Short-period Body Waves Research Articles

Faulting process of the May 17, 1976 Gazli, USSR earthquake

The May 17, 1976 Gazli earthquake (m_b = 6.2, M_s = 7.0) has both seismological and engineering importance. Waveform modeling of long-period IDA Rayleigh waves, and WWSSN long- and short-period body waves indicates a thrust mechanism with strike N40°E, dip 54°SE, and rake of 78°. The moment is estimated to be 1.6 × 10^(26) dyne-cm; the fault area, 150 km^2; the average dislocation, 3.3 meters; and the average stress drop, 200 bars. Strong directivity effects due to a propagating rupture are present in both the long- and short-period teleseismic P waves. The data are not adequately explained by a simple point source, and require a three-dimensional, finite-source model. The relative amplitudes of P and pP at both long and short periods are used to deduce that the rupture started at a depth of 15 km and propagated almost entirely unilaterally upward, with an average rupture velocity of 2.0 to 2.5 km/sec. Simultaneous deconvolution of long- and short-period P waves reveals a complex, azimuthally dependent, source-time function. Finite-source models further suggest that a major part of the faulting was concentrated at depth, below 7 km. The strong-ground motion within 10 km of the fault is characterized by unusually high-amplitude (1300 cm/sec^2), high-frequency (10 Hz) accelerations and low-amplitude (10 cm), low-frequency (0.8 Hz) displacements. Synthetic Wood-Anderson records computed from the accelerograms give a local magnitude, M_L, of 6.35. The duration and character of the strong-ground motion is consistent with a rupture propagating upward in the general direction of the station.

New calibrating functions for short-period body waves of Friuli earthquakes

Оnре¶rt;елены кaлuбрaцuонные функцuu ¶rt;ля короmкоnерuо¶rt;ных общемных волн Рn, n, Рg, Sg, коmорые рaсnросmрaняюmся в северном нanрaвленuu ¶rt;о эnuценmрaлщных рaссmоянuŭ с2.6° ¶rt;о 4.7° оm очa a в облaсmu Фрuулu (Иmaлuя). Покaэaно, чmо mолько зaвuсuмосmь σSg(D) осmamочно усmоŭчuвa ¶rt;ля бысmро о оnре¶rt;еленuя мa нumу¶rt; больщuнсmеa слaбых блuзкuх емлеmрясенuŭ.

A slow earthquake

Anomalous earthquakes such as creep events, tsunami earthquakes and silent earthquakes have been reported in the recent literature. In this paper we discuss an anomalous “slow earthquake” that occurred on June 6, 1960 in southern Chile. Although the surface-wave magnitude of this event is only 6.9, it excited anomalously large long-period multiple surface waves with a seismic moment of 5.6 · 10 27 dyn cm. The Benioff long-period seismogram of this earthquake recorded at Pasadena shows an extremely long, about 1.5–2 h coda of Rayleigh waves, with a period of 10–25 s. The coda length for other events with a comparable magnitude which occurred in the same region is about 10 min. This observation suggests that the long coda length is due to a long source rupture process which lasted at least 1 h. Although at least 15 distinct events can be identified in the coda, no short-period body waves were recorded corresponding to these, except for the first one. These results suggest that a relatively small ( M s ≅ 6.9) earthquake triggered a series of slow events; the duration of the whole sequence being longer than 1 h. This event probably occurred on a transform fault on the extension of the Chile Rise and provides important information regarding the nature of the transform fault.

Magnitudes and travel-time residuals of the Friuli earthquakes between May and August 1976

The records of the first series of the North-Italian aftershock series from the period May–August 1976 at the stations Průhonice and Kasperske Hory were analyzed from the point of view of the verification of the travel-time tables used for the interpretation of near earthquake seismograms and of the determination of the short-period body wave magnitudes for near shocks. Also the time-energy distribution was investigated for a 90 days period.

T* for S waves with a continental ray path

abstract The purpose of this study was to determine t* for S waves with ray paths under the continental United States. The data set consists of long- and short-period body waves from the Borrego Mountain earthquake as observed in the northeastern U.S. The P wave forms are dominated by the sP phase and the SH wave forms by the sS. It is assumed that there are no losses in pure compression so that the relative attenuation rate of P and S waves is known. The initial source radiation is determined from the sP phase and the value of tβ* from the spectral content of the S wave. The results indicate that tβ* is 5.2 ± 0.7 sec along this ray path. Long- and short-period body waves from some deep South American events are used to test for lateral asymmetry of the Q distribution under the U.S. No lateral amplitude variation exists in this data, but this result is difficult to correlate with many previous results. The t°* value for a 600-km deep earthquake appears to be about 3 sec. A comparison of these values with values computed from current models of the Earth's Q distribution indicates that the models are slightly too high in Q overall and that more of the total body-wave attenuation occurs above 600 km than is indicated by the models.

