Long-period Amplitude Research Articles

Oscillations of global sea-level elevation during the Paleogene correspond to 1.2-Myr amplitude modulation of orbital obliquity cycles

The mechanisms involved in the formation of 1- to 2-Myr-long third-order stratigraphic sequences through the Phanerozoic have been extensively debated, but the main underlying forcings remain uncertain. In this study, we investigated third-order sequences of Paleocene-Eocene age in the East China Sea Shelf Basin, identifying a prominent ∼1.2-Myr periodicity in the 405-kyr-tuned gamma ray profile of the WP-1 borehole. We infer that this ∼1.2-Myr cycle, which corresponds to long-period amplitude modulation of obliquity, was the main driver of third-order sea-level changes and sequence development. This modulation entails variation in the range of obliquity amplitude between a maximum of 2.4° (i.e. 24.5°–22.1°) and a minimum of 0.3° (i.e. 23.3°–23.0°) as a result of interactions among the three primary parameters of Earth's orbit (i.e., eccentricity, obliquity, and precession). We anchored our floating astronomical time scale (ATS) to the T42 seismic horizon, a major sequence boundary between the Lin Feng and Mingyuefeng formations that corresponds to the top of Nannofossil Zone NP8 (reported age of 57.7 Ma). This yields a fixed ATS for the late Paleocene-to-middle Eocene interval spanning 60.84 to 48.71 Ma. Finally, we evaluated our findings relative to the third-order sequence stratigraphy of the East China Sea Shelf Basin and to records of early Cenozoic eustasy and global events.

Astrochronology of the Early Turonian–Early Campanian terrestrial succession in the Songliao Basin, northeastern China and its implication for long-period behavior of the Solar System

The first complete Early Turonian–Early Campanian lacustrine succession has been recovered from the SK-I south (SK-Is) borehole in the Songliao Basin (SLB), northeastern China. We conducted a detailed cyclostratigraphic study of natural gamma-ray (GR) log, thorium (Th) log, and magnetic susceptibility (MS) data from this core. Spectral analysis of the upper Quantou Formation (K2q3+4), Qingshankou Formation (K2qn), Yaojia Formation (K2y), and lower Nenjiang Formation (K2n1+2) reveals a hierarchy of meter- to decameter-scale cycling in the data. The wavelength ratios of the cycles in these stratigraphic units are ~20:5:2:1, corresponding with those of Milankovitch cycle periods of 405kyr (long eccentricity):100kyr (short eccentricity):37kyr (obliquity):20kyr (precession), indicating astronomical control on sedimentation. An astronomical time scale (ATS) was established by tuning interpreted 405kyr cycles to a 405kyr orbital eccentricity target curve, and to four SIMS U–Pb zircon radioisotope ages. This ‘absolute’ ATS provides precise numerical ages for stratigraphic boundaries, biozones, geological and geophysical events, and serves as a basis for correlation of strata and events between marine and terrestrial systems. The ages of the C33r/C34n geomagnetic polarity boundary in K2n2 and three short reversal events in K2y are estimated as 83.633Ma, 84.819–84.862Ma, 84.982–85.092Ma and 85.240–85.629Ma, respectively. Long-period amplitude modulations in the obliquity and eccentricity bands of the 405-kyr-tuned GR-Th series provide strong evidence that long-period orbital forcing influenced climate change and depositional processes in the SLB. The extracted amplitude modulations provide evidence that the orbits of Earth and Mars were not in secular resonance, and were undergoing chaotic interactions during this time, although the modulations do not match those of recent astronomical models.

