Observations of PKKPab Diffraction Waves Well Beyond Cutoff Distance

2021 ◽  
Author(s):  
Yulin Chen ◽  
Sidao Ni ◽  
Baolong Zhang
Keyword(s):  
Epicentral Distance ◽  
P Wave ◽  
Source Depth ◽  
Large Area ◽  
Core Mantle Boundary ◽  
Velocity Models ◽  
Wave Velocities ◽  
The Earth ◽  
Cutoff Distance ◽  
Lowermost Mantle

Abstract The core mantle boundary (CMB) features the most dramatic contrast in the physical properties within the Earth and plays a fundamental role in the understanding of the dynamic evolution of the Earth’s interior. Seismic core phases such as PKKP sample large area of the lowermost mantle and the uppermost core, thus providing valuable information of the velocity structures on both sides of the CMB. Diffraction Waves Well Beyond Cutoff Distance (PKKPab) is one branch of the triplicated PKKP that can be observed beyond its ray theoretical cutoff distance as a result of diffraction along the CMB. The travel time and slowness of the diffracted PKKPab (denoted as PKKPabdiff) can be used to constrain the P-wave velocities at the lowermost mantle, thus have been investigated in numerous studies. Previous results (Rost and Garnero, 2006) suggest that most of the observations of the PKKPabdiff waves are in the epicentral distance range of 95°–105° (minor arc convention) (PKKPabdiff diffraction length less than 10°). However, high-frequency (∼1 Hz) synthetic seismograms show that the PKKPabdiff waveforms could be observable at distance down to 65°, which indicates that the PKKPabdiff signals could be detected at distances less than 95° in observations. To explore the distance ranges in which PKKPabdiff is observable, we collected global three-component broadband waveforms from 246 events with source depth deeper than 100 km and magnitude above M 6 from 2007 to 2017 available at the Incorporated Research Institutions for Seismology Data Management Center. We analyzed the slowness, polarization, and amplitude of the candidate PKKPabdiff signals, and found 95 events with clear PKKPabdiffsignals, with nearly 60% of the events show PKKPabdiff diffraction lengths greater than 10°, and the longest diffraction distance is beyond 20°. These newly identified PKKPabdiff waves would substantially augment the dataset of core phases for improvements of the CMB velocity models.

Shallow velocity structure and poisson's ratio at the Tarzana, California, strong-motion accelerometer site

1996 ◽  
Vol 86 (6) ◽  
pp. 1704-1713 ◽  
Author(s):  
R. D. Catchings ◽  
W. H. K. Lee
Keyword(s):  
Urban Areas ◽  
Velocity Structure ◽  
Strong Motion ◽  
P Wave ◽  
S Wave ◽  
Near Surface ◽  
Velocity Models ◽  
Wave Velocities ◽  
Poisson's Ratios ◽  
Poisson’S Ratios

Abstract The 17 January 1994, Northridge, California, earthquake produced strong ground shaking at the Cedar Hills Nursery (referred to here as the Tarzana site) within the city of Tarzana, California, approximately 6 km from the epicenter of the mainshock. Although the Tarzana site is on a hill and is a rock site, accelerations of approximately 1.78 g horizontally and 1.2 g vertically at the Tarzana site are among the highest ever instrumentally recorded for an earthquake. To investigate possible site effects at the Tarzana site, we used explosive-source seismic refraction data to determine the shallow (<70 m) P-and S-wave velocity structure. Our seismic velocity models for the Tarzana site indicate that the local velocity structure may have contributed significantly to the observed shaking. P-wave velocities range from 0.9 to 1.65 km/sec, and S-wave velocities range from 0.20 and 0.6 km/sec for the upper 70 m. We also found evidence for a local S-wave low-velocity zone (LVZ) beneath the top of the hill. The LVZ underlies a CDMG strong-motion recording site at depths between 25 and 60 m below ground surface (BGS). Our velocity model is consistent with the near-surface (<30 m) P- and S-wave velocities and Poisson's ratios measured in a nearby (<30 m) borehole. High Poisson's ratios (0.477 to 0.494) and S-wave attenuation within the LVZ suggest that the LVZ may be composed of highly saturated shales of the Modelo Formation. Because the lateral dimensions of the LVZ approximately correspond to the areas of strongest shaking, we suggest that the highly saturated zone may have contributed to localized strong shaking. Rock sites are generally considered to be ideal locations for site response in urban areas; however, localized, highly saturated rock sites may be a hazard in urban areas that requires further investigation.

