Statistical analysis of irregular wave-guide influences on regional seismic discriminants in China: Additional results for Pn/Sn, Pn/Lg, and Pg/Sn

1998 ◽  
Vol 88 (6) ◽  
pp. 1504-1510
Author(s):  
Guangwei Fan ◽  
Thorne Lay
Keyword(s):  
Crustal Structure ◽  
Seismic Waves ◽  
Low Frequency ◽  
Multivariate Regression Analysis ◽  
Crustal Thickness ◽  
Western China ◽  
The Tibetan Plateau ◽  
Sediment Thickness ◽  
Amplitude Ratios ◽  
Short Period

Abstract Reducing scatter in measurements of regional-phase amplitude ratios is desirable for seismic discrimination applications and also provides insight into crustal structure controls on energy partitioning of regional seismic waves. Our previous analysis (Fan and Lay, 1998) of the regional seismic discriminant, Pg/Lg, indicated that variations in crustal structure cause path-specific fluctuations of those amplitude ratios for earthquakes recorded at broadband station WMQ in western China. In this study, we extend our multivariate regression analysis to Pn/Sn, Pn/Lg, and Pg/Sn amplitude ratios recorded at WMQ in frequency bands of 0.75 to 1.5 Hz, 1.5 to 3.0 Hz, and 3.0 to 6.0 Hz for the suite of path-specific parameters: path length, mean path elevation, variance of topography along the path, rms topographic slope variations, mean crustal thickness, and mean sediment thickness. Optimal three- and four-parameter models all achieve reductions in variance of the measurements relative to conventional distance corrections. At low frequency, the improvements for ratios involving Pg can be more than a factor of 2. For all short-period amplitude ratios, mean path elevation seems to play an important role. While crustal thickness and sediment thickness affect ratios involving Pg, topographic variance and surface slope variations have more influence on ratios involving Pn. Strong crustal variations associated with the structure of the Tibetan Plateau are responsible for much of the amplitude variations.

Seismic signals preceding the explosive eruption of Mount St. Helens, Washington, on 18 May 1980

1983 ◽  
Vol 73 (6A) ◽  
pp. 1797-1813
Author(s):  
Anthony Qamar ◽  
William St. Lawrence ◽  
Johnnie N. Moore ◽  
George Kendrick
Keyword(s):  
Seismic Waves ◽  
Low Frequency ◽  
Seismic Energy ◽  
B Value ◽  
Volcanic Earthquake ◽  
Seismic Signals ◽  
Volcanic Earthquakes ◽  
Short Period ◽  
Stressed Region ◽  
Mount St Helens

Abstract The intense seismic activity which preceded the 18 May 1980 eruption of Mount St. Helens, Washington, released 2 to 3 × 1018 ergs/day in earthquakes that did not correlate temporally with phreatic eruptions which occurred during the same period. Although the b value and amplitude ratios (long-period/short-period) of the earthquakes vary with time, there are no definitive precursors to the 18 May earthquake and eruption. A Mogi type II frequency-magnitude relation, with critical magnitude Mc = 4.6, constrains the characteristic dimension of the highly stressed region under Mount St. Helens to approximately 3 km, preceding the eruption. A major increase in seismic energy release and a decrease in b value around 1 April 1980 may indicate the first major influx of magma into the upper portion of the volcano. Seismic waves from low-frequency volcanic earthquake have large periods at all epicentral distances. Recordings of volcanic earthquakes from 2 to 4 April 1980 at sites 4 to 9 km from Mount St. Helens show two predominant periods of 0.55 and 1.0 sec. We speculate that seismic signals from the low-frequency volcanic earthquakes have a tectonic origin, but may be modified by pressure oscillations in nearby magma.

Statistical analysis of irregular wave-guide influences on regional seismic discriminants in China

1998 ◽  
Vol 88 (1) ◽  
pp. 74-88 ◽  
Author(s):  
Guangwei Fan ◽  
Thorne Lay
Keyword(s):  
Wave Energy ◽  
P Wave ◽  
Western China ◽  
The Tibetan Plateau ◽  
Large Scatter ◽  
S Wave ◽  
Low Frequencies ◽  
Short Period ◽  
Propagation Effects ◽  
Regional Phases