Evaluation of models for earthquake source spectra in eastern North America

AbstractThere have been several relations proposed in the last few years to describe the amplitudes of ground motion in eastern North America (ENA). These relations differ significantly in their assumptions concerning the amplitude and shape of the spectrum of energy radiated from the earthquake source. In this article, we compare ground motions predicted for these source models against the sparse ENA ground-motion database. The source models evaluated include the two-corner models of Boatwright and Choy (1992), Atkinson (1993a), Haddon (1996), and Joyner (1997a,b), and the one-corner model of Brune [as independently implemented by Frankel et al. (1996) and by Toro et al. (1997)]. The database includes data from ENA mainshocks of M > 4 and historical ENA earthquakes of M > 5.5, for a total of 110 records from 11 events of 4 ≦ M ≦ 7.3, all recorded on rock. We also include 24 available rock records from 4 large earthquakes in other intraplate regions; conclusions are checked to determine whether they are sensitive to the addition of these non-ENA data.The Atkinson source model, as implemented in the ground-motion relations of Atkinson and Boore (1995), is the only model that provides unbiased ground-motion predictions over the entire period band of interest, from 0.1 to 10 sec. The source models of Frankel et al. (1996), Toro et al. (1997), and Joyner (1997a,b) all provide unbiased ground-motion estimates in the period range from 0.1 to 0.5 sec but overestimate motions at periods of 1 to 10 sec. The Haddon (1996) source model overpredicts motions at all periods, by factors of 2 to 10. These conclusions do not change significantly if data from non-ENA intraplate regions are excluded, although the tendency of all models toward overprediction of long-period amplitudes becomes more pronounced.The tendency of most proposed ENA source models to overestimate long-period motions is further confirmed by an evaluation of the relationship between Ms, a measure of the spectrum at 20-sec period, and moment magnitude. A worldwide catalog of shallow continental earthquakes (Triep and Sykes, 1996) is compared to the Ms-M relations implied by each of the source models. The Atkinson source model is consistent with these data, while other proposed ENA models overpredict the average Ms for a given M.The implications of MMI data from historical earthquakes are also addressed, by exploiting the correlation between felt area and high-frequency source spectral level. High-frequency spectral amplitudes, as specified by the Atkinson and Boore (1995), Frankel et al. (1996), Toro et al. (1997), and Joyner (1997a,b) source models, equal or exceed the levels inferred from the felt areas of most of the large ENA events, with the noteable exception of the Saguenay earthquake. By contrast, high-frequency spectral amplitudes specified by the Haddon (1996) source model agree with the felt area of the Saguenay earthquake but overpredict the felt areas of nearly all other large events. In general, models that fit the Saugenay data—be it intensity data, strong-ground-motion data, regional seismographic data, or telescismic data—will not fit the data from the remaining earthquakes.A source model derived from the California database, suitably modified for regional differences in crustal properties, is also evaluated. This model is not significantly different from the Atkinson model for ENA. There is an important practical application of this similarity, which we develop as an engineering tool: Empirical ground-motion relations for California may be modified to predict ENA ground motions from future large earthquakes.

Regional moments, energy levels, and a new discriminant

Teleseismic observations of explosions tend to be richer in short-period energy than are earthquakes, thus the effectiveness of themb∶Ms discriminant. At regional distances the same basic separation occurs for smaller events in terms ofML∶M0 (Woodset al., 1993) andmb∶M0 (Patton andWalter, 1993). While these studies demonstrate the basic differences in excitation, they suffer in practical application because of the detailed information required in the retrieval ofM0. In this paper, we introduce a new method of discrimination, based on the energy strength (ME) from broadband regional records that appears to be effective and efficient. In this method all events are processed as earthquakes, and explosions are distinguished by their stronger energy levels relative to their long-period amplitudes. Results from 29 events recorded by TERRAscope, sampling 15 explosions from NTS and 14 earthquakes from the southwestern United States, are represented, indicating complete separation (45 data points).ML=3.6 is the smallest event examined to date but the method can probably be extended to even smaller levels in calibrated regions.

A lower mantle S-wave triplication and the shear velocity structure of D''

Summary. A lower mantle S-wave triplication detected with short- and longperiod WWSSN and CSN recordings indicates a substantial shear velocity discontinuity near 280 km above the core-mantle boundary. The triplication can be observed in rotated SH seismograms from intermediate and deep focus events throughout the distance range from 70 to 95. Three distinct source region-receiver array combinations that have been investigated in detail demonstrate consistent travel time and relative amplitude behaviour of the triplication, with slight systematic shifts in the triplication indicating up to 40km variations in the depth of the discontinuity. Modelling of the observations with synthetic seismograms produced with the Cagniard de Hoop and reflectivity methods constrains the shear velocity increase to be 2.75 k 0.25 per cent, comparable to upper mantle discontinuities. Short-period observations indicate that the velocity increase may be a sharp first-order discontinuity, or may extend over a transition zone no more than 50 km thick. The shear velocity gradient below the discontinuity, within the D layer, is not well-constrained by the SH data, but slightly positive or near zero velocity gradients are consistent with the long-period amplitude ratios of ScSH/SH.

