The Broach earthquake of March 23, 1970

1972 ◽  
Vol 62 (1) ◽  
pp. 47-61
Author(s):  
Harsh K. Gupta ◽  
Indra Mohan ◽  
Hari Narain
Keyword(s):  
Main Shock ◽  
Fault Plane ◽  
Geological Structure ◽  
P Wave ◽  
Field Observations ◽  
B Values ◽  
Macroseismic Effects ◽  
Godavari Valley ◽  
First Motion ◽  
Largest Aftershock

Abstract The recent seismicity of the Broach region has been studied and correlated with the regional geological structure. The macroseismic effects are briefly described. Analysis of the first motion of P-wave data indicates the plane striking N 92°E to be the fault plane as supported by field observations also. The present seismic activity is found to be similar to the recent Godavari Valley earthquake sequence of April 1970 and different from the earthquakes in the Koyna region on the basis of b values, foreshock-aftershock pattern, and the ratio of the largest aftershock to the main shock magnitude.

Control of rupture by fault geometry during the 1966 parkfield earthquake

1981 ◽  
Vol 71 (1) ◽  
pp. 95-116 ◽  
Author(s):  
Allan G. Lindh ◽  
David M. Boore
Keyword(s):  
Main Shock ◽  
Fault Plane ◽  
San Andreas Fault ◽  
Strong Motion ◽  
P Wave ◽  
Fault Geometry ◽  
Dynamic Rupture ◽  
San Andreas ◽  
Hill Station ◽  
Parkfield Earthquake

abstract A reanalysis of the available data for the 1966 Parkfield, California, earthquake (ML=512) suggests that although the ground breakage and aftershocks extended about 40 km along the San Andreas Fault, the initial dynamic rupture was only 20 to 25 km in length. The foreshocks and the point of initiation of the main event locate at a small bend in the mapped trace of the fault. Detailed analysis of the P-wave first motions from these events at the Gold Hill station, 20 km southeast, indicates that the bend in the fault extends to depth and apparently represents a physical discontinuity on the fault plane. Other evidence suggests that this discontinuity plays an important part in the recurrence of similar magnitude 5 to 6 earthquakes at Parkfield. Analysis of the strong-motion records suggests that the rupture stopped at another discontinuity in the fault plane, an en-echelon offset near Gold Hill that lies at the boundary on the San Andreas Fault between the zone of aseismic slip and the locked zone on which the great 1857 earthquake occurred. Foreshocks to the 1857 earthquake occurred in this area (Sieh, 1978), and the epicenter of the main shock may have coincided with the offset zone. If it did, a detailed study of the geological and geophysical character of the region might be rewarding in terms of understanding how and why great earthquakes initiate where they do.

Aftershocks of the February 21, 1973 Point Mugu, California earthquake

1976 ◽  
Vol 66 (6) ◽  
pp. 1931-1952
Author(s):  
Donald J. Stierman ◽  
William L. Ellsworth
Keyword(s):  
Fault Zone ◽  
Main Shock ◽  
Fault Plane ◽  
Body Wave ◽  
P Wave ◽  
Fault System ◽  
Wave Radiation ◽  
Fault Plane Solutions ◽  
Complex Deformation ◽  
Transverse Ranges

abstract The ML 6.0 Point Mugu, California earthquake of February 21, 1973 and its aftershocks occurred within the complex fault system that bounds the southern front of the Transverse Ranges province of southern California. P-wave fault plane solutions for 51 events include reverse, strike slip and normal faulting mechanisms, indicating complex deformation within the 10-km broad fault zone. Hypocenters of 141 aftershocks fail to delineate any single fault plane clearly associated with the main shock rupture. Most aftershocks cluster in a region 5 km in diameter centered 5 km from the main shock hypocenter and well beyond the extent of fault rupture estimated from analysis of body-wave radiation. Strain release within the imbricate fault zone was controlled by slip on preexisting planes of weakness under the influence of a NE-SW compressive stress.

