Diffraction from the PKP caustic B

1974 ◽  
Vol 64 (1) ◽  
pp. 33-43 ◽  
Author(s):  
Goetz G. R. Buchbinder
Keyword(s):  
Transition Zone ◽  
Inner Core ◽  
Focal Depth ◽  
Travel Times ◽  
Depth Data ◽  
Long Period ◽  
Short Period ◽  
Amplitude Decrease

abstract Long-period energy preceding the PKPDF branch on long-period seismograms was identified as resulting from diffraction from the PKP caustic B. This conclusion is based on the measurements of the travel times, and, therefore, the slope of the travel times, on the amplitude decrease, and the very rapid dispersion of the periods of one earthquake of intermediate focal depth. Data from three deepfocus earthquakes support this interpretation. Thus, the long-period precursors are not due to the same cause as short-period precursors and are not produced by discontinuities in the transition zone between the inner and outer cores nor by structure in the inner core.

A velocity structure of the Earth's core

1971 ◽  
Vol 61 (2) ◽  
pp. 429-456 ◽  
Author(s):  
Goetz G. R. Buchbinder
Keyword(s):  
Transition Zone ◽  
Inner Core ◽  
Velocity Structure ◽  
Velocity Model ◽  
Outer Core ◽  
Travel Times ◽  
Amplitude Ratios ◽  
The Core ◽  
Short Period ◽  
Core Boundary

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.

Shear velocity from differential travel times of short-period ScS-P in New Hebrides, Fiji-Tonga, and Banda Sea regions

1975 ◽  
Vol 65 (6) ◽  
pp. 1787-1796
Author(s):  
Mansur A. Choudhury ◽  
Georges Poupinet ◽  
Guy Perrier
Keyword(s):  
Upper Mantle ◽  
Focal Depth ◽  
Shear Velocity ◽  
Mean Value ◽  
Travel Times ◽  
Velocity Models ◽  
Depth Dependence ◽  
Short Period ◽  
New Hebrides ◽  
The Antarctic

abstract Behavior of P, S and ScS residuals as well as those of differential travel times of ScS-P from the Jeffreys-Bullen tables are analyzed. The phases have been read from short-period records of the Antarctic station, Dumont d'Urville (DRV); the earthquakes originating in New Hebrides, Fiji-Tonga, and Banda Sea regions. P residuals from all regions show a mean value of about −1 sec. On the contrary, S and ScS residuals, well correlated among themselves, show important regional as well as focal-depth dependence. ScS-P residuals from shallow and intermediate shocks are largely positive for New Hebrides and largely negative for Banda Sea; those from intermediate shocks are moderately positive for Fiji-Tonga. The anomalies disappear at depths greater than about 200 km. Upper mantle shear velocity models are presented for the three regions. The models are discussed in relation to a sinking lithosphere.

scholarly journals Wave velocities in the earth's core

1958 ◽  
Vol 48 (4) ◽  
pp. 301-314
Author(s):  
B. Gutenberg
Keyword(s):  
Travel Time ◽  
Transition Zone ◽  
Southern California ◽  
Inner Core ◽  
Travel Times ◽  
Earth’S Core ◽  
Earth's Core ◽  
The Core ◽  
Wave Velocities

Abstract More than 700 seismograms of 39 shocks recorded mainly in southern California at epicentral distances between 105 and 140 degrees are used to investigate records of phases which have penetrated the earth's core. Properties of PKIKP, SKP, SKIKP, PKS, and PKIKS are discussed. Portions of travel-time curves of these phases are revised. Travel times of waves starting and ending at the surface of the core, and wave velocities in the core, are recalculated. Between about 1,500 and 1,200 km. from the earth's center in the transition zone from the liquid outer to the probably solid inner core, waves having lengths of the order of 10 km. travel faster than longer waves. This is probably caused by a rather rapid increase in viscosity toward the earth's center in this transition zone.

