The Lake Keowee, South Carolina Earthquakes of February through July 1986

1987 ◽  
Vol 59 (2) ◽  
pp. 63-70 ◽  
Author(s):  
Steven D. Acree ◽  
Jill R. Acree ◽  
Pradeep Talwani
Keyword(s):  
South Carolina ◽  
Focal Mechanism ◽  
Horizontal Stress ◽  
Early Morning ◽  
Free Solution ◽  
Epicentral Area ◽  
Focal Mechanism Solutions ◽  
Duration Magnitude ◽  
First Motion ◽  
Maximum Horizontal Stress

Abstract In the early morning of 13 February 1986, an earthquake with a duration magnitude (MD) of 3.2 rumbled through northwestern South Carolina. The event was centered near Lake Keowee in Oconee County in a region of prior low level seismicity. Approximately eighty aftershocks with magnitudes ranging from −1.0 to 2.0 were recorded during the next six days. The locations of five aftershocks were accurately determined, utilizing data from portable seismographs deployed in the epicentral area. Depths of the two earthquakes with a location quality of B or better were between 3 and 4 km. First motion focal mechanism solutions for the mainshock suggest oblique slip along a plane striking northeast or northwest. The P axis was oriented northeast-southwest in support of the directions obtained from mechanisms of other local earthquakes and from direct measurements of the maximum horizontal stress in the regions. A second mainshock (MD = 2.8) occurred in the vicinity of Lake Keowee on 11 June 1986 and was followed by over sixty earthquakes during the next five weeks. Focal mechanism solutions from first motion data obtained for the mainshock resemble those of the 13 February event and suggest oblique slip along a northeast or northwest striking plane. Depths of the best located aftershocks were approximately 1 km. Two tests were applied to the data to assess the reliability of the depth estimates. These involve the determination that the plot of RMS travel time residual versus fixed solution depth exhibits a single, sharp RMS minimum at the depth obtained from a free solution (depth uniqueness) and that the final free solution depth is not dependent upon the choice of starting depth (depth stability). Free solution depths obtained for the majority of the better located aftershocks were found to be unique and stable at depths between 1 and 4 km. A northeast trending anomaly is prominent in the geophysical data for this area. This anomaly is interpreted to result from an abrupt, lateral change in lithology along a shallow, northeast striking plane. The earthquakes do not appear to be associated with this feature. Instead, these earthquakes appear to be associated with a shallow body and may represent slip along northeast or northwest striking joints. The proximity of these earthquakes to Lake Keowee suggests the possibility of reservoir triggering. No correlation between seismicity and reservoir level is evident prior to the February events. Rapid fluctuations in water level did precede the events in June and July, providing possible triggering mechanisms.

PRESENT-DAY STATE-OF-STRESS OF SOUTHEAST AUSTRALIA

2006 ◽  
Vol 46 (1) ◽  
pp. 283 ◽  
Author(s):  
E. Nelson ◽  
R. Hillis ◽  
M. Sandiford ◽  
S. Reynolds ◽  
S. Mildren
Keyword(s):  
Focal Mechanism ◽  
Horizontal Stress ◽  
Strike Slip ◽  
Focal Mechanism Solutions ◽  
Southeast Australia ◽  
State Of Stress ◽  
Earthquake Focal Mechanism ◽  
Minimum Horizontal Stress ◽  
Maximum Horizontal Stress ◽  
Gippsland Basin

There have been several studies, both published and unpublished, of the present-day state-of-stress of southeast Australia that address a variety of geomechanical issues related to the petroleum industry. This paper combines present-day stress data from those studies with new data to provide an overview of the present-day state-of-stress from the Otway Basin to the Gippsland Basin. This overview provides valuable baseline data for further geomechanical studies in southeast Australia and helps explain the regional controls on the state-of-stress in the area.Analysis of existing and new data from petroleum wells reveals broadly northwest–southeast oriented, maximum horizontal stress with an anticlockwise rotation of about 15° from the Otway Basin to the Gippsland Basin. A general increase in minimum horizontal stress magnitude from the Otway Basin towards the Gippsland Basin is also observed. The present-day state-of-stress has been interpreted as strike-slip in the South Australian (SA) Otway Basin, strike-slip trending towards reverse in the Victorian Otway Basin and borderline strike-slip/reverse in the Gippsland Basin. The present-day stress states and the orientation of the maximum horizontal stress are consistent with previously published earthquake focal mechanism solutions and the neotectonic record for the region. The consistency between measured present-day stress in the basement (from focal mechanism solutions) and the sedimentary basin cover (from petroleum well data) suggests a dominantly tectonic far-field control on the present-day stress distribution of southeast Australia. The rotation of the maximum horizontal stress and the increase in magnitude of the minimum horizontal stress from west to east across southeast Australia may be due to the relative proximity of the New Zealand segment of the plate boundary.

