Spectral and magnitude characteristics of anomalous Eurasian earthquakes

1977 ◽  
Vol 67 (2) ◽  
pp. 463-478
Author(s):  
So Gu Kim ◽  
Otto W. Nuttli
Keyword(s):  
Focal Mechanism ◽  
Rayleigh Waves ◽  
Main Shock ◽  
Focal Depth ◽  
P Wave ◽  
Wave Spectra ◽  
P Waves ◽  
Aftershock Sequences ◽  
Short Period ◽  
Amplitude Spectra

Abstract A number of main shock-aftershock sequences in the Eurasian interior contain some aftershocks whose mb:MS values are close to those of underground explosions. This paper is concerned with a study of the amplitude spectra of the P waves and Rayleigh waves for earthquakes of those main shock-aftershock sequences. It is found that for any given sequence studied, there is little if any variation in focal depth or focal mechanism. This rules out variations in these quantities as being the cause of anomalous mb:MS values. A study of the P-wave spectra establishes that one or both of the corner periods of anomalous earthquakes are smaller than those of non-anomalous earthquakes of the same moment. Thus the cause of anomalous mb:MS values of the earthquakes studied is a relative enrichment of the short-period portion of the spectrum of the anomalous events, which cannot be attributed to focal depth or focal mechanism.

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.

Inversion of regional surface-wave spectra for source parameters of aftershocks from the Loma Prieta earthquake

1991 ◽  
Vol 81 (5) ◽  
pp. 1726-1736
Author(s):  
Susan L. Beck ◽  
Howard J. Patton
Keyword(s):  
Surface Wave ◽  
Surface Waves ◽  
Focal Mechanism ◽  
Source Parameters ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
P Wave ◽  
Wave Spectra ◽  
Short Period ◽  
First Motion

Abstract Surface waves recorded at regional distances are used to study the source parameters for three of the larger aftershocks of the 18 October 1989, Loma Prieta, California, earthquake. The short-period P-wave first-motion focal mechanisms indicate a complex aftershock sequence with a wide variety of mechanisms. Many of these events are too small for teleseismic body-wave analysis; therefore, the regional surface-waves provide important long-period information on the source parameters. Intermediate-period Rayleigh- and Love-wave spectra are inverted for the seismic moment tensor elements at a fixed depth and repeated for different depths to find the source depth that gives the best fit to the observed spectra. For the aftershock on 19 October at 10:14:35 (md = 4.2), we find a strike-slip focal mechanism with right lateral motion on a NW-trending vertical fault consistent with the mapped trace of the local faults. For the aftershock on 18 October at 10:22:04 (md = 4.4), the surface waves indicate a pure reverse fault with the nodal planes striking WNW. For the aftershock on 19 October at 09:53:50 (md = 4.4), the surface waves indicate a strike-slip focal mechanism with a NW-trending vertical nodal plane consistent with the local strike of the San Andreas fault. Differences between the surface-wave focal mechanisms and the short-period P-wave first-motion mechanisms are observed for the aftershocks analyzed. This discrepancy may reflect the real variations due to differences in the band width of the two observations. However, the differences may also be due to (1) errors in the first-motion mechanism due to incorrect near-source velocity structure and (2) errors in the surface-wave mechanisms due to inadequate propagation path corrections.

The Sharpsburg, Kentucky, earthquake of 27 July 1980

1982 ◽  
Vol 72 (4) ◽  
pp. 1219-1239 ◽  
Author(s):  
Robert B. Herrmann ◽  
Charles A. Langston ◽  
James E. Zollweg
Keyword(s):  
Surface Wave ◽  
Focal Mechanism ◽  
Seismic Moment ◽  
Wave Solution ◽  
Focal Depth ◽  
The United States ◽  
P Wave ◽  
Nodal Plane ◽  
Epicentral Zone ◽  
Short Period

