAL3 consists of 55 three-component short-period instruments located in central and southern New South Wales (NSW) with an average station spacing of approximately
ABSTRACT PKP waveforms are the main source of information regarding lowermost mantle and core sei... more ABSTRACT PKP waveforms are the main source of information regarding lowermost mantle and core seismological structures. An extension of our previously developed Simulated Annealing algorithm to analyze body waves is presented. It allows us to resolve the interference between the different PKP phases and their corresponding depth phases (pPKP and sPKP), which makes possible to process shallow earthquakes previously discarded, and thus to improve the sampling of the deep Earth. The algorithm is applied to various PKP waveform data sets in order to retrieve differential travel times and amplitude ratios of the core phases, and to demonstrate its efficiency. We present a first order analysis of differential travel times, amplitude ratios and waveforms of this new data base of PKP core phases in order to constrain the inner core seismic anisotropy, attenuation and deep structure. The introduction of shallow event data confirms the hemispherical pattern of the inner core anisotropy and the increase of the inner core quality factor with depth. An anti-correlation is found at all epicentral distances between differential travel time residuals and amplitude ratios. This anti-correlation can only be explained by a high attenuation of fast rays inside the inner core, produced by inner core fabric. Amplitude ratios and waveforms of core phases are also analyzed tin order to detect inner core discontinuities below the inner core boundary.
We report the first observational evidence that the complex rotational dynamics of the Earth'... more We report the first observational evidence that the complex rotational dynamics of the Earth's inner core appear to be in close relationship with the geomagnetic field. We infer from a newly observed collection of earthquake doublets that the Earth's inner core" shuffles", exhibiting both prograde and retrograde rotation in the reference frame of the mantle. Evidence for a complex pattern in the rotation of the inner core characterized by episodes of both prograde and retrograde motion is presented. A key feature of the new ...
Geophysical Journal International, 2010
We present a significant addition to the data set of traveltimes of seismic PKP waves that sample... more We present a significant addition to the data set of traveltimes of seismic PKP waves that sample the Earth's lowermost mantle and core along the Earth's rotation axis. Recorded at permanent Global Seismic Network (GSN) and temporary SSCUA deployment broadband seismographic stations in Antarctica, the new data improve the previously poor and biased coverage that underlies the seismic constraints on recent models of inner core structure and anisotropy. On the one hand, new differential PKP traveltime measurements improve the sampling of predominantly the eastern inner core hemisphere. PKPab-df and PKPbc-df differential traveltime residuals, with respect to the spherically symmetric model ak135, are consistently smaller than two seconds along the north-south paths sampled. Axially symmetric models of inner core seismic anisotropy with fast axis parallel to the Earth's rotation axis require a weak anisotropy of (0.7 ± 0.1) per cent to be consistent with our PKPbc-df observations. PKPbc-df residuals from the quasi-eastern hemisphere indicate (0.4 ± 0.1) per cent anisotropy. If only PKPbc-df observations from the top 200 km of this hemisphere are considered, this is reduced to (0.1 ± 0.2) per cent, consistent with an isotropic layer. On the other hand, new absolute PKP traveltime measurements add to the sampling of both hemispheres of the inner core, but it is difficult to use them with more confidence to assess structure of the core since they are affected by crustal and mantle structure and source uncertainties. The newly collected data set also increases constraints on D" structure beneath the South Pole. In contrast to previous inferences based on data from northern stations, we find no evidence of a velocity heterogeneity in the outer core near the inner core boundary associated with the cylinder tangent to the inner core in the southern hemisphere.
Bulletin of the Seismological Society of America, 2009
A volcanic earthquake with M w 5:6 occurred beneath the Bárdarbunga caldera in Iceland on 29 Sept... more A volcanic earthquake with M w 5:6 occurred beneath the Bárdarbunga caldera in Iceland on 29 September 1996. This earthquake is one of a decade-long sequence of M 5 events at Bárdarbunga with non-double-couple mechanisms in the Global Centroid Moment Tensor catalog. Fortunately, it was recorded well by the regional-scale Iceland Hotspot Project seismic experiment. We investigated the event with a complete moment tensor inversion method using regional long-period seismic waveforms and a composite structural model. The moment tensor inversion using data from stations of the Iceland Hotspot Project yields a non-double-couple solution with a 67% vertically oriented compensated linear vector dipole component, a 32% doublecouple component, and a statistically insignificant (2%) volumetric (isotropic) contraction. This indicates the absence of a net volumetric component, which is puzzling in the case of a large volcanic earthquake that apparently is not explained by shear slip on a planar fault. A possible volcanic mechanism that can produce an earthquake without a volumetric component involves two offset sources with similar but opposite volume changes. We show that although such a model cannot be ruled out, the circumstances under which it could happen are rare.