Source complexity of small earthquakes near Matsushiro, Japan
4
Citation
37
Reference
10
Related Paper
Keywords:
Seismogram
Seismometer
Coda
Seismic moment
Thirty-three seismograms from nine large quarry blasts ranging in size from 50,000 to 2,138,000 lb of explosives were analyzed for possible reflections from inhomogeneities in the earth's upper mantle. Of the 33 seismograms, four were obtained at temporary seismograph stations positioned between 90 and 243 km from the explosions and an array of three to four seismometers was used at each of the stations. The remaining twenty-nine seismograms were obtained from ten permanent seismograph stations located between 76 and 1,009 km from the explosions. Seven of these latter seismograms were obtained from the seismograph station at Salt Lake City, Utah, and six were obtained from the seismograph station at Eureka, Nevada. Each arrival on these 13 seismograms was noted and then correlated to determine which arrivals were common to all seismograms having nearly constant epicentral distances.
Of the nine quarry blasts recorded, seven were detonated at Promontory, Utah, and two were detonated at Lakeside, Utah, which lies about 33 km west of Promontory. This multiplicity of blasts resulted in two groups of seismograms for both the Salt Lake City and Eureka stations with one group at each station having a different epicentral distance from the other group at the same station. A comparison was made between the seismograms of each station based on the apparent velocity of the arrivals across this difference in epicentral distance. Seismic arrivals having apparent velocities that would be representative of deep reflections were selected from the aforementioned arrivals common to most records.
The remaining 16 seismograms, which were from eight permanent seismograph stations located at epicentral distances in excess of 500 km, were used to check the results from the analysis of the Salt Lake City, Eureka, and temporary stations.
Times of possible reflected events are presented which could result from energy reflected at discontinuities in the upper mantle at depths of about 190, 520, and 910 km. The depths were computed using average velocities based on velocity-depth curves given by Jeffreys and Gutenberg (Jacobs 1953, p. 187) for the deeper portions of the upper mantle and assuming that linear ray paths pertained.
Seismogram
Seismometer
Promontory
Classification of discontinuities
Seismic energy
Epicenter
Microseism
Cite
Citations (30)
A finite-difference algorithm is used to generate synthetic seismograms for waves propagating through two-dimensional random media. The media have a significant component of their material properties varying randomly over length scales smaller than the seismic wavelength and are meant to approximate the heterogeneity of the crust and upper mantle. The finite-difference technique retains all multiply scattered and diffracted waves, and also accounts for transmission losses.
The synthetic seismograms clearly exhibit coda and apparent attenuation caused by scattering. For a medium with a white wavenumber spectrum of velocity fluctuations, the coda is higher frequency than the initial pulse. The apparent attenuation is greatest when the scatterer size is comparable to the seismic wavelength. The spectra of the coda generally increase in frequency as the scatterers decrease in size. Examples demonstrate how scattering can produce spectra with broad peaks and sharp fall-offs that can make the determination of the source spectra and corner frequencies of small earthquakes extremely difficult.
Coda
Seismogram
Wavenumber
Cite
Citations (119)
Coda
Seismogram
Cite
Citations (0)
Using an indirect boundary integral method, seismograms are computed for elastic media with localized heterogeneities. Models are two-dimensional homogeneous full spaces (P and SV waves) with many circular cavities as heterogeneities or scatterers. Heterogeneities are localized within a depth range, forming a relatively thin random layer. Seismograms are obtained for receivers at depth 0 km (surface) and for several focal depths, with sources radiating S waves isotropically. Seismograms are composed of the direct S wave and all the possible scattered waves by the heterogeneities, exhibiting late arrivals or coda waves. The coda wave amplitude, or coda energy level, and its duration vary for events with different focal depths. As the focal depth increases and the source gets closer to the layer of localized heterogeneities, the coda level becomes small. When the source is within the heterogeneous layer, however, the coda level becomes larger than for a case of the source either above or below the heterogeneous layer. This local enhancement of coda takes place clearly only in the frequency range for which the scattering is the most effective, that is, when the non-dimensional frequency kd takes values from 2 to 3, where k is the wavenumber and d is the size of each heterogeneity. Such enhancement of coda is not observed when the density of cavities, or strength of the heterogeneities, is reduced. Coda level becomes locally large for a source within the heterogeneous layer only in the case that the heterogeneities are strong enough to excite multiply scattered waves, as compared with the singly scattered ones. Robinson (1987) studied the temporal variation of coda-duration magnitude relative to event magnitude based on P and S wave amplitudes using the seismic network of the Wellington, New Zealand, region. He found that coda-duration magnitude relative to amplitude magnitude decreases with focal depth and becomes large locally for events in the depth range from 65 and 75 km. Our synthetic seismograms explain his results well, implying that there must be a region of localized, strong heterogeneity at depths around 70 km. The effective size of the heterogeneous region may be of about several kilometers because the observation was performed with 1 Hz seismometers. This localized heterogeneous layer is probably associated with the subducting Pacific plate underneath the Wellington region.
Coda
Seismogram
Wavenumber
Cite
Citations (2)
A method based on the coda attenuation law: Q=Q0(f/f0)v leads to the determination of the lateral variation of coda-Q in the southern part of the Iberian Peninsula using seismograms belonging to the seismological network of the Cartuja Observatory, located in Granada. The lateral variation of Q0 (Q value corresponding to a reference frequency f0 of 1 Hz) and its frequency dependence for the 1 to 5 Hz frequency range are, in general, in agreement with coda-Q values for frequencies less than about 1 Hz, previously determined in the region under study. To determine the coda-Q values analytical functions have been used to fit the magnification curves of the vertical component short-period seismographs belonging to the Cartuja network. The problem is solved by using least-squares techniques and non-linear inversion. The determined coda-Q0 values and its frequency dependence correlate well with several known geophysical parameters in the southern part of the Iberian Peninsula.