Abstract Short-period regional phases play an important role in identifying low-magnitude seismic events in the context of monitoring the Comprehensive Test Ban Treaty. Amplitude ratios of regional phases comprised mainly of P-wave energy (Pn, Pg) to those comprised mainly of S-wave energy (Sn, Lg) effectively discriminate between explosions and earthquakes in many regions, particularly at frequencies higher than 3 Hz. At lower frequencies, discrimination is usually poor due to large scatter that causes overlapping of event populations. Scatter in regional discriminant measures such as Pg/Lg ratios is caused by both source and propagation effects, and reducing the scatter imparted by the latter is essential to improving the discriminant performance when events do not share identical paths. Regional phases experience distance-dependent amplitude variations due to effects such as critical angle amplification, geometric spreading, and attenuation. Discriminant measures are usually corrected for empirically determined distance trends for a given region, but large scatter persists after such corrections. This study seeks to develop more sophisticated empirical corrections for path properties in order to further reduce the scatter in regional discriminant measures caused by propagation effects. Broadband seismic waveforms recorded at station WMQ, in western China, demonstrate that regional Pg/Lg ratios show significant distance dependence for frequencies less than 6 Hz. However, variations in crustal structure cause additional path-specific amplitude fluctuations that are not accounted for by regionally averaged distance corrections. Blockage of Lg phases on paths traversing the margins of the Tibetan Plateau is one such effect. Regression analysis demonstrates that Pg/Lg ratios measured at WMQ display significant correlations with path-specific properties such as mean elevation, topographic roughness, basement depth, and crustal thickness. Multiple regressions using optimal combinations of parameters yield corrections that reduce variance in Pg/Lg measurements for frequencies less than 3 Hz by a factor of 2 or more relative to standard distance corrections. This should systematically enhance the performance of the Pg/Lg discriminant at low frequencies. The method presented here can be used for all regions and all short-period regional discriminants. It is likely that the extraordinary crustal heterogeneity in western China represents an extreme case of path-dependent effects.

scholarly journals Estimation of permeability and saturation based on imaginary component of complex resistivity spectra: A laboratory study

2020 ◽  
Vol 12 (1) ◽  
pp. 299-306
Author(s):  
Jiang Jia ◽  
Shizhen Ke ◽  
Junjian Li ◽  
Zhengming Kang ◽  
Xuerui Ma ◽  
...  
Keyword(s):  
Water Saturation ◽  
Pore Space ◽  
Evaluation Model ◽  
Low Frequency ◽  
Petroleum Exploration ◽  
Western China ◽  
Imaginary Component ◽  
Estimation Model ◽  
Complex Resistivity ◽  
Linear Relationships

AbstractLow-frequency resistivity logging plays an important role in the field of petroleum exploration, but the complex resistivity spectrum of rock also contains a large amount of information about reservoir parameters. The complex resistivity spectra of 15 natural sandstone cores from western China, with different water saturations, were measured with an impedance analyzer. The pore space of each core was saturated with NaCl solution, and measurements were collected at a frequency range of 40–15 MHz. The results showed a linear relationship between the real resistivity at 1 kHz and the maximum values of imaginary resistivity for each core with different water saturations. The slopes of the linear best-fit lines had good linear relationships with the porosity and the permeability of cores. Based on this, a permeability estimation model was proposed and tested. In addition, the maxima of imaginary resistivity had power exponential relationships with the porosity and the water saturation of the cores. A saturation evaluation model based on the maxima of imaginary resistivity was established by imitating Archie’s formula. The new models were found to be feasible for determining the permeability and saturation of sandstone based on complex resistivity spectrum measurements. These models advance the application of complex resistivity spectrum in petrophysics.

Frequency Response Analysis of Piecewise Nonlinear Vibration Isolator

2005 ◽  
Author(s):  
Patrick Stahl ◽  
G. Nakhaie Jazar
Keyword(s):  
Low Frequency ◽  
High Amplitude ◽  
Relative Displacement ◽  
Response Analysis ◽  
Frequency Response Analysis ◽  
State Behavior ◽  
Vibration Isolators ◽  
Short Period ◽  
Secondary Suspension ◽  
Low Amplitude

Non-smooth piecewise functional isolators are smart passive vibration isolators that can provide effective isolation for high frequency/low amplitude excitation by introducing a soft primary suspension, and by preventing a high relative displacement in low frequency/high amplitude excitation by introducing a relatively damped secondary suspension. In this investigation a linear secondary suspension is attached to a nonlinear primary suspension. The primary is assumed to be nonlinear to model the inherent nonlinearities involved in real suspensions. However, the secondary suspension comes into action only during a short period of time, and in mall domain around resonance. Therefore, a linear assumption for the secondary suspension is reasonable. The dynamic behavior of the system subject to a harmonic base excitation has been analyzed utilizing the analytic results derived by applying the averaging method. The analytic results match very well in the transition between the two suspensions. A sensitivity analysis has shown the effect of varying dynamic parameters in the steady state behavior of the system.