Revised velocities in the Earth's core

1973 ◽  
Vol 63 (3) ◽  
pp. 1073-1105 ◽  
Author(s):  
Anthony Qamar
Keyword(s):  
Transition Zone ◽  
Inner Core ◽  
Velocity Model ◽  
Outer Core ◽  
Travel Times ◽  
Zone Boundary ◽  
Underground Nuclear Explosions ◽  
Nuclear Explosions ◽  
Core Boundary ◽  
Velocity Jump

abstract Travel times and amplitudes of PKP and PKKP from three earthquakes and four underground nuclear explosions are presented. Observations of reflected core waves at nearly normal angles of incidence provide new constraints on the average velocities in the inner and outer core. Interpretation of these data suggests that several small but significant changes to Bolt's (1962) core velocity model (T2) are necessary. A revised velocity model KOR5 is given together with the derived travel times that are consistent with the 1968 tables for P. Model KOR5 possesses a velocity in the transition zone which is 112 per cent lower than that in model T2. In addition, KOR5 has a velocity jump at the transition zone boundary (r = 1782 km) of 0.013 km/sec and a jump at the inner core boundary (r = 1213 km) of 0.6 km/sec. These values are, respectively, 1/20 and 2/3 of the corresponding model T2 values.

Definition and identification of seismic events in the USSR in 1971

1974 ◽  
Vol 64 (3-1) ◽  
pp. 607-636
Author(s):  
Ola Dahlman ◽  
Hans Israelson ◽  
Atle Austegard ◽  
Gunnel Hörnström
Keyword(s):  
Focal Depth ◽  
Seismic Monitoring ◽  
Seismic Events ◽  
Long Period ◽  
The Earth ◽  
Short Period ◽  
Complete Test ◽  
Period Data ◽  
Test Ban ◽  
Test Ban Treaty

abstract Seismic events reported to have occurred in the USSR in 1971 are studied to assess the seismic monitoring problem as it may occur in the context of a complete test-ban treaty. Available epicenter data of a total of 199 events, 180 earthquakes and 19 explosions, are presented. Focal depth estimates reported by the National Oceanic and Atmospheric Administration, U.S., and the Institute of Physics of the Earth, Moscow, are compared. Identification parameters determined using short- and long-period data from Hagfors Observatory and supplementary short-period data from the Yellowknife array station in Canada are presented. To study the combined operative efficiency and applicability of available identification parameters, the reported depth estimates and the identification data are assessed in a defined way.

Surface-wave magnitudes of Eurasian earthquakes and explosions

1975 ◽  
Vol 65 (3) ◽  
pp. 693-709 ◽  
Author(s):  
Otto W. Nuttli ◽  
So Gu Kim
Keyword(s):  
Surface Wave ◽  
Focal Mechanism ◽  
Rayleigh Waves ◽  
Body Wave ◽  
Focal Depth ◽  
Vertical Component ◽  
P Waves ◽  
Long Period ◽  
Short Period ◽  
Continental Interior

abstract Body-wave magnitudes, mb, and surface-wave magnitudes, MS, were determined for approximately 100 Eurasian events which occurred during the interval August through December 1971. Body-wave magnitudes were determined from 1-sec P waves recorded by WWSSN short-period, vertical-component seismographs at epicentral distances greater than 25°. Surface-wave magnitudes were determined from 20-sec Rayleigh waves recorded by long-period, vertical-component WWSSN and VLPE seismographs. The earthquakes had mb values ranging from 3.6 to 5.7. Of 96 presumed earthquakes studied, 6 lie in or near the explosion portion of an mb:MS plot. The explosion mb:MS curve was obtained from seven Eurasian events which had mb values ranging from 5.0 to 6.2 and MS values from 3.2 to 5.1. All six anomalous earthquakes were located in the interior of Asia, in Tibet, and in Szechwan and Sinkiang provinces of China. In general, oceanmargin earthquakes were found to have more earthquake-like mb:MS values than those occurring in the continental interior. Neither focal depth nor focal mechanism can explain the anomalous events.

scholarly journals Reconciling PP and P′P′ precursor observations of a complex 660 km seismic discontinuity

2013 ◽  
Vol 194 (2) ◽  
pp. 834-838 ◽  
Author(s):  
Elizabeth A. Day ◽  
Arwen Deuss
Keyword(s):  
Phase Change ◽  
Transition Zone ◽  
High Frequency ◽  
Data Types ◽  
Mantle Transition Zone ◽  
Long Period ◽  
Short Period ◽  
Period Data ◽  
Seismic Discontinuity

Abstract High frequency precursors to P′P′ almost invariably observe a narrow 660 km discontinuity, whereas PP precursor studies at long periods struggle to detect a reflection from the ‘660’ despite its apparent sharpness to P′P′. To investigate these contradictory observations we compare PP and P′P′ precursors in the same region. Using short period P′P′ precursors we observe a sharp 660 km discontinuity, which appears to vary in depth substantially. The apparent topography on the ‘660’ is too large to originate solely from thermal variations, regardless of its cause, therefore indicating chemical variations at the base of the mantle transition zone. Long period P′P′ precursors show no ‘660’ as they are sensitive to a larger area and thus average out the apparent topography, in agreement with long period PP precursors. Instead, we see some evidence in both long period data types for a reflection from 720 km depth, which is likely to correspond to a phase change in the garnet system.

