Short-period Seismograms Research Articles

Inversion of teleseismic body waves for the moment tensor of the 1978 Thessaloniki, Greece, earthquake

abstractSeismograms from WWSSN and Canadian network stations were modeled to determine the source parameters of the 20 June 1978 Thessaloniki, Greece, earthquake (Ms = 6.4). The depth of the initial rupture was constrained to 11 ± 1 km by comparison of the arrival times of surface reflections with synthetic short-period seismograms. A focal sphere plot of first motion polarities provided little constraint on other focal parameters, except to indicate that predominantly normal faulting was involved. A generalized inverse technique utilizing the moment tensor formalism was applied to teleseismic P and SH waves for six increments of depth. The moment tensor obtained indicated a nearly horizontal, N-trending tension axis and a nearly vertical compression axis, and yielded the following double-couple source parameters: strike 280° ± 7°; dip 55° ± 3°; rake −65° ± 5°; seismic moment 5.7 × 1025 dyne-cm; and a skewed triangular source time function with a rise time of about 1 sec and duration of 6 to 8 sec. Due to indications of multiple or finite source effects for this event, and the assumption in the moment tensor formalism of a point source, a low-pass filter was applied to the data and the inversions were repeated. The results were nearly identical with those of the original inversion, suggesting that any individual sources had similar mechanisms, or that the point source model is sufficient for this earthquake.

Short-period PKP phases and the anelastic mechanism of the inner core

Short-period seismograms are synthesized for PKP phases in anelastic Earth models. The synthetics were constructed using a synthetic technique valid at grazing incidence, a source-time function appropriate for deep-focus earthquakes, and an instrument response for either a short-period WWSSN or SRO seismograph. The agreement between predicted and observed amplitudes and spectral ratios requires neither a low- Q α zone at 0.2–2 Hz nor a low or negative P-velocity gradient at the bottom of the outer core. Thin low- Q α zones beneath the inner core boundary fit spectral ratio data that sample the upper 200 km of the inner core but fail to fit data that sample the lower inner core. Only a model having Q α −1 ϵ[0.003, 0.004] at 0.2–2 Hz, nearly constant with depth in the inner core, satisfies all of the spectral ratio and amplitude data. The assumption of a bulk viscosity of 10-10 3 Pa s for the liquid phase of a partially molten inner core combined with the observation of low shear attenuation in the inner core at frequencies less than 0.005 Hz limit the physical parameters associated with two possible attenuation mechanisms: (1) fluid flow and viscous relaxation due to ellipsoidally shaped inclusions of melt, and (2) the solid-liquid phase transformation induced by the stress change during the passage of a seismic wave. Both mechanisms require an order of 0.1% partial melt to reproduce the observed Q α −1. In the outer core, the time constant of the mechanism of phase transformation is predicted to be 10 4–10 6 s. Confirmation of small shear attenuation in the inner core in the frequency band of seismic body waves would favor the mechanism of phase transformation.

Crustal structure from a seismic refraction profile across southern British Columbia

A partially reversed seismic refraction profile utilizing mine blasts as sources was recorded across southern British Columbia from Sparwood to the Highland Valley. The westwardly directed profile consisted of 32 short period seismograms covering 440 km, while the reversed line extended 330 km with 41 seismograms. From a starting model based on first arrival times and previous geological and geophysical data, a seismic structural section is developed using both synthetic seismograms and a program for ray tracing through inhomogeneous media.The refraction data indicate that the M-discontinuity dips to the east from an approximate depth of 30 km east of the Highland Valley to in excess of 40 km beneath the Purcell Anticlinorium. Undulations of about 165 km wavelength and several kilometres amplitude characterize the crust–mantle boundary. The Pn velocity is 7.8 km/s. Above the M-discontinuity, secondary arrivals are interpreted to be from a lower crustal layer of thickness near 12 km and velocity 6.9 km/s. The upper boundary of this layer also dips gently to the east.The seismic structure of the upper crust correlates closely with the regional geology as evidenced by traveltime and amplitude anomalies where the profile crosses the Rocky Mountain Trench and the Interior Plateau – Eastern Metamorphic Belt boundaries. The crustal P and S phases in the Interior Plateau yield a relatively low value of Poisson's ratio of 0.23. The detailed data close to the Highland Valley indicate significant velocity heterogeneity. For the Guichon Creek batholith, the inner Bethlehem phase is found to have a higher velocity than the surrounding Highland Valley phase.

Magnitude scale structure from filtered broadbandP-wave records

A series of Hokkaido events, recorded by the FBV Broadband Seismograph System at the KHC Seismic Station, is used to study the structure of the earthquake magnitude scale on the basis of maximum velocity amplitudesA vmax of teleseismicPwaves in different period bands. Amplitude-periodband (APB) diagrams are constructed for each event. According to the shape of the APB diagrams the events investigated can be divided into three types: (a) events with largestA vmax values in the intermediate period range (periods ofA vmax from 2.2 to 23 sec), (b) events with largestA vmax values in the short-period range (periods ofA vmax from 1 to 2 sec), (c) events exhibiting anomalous APB diagrams. Type (a) events seem to represent the process of wave generation that prevails for shallow earthquakes. Type (b) events approach to explosive-like generation of seismic waves. The nature of the exceptionally occurring type (c) events must be clarified in further investigations. The influence of the type of earthquake on the magnitude values estimated on the basis of standard class A and B (short-period and intermediate-band) seismograms is demonstrated. It appears that for estimating correct values of earthquake magnitudes complementary information on the process of seismic wave generation in the focus is necessary. At teleseismic distances this information can be obtained from either APB diagrams or amplitude spectra ofP waves recorded, e.g., by broadband velocity sensing instruments.

