A computer model study of the propagation of the long‐range Pn phase

Gettrust, Joseph F.; Frazer, L. Neil

doi:10.1029/gl008i007p00749

Cited by 32 publications

(23 citation statements)

References 15 publications

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“…We now review some earlier seismic velocity models for the area of the OSS IV site. The model used by Gettrust & Frazer (1981) to compute Po synthetics was consistent with prior refraction studies in this area. As very little information about Q was available at that time, Gettrust & Frazer (1981) used a constant Qp of 5000 and Q, of 2500 throughout the crust and lithosphere.…”

Section: Modelling Of P O L S O D a T A Recorded At O S S I Vmentioning

confidence: 88%

“…However none could successfully model the long coda duration that characterizes these phases. Gettrust & Frazer (1981) first used the reflectivity technique to correctly model the traveltime as well as the first 15-20 s of the Po wavetrain up to a distance of 900 km, for a typical oceanic model obtained from seismic refraction studies in the Pacific. Though their computation was over-simplified, as they included only one free surface multiple, it matched the data well.…”

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confidence: 99%

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Po/So Synthetics For A Variety of Oceanic Models and Their Implications For the Structure of the Oceanic Lithosphere

Mallick¹,

Frazer²

1990

Geophysical Journal International

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S U M M A R Y Polso synthetic seismograms were computed for a variety of oceanic structures in order to model data from an t?zb = 5.7 earthquake, recorded during the OSS IV seismic experiment. Satisfactory modelling of Polso waveshapes and frequency content was achieved with no lateral heterogeneity, but with a small random vertical heterogeneity in the mantle, superimposed on a mean velocity structure which is consistent with seismic refraction data. The random heterogeneity in the P and S velocities appears to be about 2 per cent with a scale length of 5 km. This kind of heterogeneity is easily achieved by varying slightly the major mineral components in either the peridotite or eclogite mantle mineralogy and can be another cause of observed upper mantle anisotropy.Using a step-function source time behaviour, our numerical modelling also indicates that in the upper mantle Q p is roughly proportional to rising from about 450 at f = 1 Hz to about 800 at f = 15 Hz, and that Q, is roughly proportional to rising from about 900 at f = 1 H z to about 1800 at f = 15Hz. A slight decrease in Q, from these values in any small zone of the upper mantle reduces So amplitudes drastically and we believe this is why So is sometimes missing from observed seismograms.The computed synthetic record sections were also used to examine the relation between intrinsic Q and the apparent Q inferred from data in earlier studies. Apparent Q was found to be lower than intrinsic Q below about 5 H z and higher than intrinsic Q above 5 Hz. When these differences are accounted for, the mantle Q's of this study agree with the mantle Q's of Butler et al. (1987) to within 10 per cent. There can no longer be any doubt that oceanic upper mantle Q's are very large, above 5 Hz, and that Q, is about twice as large as Qp.

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Section: Modelling Of P O L S O D a T A Recorded At O S S I Vmentioning

confidence: 88%

mentioning

confidence: 99%

Po/So Synthetics For A Variety of Oceanic Models and Their Implications For the Structure of the Oceanic Lithosphere

Mallick¹,

Frazer²

1990

Geophysical Journal International

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“…Gettrust and Frazer () suggested that long range Pn require the oceanic upper mantle having a stable velocity‐depth structure along the propagation path. Further, Sereno and Orcutt () showed that oceanic lithosphere is an efficient waveguide for high‐frequency seismic energy and that Pn phase can be explained as refractions from the lower oceanic lithosphere.…”

Section: Methodsmentioning

confidence: 99%

Regional Pn Body‐Wave Magnitude Scale m_b(Pn) for Earthquakes Along the Northern Mid‐Atlantic Ridge