Characterization of global seismograms using an automatic-picking algorithm

1994 ◽  
Vol 84 (2) ◽  
pp. 366-376
Author(s):  
Paul S. Earle ◽  
Peter M. Shearer
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
Low Frequency ◽  
Travel Times ◽  
Comparable Amount ◽  
Arrival Times ◽  
Long Period ◽  
Automatic Phase ◽  
Short Period ◽  
Sample Rate

Abstract An automatic phase picker is useful for quickly identifying and timing phase arrivals in large seismic data bases. We have developed an automatic phase picker that is sensitive to small changes in amplitude and applied it to over 7 yr of global data distributed by the National Earthquake Information Center (NEIC). Our phase-picking algorithm is based on a short-term-average to long-term-average ratio (STA/LTA) taken along an envelope function generated from the seismogram. The algorithm returns arrival times and corresponding pick qualities. The procedure requires few input parameters and is easily adapted to various types of data. We produce global travel-time plots from both high-frequency (20- or 40-Hz sample rate) and low-frequency (1-Hz sample rate) data. These plots clearly image the predominant high- and low-frequency phases in the NEIC data base. Picks made from the long-period seismograms are less precise, but they reveal far more phase arrivals than the short-period picks. A number of phases resulting from reflections and phase conversions at upper mantle discontinuities can be identified in the low-frequency picks; however, a search of the short-period picks for upper mantle discontinuity phases, between P and PP and prior to P′P′, has so far been unsuccessful. In the long-period S and SS picks, we observe a discrepancy in SV and SH travel times, a possible result of upper mantle anisotropy. To check the accuracy and consistency of our algorithm, we present comparisons between hand-picked times and automatic-picked times for identical seismograms. Travel-time residuals from the short-period automatic picks and data reported to the International Seismological Centre (ISC) picks exhibit a comparable amount of scatter. Histograms of the ISC residuals and automatic-pick residuals are similar in shape and width for P and PcP. These observations suggest that human picking errors are not a major contributor to the scatter observed in ISC travel times, although direct comparisons between ISC reported picks and automatic picks on particular seismograms occasionally identify operator mispicks.

SKP and related phases*

1956 ◽  
Vol 46 (3) ◽  
pp. 185-201 ◽  
Author(s):  
R. D. Forester
Keyword(s):  
Inner Core ◽  
Energy Content ◽  
Focal Point ◽  
Vertical Component ◽  
Continuous Increase ◽  
Long Period ◽  
Amplitude Data ◽  
Short Period ◽  
Additional Support ◽  
Period Instruments

Abstract For shallow shocks, multiple SKP phases are observed after the initial SKP motion as long as 54 seconds on short-period instruments and as long as 87 seconds on long-period instruments. Amplitude data indicate that each multiple phase has a focal point similar to that of the initial SKP phase. The focal point for waves having periods of 1 to 5 seconds occurs at 131½°, and that for waves having periods of 5 to 10 seconds is broadly defined between 130° and 131°. Short-period SKP waves extend from 129° to at least 140°; long-period SKP waves, from 125° to 145°. The long-period waves are believed to be diffracted from the caustic in accordance with Airy's hypothesis. For all types of SKP phases the energy content of the short-period waves is several times less than that of the long-period waves. For the vertical component the agreement between theoretical and observed values of energy of long-period waves is good. For the horizontal component the observation of too little energy is not satisfactorily explained. SKP″, the SKP phase associated with the inner core, is observed between 114° and 125°. It records with very short periods. The observations of SKP″ present additional support for the hypothesis of large but continuous increase of velocity at the transitional boundary of the inner core.

Discussion following the presentation by F. Deubner

1977 ◽  
Vol 36 ◽  
pp. 69-74
Keyword(s):  
List Type ◽  
The Sun ◽  
Type Number ◽  
Harmonic Motion ◽  
Locally Excited ◽  
Long Period ◽  
Coherent Wave ◽  
Short Period ◽  
Global Modes

The discussion was separated into 3 different topics according to the separation made by the reviewer between the different periods of waves observed in the sun :1) global modes (long period oscillations) with predominantly radial harmonic motion.2) modes with large coherent - wave systems but not necessarily global excitation (300 s oscillation).3) locally excited - short period waves.

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