Some Geophysical Applications of Autoregressive Spectral Estimates

This paper discusses the practical application of autoregressive spectral analysis to three different geophysical data sets. In all cases the amount of available data was limited so that autoregressive methods might give more detailed spectra than those obtainable by the classical windowed spectral estimate methods. For each series, the Burg technique, which guarantees positive-definite autocorrelation functions, is used to determine the prediction error coefficients. The degree of spectral instability known to result from the use of Burg's algorithm is not crucial to our results. Algorithms due to Akaike and Parzen are applied to the time series to aid in order-number determination. For the first example, autoregressive spectral analysis of a complex time series is applied to the log spectrum of short-period seismograms. The resultant spectrum, the so-called complex cepstrum, allows one to determine the depth below the earth's surface at which the seismic event originated. The other examples concern the analysis of real time series due to two somewhat unusual data sets. One of these is the analysis of the time rate at which earthquakes occur. The purpose is to determine if periodicities exist that correspond to known astronomical and terrestrial rotational periods. The other is a study of biological and chemical parameters measured in core samples of oceanbottom sediments where displacement down the core is calibrated in geological time. The measurements directly infer the amount of ice on the earth's surface.

Upper mantle P-wave model catalogue

This note is a description of a catalogue that contains tables of parameters and synthetic seismograms for 50 upper-mantle P-wave velocity–depth profiles. The table for each model contains values of ray parameter, epicentral distance, travel time, velocity, and bottoming depth for a number of representative rays. Short-period synthetic seismograms are computed at 1 °intervals from 10° to 30° by the quantized ray theory algorithm and are vertical component traces for a surface focus point source. The catalogue is designed as a comprehensive reference and so includes a wide variety of mantle models.

Application of synthetic seismograms to the study of low-magnitude earthquakes and crustal structure in the northern Red Sea region

abstract Previous theoretical results are used to produce synthesized seismograms for realistic crustal models and earthquake dislocation sources. Short-period vertical displacement seismograms at Δ = 240 km are calculated. They show strong resemblance to observed recordings, at least for the first 55 sec after the P n arrival. These results are then used for the study of low-magnitude earthquakes from a known source and the tectonics of the source region south of the Sinai peninsula. The source-mechanism found by Ben-Menahem and Aboodi (1969) and the crustal structures given by Niazi (1968) and Drake and Girdler (1964) are consistent with the time series of many short-period seismograms recorded at Eilat since 1969.

Determination of source parameters of small earthquakes from P-wave rise time

abstract A simple method of estimating source dimensions and stress drops of small earthquakes is presented. The basic measurement is the time from the first break to the first zero crossing on short-period seismograms. Graphs relating these measurements to rise time as a function of Q and instrument response permit an estimate of earthquake source parameters without the calculation of spectra. Tests on data from Rangely, Colorado, and Hollister, California, indicate that the method gives reasonable results.

Earthquake mbvs MsRelations and Source Multiplicity

A relation Ms = 1·07mb − 0·23 has been obtained from about 900 earthquake magnitude values reported by CGS during May 1968 to January 1971. Hyderabad observatory seismograms were examined and it was found that all earthquakes lying above the line Ms = 1·07mb + 0·27 are complex multiple events and the ones lying below the line Ms = 1·07 mb − 0·73 are simple events. For complex earthquakes clear later phases are observed on short-period seismograms following the initial P-phase. On the long-period seismograms the initiation is some times later than on short-period seismograms, often characterized by slow build up of energy and is followed by later unaccountable larger phases within a few tens of seconds. Simple earthquakes have considerably small energies in surface waves and long-period body waves. Variation of source time function associated with the increase of magnitude only does not seem to explain the exceptionally high Ms found for some earthquakes and multiplicity of the source is suggested to be the cause of this. The difference in the observed Ms values and the ones obtained corresponding to the observed mb values could be used as a measure of source complexity.

