The complex, static displacement of a very long period seismic signal observed at Soufrière Hills volcano, Montserrat, WI
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Seismometer
Seismogram
Synthetic seismogram
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.
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Promontory
Classification of discontinuities
Seismic energy
Epicenter
Microseism
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Abstract The instrumental group delay dθ/dω is considered here. First, these delays were calculated for three different recording systems that were used in a precise travel-time monitoring experiment where the delays varied between 10 and 40 msec for the high frequencies of the seismograms involved. A technique is demonstrated by which these delays may be readily accounted for and by which instrumental malfunctions can be readily detected. Second, two of these systems are also currently used for the recording of short-period teleseisms; at the 1-sec period, the group delays are from 0.3 to 0.4 sec, which is significant and must be accounted for. This is particularly important when these systems are used in connection with data from other systems that have different delays, such as the World-Wide Seismograph Station Network and Canadian Seismograph Network stations. Neglecting these delays will create serious problems in seismological tomography and earthquake catalogs. Third, for long-period phases recorded by the SRO-type instruments, the delays for the 10- to 20-sec periods are 6 to 12 sec; again, these are significant and must be accounted for.
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Abstract If energy emitted by a seismic source such as an earthquake is recorded on a suitable backbone array of seismometers, source‐receiver interferometry (SRI) is a method that allows those recordings to be projected to the location of another target seismometer, providing an estimate of the seismogram that would have been recorded at that location. Since the other seismometer may not have been deployed at the time at which the source occurred, this renders possible the concept of “retrospective seismology” whereby the installation of a sensor at one period of time allows the construction of virtual seismograms as though that sensor had been active before or after its period of installation. Here we construct such virtual seismograms on target sensors in both industrial seismic and earthquake seismology settings, using both active seismic sources and ambient seismic noise to construct SRI propagators , and on length scales ranging over 5 orders of magnitude from ∼40 m to ∼2500 km. In each case we compare seismograms constructed at target sensors by SRI to those actually recorded on the same sensors. We show that spatial integrations required by interferometric theory can be calculated over irregular receiver arrays by embedding these arrays within 2‐D spatial Voronoi cells, thus improving spatial interpolation and interferometric results. The results of SRI are significantly improved by restricting the backbone receiver array to include approximately those receivers that provide a stationary‐phase contribution to the interferometric integrals. Finally, we apply both correlation‐correlation and correlation‐convolution SRI and show that the latter constructs fewer nonphysical arrivals.
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Synthetic seismogram
Seismic Noise
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Interpolation
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The Geiyo earthquake occurred on June 2 in 1905 in the western Seto Inland Sea between the Honshu and Shikoku Islands, Japan. The seismograms of the earthquake obtained at the stations of Central Meteorological Observatory were newly found at Earthquake Research Institute of the University of Tokyo. They are recorded by the Omori seismometers and tromometers, which are superior to former seismometers with respect to continuous recording. For the estimation of the magnitude and source mechanism from the seismograms, we digitize 3 records, which are Hongo EW component, Hitotsubashi EW component and Tokyo tromometer. We have to know the response of the Omori seismometer to estimate the ground motion during the earthquake. In order to know the frequency characteristics of the seismometers, we calculate their Fourier amplitude spectra. The spectra of the Hongo EW component and Hitotsubashi EW component show clear peaks which may be considered as the natural periods of the seismometers. The natural periods of Hongo EW component and Hitotsubashi EW component are about 60s and 25s, respectively. The damping constant estimated from the free oscillation record of Omori seismometer at Ishinomaki observatory is less than 0.01, and the friction is 1.7mm.
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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.
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Abstract The Earthquake Research Institute (ERI) of the University of Tokyo maintains archives of analog seismograms and mareograms. The main collection is ∼236,000 Japanese historical seismograms recorded at the University of Tokyo (at various buildings and using various instruments around Hongo [Tokyo] with a total of 189,000 records from 1881 to 1993), at the Tsukuba observatory (∼11,000 records from 1921 to 1986), and at the Wakayama seismological network (∼12,650 records from 1928 to 1968). Seismograms recorded by temporal stations at various locations in Japan for several years, typically following large earthquakes, are also included. Different types of instruments were used to record the data. The oldest record from a large earthquake is from the 1891 Nobi earthquake recorded at Hongo on a circular seismogram using an Ewing-type seismograph. Teleseismic seismograms include those from the 1899 Alaska earthquake at Hongo on an Omori-type seismograph. Imamura-type and Omori-type tremometers and strong-motion seismographs had also been used for a long time. While these seismograms were microfilmed by the 1990s, the original smoked paper records have also been archived. Foreign seismogram collections include those from earthquakes in Taiwan between 1904 and 1917 recorded in both Japan and Taiwan and those from the Canadian Seismograph Network between 1981 and 1989. For the Worldwide Standardized Seismograph Network stations, almost all (∼5,000,000) microfilm records at 167 stations from 1963 to 1988 are archived. High-resolution image scanning of analog daily seismograms at the Wakayama microearthquake network is currently being performed, and the scans are provided using Leaflet software so that the users can easily access and enlarge parts of seismograms. The tsunami waveform archive contains ∼3100 records on Japanese tide gauges from large earthquakes between 1911 and 1996. The available data, with dates and types of instruments, can be searched from the database through the website of the ERI.
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