Observation of 1- to 5-second microtremors and their application to earthquake engineering. Part II. Evaluation of site effect upon seismic wave amplification due to extremely deep soil deposits

abstractThe usefulness of long-period microtremor observation for earthquake engineering problems at extremely deep soil deposit site was examined in this study. Microtremor observations were made in the Niigata Plain, Japan, and in Los Angeles, California, where depths to the basement rock are several kilometers. These two locations were selected because strong-motion records obtained during the Niigata earthquake of 1964 and the San Fernando earthquake of 1971 contain large long-period amplitudes. Understanding why these predominate long-period motions were recorded is important for the evaluation of seismic input motions for large-scale structures.Long-period microtremor observations were carried out in both areas in order to evaluate the characteristics of deep soil. Observations were performed simultaneously at a number of stations to distinguish the nature of deep soil deposit. The result of Fourier analysis showed that amplitude of microtremors in long-period range increases systematically from the point of basement rock outcrop to deep deposit site, with the increase corresponding to the depth-to-basement rock. This relationship coincides with that observed in the strong-motion earthquake records. These analyses indicated that simultaneous observation of long-period microtremors at a number of stations can provide insight to the characteristic of deep soil amplification and, therefore, permit estimation of seismic input motions for large-scale structures.

The structure of the lowermost mantle determined by short-period P-wave amplitudes

Summary. The seismic velocities in the D″ region (lowermost 200 km of the mantle) are recognized to be anomalously low, though the details of the velocity structure are not known. The structure of D″ is important, in particular whether a smooth velocity model is appropriate or not. A smooth decrease in the seismic velocities would be consistent with a thermal boundary layer at the base of the mantle. We have used the amplitudes of short-period (T= 1s) P- and diffracted P-waves to investigate the internal structure of D″. A short-period amplitude data set is obtained by using underground nuclear events as sources and applying receiver corrections to the amplitudes. Receiver effects are largely responsible for the factor of ∼8 scatter in the amplitudes of the North American WWSSN stations. Applying receiver corrections reduces the scatter to a factor of ∼2, thereby providing a quantitatively useful amplitude profile into the core shadow. Using Soviet events and North American WWSSN stations, the D″ layer beneath the north polar region is well sampled. The core shadow (at T= 1 s) begins sharply at a distance of Δ= 95.5 and the slope of the amplitude decay is well defined. Also, the amplitudes decrease slightly from Δ∼87 to ∼90, then increase to Δ∼95. Synthetic seismograms are used to test various earth models, with the important conclusion that the amplitudes from smooth D″ models with a small velocity gradient in D″ decay too slowly in the shadow. This mismatch cannot be satisfactorily explained by random forward scattering or a thin low Q layer within D″. Anelastic calculations show that a thin low Q layer in D″ decreases the amplitudes before the shadow, with little effect on the decay slope in the shadow. All of the features of the observed amplitude profile can be explained as the interference effects of a model that has a low-velocity zone in the upper part of D″ followed by a normal velocity gradient in the lower part of D″. This type of model (POLAR series) also explains the scatter often observed in dT/dΔ beyond Δ∼90. The interference effects and required velocity changes in D″ are small, and long-period amplitudes will respond only to the averaged velocity gradient in D″. The POLAR models imply a compositional and/or phase change at the top of D″. Thus, the preferred seismological model does not allow the D″ region to be interpreted as a single thermal boundary layer between the mantle and core.