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.

Focal mechanism and stress field in the northeastern Tibetan Plateau: insight into layered crustal deformations

2019 ◽  
Vol 218 (3) ◽  
pp. 2066-2078 ◽  
Author(s):  
Cunrui Han ◽  
Zhouchuan Huang ◽  
Mingjie Xu ◽  
Liangshu Wang ◽  
Ning Mi ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Stress Field ◽  
Focal Mechanism ◽  
Horizontal Stress ◽  
P Wave ◽  
The Tibetan Plateau ◽  
Linear Inversion ◽  
Focal Mechanism Solutions ◽  
Stress Orientations ◽  
Ne Tibetan Plateau

SUMMARY Focal mechanism solutions (FMSs) reflect the stress field underground directly. They provide essential clue for crustal deformations and therefore improve our understanding of tectonic uplift and expansion of the Tibetan Plateau. In this study, we applied generalized Cut and Paste and P-wave first-motion methods to determine 334 FMSs (2.0 ≤ Mw ≤ 6.4) with the data recorded by a new temporary network deployed in the NE Tibetan Plateau by ChinArray project. We then used 1015 FMSs (including 681 published FMSs) to calculate the regional stress field with a damped linear inversion. The results suggest dominant thrust and strike-slip faulting environments in the NE Tibetan Plateau. From the Qilian thrust belt to the Qinling orogen, the maximum horizontal stress orientations (${S_\mathrm{ H}}$) rotate clockwise from NNE to NE, and further to EW, showing a fan-shaped pattern. The derived minimum horizontal stress orientations (${S_\mathrm{ h}}$) are parallel to the aligned fabrics in the mantle lithosphere indicated by shear wave splitting measurements, suggesting vertically coherent deformation in the NE Tibetan Plateau. Beneath the SW Qinling adjacent to the plateau, however, the stress orientations in the shallow and deep crust are different, whereas the deep crustal stress field indicates possible ductile crustal flow or shear.

Shaking in the Southeastern United States: Examining Earthquakes and Blasts in the Central Georgia–South Carolina Seismic Region

2021 ◽  
Author(s):  
Rachel E. Marzen ◽  
James B. Gaherty ◽  
Donna J. Shillington ◽  
Won-Young Kim
Keyword(s):  
United States ◽  
South Carolina ◽  
Southeastern United States ◽  
Horizontal Stress ◽  
Water Levels ◽  
Spectral Amplitude ◽  
Seismic Zone ◽  
Seismic Region ◽  
Temporal Cluster ◽  
Maximum Horizontal Stress

Abstract Seismicity in the southeastern United States is relatively poorly characterized and thus not well understood. Structures and heterogeneities from multiple stages of Appalachian orogenesis, continental rifting, and magmatism as well as rivers and reservoirs may be influencing seismic activity in the region, but previous constraints are limited. The addition of seismic stations from the U.S. Transportable Array and the Southeastern Suture of the Appalachian Margin Experiment Array in 2012–2014 provide an opportunity to characterize seismicity in the central Georgia–South Carolina region. We develop a seismic catalog of >1000 events from March 2012 to May 2014 within or near the instrument array boundaries 30.1°–35.2°N, 80.9°– 85.7°W. Many of the events detected were industrial blasts, so multiple strategies were tested to discriminate between earthquakes and blasts based on event locations, timing, and spectral amplitude of the P and S arrivals. Based on this analysis, ∼10% of the events in the catalog were classified as earthquakes. Most earthquakes southeast of the eastern Tennessee seismic zone are located in the Carolina terrane, particularly where the Carolina terrane intersects major rivers or reservoirs. One prominent region of seismicity along the Savannah River near Thurmond Lake corresponds with an ∼4.5  m rise in water levels in 2013. A temporal cluster of earthquakes in April 2013 was followed by increased levels of ambient seismicity preceding the nearby Mw 4.1 earthquake in 2014. Focal mechanisms based on first-motion polarities indicate strike-slip to oblique-thrust motion on structures trending approximately north–south or east–west, and a maximum horizontal stress orientation consistent with the regional trend of ∼N60°E, implying that seismicity may reactivate more optimally oriented structures in the Carolina terrane that are oblique to the trend of the Appalachians. Seismicity in central Georgia appears to be controlled by a complex interaction between preexisting crustal structure and hydrologic variability.

scholarly journals Stress filed data visualization by using earthquake focal mechanism and shear wave splitting results in the Makran region northern Iran

2021 ◽  
Vol 34 (02) ◽  
pp. 825-834
Author(s):  
Shahrokh Pourbeyranvand
Keyword(s):  
Shear Wave ◽  
Data Visualization ◽  
Focal Mechanism ◽  
Horizontal Stress ◽  
Shear Wave Splitting ◽  
Northern Iran ◽  
Earthquake Focal Mechanism ◽  
Study Results ◽  
Wave Splitting ◽  
Maximum Horizontal Stress