Transient and impulse responses of a one-dimensional linearly attenuating medium -- II. A parametric study

We investigate one-dimensional waves in a standard linear solid for geophysically relevant ranges of the parameters. The critical parameters are shown to be T*= t_u/Q_m where t_u is the travel time and Q_m the quality factor in the absorption band, and τ^(-1)_m, the high-frequency cut-off of the relaxation spectrum. The visual onset time, rise time, peak time, and peak amplitude are studied as functions of T* and τ_m. For very small τ_m, this model is shown to be very similar to previously proposed attenuation models. As τ_m grows past a critical value which depends on T*, the character of the attenuated pulse changes. Seismological implications of this model may be inferred by comparing body wave travel times with a ‘one second’ earth model derived from long-period observations and corrected for attenuation effects assuming a frequency independent Q over the seismic band. From such a comparison we speculate that there may be a gap in the relaxation spectrum of the Earth's mantle for relaxation times shorter than about one second. However, observational constraints from the attenuation of body waves suggest that such a gap might in fact occur at higher frequencies. Such a hypothesis would imply a frequency dependence of Q in the Earth's mantle for short-period body waves.

Single isotropic scattering model including wave conversions simple theoretical model of the short period body wave propagation.

The elastic energy propagation in a three dimensional elastic medium, in which scatterers are distributed homogeneously and randomly, is studied from the point of view of the scattering theory. The isotropic scattering is assumed and the scattering process is considered up to the first order. It is assumed that not only the P-wave to P-wave scattering and the S-wave to S-wave scattering but also the S-wave to P-wave conversion scattering and the P-wave to S-wave conversion scattering take place. Green functions corresponding to these single isotropic scatterings are obtained and their properties are exhibited when the elastic energy is radiated spherically from a source. A space-time distribution of the energy density of the single scattered elastic waves is obtained. This model is applied to the short period body wave propagation of a small local earthquake near a hypocenter in the earth, when heterogeneities are interpreted as the scatterers. The temporal development of the seismic energy density, which is closely related to the seismogram, may be explained qualitatively.

Seismic-wave transmission across the Caribbean plate: High attenuation on concave side of Lesser Antilles Island Arc

High attenuation of short-period body waves and extremely low surface-wave group velocities have been found for seismic paths that traverse the crust and upper mantle beneath the concave side of the Lesser Antilles island arc (eastern Caribbean). The observations can be explained in terms of the currently accepted models of lithospheric plate subduction at other island arcs such as Fiji-Tonga, Marianas and the Aegean, a characteristic of which is the existence of an abnormally low Q zone in the crust and upper mantle above the down-going slab. The Greater Antilles island arc appears to be tectonically distinct, as subduction is not evident there and no anomalous low-Q zone exists south of Puerto Rico and Hispaniola. The Aves swell is probably not related tectonically to the low-Q zone, or to the subduction process, at least presently.

Upper mantle structure in western Canada

Several upper mantle velocity distributions in western Canada are determined from short-period (1–10 Hz) body wave (P) observations. With the use of quantized ray theory, synthetic seismograms were constructed for laterally homogeneous earth models to satisfy the absolute travel time of first arrivals and the differential travel times and relative amplitudes of later arrivals. On the basis of travel time and amplitude observations, three models were obtained that correspond to three different regions. All of the three models show the presence of a low-velocity zone and discontinuities at 410 and 650 km. Of the three models, models WCA and WCB show the presence of a discontinuity in slope at about 504 km, but this is absent in the model WCC. Models WCA and WCB differ in their thickness of the lithosphere, the absolute velocity below the base of low-velocity zone, and the gradient of the velocity between the base of the low-velocity zone and 300 km of depth. Beyond 300 km, models WCA and WCB have the same velocity distribution. An extremal inversion determination of the depth limits to the discontinuities at 410 and 650 km indicates a maximum uncertainty of about ±30 km in depth to the discontinuities. An analysis of reduced delay times for the reflection from the 410-km discontinuity indicates a local down-buckling of the 410-km discontinuity by 30 km under the Cordillera.

Mantle Structure and Short Period Body Wave Spectra at Intermediate Epicentral Distances

Summary The existence of numerous primary upper mantle travel-time branches contributes significantly to the character of short period body wave spectra observed at intermediate (10°–35°) epicentral distances. The effects can be predicted through the computation of synthetic mantle response functions by the Cagniard-de Hoop algorithm. This method allows definition of the changes in the mantle response as a function of epicentral distance.