abstract The Sharpsburg, Kentucky, earthquake was the second largest earthquake to have occurred in the United States, east of the Continental Divide, in the past 20 yr, having a seismic moment of 4.1 × 1023 dyne-cm. A surface-wave focal mechanism study defines a nodal plane striking N30°E, dipping 50°SE, and a nearly vertical nodal plane striking N60°W. P-wave first motion data indicate right-lateral motion on the nodal plane striking N30°E, with the pressure axes oriented east-west. These angles can be varied by ±10° without affecting the fit to the surface-wave data. The surface-wave solution is reinforced by a modeling of long-period seismograms at regional distances. The P, pP, and sP polarities and amplitudes from the short-period vertical component array stack at NORSAR are used together with six unambiguous short-period P-wave first motions recorded in North America to test whether it is possible to constrain focal mechanism solutions with such data. These solutions are compatible with the surface-wave solution. Waveform modeling of the NORSAR data suggests a source pulse duration of 1.0 sec and constrains the depth to 12.0 km. To match mb estimates from NORSAR and Canadian stations, t*, for teleseismic P, must be 0.7 and 0.5, respectively, when the synthetics are scaled using the surface-wave seismic moment. In spite of extensive coverage of the epicentral zone, fewer than 70 aftershocks were recorded. The largest aftershock and an mbLg = 2.2. Aftershock locations suggest that the nodal plane striking N30°E is the fault plane. An aftershock area of 30 to 50 km2 implies a stress drop of 2.8 to 6 bars and a dislocation of 2.0 to 3.4 cm. Because of the variety of studies performed, this earthquake is presently the best-studied eastern North American seismic event with well-constrained estimates of focal depth, focal mechanism and seismic moment, and indications of the duration of the source time function and upper mantle P-wave t*.

Estimates of velocity structure and source depth using multiple P waves from aftershocks of the 1987 Elmore Ranch and Superstition Hills, California, earthquakes

1991 ◽  
Vol 81 (2) ◽  
pp. 508-523
Author(s):  
Jim Mori
Keyword(s):  
Velocity Structure ◽  
Velocity Model ◽  
P Wave ◽  
Sediment Layer ◽  
Source Depth ◽  
Imperial Valley ◽  
P Waves ◽  
Main Shocks ◽  
Aftershock Sequences ◽  
Event Record

Abstract Event record sections, which are constructed by plotting seismograms from many closely spaced earthquakes recorded on a few stations, show multiple free-surface reflections (PP, PPP, PPPP) of the P wave in the Imperial Valley, California. The relative timing of these arrivals is used to estimate the strength of the P-wave velocity gradient within the upper 5 km of the sediment layer. Consistent with previous studies, a velocity model with a value of 1.8 km/sec at the surface increasing linearly to 5.8 km/sec at a depth of 5.5 km fits the data well. The relative amplitudes of the P and PP arrivals are used to estimate the source depth for the aftershock distributions of the Elmore Ranch and Superstition Hills main shocks. Although the depth determination has large uncertainties, both the Elmore Ranch and Superstition Hills aftershock sequences appear to have similar depth distribution in the range of 4 to 10 km.

scholarly journals The comparative analysis of 2018 Alaska earthquakes from surface wave records

2020 ◽  
Vol 2 (1) ◽  
pp. 76-84
Author(s):  
Anastasiya Fomochkina ◽  
Boris Bukchin
Keyword(s):  
Surface Wave ◽  
Focal Mechanism ◽  
Seismic Moment ◽  
Optimal Solution ◽  
Seismic Source ◽  
P Wave ◽  
Wave Spectra ◽  
Earthquake Intensity ◽  
Second Moments ◽  
First Arrival

We consider the source of an earthquake in an approximation of instant point shift dislocation. Such a source is given by its depth, the focal mechanism determined by three angles (strike, dip, and slip), and the seismic moment characterizing the earthquake intensity. We determine the source depth and focal mechanism by a systematic exploration of 4D parametric space, and seismic moment - by solving the problem of minimization of the misfit between observed and calculated surface wave spectra for every combination of all other parameters. As is well known, the focal mechanism cannot be uniquely determined from the surface wave’s amplitude spectra only. We used P-wave first arrival polarities to select the optimal solution. Ana-lyzing the surface wave spectra at shorter periods, we describe the source in an approximation of the stress glut second moments. Using these moments we determine integral estimates of the geometry, the duration of the seismic source, and rupture propagation. The results of the application of this technique for two Alaska earthquakes that occurred in 2018 (with Mw7.9 in January and with Mw7.1 in November) are presented. The possibility of the fault plane identification, which based on the obtained estimates of the focal mechanisms and second mo-ments, is analyzed for both events. Bilateral model of the source is constructed.