Kim

Ottemöller

2017

JGR Solid Earth

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We obtained a robust regional body wave magnitude scale mb(Pn) for earthquakes along the Northern mid‐Atlantic Ridge by using Pn wave recorded at continental stations. The new magnitude scale is mb(Pn) = log10 A [nm] − 1.86 log10 (100/Δ) [km] + C + 1.62, where A is the zero‐to‐peak amplitude of Pn wave on the simulated vertical Wood‐Anderson seismogram in nanometers, Δ is the epicentral distance in kilometers, 1.86 represents amplitude attenuation, C is the station correction, and 1.62 anchors the mb(Pn) to the moment magnitude (Mw), plus amplitude loss in the two crustal legs. By using moment magnitude as reference, we obtained the event magnitude adjustments (EMAs), which represent the difference between the known long‐period Mw and the short‐period mb(Pn). EMAs correlate with the source property, where ridge events show strong long‐period waves but poor high‐frequency signals that lead to mb(Pn) < Mw and positive EMA of +0.08 ± 0.26 magnitude units (m.u.), whereas transform fault earthquakes show relatively strong high‐frequency waves that lead to mb(Pn) > Mw and negative adjustment of −0.20 ± 0.24 m.u. Almost all normal faulting events are along spreading ridges, whereas strike‐slip events are along the fracture zones and transform faults; hence, we developed source‐specific magnitude adjustments (SSMAs), which range from +0.27 m.u. for earthquakes along the ridges, to −0.26 m.u. for earthquakes along the transform faults. Using SSMA, we obtained a regional Pn magnitude that is consistent with Mw and mb(P). The regression relationship is Mw = 0.91 mb(Pn)[SSMA] + 0.46 with a standard deviation of ±0.15 m.u.

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“…Descriptions of the earth's mantle by its velocity fluctuations rather than by its average velocity are mostly restricted to interpretation of studies of wave fluctuation beneath seismological arrays (WU and FLATTÉ , 1990), to modelling wave forms for propagation in oceanic lithosphere (e.g., GETTRUST and FRAZER, 1981;MALLICK and FRAZER, 1990) and more recently to the understanding of a high-frequency phase observed in Russian long-range data with nuclear explosions as the source (RYBERG et al, 1995). Vol.…”

Section: Introductionmentioning

confidence: 99%

Scales of Heterogeneities in the Continental Crust and Upper Mantle

Tittgemeyer¹,

Wenzel²,

Ryberg

et al. 1999

Pure appl. geophys.

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A seismological characterization of crust and upper mantle can refer to large-scale averages of seismic velocities or to fluctuations of elastic parameters. Large is understood here relative to the wavelength used to probe the earth.In this paper we try to characterize crust and upper mantle by the fluctuations in media properties rather than by their average velocities. As such it becomes evident that different scales of heterogeneities prevail in different layers of crust and mantle. Although we cannot provide final models and an explanation of why these different scales exist, we believe that scales of inhomogeneities carry significant information regarding the tectonic processes that have affected the lower crust, the lithospheric and the sublithospheric upper mantle.We focus on four different types of small-scale inhomogeneities: (1) the characteristics of the lower crust, (2) velocity fluctuations in the uppermost mantle, (3) scattering in the lowermost lithosphere and on (4) heterogeneities in the mantle transition zone.

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A computer model study of the propagation of the long‐range Pn phase

Cited by 32 publications

References 15 publications

Po/So Synthetics For A Variety of Oceanic Models and Their Implications For the Structure of the Oceanic Lithosphere

Po/So Synthetics For A Variety of Oceanic Models and Their Implications For the Structure of the Oceanic Lithosphere

Regional Pn Body‐Wave Magnitude Scale m_b(Pn) for Earthquakes Along the Northern Mid‐Atlantic Ridge

Scales of Heterogeneities in the Continental Crust and Upper Mantle

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A computer model study of the propagation of the long‐range Pn phase

Cited by 32 publications

References 15 publications

Po/So Synthetics For A Variety of Oceanic Models and Their Implications For the Structure of the Oceanic Lithosphere

Po/So Synthetics For A Variety of Oceanic Models and Their Implications For the Structure of the Oceanic Lithosphere

Regional Pn Body‐Wave Magnitude Scale mb(Pn) for Earthquakes Along the Northern Mid‐Atlantic Ridge

Scales of Heterogeneities in the Continental Crust and Upper Mantle

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Regional Pn Body‐Wave Magnitude Scale m_b(Pn) for Earthquakes Along the Northern Mid‐Atlantic Ridge