Tim Long | Georgia Institute of Technology (original) (raw)
Papers by Tim Long
April 4, 2009 at 20:45 Universal Time (4:45 PM local time) the vicinity of Lake Sinclair received... more April 4, 2009 at 20:45 Universal Time (4:45 PM local time) the vicinity of Lake Sinclair received a magnitude 3.1 gentle reminder that earthquakes do occur in central Georgia. From a historical perspective the vicinity of Lake Sinclair is the second most active seismic zone in Georgia, after the line of activity extending from Fort Payne Alabama to Chattanooga, Tennessee, across northwest Georgia. Because the central Georgia earthquakes are typically shallow, on the order of 1 km or less, they are more often felt than the 8 to 30 km deep northwest Georgia earthquakes. A magnitude 2.5 or larger earthquake is felt in central Georgia on average every two years. Approximately 50 events per year of magnitude greater than 0.5 could be detected by sensitive seismometers located in the Lake Sinclair epicentral zone. During the construction of Wallace Dam and the filling of Lake Oconee, Georgia Tech maintained a 4-station seismic network to monitor earthquakes near Lake Sinclair and Lake Oco...
Bulletin of the Seismological Society of …, 1969
Page 1. 8u11et1n 0f the 5e15m01091ca1 50c1ety 0f Amer1ca. V01. 59, N0. 1, pp. 131-146. Fe6ruary, ... more Page 1. 8u11et1n 0f the 5e15m01091ca1 50c1ety 0f Amer1ca. V01. 59, N0. 1, pp. 131-146. Fe6ruary, 1969 7RAN5M15510N AND A77ENUA710N 0F 7HE PR1MARY 5E15M1C WAVE, 100 70 600 KM 8Y L~1~AN9 71~07m L0~6* AND j05EP~ W. 8En6, JR.$ ...
Seismological Research Letters, 1993
Time-frequency analysis techniques, induding the classical use of zero crossings to measure perio... more Time-frequency analysis techniques, induding the classical use of zero crossings to measure period, have been widely used in seismology for the estimation of surface wave group velocities. Group velocity estimation by the short-time Fourier transform and the multiple filter techniques are equivalent. Although these techniques are used most often, their resolution is limited. The resolution is controlled by the window length in the short time Fourier transform and the filter band width in the multiple filter technique. The moving-window autoregressive spectral estimation provides the highest resolution with the shortest possible window length by predicting the properties of the signal outside the analysis window; however, high resolution is obtained at the expense of uncertainty in the amplitude. Recently, the Wigner distribution has been introduced as a tool for mapping dispersed surface waves into the time-frequency domain. Resolution of the Wigner distribution is comparable to tha...
... E80_SD8) Project Co-Directors: Tim Long (School of Earth and Atmospheric Sciences) NevaRose (... more ... E80_SD8) Project Co-Directors: Tim Long (School of Earth and Atmospheric Sciences) NevaRose (CEISMC) Middle Grade Science Teacher Workshop in Earthquakes and Seismometers: A Way to Enhance Earth Science Teaching Summary From June 23, to June 27,1997 ...
Bulletin of the Seismological Society of America
Trains moving through the Rio Grande Valley at Socorro are a known isolated source of short-perio... more Trains moving through the Rio Grande Valley at Socorro are a known isolated source of short-period microseisms. Power spectra at five different locations indicate that the train noise is confined to a narrow band (< 3 cps) of frequencies. In general, the frequency limits and peaks of the power spectra shift to lower values with an increase in distance from the source. Peaks in the power spectra, which range from 1.80 to 4.75 cps, do not correlate with mechanical interactions of the train with the track. Particle motions for randomly selected samples of train noise are extremely complex and no dominant component of motion—Rayleigh or Love—can be identified. However, for samples of noise showing the same regular frequency on all components of ground motion, a strong fundamental mode Rayleigh wave can generally be found. Phase-velocity measurements at a distance of about 17000 feet from the train indicate that a major amount of the train noise at Socorro is being propagated as a fun...