Geophysical and tectonic features of the Aegean Arc

Several geophysical and tectonic features of the Aegean Island arc are described. An attempt has been made to interpret these features by means of a model of the area that includes a lithospheric plate that underthrusts the arc and moves from the Mediterranean to Europe. The upper boundary of this plate is determined by a zone of intermediate earthquakes. These earthquakes are located close to a surface of amphitheatrical shape that is dipping under the arc at a mean angle of about 30°. There is an aseismic zone in the upper mantle under the inner part of the arc. The absorption of short-period body waves in this zone explains the observed abnormal distribution of the seismic energy in the area. The results of this work are in general agreement with the ‘new global tectonics’.

Crustal transfer ratios of short- and long-period body waves recorded at Yellowknife

A feasibility study into the spectral ratio technique of deducing crustal structure at Yellowknife indicates that (a) through the use of long-period teleseismic body waves, the crustal thickness can be determined to within 4 km and the velocity to within 0.6 km/sec of the values obtained by a reversed refraction profile and (b) through the use of short-period body waves, a thin (a few km thick) layer near the surface with a velocity contrast of about 10 per cent with that of the underlying layer can be detected. Theoretical considerations indicate that long-period body waves are not modified, to any significant extent, by absorption in the crust (representative Q values 75-200) but that short-period body waves are. However, estimates of a mean Q for the crust at Yellowknife through the use of short-period body waves are inconclusive, due mainly to inconclusive data—that is, the spectral ratio curves for the different events shown do not agree. The advantages of incorporating multiple phases, such as pP and PcP , into the spectral ratio calculations for long-period body phases are more than offset by the increased intractability in deducing gross crustal features. In the present study, the theoretical composite spectral ratios are restricted to the case where equal amplitudes are assumed for P , pP and PcP , whereas, in the experimental seismograms selected, pP and PcP have smaller amplitudes.

Seismic Study of an Oceanic Ridge Earthquake Swarm in the Gulf of California

Detailed seismic investigation of an unusually intense earthquake swarm which occurred in the northern Gulf of California during March 1969 has provided new information about seismic processes which occur on actively spreading oceanic ridges and has placed some constraints on the elastic wave velocities beneath them. Activity during this swarm was similar to that of a foreshock-mainshock-aftershock sequence, but with a ‘mainshock’ composed of over 70 events with magnitudes between 4 and 5.5 occurring in a 6-hr period about a day after swarm activity was initiated. ‘Aftershocks’, including many events greater than magnitude 5, continued for over two weeks. Near-source travel-time data indicate all sources located are within 5–10 km of each other and that hypocentres are confined to the upper crust. Teleseismic P-delays for rays travelling beneath this ridge may be interpreted in terms of an upper mantle with compressional velocities 5–10 per cent less than normal mantle to a depth of 200 km. Average apparent stresses for all swarm events studied are very similar, show no consistent pattern as a function of time, and are close to values obtained from other ridges. The focal mechanism solution shows a large component of normal faulting. An apparent non-orthogonality of nodal planes common to this mechanism solution and to normal faulting events on other ridges disappears when the indicated low upper mantle velocities beneath the source are taken into account. A survey of recent seismicity (post 1962) in the northern Gulf suggests seismic coupling across about 200 km between adjacent inferred spreading ridge segments. Surface waves from these Gulf Swarm earthquakes have amplitudes from one to two orders of magnitude greater than Northern Baja California events with similar short period body wave excitation.

Great Circle Rayleigh and Love Wave Dispersion from 100 to 900 Seconds

Rayleigh and Love wave phase velocities for great circle paths from ordinary seismograms of World Wide Standard Seismographic Network /WWSSN/ stations

Travel times and amplitudes of principal body phases recorded from gnome

abstract gnome, an undergound nuclear explosion in salt near Carlsbad, New Mexico, produced seismic waves which were recorded widely throughout the United States and at a few foreign stations. The travel times of P were strongly dependent on the path of propagation, and were as much as 12 seconds earlier in the eastern United States than at equivalent distances in the western part of the United States. At the few stations more distant than 25°, P was about 2 seconds earlier than predicted by the Jeffreys-Bullen table for surface focus. Amplitudes of Pn were similarly dependent upon the path of propagation; although the measurements showed great scatter, amplitudes to the east were generally larger than those to the west. Pn travel time and amplitude anomalies suggest a systematic relationship to crustal thickness. There is evidence from the difference in the speeds and attenuation rates that Lg and P are not transmitted along analogous paths through the crust. Short period body waves were two or three times larger than expected from an explosion of the same energy in tuff at the Nevada Test Site. Surface waves, however, were relatively weak compared with explosions of similar yield in tuff.