Attenuation of short period seismic waves at Etna as compared to other volcanic areas

1987 ◽  
Vol 125 (6) ◽  
pp. 1039-1050 ◽  
Author(s):  
E. Del Pezzo ◽  
S. Gresta ◽  
G. Patané ◽  
D. Patané ◽  
G. Scarcella
Keyword(s):  
Seismic Waves ◽  
Short Period

Characteristics of Crustal Structure and Their Relation with Major Earthquakes in Western China

2015 ◽  
Vol 58 (1) ◽  
pp. 83-95
Author(s):  
JIANG Di-Di ◽  
JIANG Wei-Wei ◽  
XU Yi ◽  
HAO Tian-Yao ◽  
HU Wei-Jian ◽  
...  
Keyword(s):  
Crustal Structure ◽  
Western China

A periodic seismic isolation foundation with an extremely broad low-frequency attenuation zone: Theoretical analysis and experimental verification

2021 ◽  
pp. 136943322110646
Author(s):  
Peng Zhou ◽  
Shui Wan ◽  
Xiao Wang ◽  
Yingbo Zhu ◽  
Muyun Huang
Keyword(s):  
Seismic Isolation ◽  
Seismic Waves ◽  
Periodic Structures ◽  
Finite Size ◽  
Low Frequency ◽  
Bridge Structure ◽  
Engineering Structures ◽  
P Waves ◽  
New Type ◽  
Dominant Frequencies

The attenuation zones (AZs) of periodic structures can be used for seismic isolation design. To cover the dominant frequencies of more seismic waves, this paper proposes a new type of periodic isolation foundation (PIF) with an extremely wide low-frequency AZ of 3.31 Hz–17.01 Hz composed of optimized unit A with a wide AZ and optimized unit B with a low-frequency AZ. The two kinds of optimized units are obtained by topology optimization on the smallest periodic unit with the coupled finite element-genetic algorithm (GA) methodology. The transmission spectra of shear waves and P-waves through the proposed PIF of finite size are calculated, and the results show that the AZ of the PIF is approximately the superposition of the AZs of the two kinds of optimized units. Additionally, shake tests on a scale PIF specimen are performed to verify the attenuation performance for elastic waves within the designed AZs. Furthermore, numerical simulations show that the acceleration responses of the bridge structure with the proposed PIF are attenuated significantly compared to those with a concrete foundation under the action of different seismic waves. Therefore, the newly proposed PIF is a promising option for the reduction of seismic effects in engineering structures.

Crustal structure of the Tibetan Plateau: A surface-wave study by a moving window analysis

1977 ◽  
Vol 67 (3) ◽  
pp. 735-750
Author(s):  
Kin-Yip Chun ◽  
Toshikatsu Yoshii
Keyword(s):  
Tibetan Plateau ◽  
Crustal Structure ◽  
The Tibetan Plateau ◽  
Moving Window ◽  
Low Velocity ◽  
Period Range ◽  
Window Analysis ◽  
Dispersion Data ◽  
Moving Window Analysis ◽  
Group Velocities

abstract Group velocities of fundamental-mode Rayleigh and Love waves are analyzed to construct a crustal structure of the Tibetan Plateau. A moving window analysis is employed to compute group velocities in a wide period range of 7 to 100 sec for 17 individual paths. The crustal models derived from these dispersion data indicate that under the Tibetan Plateau the total crustal thickness is about 70 km and that the crustal velocities are generally low. The low velocities are most probably caused by high temperatures. A low-velocity zone located at an intermediate depth within the crust appears to be strongly demanded by the observed dispersion data. The main features of the proposed crustal structure will place stringent constraints on future tectonic models of the Tibetan Plateau which is generally regarded as a region of active deformation due to the continent-continent collision between India and Asia.

Short-period Rayleigh-wave dispersion across the Tibetan Plateau

1975 ◽  
Vol 65 (5) ◽  
pp. 1051-1057 ◽  
Author(s):  
W. P. Chen ◽  
P. Molnar
Keyword(s):  
Tibetan Plateau ◽  
Simple Wave ◽  
Wave Dispersion ◽  
New Delhi ◽  
The Tibetan Plateau ◽  
Period Range ◽  
Short Period ◽  
Wave Forms ◽  
Wave Trains ◽  
Rayleigh Wave Dispersion

Abstract Well-dispersed Rayleigh waves within the period range of 4 to 11 sec are observed at New Delhi (NDI) and Shillong (SHL), India, for seven earthquakes near and in the Tibetan Plateau from 1963 to 1971. The dispersion curves and the simply dispersed wave forms suggest a prominent overlying wave guide, probably sediments, in the Tibetan area. The thickness of such sediments is most likely between 2.5 and 7.0 km. The simple wave trains, without much distortion due to multipathing, are consistent with a relatively inert, recent tectonism in Tibet.

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