Body wave amplitude patterns and upper mantle attenuation variations across North America

Summary. A broad band amplitude study of P- and SH-waves from deep South American events recorded at WWSSN stations in the United States provides constraints on upper mantle variations in attenuation. The events were selected to minimize contamination due to source complexity, radiation pattern, and source structure. The prevalent feature in the short- and longperiod bands is that the SH amplitudes show the same regional variations as the P amplitudes, with the regional variations of short periods being similar, but enhanced relative to the variations at long periods. The amplitudes show some systematic regional difference between the East Coast and Rocky Mountain provinces, and both short and long periods are enhanced at the Gulf Coast and midwestern stations, probably due to thick sedimentary bed receiver structure. Since detailed receiver structures for individual stations are poorly known, we used the working hypothesis that the regional amplitude patterns are caused by lateral variations in upper mantle attenuation. Both a constant Q and a frequency-dependent attenuation operator were considered, with the relative amplitudes and waveforms in short- and long-period bands used to estimate the acceptable range of At* in the constant Q model, or the acceptable range of the parameter r, , which describes the high-frequency roll-off of the absorption band in the frequency-dependent model. Time domain modelling indicates that the short-period P and SH amplitudes and waveforms in the 1-10s period range do not unambiguously demand frequency-dependent attenuation, allowing for the uncertainty in source frequency content and long-period absorption band amplitude. The short-period P amplitudes can be fit with a range in At; = 0.5 s or a range in r, from 0.001 to 0.25s, with either single parameter variation consistent with the general short-period SH amplitude behaviour. A technique for determining the absolute value of t* and possible r, values appropriate for the 5-20s band was applied to selected data. A value of t* = 0.8 s and small 7, were found for travel paths from the deep events to North America, and lower t* or large r,, values were found for paths to southern African stations. Longperiod amplitude variations cannot be explained by frequency-dependent

Search of single long-period oscillations in the daily temperatures

Long-period oscillations have been searched in the time series of the daily minimum, mean and maximum temperatures measured in Modena, Italy, during the years 1869–1976. By assuming only one oscillation present, the amplitude and its statistical uncertainty have been estimated by means of a fitting procedure which takes into account the stochastic variability of the daily temperatures. The period of the long-term oscillations has been chosen to be an integer number of years (from 2 up to 36). This choice and the fit performed over a suitable number of years (the maximum possible multiple of the period) ensure a numerical stable solution and a negligible correlation between the long-period oscillation and the annual and seasonal oscillations present in the time series. The instrumental uncertainty of the long-period amplitude has been also evaluated. Within the overall uncertainty, no reliable evidence exists on single long-period oscillations.

やや長周期の微動観測と地震工学への適用 (3)

Continued to the previous papers observations of the long-period microtremors were carried out in Aomori and Miyako cities for a farther comparative research of their characteristics to the SMAC type accelerograph records in Tokachi-oki earthquake of 1968. The purpose we extended the observations from Hachinohe to these areas is following. The very large acceleration amplitudes in 1- to 5-sec in period were recorded more clearly in Aomori than in Hachinohe, but no significant indication was seen on the accelerograph record in Miyako. Still the differences were seemed too extreme to be explained by the epicentral distances. In Hachinohe we successfully gave an interpretation that the predominancy of the earthquake motions in the long period was considerably due to a large amplification effect by the deep soils. But we did not know if this was true in general.The observation and the analysis of the microtremors were performed in 1975 with a similar manner to those reported in the previous papers.The SMAC site in Aomori is situated on the ground of which depth to the bed rock is around 600m, and the predominant period of microtremors, obtained by a series of careful observations from bed rock to the SMAC site, was 3.0sec. This is nothing but a predominant period in the strong motions of the Tokachi-oki earthquake.On the contrary to this, the SMAC accelerograph in Miyako has been installed above the hard rock with a very thin superficial layer, and no predominant period in the long-period range was observed in the microtremors. This must be the same reason why there were no long-period amplitudes in the acceleration records in Miyako.Combining these results with that in Hachinohe we can conclude that the correspondency of the predominancy and the predominant period between the microtremors and the strong motion records is very remarkable and thus the observation of the long-period microtremors is a good alternative to estimate the long-period input motions to large-scale structures by future earthquakes.Finally a short recommendation on the observation schema of long-period microtremors was added. The problem left untouched is the wave-theoretical considerations of the microtremors, which will be scheduled before long.

