The July 1986 Oceanside (M_L = 5.3) earthquake sequence in the Continental Borderland, Southern California

Abstract

An earthquake of M_L = 5.3 occurred at 32°58.7′N, 117°51.5′W southwest of Oceanside in San Diego County at 13:47 13 July 1986 (UT). This main shock was followed by an extensive aftershock sequence, with 55 events of M_L ≧ 3.0 during July 1986. The epicenters of the main shock and aftershocks are located at the northern end of the San Diego Trough-Bahia Soledad fault zone (SDT-BS) where it changes strike from northwest to a more westerly direction through a left offset or a bend in the fault. The northwest-striking SDT-BS is one of three strike-slip fault systems that constitute the offshore Agua Blanca fault system. The spatial distribution of the aftershocks indicates a unilateral 7- to 9-km long rupture to the east-southeast away from the epicenter of the main shock. The focal mechanism of the main shock also has an east-southeast striking and south-dipping plane with mostly reverse movement on it. Focal mechanisms of the M_L ≧ 3.0 aftershocks show both reverse and strike-slip movement. The reverse focal mechanisms indicate that this sequence may have occurred on a thrust fault that provides for a left stepping offset or a bend in the San Diego Trough fault as movement is transferred to the west along the Santa Cruz-Catalina Island escarpment. Some of the aftershocks that are located to the southeast of the main shock and have strike-slip focal mechanisms suggest activation of the northwest-trending San Diego Trough fault. A stress inversion of the focal mechanism data shows that the maximum principal stress determined from the focal mechanisms of the main shock and 22 aftershocks that occurred within 36 hours of the main shock has an azimuth of S30°W plunging 18°. The maximum principal stress determined from 30 aftershocks that occurred from 15 July to 2 October 1986 has an azimuth of S20°W, plunging 18°. The φ-values (the measure of the relative sizes of the principal stresses) are approximately 0.07 and 0.1, respectively, indicating that the intermediate and minimum principal stress are of similar magnitude. The results of the stress inversion, and the focal mechanisms that showed reverse faulting, suggest that the Inner Continental Borderland offshore from Oceanside is not currently a pure-strike-slip tectonic regime but rather a strike-slip mixed with reverse faulting regime. When the 52 aftershock focal mechanisms are divided into four groups and the stress inversion is repeated, the change in stress can be described as a progressive counter-clockwise rotation of 14° of the orientation of the maximum principal stress to a more southerly direction, with the greatest change in stress orientation observed shortly after the main shock. The abundance of aftershocks may be related to the large temporal variation in stress orientation that, in turn, may have resulted from the small stress drop of the main shock.