Bulletin of the Seismological Society of America
Aftershock and foreshock activity within 12 hr of the July 13, 1971 earthquake near Seneca, South... more Aftershock and foreshock activity within 12 hr of the July 13, 1971 earthquake near Seneca, South Carolina, indicates a b value of 0.9 at ML = 3.0. Approximately 40 events recorded in a 5-day aftershock survey near Seneca indicate a b value of 1.7 at ML = 0.5. A sequence of over 40 events occurring west of McCormick, South Carolina, indicates a b value of 1.3 at ML = 2.4. The McCormick sequence was active for 4 months. Unlike the Seneca region, the McCormick region has a history of earthquake activity. Examinations of other published southeastern b values suggest that southeastern United States earthquakes originate from conditions of ambient stress which vary with epicentral region or magnitude.
Bulletin of the Seismological Society of America
Seven temporary and three permanent seismograph stations recorded seismic waves to a distance of ... more Seven temporary and three permanent seismograph stations recorded seismic waves to a distance of about 270 km from a 110,000 pound quarry blast detonated near Depoe Bay, Oregon. The recording stations were in a north-south line along the northern coast range of Western Oregon and Washington. The travel-time data indicate an apparent shallow crustal thickness (about 16 km) for this region. The time versus distance data were not continuous beyond 130 km from the source which may have resulted from any combination of the following causes: (1) insufficient source energy; (2) lateral geological variations; and/or (3) a subcrustal negative velocity gradient.
Seismological Research Letters
Wavelets scattered from a spherical heterogeneity were computed from solutions of the wave equati... more Wavelets scattered from a spherical heterogeneity were computed from solutions of the wave equation for a plane wave incident on a spherical inhomogeneity. The solution is expressed as an orthogonal function expansion that is asymptotically correct for all wavelengths. In this paper we computed the spectral response and corresponding time-domain impulse response for wavelengths greater than one tenth the radius. The frequency content and amplitude of the scattered wave from an incident compressional wave systematically varies with scattering angle. A comparison of the scattered waves with arrival times and amplitudes computed using ray theory shows that the variations in response with scattering angle are closely tied to arrival times and amplitudes of the internally reflected and refracted waves. Use of the minimum phase criterion in order to generate a wavelet does not replicate the wavelet shape, but can be used to generate wavelets where the spectral amplitudes are the principal...
Seismological Research Letters
Time-frequency analysis techniques, including the classical use of zero crossings to measure peri... more Time-frequency analysis techniques, including the classical use of zero crossings to measure period, have been widely used in seismology for the estimation of surface wave group velocities. Group velocity estimation by the short-time Fourier transform and the multiple filter techniques are equivalent. Although these techniques are used most often, their resolution is limited. The resolution is controlled by the window length in the short time Fourier transform and the filter band width in the multiple filter technique. The moving-window autoregressive spectral estimation provides the highest resolution with the shortest possible window length by predicting the properties of the signal outside the analysis window; however, high resolution is obtained at the expense of uncertainty in the amplitude. Recently, the Wigner distribution has been introduced as a tool for mapping dispersed surface waves into the time-frequency domain. Resolution of the Wigner distribution is comparable to th...
Seismological Research Letters
Traditional paradigms of continental seismicity assert the stationarity of the earthquake process... more Traditional paradigms of continental seismicity assert the stationarity of the earthquake process and a causal association of earthquakes with active faults, increasing levels of stress, and crustal structures, in a framework of Plate Tectonics. I propose, instead, that the seismicity associated with a magnitude six or greater intraplate continental earthquake is a transient phenomenon responding to a perturbation in crustal strength independent of existing faults and crustal structures. Regional plate stress may still provide the driving energy, but the causative stress is released by a perturbation in crustal strength in the vicinity of a major earthquake. The timing of a major earthquake and the characteristics of the associated seismicity may then be described by a sequence of five phases which are as follows: (1) Initiation. A major intraplate continental earthquake is initiated with a disturbance in the hydraulic or thermal properties of the crust below the epicenter. Such dis...
