ALBERT

Description: A SPOT satellite image of the Charleston, South Carolina, area revealed a linear feature, the Woodstock lineament, that may be the surface expression of a seismogenic fault of the 1886 Charleston earthquake. An enhancement of the SPOT image reveals a linear, 5 km wide band on the east side of the lineament that extends at least 65 km in a north-northeast direction and coincides with a part of the axis of the highest intensity isoseismal contours of the Charleston earthquake. Corroborative geologic, geodetic, and geophysical data suggest that the lineament and associated band may be the result of ongoing tectonic activity on the Woodstock fault inferred from seismicity.

Description: In intraplate settings in eastern North America, paleoseismological studies have been used to obtain recurrence of earthquake at three locations. As causative faults are usually inaccessible at these locations the effects of paleoearthquakes have been studied. While calculating recurrence intervals we have tacitly assumed that the earthquakes at each locality repeatedly occurred on the same faults. Indicators of paleoearthquakes that were studied include deformed soft sediments due to seismically induced liquefaction (New Madrid and Charleston, SC, seismic zones), warped lake sediments and anomalous silt layers in otherwise organic rich sediments (Charlevoix, Canada seismic zone). The resulting recurrence intervals for New Madrid (≈ 600 years) and Charleston (1500–2000 years) are in general agreement with those obtained from statistical studies. At Charlevoix it was estimated that a MM intensity VI event (sufficient to disturb varves) occurs every ≈ 400 years.

Description: 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.

Description: The tectonic cause of intraplate earthquakes has remained enigmatic. As newer data became available, several common features were apparent for intraplate earthquakes occurring in a wide variety of geologic terranes. These have been incorporated in the intersection model. Seismicity occurs near the intersection of, and by the reactivation of, preexisting zones of weakness. The intersections are the foci of anomalous stress build up in response to the ambient stress field due to plate tectonic forces. This anomalous stress buildup is relieved by strike slip motion on a suitably oriented fault and, due to kinematic adjustment, is followed by vertical or horizontal movement on the intersecting fault. The intersection model has several elements that complement other models proposed to explain intraplate earthquakes.