Variation of source properties: The Inangahua, New Zealand, aftershocks of 1968

abstract A theoretical relationship between seismic moment and local magnitude ML is derived from the relationship between magnitude ML and source dimension given by Randall (1973). For a circular fault of radius smaller than about 0.5 km, the magnitude ML is proportional to the logarithm of the seismic moment Mo, and these values alone cannot specify other source parameters. For greater radii the values of Mo and ML define Brune's (1970) far-field spectrum and in these cases other source characteristics can be readily obtained. The seismic moment can be estimated from the long-period amplitudes, and therefore the moment-magnitude relation provides a convenient method for determination of the source properties. The relationship between the logarithm of the various source parameters and seismic moment is considered for a number of regions and earthquake sequences. It appears to be of linear form and, furthermore, it seems that the same slope coefficient can be used in different regions. Source properties show regional differences, and the most suitable parameter to describe these differences is the average displacement. Besides the regional variations, there seems to be a time variation of source properties. This is the case for the Inangahua aftershock sequence, during which the variation of the displacement residuals correlates with the variation of the coefficient b, which defines the frequency-magnitude relation.

Seismic Source Descriptions of Underground Explosions and a Depth Discriminate

Synthetic seismograms of both body waves and Rayleigh waves are used to determine the radiation field of a few large contained underground explosions. A number of possible source descriptions are investigated. A reduced displacement potential of the form, o(t) = o_0t^ξ exp(-ηt), fits the long- and short-period data. The source parameters appropriate for the Boxcar event are ξ = 0·5 and η = 0·15. Synthetic PL and Rayleigh waves are compared with observations from a number of different size events to determine the dependence of η on yield. The amplitude of the long period synthetic body wave responses at ranges greater than about 12° increases rapidly as the source depth is increased. Thus the difference in spectral properties of explosions and earthquakes can be largely explained by the depth effect. The theoretical ratio SP/LP, that is the short period divided by the long-period amplitude, is computed from 12 to 25° for the Johnson upper mantle model and the Boxcar source. A study of an earthquake which cannot be distinguished from an explosion using the m_b vs. M_s criterion is investigated by the SP/LP discriminate.

Regional variations of source properties in southern California estimated from the ratio of short- to long-period amplitudes

abstract The ratio of short- to long-period amplitude is expressed in terms of apparent stress, rigidity times energy over moment (μEG/M0), for 277 earthquakes in California. A map showing the apparent stresses is compiled. In general, the Mendocino and San Andreas faults as well as the Gulf of California area are regions of large surface-wave excitation and little short-period radiation (low apparent stress). Away from the main fault zones, the apparent stresses tend to be higher. Regions of conspicuously low surface-wave excitation (high apparent stress) are the Laguna Salada-Sierra Juarez region in northern Baja California, the California-Nevada border region north of Bishop, and the region associated with the bend of the San Andreas between San Bernardino and San Gorgonio Mountain. A detailed comparison of earthquakes with accurately-known depths at Parkfield and Borrego Mountain indicates two important differences in apparent stresses between these two source regions. The apparent stress at all depths is larger at Borrego Mountain than at Parkfield, and it increases with depth at Borrego Mountain, whereas it remains constant at all depths at Parkfield. The explanation for the variation of surface-wave excitation (apparent stress) is not known for certain, but it could be related to variations in true stress.

Diffraction ofPby the core: A study of long-period amplitudes near the edge of the shadow

Abstract : Theoretical expressions for the amplitude of P-waves in the neighborhood of the core shadow boundary have been formulated and evaluated by computer. Comparison with 2.5 second data of Sacks confirms his determination of the shadow position at 96.5 degrees. A square root frequency scaling law is found for the high frequency diffraction pattern, and permits use of the Fresnel integral function at short periods. Long-period amplitude data for three paths have been spectrum analyzed and compared against the theoretical amplitude dropoff going into the shadow. The agreement is about as good as the data will permit, and no indication is found at these periods of a low-amplitude defocused region at 90 degrees to 96 degrees, after diffraction has been accounted for.