Source mechanism of the Truckee, California, earthquake of September 12 1966

1970 ◽  
Vol 60 (4) ◽  
pp. 1199-1208 ◽  
Author(s):  
Yi-Ben Tsai ◽  
Keiiti Aki
Keyword(s):  
Seismic Moment ◽  
Additional Data ◽  
Focal Depth ◽  
P Wave ◽  
Eastern United States ◽  
Time Data ◽  
Travel Time Data ◽  
Amplitude Spectra ◽  
Plane Solution ◽  
Minimum Estimate

abstract With additional data on P wave polarities, the existing fault plane solution for the Truckee, California, earthquake of September 12 1966 (Ryall, Van Wormer and Jones, 1968) is revised to have the following strike and dip angles for its nodal planes: Strike Dip Plane A N 44°E 80°SE Plane B N 134°E 90° Nodal plane A is shown to agree very well with the aftershock zone determined by Greensfelder (1968). The observed Rayleigh and Love wave amplitude spectra from four WWSSN stations in the eastern United States are consistent with the revised solution, but not with the original one by Ryall et al. From these data, the focal depth and seismic moment of the earthquake are determined as 10 km and 0.83 × 1025 dyne-cm, respectively. The focal depth so obtained is the same as that determined from the near station travel time data by Ryall et al. The seismic moment is used to give a minimum estimate of about 30 cm for the average dislocation of the fault.

Short-period P-wave attenuation along various paths in North America as determined from P-wave spectra of the SALMON nuclear explosion

1976 ◽  
Vol 66 (5) ◽  
pp. 1609-1622 ◽  
Author(s):  
Zoltan A. Der ◽  
Thomas W. McElfresh
Keyword(s):  
United States ◽  
North America ◽  
Wave Attenuation ◽  
Nuclear Explosion ◽  
P Wave ◽  
Western United States ◽  
Source Spectrum ◽  
Wave Spectra ◽  
Short Period ◽  
Q Values

abstract Average Q values were determined for ray paths to various LRSM stations from the SALMON nuclear explosion by taking ratios of observed P-wave spectra to the estimated source spectrum. Most Q values for P-wave paths throughout eastern North America are in the range 1600 to 2000 while those crossing over into the western United States are typically around 400 to 500. These differences in Q for intermediate distances can sufficiently explain the differences in the teleseismic event magnitudes observed, 0.3 to 0.4 magnitude units, in the western versus the eastern United States, if one assumes that the low Q layer under the western United States is located at depths less than 200 km.

Propagation of high-frequency (4- to 50-Hz) P waves in the northeastern United States

1995 ◽  
Vol 85 (4) ◽  
pp. 1244-1248
Author(s):  
Eric P. Chael ◽  
Patrick J. Leahy ◽  
Jerry A. Carter ◽  
Noël Barstow ◽  
Paul W. Pomeroy
Keyword(s):  
United States ◽  
Decay Rate ◽  
High Frequency ◽  
P Wave ◽  
Wave Spectra ◽  
Northeastern United States ◽  
P Waves ◽  
Geometric Spreading ◽  
Frequency Independent

Abstract We have measured the decay rate of high-frequency (4- to 50-Hz) P waves in the northeastern United States. We analyzed signals from 28 explosions of a 1988 USGS/AFGL/GSC refraction survey recorded at distances between 30 and 400 km. Over this range, the decay rate steadily increases from Δ−2 at 10 Hz to Δ−4 at 45 Hz. If one assumes geometric spreading of Δ−1.3, then the remaining decay is consistent with a nearly frequency-independent Q of about 1000. The results provide a useful parameterization for predicting P-wave spectra at near-regional ranges.