The Godavari Valley earthquake sequence of April 1969

1970 ◽  
Vol 60 (2) ◽  
pp. 601-615 ◽  
Author(s):  
Harsh K. Gupta ◽  
Indra Mohan ◽  
Hari Narain
Keyword(s):  
Main Shock ◽  
Focal Depth ◽  
Earthquake Sequence ◽  
Total Strain ◽  
Type I ◽  
Aftershock Area ◽  
Macroseismic Effects ◽  
Godavari Valley

abstract The Godavari Valley earthquake sequence of April 1969 has been studied in detail. The (Sg − Pg) and the (Pg − Pn) intervals have been used for estimating the extent of aftershock area and focal depth variations respectively. The main shock of magnitude 5.7 was followed by a number of aftershocks which are related by the function Log N = a + b M. The value of b is found to be −0.51. The main shock of April 13 accounted for 70 per cent of the total strain released. This sequence belongs to Type I of Mogi's classification. The macroseismic effects are also discussed briefly.

Common features of the reservoir-associated seismic activities

1972 ◽  
Vol 62 (2) ◽  
pp. 481-492 ◽  
Author(s):  
Harsh K. Gupta ◽  
B. K. Rastogi ◽  
Hari Narain
Keyword(s):  
Main Shock ◽  
Detailed Examination ◽  
Factors Affecting ◽  
B Values ◽  
Common Features ◽  
Rate Of Increase ◽  
Tremor Frequency ◽  
Earthquake Sequences ◽  
Largest Aftershock ◽  
Frequency Magnitude

abstract A detailed examination of the behavior of earthquakes associated with over a dozen artificial lakes shows that, in all cases, the tremors were initiated or their frequency increased considerably following the lake filling and that their epicenters were mostly located within a distance of 25 km from the lakes. Among the factors affecting the tremor frequency are the rate of increase of water level, duration of loading, maximum levels reached, and the period for which the high levels are retained. The study of these reservoir-associated earthquake sequences reveals that the ratio of the largest aftershock to the main shock is high (about 0.9), and the b values are also high in the frequency-magnitude relation, which is contrary to the normal earthquakes of the concerned regions.

Earthquake/earth tide correlation and other features of the Susanville, California, earthquake sequence of June-July 1976

1980 ◽  
Vol 70 (5) ◽  
pp. 1583-1593
Author(s):  
Amy S. Mohler
Keyword(s):  
Shear Stress ◽  
Compressive Stress ◽  
Sierra Nevada ◽  
Fault Plane ◽  
Earth Tide ◽  
P Wave ◽  
First Motion ◽  
Entire Sequence ◽  
June July ◽  
Earthquake Sequences

abstract An earthquake of magnitude ML 4.5 occurred on June 20, 1976 in an area of complex faulting in northeastern California, near the intersection of the Sierra Nevada, Modoc Plateau, Cascade Range, and Basin and Range geological provinces. P-wave first motion plots for larger aftershocks of this earthquake indicate maximum and minimum compressive stress, respectively, in north-south and east-west directions, with predominantly strike-slip motion. Focal depths for these events ranged from 7 to 15 km, consistent with other earthquake sequences in the region. Origin times of more than 4,700 aftershocks for the period between June 20 and July 1 are compared with the phase of solid-earth tidal components appropriate for normal and shear stress on northeast- and northwest-trending fault planes. Based on this comparison, approximately 20 per cent more earthquakes occurred at times when the normal compressive stress on the fault plane was decreasing, and the shear stress was increasing in the sense of slip on the fault plane. This correlation may be explained by two large bursts of aftershocks that occurred at times when tidal stresses were favorable for motion on the fault plane, rather than continuous triggering of small events during the entire sequence.