P-wave amplitude variation with epicentral distance and the magnitude relations

1975 ◽  
Vol 65 (4) ◽  
pp. 915-926
Author(s):  
K. L. Kaila ◽  
Dipankar Sarkar
Keyword(s):  
Lower Mantle ◽  
Wave Amplitude ◽  
Epicentral Distance ◽  
P Wave ◽  
Amplitude Variation ◽  
Distance Range ◽  
Nuclear Explosions ◽  
The Earth ◽  
Shallow Earthquakes ◽  
Discontinuous Nature

Abstract Investigation of the variation of P-wave amplitudes and d2T/dΔ2 with epicentral distance reveals that the amplitudes in the distance range up to 100° can be represented by seven discontinuous curves. The discontinuous nature of the amplitude curves can be explained due to the possible existence of first- or second-order velocity discontinuities in the upper and lower mantle of the Earth. Seven magnitude relations corresponding to these amplitude curves are given, which yield consistent magnitudes for nuclear explosions as well as for shallow earthquakes.

Focal depth determination from the signal character of long-period P waves

1976 ◽  
Vol 66 (4) ◽  
pp. 1221-1232
Author(s):  
Robert B. Herrmann
Keyword(s):  
Epicentral Distance ◽  
Focal Depth ◽  
Time Function ◽  
P Wave ◽  
Source Time Function ◽  
P Waves ◽  
Long Period ◽  
The Earth ◽  
Resolution Capability ◽  
Earth Structures

abstract The shape of long-period teleseismic P-wave signals is a function of many factors, among which are focal depth, focal mechanism, the source time function, and the earth structures at both the source and receiver. The effect of focal depth is quite pronounced, so much so, that focal depths should be able to be determined to within 10 km on the basis of the long-period P-wave character. This resolution capability is demonstrated for events occurring in continental and oceanic crust as observed by seismographs in the 30° to 80° epicentral distance range.

scholarly journals Minimum 1D velocity models in Central and Southern Italy: a contribution to better constrain hypocentral determinations

1997 ◽  
Vol 40 (4) ◽  
Author(s):  
C. Chiarabba ◽  
A. Frepoli
Keyword(s):  
Southern Italy ◽  
Deep Structure ◽  
P Wave ◽  
One Dimensional ◽  
Velocity Models ◽  
The Past ◽  
Wave Velocities ◽  
Velocity Gradients ◽  
P Wave Velocity ◽  
Data Yield

We computed one-dimensional ( I D) velocity models and station corrections for Centrai and Southern Italy, in- verting re-picked P-wave alTival times recorded by the Istituto Nazionale di Geofisica seismic network. The re-picked data yield resolved P-wave velocity results and proved to be more suited than bulletin data for de- tailed tomographic studies. Using the improved velocity models, we relocated the most significant earthquakes which occurt.ed in the Apennines in the past 7 years, achieving constrained hypocentral determinations for events within most of the Apenninic belt. The interpretation of the obtained lD velocity models allows us to infer interesting features on the deep structure of the Apennines. Smooth velocity gradients with depth and low P-wave velocities are ob,'ierved beneath the Apennines. We believe that our results are effective to constrain hypocentral locations in Italy and may represent a first step towards more detailed seismotectonic analyses.

scholarly journals Elastic reflection-based waveform inversion with a nonlinear approach

Geophysics ◽  
2017 ◽  
Vol 82 (6) ◽  
pp. R309-R321 ◽  
Author(s):  
Qiang Guo ◽  
Tariq Alkhalifah
Keyword(s):  
Wave Velocity ◽  
Equivalent Stress ◽  
Waveform Inversion ◽  
P Wave ◽  
S Wave ◽  
Velocity Models ◽  
Wave Velocities ◽  
Full Waveform ◽  
Highly Nonlinear ◽  
Elastic Reflection