Aftershocks and surface faulting associated with the intraplate Guinea, West Africa, earthquake of 22 December 1983

1987 ◽  
Vol 77 (5) ◽  
pp. 1579-1601
Author(s):  
C. J. Langer ◽  
M. G. Bonilla ◽  
G. A. Bollinger
Keyword(s):  
West Africa ◽  
Focal Mechanism ◽  
Main Shock ◽  
Strike Slip ◽  
Fault System ◽  
Epicentral Area ◽  
Lateral Strike Slip ◽  
Focal Mechanism Solutions ◽  
Short Period ◽  
East West

Abstract This study reports on the results of geological and seismological field studies conducted following the rare occurrence of a moderate-sized West African earthquake (mb = 6.4) with associated ground breakage. The epicentral area of the northwestern Guinea earthquake of 22 December 1983 is a coastal margin, intraplate locale with a very low level of historical seismicity. The principal results include the observation that seismic faulting occurred on a preexisting fault system and that there is good agreement among the surface faulting, the spatial distribution of the aftershock hypocenters, and the composite focal mechanism solutions. We are not able, however, to shed any light on the reason(s) for the unexpected occurrence of this intraplate earthquake. Thus, the significance of this study is its contribution to the observational datum for such earthquakes and for the seismicity of West Africa. The main shock was associated with at least 9 km of surface fault-rupture. Trending east-southeast to east-west, measured fault displacements up to ∼13 cm were predominantly right-lateral strike slip and were accompanied by an additional component (5 to 7 cm) of vertical movement, southwest side down. The surface faulting occurred on a preexisting fault whose field characteristics suggest a low slip rate with very infrequent earthquakes. There were extensive rockfalls and minor liquefaction effects at distances less than 10 km from the surface faulting and main shock epicenter. Main shock focal mechanism solutions derived from teleseismic data by other workers show a strong component of normal faulting motion that was not observed in the ground ruptures. A 15-day period of aftershock monitoring, commencing 22 days after the main shock, was conducted. Eleven portable, analog short-period vertical seismographs were deployed in a network with an aperture of 25 km and an average station spacing of 7 km. Ninety-five aftershocks were located from the more than 200 recorded events with duration magnitudes of about 1.5 or greater. Analysis of a selected subset (91) of those events define a tabular aftershock volume (26 km long by 14 km wide by 4 km thick) trending east-southeast and dipping steeply (∼60°) to the south-southwest. Composite focal mechanisms for groups of events, distributed throughout the aftershock volume, exhibit right-lateral, strike-slip motion on subvertical planes that strike almost due east. Although the general agreement between the field geologic and seismologic results is good, our preferred interpretation is for three en-echelon faults striking almost due east-west.

WIDE‐BAND EXTRACTION OF MANTLE P WAVES FROM AMBIENT NOISE

Geophysics ◽  
1964 ◽  
Vol 29 (5) ◽  
pp. 672-692 ◽  
Author(s):  
Milo Backus ◽  
John Burg ◽  
Dick Baldwin ◽  
Ed Bryan
Keyword(s):  
Ambient Noise ◽  
Wide Band ◽  
P Wave ◽  
Cumberland Plateau ◽  
P Waves ◽  
Single Output ◽  
Short Period ◽  
Multichannel Filtering ◽  
On Line ◽  
Element Array

The spatial correlation characteristics of ambient short‐period (0.5 to 5 cps) noise at Ft. Sill, Oklahoma, and on the Cumberland Plateau in Tennessee were investigated on “permanent” arrays with 3–4 kilometer diameter. Dominant ambient noise at the two locations is spatially organized, and to first order may be treated as a combination of seismic propagating wave trains. At the Tennessee location noise energy above one cps is dominantly propagating with velocities from 3.5 to 4.5 km/sec, and must be carried in deeply trapped, high‐order modes. Generalized multichannel filtering (Burg) can be used to preserve a large class of mantle P‐wave signals, wide‐band, in a single output trace, while at the same time specifically rejecting ambient noise on the basis of its organization. Results of generalized multichannel filtering applied on‐line at the nineteen‐element array in Tennessee and applied off‐line are discussed.

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