Bulletin of the Seismological Society of America
Seismological Research Letters
Symposium on the Application of Geophysics to Engineering and Environmental Problems 2002, 2002
Georgia Institute of Technology Final Report, Dec 1, 1974
Seismological Research Letters, Apr 1, 1993
Time-frequency analysis techniques, including the classical use of zero crossings to measure peri... more Time-frequency analysis techniques, including the classical use of zero crossings to measure period, have been widely used in seismology for the estimation of surface wave group velocities. Group velocity estimation by the short-time Fourier transform and the multiple filter techniques are equivalent. Although these techniques are used most often, their resolution is limited. The resolution is controlled by the window length in the short time Fourier transform and the filter band width in the multiple filter technique. The moving-window autoregressive spectral estimation provides the highest resolution with the shortest possible window length by predicting the properties of the signal outside the analysis window; however, high resolution is obtained at the expense of uncertainty in the amplitude. Recently, the Wigner distribution has been introduced as a tool for mapping dispersed surface waves into the time-frequency domain. Resolution of the Wigner distribution is comparable to that of the moving-window autoregressive spectral estimation. When the spectral density at a given time contains two or more dominant frequencies, their interference causes the Wigner distribution to introduce spurious spectral peaks complicating the interpretation. The Choi-Williams distribution, in which these interference effects are minimized, can be used for such dispersed signals. However, the implementation is computationally complex and the distribution offers only a medium resolution.
Bulletin of the Seismological Society of America, Feb 1, 1968
abstract Trains moving through the Rio Grande Valley at Socorro are a known isolated source of sh... more abstract Trains moving through the Rio Grande Valley at Socorro are a known isolated source of short-period microseisms. Power spectra at five different locations indicate that the train noise is confined to a narrow band (< 3 cps) of frequencies. In general, the frequency limits and peaks of the power spectra shift to lower values with an increase in distance from the source. Peaks in the power spectra, which range from 1.80 to 4.75 cps, do not correlate with mechanical interactions of the train with the track. Particle motions for randomly selected samples of train noise are extremely complex and no dominant component of motion—Rayleigh or Love—can be identified. However, for samples of noise showing the same regular frequency on all components of ground motion, a strong fundamental mode Rayleigh wave can generally be found. Phase-velocity measurements at a distance of about 17000 feet from the train indicate that a major amount of the train noise at Socorro is being propagated as a fundamental mode Rayleigh wave. The average of over two hundred phase velocity determinations was 1400 ft/sec, a value that corresponds closely to 0.92 of the shear velocity of the surface layer at the recording location. Other observations at the same location indicate normal dispersion of fundamental mode Rayleigh waves, but the change in phase velocity with frequency (in the range of 2 to 5 cps) appears to be quite small.
April 4, 2009 at 20:45 Universal Time (4:45 PM local time) the vicinity of Lake Sinclair received... more April 4, 2009 at 20:45 Universal Time (4:45 PM local time) the vicinity of Lake Sinclair received a magnitude 3.1 gentle reminder that earthquakes do occur in central Georgia. From a historical perspective the vicinity of Lake Sinclair is the second most active seismic zone in Georgia, after the line of activity extending from Fort Payne Alabama to Chattanooga, Tennessee, across northwest Georgia. Because the central Georgia earthquakes are typically shallow, on the order of 1 km or less, they are more often felt than the 8 to 30 km deep northwest Georgia earthquakes. A magnitude 2.5 or larger earthquake is felt in central Georgia on average every two years. Approximately 50 events per year of magnitude greater than 0.5 could be detected by sensitive seismometers located in the Lake Sinclair epicentral zone. During the construction of Wallace Dam and the filling of Lake Oconee, Georgia Tech maintained a 4-station seismic network to monitor earthquakes near Lake Sinclair and Lake Oco...
Bulletin of the Seismological Society of …, 1969
Page 1. 8u11et1n 0f the 5e15m01091ca1 50c1ety 0f Amer1ca. V01. 59, N0. 1, pp. 131-146. Fe6ruary, ... more Page 1. 8u11et1n 0f the 5e15m01091ca1 50c1ety 0f Amer1ca. V01. 59, N0. 1, pp. 131-146. Fe6ruary, 1969 7RAN5M15510N AND A77ENUA710N 0F 7HE PR1MARY 5E15M1C WAVE, 100 70 600 KM 8Y L~1~AN9 71~07m L0~6* AND j05EP~ W. 8En6, JR.$ ...