The Peru-Bolivia border earthquake of August 15, 1963

1970 ◽  
Vol 60 (2) ◽  
pp. 639-646 ◽  
Author(s):  
Umesh Chandra
Keyword(s):  
Travel Time ◽  
Fault Plane ◽  
P Wave ◽  
Focal Mechanism Solution ◽  
Event Analysis ◽  
Rupture Propagation ◽  
Fault Propagation ◽  
Amplitude Data ◽  
First Motion ◽  
Deep Focus

abstract The seismograms of the deep focus Peru-Bolivia border earthquake of August 15, 1963 reveal the presence of a number of conspicuous phases occurring within 15 seconds of the first P onset. These phases cannot be explained on the basis of known travel-time curves. Accordingly, the earthquake is interpreted to have occurred in a series of jerks during the course of fault propagation, or in other words it is composed of multiple events. Only one of these events, following the first event, at which the amplitude of the recorded motion becomes suddenly very large, has been located in this study. The focal mechanism solution of this earthquake has been determined from the P wave first motion and amplitude data. Consideration of the direction of rupture propagation determined from the multiple event analysis makes it possible to identify the fault plane in the mechanism solution. The parameters of the fault plane, length and speed of rupture between the two events have been determined.

Source dynamics of the Dasht-e Bayāz earthquake of August 31, 1968

1969 ◽  
Vol 59 (5) ◽  
pp. 1843-1861
Author(s):  
Mansour Niazi
Keyword(s):  
Stress Drop ◽  
Main Shock ◽  
Mean Value ◽  
The Body ◽  
P Wave ◽  
Polarization Angle ◽  
S Wave ◽  
Long Duration ◽  
First Motion ◽  
Complicated Pattern

abstract Radiation patterns of the P-wave first motion and S-wave polarization angle of the Dasht-e Bayāz earthquake of August 31, 1968, as well as its principal aftershock which occurred about 20 hours after the main shock are studied. The main shock data are consistent with the observed left-lateral strike-slip fault which accompanied it. The radiation pattern of the aftershock differs somewhat from that of the main shock and agrees with the directions of the secondary faulting in the area. Several lines of evidence pointing to a multiple source for the main shock are presented. They include complexity of the body phases, low value of the rupture speed as studied from the analysis of the surface wave spectra, reported long duration of shaking and complicated pattern of striations produced by faulting. Energy, moment and stress drop associated with the main shock are estimated. The resulting mean value of stress drop over the faulted surface has a range of 40-100 bars. Based on the age of some well-built structures in the area, it is proposed that no earthquake as severe as the recent one has occurred near the location of the August 31, 1968 earthquake during the last 800 years.

scholarly journals Fault plane solutions of the 1993 and 1995 Gulf of Aqaba earthquakes and their tectonic implications

1997 ◽  
Vol 40 (6) ◽  
Author(s):  
A. K. Abdel-Fattah ◽  
H. M. Hussein ◽  
E. M. Ibrahim ◽  
A. S. Abu El Atta
Keyword(s):  
Fault Plane ◽  
Strike Slip ◽  
P Wave ◽  
Gulf Of Aqaba ◽  
Main Trend ◽  
Gulf Of Suez ◽  
Normal Faulting ◽  
Aftershock Distribution ◽  
Lateral Strike Slip ◽  
Largest Aftershock

The stereographic projection of P-wave first motions for the 3 August 1993 Gulf of Aqaba earthquake, its largest aftershock (16 h 33 min), and for the 22 November 1995 earthquake were constructed using the polarity readings of regional and teleseismic stations. The focal mechanism solutions of the 3 August 1993 mainshock and its largest aftershock represent a normal faulting mechanism with some left lateral strike slip component. The nodal planes selected as the fault imply high similarity in strike and dip. They are related to a local fault striking NW-SE and dipping to the SW. The selected fault planes are in good agreement with the aftershock distribution. For the main shock of the 22 November 1995, the fault plane solution displays the same mechanism (normal faulting with left lateral strike slip component) with a plane striking N-S and dipping to the west. The fault plane is greatly conformable with the direction of the regional tectonics and also with the aftershock distribution. The main trend of the extension stress pattern is in a NE-SW direction, corresponding to the rifting direction of the Gulf of Suez and may be related to the paleostress along the Gulf of Suez and Aqaba during the Middle to Late Miocene.

