Showing papers on "Hypocenter published in 1972"

Seismicity and tectonics of western Venezuela

Abstract: New seismicity data on western Venezuela and northeastern Colombia are presented. Teleseismically recorded earthquakes from 1930 through 1970 have been relocated by Joint Hypocenter Determination (JHD) or with source-station adjustments calculated by JHD. Additionally, 540 days of recording have been obtained with local seismographs installed near the Bocono Fault. The most intense shallow activity occurred north and south of the Tachira Depression along the eastern flank of the Cordillera Oriental of Colombia. The Bocono Fault Zone is seismically active; small shallow shocks were recorded in it by the local stations. Shallow earthquakes also occur in the Cordillera de Merida away from the Bocono Fault. The new hypocenters for the intermediate-depth Bucaramanga earthquakes define a smaller source volume than defined by previously computed hypocenters. A previously inferred southward-dipping seismic zone near Bucaramanga is probably spurious, a consequence of correlation between errors in latitude and errors in depth. If one assumes that these intermediate-depth earthquakes lie on a single lithospheric slab, that slab strikes approximately north and dips to the east. The distribution of hypocenters and focal mechanisms support the platetectonic hypothesis that the present tectonics of northwestern Venezuela are a result of eastward motion of the Caribbean plate with respect to the South American plate. The principal interface between these two plates may have changed within the last 5 m.y. from a zone of underthrusting west of the Sierra de Perija to the predominantly right-lateral Bocono Fault Zone.

Application of prediction analysis to hypocenter determination using a local array

Abstract: Prediction analysis provides an efficient means of estimating errors in local earthquake hypocenter determinations using a dense array of seismometers. The standard errors of the hypocentral coordinates and the origin time due to random errors in the data and model parameters may be calculated rigorously for a least-squares hypocenter inversion procedure using arbitrary earth models. Contour maps of the estimated standard errors in the hypocenter parameters are plotted for two array configurations and for several earth models and event depths. These maps are useful in predicting the relative accuracy and difficulty of hypocenter location as a function of position in the array, and in selecting events with small relative error to be used in velocity-structure refinement in the vicinity of the array. In particular, the method may be used to provide a means of optimizing array geometry to provide maximum hypocenter control in specific target regions. Tests were made using hypothetical data with prescribed error distributions showing good agreement with the error analysis.

Foreshock, Main Shock, and Larger Aftershocks of the Borrego Mountain Earthquake

Abstract: The Borrego Mountain earthquake, magnitude 6.4, occurred at 02:28:59.1 G.m.t. on April 9, 1968 and has been assigned a hypocenter at 33°11.4' N., 116°07.7' W., h=11.1 km. The focal-mechanism solution indicates right-lateral slip on a fault striking N. 48° W. and dipping 83° NE., which is consistent with the field observations of faulting and the regional tectonic framework. A single foreshock of magnitude 3.7 preceded the main shock by one minute, but no other precursory activity has been identified. During the year following the event, 135 aftershocks of magnitude 3.0 and greater have been identified and located, outlining a broad zone of activity centered on but displaced 2-3 km northeast of the 33-km-long surface rupture on the Coyote Creek fault. Fracturing at depth during the aftershock period evidently occurred throughout the width of the San Jacinto fault zone, but initial surface faulting was localized along the Coyote Creek fault at the zone's southwestern margin. The area of aftershock activity enlarged progressively with time, and the region of the original epicenter became relatively inactive late in the aftershock period, leading to a doughnut-shaped epicentral distribution of late aftershocks. Inasmuch as the epicenter of the main shock was roughly midway along the zone of aftershock activity, the faulting presumably was bilateral. This kind of faulting is unusual in California.

Computer Determination of Earthquake Focal Depth

Abstract: This paper is a continuation of [1], which described a method of determining a distant hypocenter from the arrival times of P waves.

An Application of Ray-Tracing to Seismic Event Location

Abstract: : The objective of the study was to determine whether source modeling could improve teleseismic event locations near island arc structures, using models defined by available information about the underthrusting plates of these regions. The authors describe a location method based on ray-tracing, using a crude source region plate model. In the case of the Aleutian-Alaskan region, this method predicts that all teleseismic locations should be shifted along the perpendicular to the Aleutian arc near the source location. The magnitude of the shift is estimated by a method which involves an initial hypocenter estimate, one calibration event anywhere along the island arc, and ray-tracing calculations of time residuals for sources near the estimated hypocenter. The conclusion is that the technique shows promise of being able to remove location bias in island arc structures and should be investigated further.

Crustal Deformation Propagation

Abstract: : Tilt steps similar to strain steps have been observed for earthquakes with magnitudes ranging from 2 to 8 and distances ranging anywhere from 10 km to 12,600 kilometers. Tilt-step propagation velocities from the hypocenter to the station have been observed from 1.2 to 3.8 km/sec, and those corresponding to the arrival time of teleseismic S waves. Tilt directions, amplitudes and velocities observed at several stations simultaneously for the same earthquake are internally consistent and are likely to depend on the tectonic environment of the station in addition to the focal mechanism. The low velocity of 1.2 km/ sec is consistent with a plastic wave propagation. Other velocities are close to the Rayleigh wave short-period group velocity over continental paths.