Seismological Research Letters, 1993
Time-frequency analysis techniques, induding the classical use of zero crossings to measure perio... more Time-frequency analysis techniques, induding the classical use of zero crossings to measure period, have been widely used in seismology for the estimation of surface wave group velocities. Group velocity estimation by the short-time Fourier transform and the multiple filter techniques are equivalent. Although these techniques are used most often, their resolution is limited. The resolution is controlled by the window length in the short time Fourier transform and the filter band width in the multiple filter technique. The moving-window autoregressive spectral estimation provides the highest resolution with the shortest possible window length by predicting the properties of the signal outside the analysis window; however, high resolution is obtained at the expense of uncertainty in the amplitude. Recently, the Wigner distribution has been introduced as a tool for mapping dispersed surface waves into the time-frequency domain. Resolution of the Wigner distribution is comparable to tha...
... E80_SD8) Project Co-Directors: Tim Long (School of Earth and Atmospheric Sciences) NevaRose (... more ... E80_SD8) Project Co-Directors: Tim Long (School of Earth and Atmospheric Sciences) NevaRose (CEISMC) Middle Grade Science Teacher Workshop in Earthquakes and Seismometers: A Way to Enhance Earth Science Teaching Summary From June 23, to June 27,1997 ...
Bulletin of the Seismological Society of America
Trains moving through the Rio Grande Valley at Socorro are a known isolated source of short-perio... more Trains moving through the Rio Grande Valley at Socorro are a known isolated source of short-period microseisms. Power spectra at five different locations indicate that the train noise is confined to a narrow band (< 3 cps) of frequencies. In general, the frequency limits and peaks of the power spectra shift to lower values with an increase in distance from the source. Peaks in the power spectra, which range from 1.80 to 4.75 cps, do not correlate with mechanical interactions of the train with the track. Particle motions for randomly selected samples of train noise are extremely complex and no dominant component of motion—Rayleigh or Love—can be identified. However, for samples of noise showing the same regular frequency on all components of ground motion, a strong fundamental mode Rayleigh wave can generally be found. Phase-velocity measurements at a distance of about 17000 feet from the train indicate that a major amount of the train noise at Socorro is being propagated as a fun...
Bulletin of the Seismological Society of America
Aftershock and foreshock activity within 12 hr of the July 13, 1971 earthquake near Seneca, South... more Aftershock and foreshock activity within 12 hr of the July 13, 1971 earthquake near Seneca, South Carolina, indicates a b value of 0.9 at ML = 3.0. Approximately 40 events recorded in a 5-day aftershock survey near Seneca indicate a b value of 1.7 at ML = 0.5. A sequence of over 40 events occurring west of McCormick, South Carolina, indicates a b value of 1.3 at ML = 2.4. The McCormick sequence was active for 4 months. Unlike the Seneca region, the McCormick region has a history of earthquake activity. Examinations of other published southeastern b values suggest that southeastern United States earthquakes originate from conditions of ambient stress which vary with epicentral region or magnitude.
Bulletin of the Seismological Society of America
Seven temporary and three permanent seismograph stations recorded seismic waves to a distance of ... more Seven temporary and three permanent seismograph stations recorded seismic waves to a distance of about 270 km from a 110,000 pound quarry blast detonated near Depoe Bay, Oregon. The recording stations were in a north-south line along the northern coast range of Western Oregon and Washington. The travel-time data indicate an apparent shallow crustal thickness (about 16 km) for this region. The time versus distance data were not continuous beyond 130 km from the source which may have resulted from any combination of the following causes: (1) insufficient source energy; (2) lateral geological variations; and/or (3) a subcrustal negative velocity gradient.
Seismological Research Letters
Wavelets scattered from a spherical heterogeneity were computed from solutions of the wave equati... more Wavelets scattered from a spherical heterogeneity were computed from solutions of the wave equation for a plane wave incident on a spherical inhomogeneity. The solution is expressed as an orthogonal function expansion that is asymptotically correct for all wavelengths. In this paper we computed the spectral response and corresponding time-domain impulse response for wavelengths greater than one tenth the radius. The frequency content and amplitude of the scattered wave from an incident compressional wave systematically varies with scattering angle. A comparison of the scattered waves with arrival times and amplitudes computed using ray theory shows that the variations in response with scattering angle are closely tied to arrival times and amplitudes of the internally reflected and refracted waves. Use of the minimum phase criterion in order to generate a wavelet does not replicate the wavelet shape, but can be used to generate wavelets where the spectral amplitudes are the principal...