In this study, a new method is introduced for stress data visualization. As an example, the SHmax variations in the Makran region are mapped by using this new approach. Maximum horizontal stress directions in the study area are extracted from earthquake focal mechanism data. The anisotropy study results in terms of shear wave splitting fast direction axes are added to the dataset to show its effect. The results show substantial variation in SHmax directions, which reflects the complicated tectonic nature of the region. Adding the shear wave splitting data improved the results' accuracy and showed the correlation between the two quantities in the study area.

scholarly journals Implications of 3D Seismic Raytracing on Focal Mechanism Determination

2019 ◽  
Vol 109 (6) ◽  
pp. 2746-2754
Author(s):  
Katharina Newrkla ◽  
Hasbi Ash Shiddiqi ◽  
Annie Elisabeth Jerkins ◽  
Henk Keers ◽  
Lars Ottemöller
Keyword(s):  
Focal Mechanism ◽  
Historical Earthquakes ◽  
Velocity Model ◽  
Waveform Data ◽  
Focal Mechanism Solutions ◽  
Velocity Models ◽  
Maximum Value ◽  
3D Velocity Model ◽  
First Motion

Abstract The purpose of this study is to investigate apparent first‐motion polarities mismatch at teleseismic distances in the determination of focal mechanism. We implement and compare four seismic raytracing algorithms to compute ray paths and travel times in 1D and 3D velocity models. We use the raytracing algorithms to calculate the takeoff angles from the hypocenter of the 24 August 2016 Mw 6.8 Chauk earthquake (depth 90 km) in central Myanmar to the stations BFO, GRFO, KONO, and ESK in Europe using a 3D velocity model of the upper mantle below Asia. The differences in the azimuthal angles calculated in the 1D and 3D velocity models are considerable and have a maximum value of 19.6°. Using the takeoff angles for the 3D velocity model, we are able to resolve an apparent polarity mismatch where these stations move from the dilatational to the compressional quadrant. The polarities of synthetic waveforms change accordingly when we take the takeoff angles corresponding to the 3D model into account. This method has the potential to improve the focal mechanism solutions, especially for historical earthquakes where limited waveform data are available.

scholarly journals First-Motion Focal Mechanism Solutions for 2015–2019 M ≥ 4.0 Italian Earthquakes

2021 ◽  
Vol 9 ◽  
Author(s):  
Maria G. Ciaccio ◽  
Raffaele Di Stefano ◽  
Luigi Improta ◽  
Maria T. Mariucci ◽  
Keyword(s):  
Focal Mechanism ◽  
Carbonate Platform ◽  
Central Italy ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
Seismic Sequence ◽  
Earthquake Parameters ◽  
Focal Mechanism Solutions ◽  
Initial Rupture ◽  
First Motion

A list of 100 focal mechanism solutions that occurred in Italy between 2015 and 2019 has been compiled for earthquakes with magnitude M ≥ 4.0. We define earthquake parameters for additional 22 seismic events with 3.0 ≤ M < 4.0 for two specific key zones: Muccia, at the northern termination of the Amatrice–Visso–Norcia 2016–2018 central Italy seismic sequence, and Montecilfone (southern Italy) struck in 2018 by a deep, strike-slip Mw 5.1 earthquake apparently anomalous for the southern Apennines extensional belt. First-motion focal mechanism solutions are a good proxy for the initial rupture and they provide important additional information on the source mechanism. The catalog compiled in the present paper provides earthquake parameters for individual events of interest to contribute, as a valuable source of information, for further studies as seismotectonic investigations and stress distribution maps. We calculated the focal mechanisms using as a reference the phase pickings reported in the Italian Seismic Bulletin (BSI). We visually checked the reference picks to accurately revise manual first-motion polarities, or include new onsets when they are not present in the BSI dataset, for the selected earthquakes within the whole Italian region, with a separate focus on the Amatrice–Visso–Norcia seismic sequence area from August 24, 2016 to August 24, 2018. For the Montecilfone area, we combined the information on the geometry and kinematics of the source of the 2018 Mw 5.1 event obtained in this study with available subsurface and structural data on the Outer Apulia Carbonate Platform to improve understanding of this intriguing strike-slip sequence. Our analysis suggests that the Montecilfone earthquake ruptured a W–E trending strike-slip dextral fault. This structure is confined within the Apulia crystalline crust and it might represent the western prolongation of the Mattinata Fault–Apricena Fault active and seismogenic structures. The calculated focal mechanisms of the entire catalog are of good quality complementing important details on source mechanics from moment tensors and confirming the relevance of systematically including manually revised and more accurate polarity data within the BSI database.

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