scholarly journals Earthquake of July 1980 in Far Western Nepal

1982 ◽  
Vol 2 (2) ◽  
Author(s):  
Vinod Singh
Keyword(s):  
Wave Motion ◽  
Fault Plane ◽  
P Wave ◽  
Main Central Thrust ◽  
Nodal Plane ◽  
Nepal Himalaya ◽  
The Earth ◽  
Main Boundary Thrust ◽  
First Motion ◽  
Plane Solution

This paper is an attempt to study the seismicity of the Nepal Himalaya and the earth-quake of July 1980 of Magnitude 6.5 in western Nepal on its regional geological framework. A new fault plane solution is obtained for this earthquake from the study of P wave motion on WWSSN Seismograms at 34 stations. The solution given by Kanamori and Given is not in conformity with the observed P wave first motion. The damaged area is elongated in the azimuth of 110°-120°, which is parallel to the strike of the nodal plane obtained in this study. From the study of the seismicity of Nepal region it is observed that most of the seismicity in recent years is confined in between the Main Central Thrust and the Main Boundary Thrust.

Analysis of near-source static and dynamic measurements from the 1979 Imperial Valley earthquake

1982 ◽  
Vol 72 (6A) ◽  
pp. 1927-1956
Author(s):  
Ralph J. Archuleta
Keyword(s):  
Particle Velocity ◽  
Large Amplitude ◽  
Main Shock ◽  
Fault Plane ◽  
P Wave ◽  
Far Field ◽  
Surface Rupture ◽  
Imperial Valley ◽  
Static Stress Drop ◽  
Vertical Accelerations

abstract Gross features of the rupture mechanism of the 1979 Imperial Valley earthquake (ML = 6.6) are inferred from qualitative analysis of near-source ground motion data and observed surface rupture. A lower bound on the event's seismic moment of 2.5 × 1025 dyne-cm is obtained by assuming that the average slip over the whole fault plane equals the average surface rupture, 40.5 cm. Far-field estimates of moment suggest an average slip over the fault plane of 105 cm, from which a static stress drop of 11 bars is obtained. An alternative slip model, consistent with the far-field moment, has 40.5 cm of slip in the upper 5 km of the fault and 120 cm of slip in the lower 5 km. This model suggests a static stress drop of 39 bars. From the lower estimate of 11 bars, an average strain drop of 32 µstrain is derived. This strain drop is four times greater than the strain that could have accumulated since the 1940 El Centro earthquake based on measured strain rates for the region. Hence, a major portion of the strain released in the 1979 main shock had been accumulated prior to 1940. Unusually large amplitude (500 to 600 cm/sec2) vertical accelerations were recorded at stations E05, E06, E07, E08, EDA of the EI Centro array, and the five stations of the differential array near EDA. Although the peak acceleration of 1705 cm/sec2 at E06 is probably amplified by a factor of 3 due to local site conditions, these large amplitude vertical accelerations are unusual in that they are evident on only a few stations, all of which are near the fault trace and at about the same epicentral range. Two possible explanations are considered: first, that they are due to a direct P wave generated from a region about 17 km north of the hypocenter, or second, that they are due to a PP phase that is unusually strong in the Imperial Valley due to the large P-wave velocity gradient in the upper 5 km of the Imperial Valley. Based on the distribution of both the horizontal and vertical offsets, it is likely that the rupture went beyond stations E06 and E07 during the main shock. By exploiting the antisymmetry of the parallel components of particle velocity between E06 and E07 and by examining polarization diagrams of the particle velocity at E06 and E07, an average rupture velocity in the basement of 2.5 to 2.6 km/sec between the hypocenter and station E06 is obtained. In addition, several lines of evidence suggest that the Imperial fault dips about 75° to the NE.

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