Coda
Seismogram
Seismometer
Frequency dependence
Peninsula
Variation (astronomy)
Epicenter
Cite
Citations (26)
Abstract Wave tests, performed in an area near Tulsa, OK, using both vertical and in-line horizontal component seismometers, show some interesting events that may be interpreted as backscattered waves from inhomogeneous regions. Normal wave tests (i.e., where seismometers record the signals generated at a fixed source location) and the corresponding transposed wave tests (i.e., where a seismometer occupies the previous source location and the source points o previous source location and the source points o the previous seismometer locations progressively were performed. As previously reported, the transposed wave test seismogram sections show significant improvement in trace-to-trace coherency over the normal wave test sections. However, in this experiment the transposed horizontal component seismogram section shows some events that apparently are not discernible on the corresponding vertical component seismograms. These are believed to be due to backscattering from randomly inhomogeneous zones where considerable conversion of compressional waves to shear waves takes place. This raises the possibility of locating geological features such as possibility of locating geological features such as unconformities, lenses, fracture zones, etc. Another interesting possibility is that scattering and conversion to shear waves may take place close to the seismometers, resulting in their successful recording on a horizontal component seismometer as opposed to a vertical component seismometer. This again raises the hope that areas that have heretofore been labeled NG may be explored using appropriate seismic techniques. The different possibilities need further research and development that would also be beneficial to static corrections in routine seismic surveys. Introduction Seismogram sections basically are geologic cross-sections displaying the variations of elastic properties of the subsurface with depth. These properties of the subsurface with depth. These seismograms are composited from a large number of single seismic traces after appropriate corrections are made for the source-receiver geometry and the near-surface layer travel times. Corrections for the near-surface have been made traditionally with the basic assumption that the elastic waves that are recorded are compressional or P-waves. However, investigations into this problem in a particularly difficult area, the clinker area of Wyoming, show that this is not so. Failure to make appropriate allowances for this effect results in a distorted structural picture of the subsurface. The presence of a very strong shear wave is demonstrated on horizontal component seismograms. These events may be interpreted in three different ways, each with economic value in exploration. The paper deals with illustrations and interpretation of paper deals with illustrations and interpretation of observations using scattering theory. SCATTERING OF ELASTIC WAVES IN RANDOMLY INHOMOGENEOUS MEDIA Lord Rayleigh pointed out that the scattered wave amplitude at distances large compared with the incident wave length is inversely proportional to the distance from the scatterer to the point of observation, directly proportional to the volume of the scatterer, and inversely proportional to the square of the wave length. Theoretical solutions are possible only when very simplifying assumptions are made and almost always the far-field solution is obtained. Knopoff and several others have computed far-field solutions for specific models and have formulated the general problem of seismic-wave scattering. Scattering in problem of seismic-wave scattering. Scattering in a randomly inhomogeneous medium involves considerable conversion of compressional wave energy to shear wave energy. Experimental observations are in agreement with the simplified models theoretically investigated by other workers. For example, Hudson derived formulas comparing scattered S-wave energy with scattered P-wave energy as a function of the P- and S-wave P-wave energy as a function of the P- and S-wave velocities for the simple surface wave scattering model he was investigating.
Seismogram
Seismometer
Component (thermodynamics)
Shear waves
Synthetic seismogram
Microseism
Cite
Citations (0)
Using an indirect boundary integral method, seismograms are computed for elastic media with localized heterogeneities. Models are two-dimensional homogeneous full spaces (P and SV waves) with many circular cavities as heterogeneities or scatterers. Heterogeneities are localized within a depth range, forming a relatively thin random layer. Seismograms are obtained for receivers at depth 0 km (surface) and for several focal depths, with sources radiating S waves isotropically. Seismograms are composed of the direct S wave and all the possible scattered waves by the heterogeneities, exhibiting late arrivals or coda waves. The coda wave amplitude, or coda energy level, and its duration vary for events with different focal depths. As the focal depth increases and the source gets closer to the layer of localized heterogeneities, the coda level becomes small. When the source is within the heterogeneous layer, however, the coda level becomes larger than for a case of the source either above or below the heterogeneous layer. This local enhancement of coda takes place clearly only in the frequency range for which the scattering is the most effective, that is, when the non-dimensional frequency kd takes values from 2 to 3, where k is the wavenumber and d is the size of each heterogeneity. Such enhancement of coda is not observed when the density of cavities, or strength of the heterogeneities, is reduced. Coda level becomes locally large for a source within the heterogeneous layer only in the case that the heterogeneities are strong enough to excite multiply scattered waves, as compared with the singly scattered ones. Robinson (1987) studied the temporal variation of coda-duration magnitude relative to event magnitude based on P and S wave amplitudes using the seismic network of the Wellington, New Zealand, region. He found that coda-duration magnitude relative to amplitude magnitude decreases with focal depth and becomes large locally for events in the depth range from 65 and 75 km. Our synthetic seismograms explain his results well, implying that there must be a region of localized, strong heterogeneity at depths around 70 km. The effective size of the heterogeneous region may be of about several kilometers because the observation was performed with 1 Hz seismometers. This localized heterogeneous layer is probably associated with the subducting Pacific plate underneath the Wellington region.
Coda
Seismogram
Wavenumber
Cite
Citations (0)