Full-waveform inversion (FWI) is a highly nonlinear problem due to the complex reflectivity of the earth, and this nonlinearity only increases under the more expensive elastic assumption. In elastic media, we need a good initial P-wave velocity and even better initial S-wave velocity models with accurate representation of the low model wavenumbers for FWI to converge. However, inverting for the low-wavenumber components of P- and S-wave velocities using reflection waveform inversion (RWI) with an objective to fit the reflection shape, rather than produce reflections, may mitigate the limitations of FWI. Because FWI, performing as a migration operator, is preferred of the high-wavenumber updates along reflectors. We have developed an elastic RWI that inverts for the low-wavenumber and perturbation components of the P- and S-wave velocities. To generate the full elastic reflection wavefields, we derive an equivalent stress source made up by the inverted model perturbations and incident wavefields. We update the perturbation and propagation parts of the velocity models in a nested fashion. Applications on the synthetic isotropic models and field data indicate that our method can efficiently update the low- and high-wavenumber parts of the models.

The origin of short-period precursors to PKP

1975 ◽  
Vol 65 (3) ◽  
pp. 765-786
Author(s):  
C. Wright
Keyword(s):  
Wave Energy ◽  
Wave Train ◽  
Outer Core ◽  
Temporal Changes ◽  
P Wave ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
Short Period ◽  
Lowermost Mantle

abstract An investigation of the origin of precursors to short-period PKP phases has been undertaken using 23 earthquakes recorded at the Yellowknife Array at distances between 123° and 143°. In particular, the pattern of slowness and azimuth changes with time has been examined for coherent bursts of energy occurring throughout the precursor wave train. These temporal changes demonstrate that the precursor energy is most satisfactorily explained by scattering from small inhomogeneities at the core-mantle boundary or in the lowermost mantle, both before P-wave energy enters the core and when it re-emerges into the mantle. Moreover, scattering before entry into the core seems to generate the larger amplitudes. The bulk of the data cannot be attributed to reflection or sharp upward refraction from velocity discontinuities within the lower part of the outer core, although there is some ambiguous evidence for a reflecting interface at a depth of about 4850 km.

Decay rate of P-wave amplitudes from nuclear explosions and the magnitude relations in the epicentral distance range 1° to 98°

1970 ◽  
Vol 60 (2) ◽  
pp. 447-460
Author(s):  
K. L. Kaila
Keyword(s):  
Decay Rate ◽  
Travel Time ◽  
Epicentral Distance ◽  
P Wave ◽  
Amplitude Curve ◽  
Distance Range ◽  
Nuclear Explosions ◽  
The Earth

abstract Study of the decay rate with epicentral distance of P-wave amplitudes from nuclear explosions in the distance range 1° to 98° has revealed that instead of the continuous amplitude curve of Gutenberg, the amplitudes are most appropriately represented by six discontinuous curves. Corresponding to these six amplitude decay curves, magnitude relations were worked out. The new magnitude relations yield consistent magnitudes for nuclear explosions as well as earthquakes independent of epicentral distance over the entire distance range of 1° to 98°. The magnitude values are however slightly shifted towards the higher side by 0.22 magnitude unit (at M = 4.5) as compared to those predicted by the Gutenberg-Richter method. Magnitudes are evaluated for Logan, Blanca and Salmon explosions using the new magnitude relations and they agree fairly well with the published magnitudes for these events determined by other workers using the Gutenberg-Richter method. These amplitude decay curves when compared with the travel-time curves of Carder (1964) do show a reasonable correspondence between the starting points of amplitude curves and the distances where the deeply refracted phases from plausible velocity discontinuities in the mantle start appearing as first arrivals on the surface of the Earth.

Shear wave velocities in the lower mantle

1969 ◽  
Vol 59 (5) ◽  
pp. 1983-1999
Author(s):  
J. W. Fairborn
Keyword(s):  
Monte Carlo ◽  
Velocity Gradient ◽  
Lower Mantle ◽  
Travel Times ◽  
Core Mantle Boundary ◽  
Velocity Models ◽  
The Core ◽  
Wave Velocities ◽  
Shear Wave Velocities ◽  
Steep Slopes

abstract The Large Aperture Seismic Array in eastern Montana was used to measure the travel times and dT/dΔ of earthquake-generated shear waves for the purpose of determining lower-mantle shear velocities. The data were limited to epicentral distances between 27 and 95 degrees. To convert uncertainties in the dT/dΔ measurements to uncertainties in the computed velocities, the data were inverted by a Monte Carlo procedure. The computer randomly generated velocity models, and those which satisfied prescribed travel time and dT/dΔ-versus-Δ limits were considered as possible models for the real earth. The dT/dΔ curve possessed anomalously steep slopes between 27-30 and 65-75 degrees, and the corresponding velocity models had increased gradients between the approximate depths of 700-800 and 1550-2100 kilometers. The slope of the dT/dΔ curve also decreased at about 88 degrees which may indicate a decrease in velocity gradient several hundred kilometers above the core-mantle boundary. For epicentral distances greater than 27 degrees the observed travel times to LASA were greater than the expected J-B times. Although vague, a systematic increase in the observed residuals appeared to exist between 40 and 60 degrees which agreed with the J-B residuals of the computed models.

