Catalog of first motion focal mechanisms, 1986-1987, Volume 3

<div> <div> <div> <p>The western region of the Pyeongnam Basin has relatively higher e&#8203;arthquake activity than the rest of the Korean Peninsula. We analyzed 48 earthquakes in the area, with a magnitude (M<sub>L</sub>) of 2.0 or more, from January 2009 to June 2019. The hypocentral parameters were re-determined using an iterative algorithm that repeats the calculation until the residual error between the observed and calculated arrival time of a seismic phase at each station is minimized. Using the hypocenters and the optimal 1-D velocity model derived from this process, the focal mechanisms were determined using the first-motion polarities of body waves. Many earthquakes are associated with left-lateral strike-slip faults, with a strike in the NW-SE direction and a normal faulting component. A stress inversion was performed using data of the pressure and tensional axes from the focal mechanisms. The maximum principal stress in the study area acts in the NW-SE direction with high angles of plunge and differs from the maximum horizontal principal stress in the rest of the Korean Peninsula. This stress perturbation is caused by the detachment of a small local stress from the regional stress field due to the presence of weak faults with low shear strength that develop in the sedimentation environment of the Pyeongnam Basin.</p> </div> </div> </div>

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Catalog of first motion focal mechanisms 1981-1983; Volume 3

Open-File Report ◽

10.3133/ofr86285c ◽

1986 ◽

Author(s):

R.E. Needham

Keyword(s):

Focal Mechanisms ◽

First Motion

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Catalog of first motion focal mechanisms 1984-1985 : Volume 1

Open-File Report ◽

10.3133/ofr86520a ◽

1988 ◽

Author(s):

R.E. Needham

Keyword(s):

Focal Mechanisms ◽

First Motion

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

Bulletin of the Seismological Society of America ◽

10.1785/bssa0810051726 ◽

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.

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