Source process of a large deep-focus earthquake and its tectonic implications — the Western Brazil earthquake of 1963

A large deep-focus earthquake ( h = 577 km) in the western Brazil region that occurred on November 9, 1963, was followed by a relatively large aftershock. The long-period P wave records of the WWSSN for the main shock reveal three successive P phases. Regarding this earthquake as a triplet, the temporal and spatial relations among the three successive events are examined on the basis of the azimuthal distribution of the difference between the arrival times of the first and the second P phases and that of the first and the third P phases. The separation between the first shock and the aftershock in space and time is determined from the time differences of the first P arrivals which are read on the short-period seismograms of the WWSSN. The second and the third foci and the focus of the aftershock are found to be all situated on one of the P nodal planes of the first shock. This result almost definitely suggests that the earthquake occurred in the form of shear faulting. The primary P wave forms are obtained from the long-period P wave signals after eliminating the influence of the seismograph, crust and mantle. The seismic moment is accurately determined as 2.2 × 10 27 dyne · cm from their time integrations. The fault process is inferred from the spatial and temporal relations among the four shocks and from the primary P wave forms: the rupture which initiated at the first focus spread out with a uniform velocity and covered a more or less fan-shaped surface. The velocity of rupture propagation, faulted area, displacement discontinuity and the stress drop are estimated to be 2.2–2.5 km/s, 510–680 km 2, 265–355 cm and 300–460 b. Some implications are discussed in the light of the plate tectonics. The predominant gap in seismic activity beneath northern Peru, western Brazil has been interpreted in terms of the detachment of the descending lithosphere. The presence of a large-scale normal faulting which extends over the entire thickness of the detached slab of lithosphere is suggested from the present results.

A velocity structure of the Earth's core

abstract Travel times and amplitudes of PKP, P2KP and higher multiple K phases are determined from a worldwide distribution of short-period seismograms. The sources are one explosion in Novaya-Zemlya and seven earthquakes, consisting of one intermediate focus event in the New Hebrides, and deep-focus events in Fiji, Java, Kermadec Islands, and Peru. The data are used to determine a new velocity model of the lowest mantle and the core. In the new velocity model 132, the velocity of the bottom of the mantle is 13.44 km/sec; the core mantle boundary is placed at 2892 ± 2 km. The velocity model of the core produces the PKP caustic B1 at 143° and the P2KP caustic B2 at −125°. A velocity discontinuity of 0.01 km/sec at a depth of 4550 km represents the top of the transition zone to account for the earliest forerunners of PKP. To account for the later forerunners a second discontinuity of 0.02 km/sec is placed at a depth of 4850 km. Since the forerunner data could not be resolved into branches, neither discontinuity is well defined. The top of the inner core boundary is placed at a depth of 5145 km with an uncertainty of at least 10 km and represents a discontinuity of 0.576 km/sec. Older core models have transition zone discontinuities an order of magnitude larger than those of model 132 with a discontinuity at the inner core boundary of about 1 km/sec. The smaller velocity discontinuities are a result of interpreting the amplitudes and travel times of PKP so that the turning points D and G are located at 120° and 140°, respectively, rather than at 110° and 125° as in previous interpretations. Amplitude ratios of PKP phases yield an inner core Q of about 400 and amplitude ratios of P3KP, P4KP and P5KP result in an outer core Q of about 4000.

Integrated and frequency-band magnitude, two alternative measures of the size of an earthquake

Abstract Integrated magnitude substitutes the r.m.s. average amplitude over a pre-selected interval for the peak amplitude in the conventional body-wave magnitude formula. Frequency-band magnitude uses an equivalent quantity in the frequency domain. Integrated magnitude exhibits less scatter than conventional body-wave magnitude for short-period seismograms. Frequency-band magnitude exhibits less scatter than body-wave magnitude or integrated magnitude for both long- and short-period seismograms. The scatter of frequency-band magnitude is probably due to real azimuthal effects, crustal-transfer-function variations, errors in compensation for seismograph response, microseismic moise and uncertainties in the compensation for attenuation with distance. To observe azimuthal variations clearly, the crustal-transfer functions and seismograph response need to be known more precisely than was the case in this experiment, because these two sources of scatter can be large enough to explain all of the observed variations.

A wide range seismogram digitizer

abstract A visual seismogram drum-digitizer has been developed with precision adequate for the seismograms generated in standard networks. The variable speed, sampling interval and optical magnification permit convenient and efficient digital conversion of a wide range of record traces for different seismological research purposes. An analysis of the precision has been made: over a dynamic range of 150 mm errors are less than 50 μ, and time intervals accurate to ±.05 sec for short-period seismograms or ±0.2 sec for long-period records.

Note on use of a Sofar geophone to determine seismicity of regional oceanic areas

abstract Daily Bermuda short period seismograms are compared with Sofar geophone records for a period of three years. Many short-period seismic signals were recorded on the geophone record that are not on the seismograms. Therefore, it is suggested that a geophone array could be most useful in determining regional seismicity in oceanic areas. The large number of geophone signals recorded gives some indication that the advantages of the expected quiet of the ocean bottom may be somewhat offset.

The T phase of shallow-focus earthquakes*

Abstract A short-period phase, 0.5 sec. period or less, traveling through the ocean with the velocity of sound in water has been identified on a large number of seismograms of earthquakes occurring at sea. This arrival was identified on the Benioff short-period seismograms of the Weston, Fordham, and Ottawa stations for a series of Dominican Republic shocks, and on the Benioff short-period seismograms of the Pasadena network of stations and the Mount Hamilton and Mineral stations for a series of major Japanese and Aleutian shocks. The existence of this phase had been noted previously by Linehan, who was at a loss to provide an explanation for them. The mechanism of propagation of the T phase is discussed and several practical applications are described.