The relationship between Vs, Vp, density and depth based on PS-logging data at K-NET and KiK-net sites

2021 ◽  
Author(s):  
Fumiaki Nagashima ◽  
Hiroshi Kawase
Keyword(s):  
Standard Deviation ◽  
Seismic Velocity ◽  
Saturated Soil ◽  
P Wave ◽  
Soil Types ◽  
Depth Range ◽  
Velocity Inversion ◽  
Velocity Models ◽  
Regression Curves ◽  
Ps Logging

Summary P-wave velocity (Vp) is an important parameter for constructing seismic velocity models of the subsurface structures by using microtremors and earthquake ground motions or any other geophysical exploration data. In order to reflect the ground survey information in Japan to the Vp structure, we investigated the relationships among Vs, Vp, and depth by using PS-logging data at all K-NET and KiK-net sites. Vp values are concentrated at around 500 m/s and 1,500 m/s when Vs is lower than 1,000 m/s, where these concentrated areas show two distinctive characteristics of unsaturated and saturated soil, respectively. Many Vp values in the layer shallower than 4 m are around 500 m/s, which suggests the dominance of unsaturated soil, while many Vp values in the layer deeper than 4 m are larger than 1,500 m/s, which suggests the dominance of saturated soil there. We also investigated those relationships for different soil types at K-NET sites. Although each soil type has its own depth range, all soil types show similar relationships among Vs, Vp, and depth. Then, considering the depth profile of Vp, we divided the dataset into two by the depth, which is shallower or deeper than 4 m, and calculated the geometrical mean of Vp and the geometrical standard deviation in every Vs bins of 200 m/s. Finally, we obtained the regression curves for the average and standard deviation of Vp estimated from Vs to get the Vp conversion functions from Vs, which can be applied to a wide Vs range. We also obtained the regression curves for two datasets with Vp lower and higher than 1,200 m/s. These regression curves can be applied when the groundwater level is known. In addition, we obtained the regression curves for density from Vs or Vp. An example of the application for those relationships in the velocity inversion is shown.

scholarly journals Seismic Structure of Local Crustal Earthquakes beneath the Zipingpu Reservoir of Longmenshan Fault Zone

2011 ◽  
Vol 2011 ◽  
pp. 1-6 ◽  
Author(s):  
Haiou Li ◽  
Xiwei Xu ◽  
Wentao Ma ◽  
Ronghua Xie ◽  
Jingli Yuan ◽  
...  
Keyword(s):  
Fault Zone ◽  
Three Dimensional ◽  
P Wave ◽  
The Tibetan Plateau ◽  
Seismic Structure ◽  
Infiltration Depth ◽  
Low Velocity ◽  
Longmenshan Fault Zone ◽  
Velocity Models ◽  
Longmenshan Fault

Three-dimensional P wave velocity models under the Zipingpu reservoir in Longmenshan fault zone are obtained with a resolution of 2 km in the horizontal direction and 1 km in depth. We used a total of 8589 P wave arrival times from 1014 local earthquakes recorded by both the Zipingpu reservoir network and temporary stations deployed in the area. The 3-D velocity images at shallow depth show the low-velocity regions have strong correlation with the surface trace of the Zipingpu reservoir. According to the extension of those low-velocity regions, the infiltration depth directly from the Zipingpu reservoir itself is limited to 3.5 km depth, while the infiltration depth downwards along the Beichuan-Yingxiu fault in the study area is about 5.5 km depth. Results show the low-velocity region in the east part of the study area is related to the Proterozoic sedimentary rocks. The Guanxian-Anxian fault is well delineated by obvious velocity contrast and may mark the border between the Tibetan Plateau in the west and the Sichuan basin in the east.

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