Seismological Research Letters
Time-frequency analysis techniques, including the classical use of zero crossings to measure peri... more Time-frequency analysis techniques, including the classical use of zero crossings to measure period, have been widely used in seismology for the estimation of surface wave group velocities. Group velocity estimation by the short-time Fourier transform and the multiple filter techniques are equivalent. Although these techniques are used most often, their resolution is limited. The resolution is controlled by the window length in the short time Fourier transform and the filter band width in the multiple filter technique. The moving-window autoregressive spectral estimation provides the highest resolution with the shortest possible window length by predicting the properties of the signal outside the analysis window; however, high resolution is obtained at the expense of uncertainty in the amplitude. Recently, the Wigner distribution has been introduced as a tool for mapping dispersed surface waves into the time-frequency domain. Resolution of the Wigner distribution is comparable to th...
Seismological Research Letters
Traditional paradigms of continental seismicity assert the stationarity of the earthquake process... more Traditional paradigms of continental seismicity assert the stationarity of the earthquake process and a causal association of earthquakes with active faults, increasing levels of stress, and crustal structures, in a framework of Plate Tectonics. I propose, instead, that the seismicity associated with a magnitude six or greater intraplate continental earthquake is a transient phenomenon responding to a perturbation in crustal strength independent of existing faults and crustal structures. Regional plate stress may still provide the driving energy, but the causative stress is released by a perturbation in crustal strength in the vicinity of a major earthquake. The timing of a major earthquake and the characteristics of the associated seismicity may then be described by a sequence of five phases which are as follows: (1) Initiation. A major intraplate continental earthquake is initiated with a disturbance in the hydraulic or thermal properties of the crust below the epicenter. Such dis...
Bulletin of the Seismological Society of America
Seismological Research Letters
Symposium on the Application of Geophysics to Engineering and Environmental Problems 2002, 2002
Georgia Institute of Technology Final Report, Dec 1, 1974
Seismological Research Letters, Apr 1, 1993
Time-frequency analysis techniques, including the classical use of zero crossings to measure peri... more Time-frequency analysis techniques, including the classical use of zero crossings to measure period, have been widely used in seismology for the estimation of surface wave group velocities. Group velocity estimation by the short-time Fourier transform and the multiple filter techniques are equivalent. Although these techniques are used most often, their resolution is limited. The resolution is controlled by the window length in the short time Fourier transform and the filter band width in the multiple filter technique. The moving-window autoregressive spectral estimation provides the highest resolution with the shortest possible window length by predicting the properties of the signal outside the analysis window; however, high resolution is obtained at the expense of uncertainty in the amplitude. Recently, the Wigner distribution has been introduced as a tool for mapping dispersed surface waves into the time-frequency domain. Resolution of the Wigner distribution is comparable to that of the moving-window autoregressive spectral estimation. When the spectral density at a given time contains two or more dominant frequencies, their interference causes the Wigner distribution to introduce spurious spectral peaks complicating the interpretation. The Choi-Williams distribution, in which these interference effects are minimized, can be used for such dispersed signals. However, the implementation is computationally complex and the distribution offers only a medium resolution.
Bulletin of the Seismological Society of America, Feb 1, 1968
abstract Trains moving through the Rio Grande Valley at Socorro are a known isolated source of sh... more abstract Trains moving through the Rio Grande Valley at Socorro are a known isolated source of short-period microseisms. Power spectra at five different locations indicate that the train noise is confined to a narrow band (< 3 cps) of frequencies. In general, the frequency limits and peaks of the power spectra shift to lower values with an increase in distance from the source. Peaks in the power spectra, which range from 1.80 to 4.75 cps, do not correlate with mechanical interactions of the train with the track. Particle motions for randomly selected samples of train noise are extremely complex and no dominant component of motion—Rayleigh or Love—can be identified. However, for samples of noise showing the same regular frequency on all components of ground motion, a strong fundamental mode Rayleigh wave can generally be found. Phase-velocity measurements at a distance of about 17000 feet from the train indicate that a major amount of the train noise at Socorro is being propagated as a fundamental mode Rayleigh wave. The average of over two hundred phase velocity determinations was 1400 ft/sec, a value that corresponds closely to 0.92 of the shear velocity of the surface layer at the recording location. Other observations at the same location indicate normal dispersion of fundamental mode Rayleigh waves, but the change in phase velocity with frequency (in the range of 2 to 5 cps) appears to be quite small.