State of Stress from Focal Mechanisms Before and After the 1992 Landers Earthquake Sequence

The state of stress in the Eastern California Shear Zone (ECSZ) changed significantly because of the occurrence of the 1992 Mw 6.1 Joshua Tree and the Mw 7.3 Landers earthquakes. To quantify this change, focal mechanisms from the 1975 Galway Lake sequence, the 1979 Homestead Valley sequence, background seismicity from 1981 to 1991, and the 1992 Landers sequence are inverted for the state of stress. In all cases, the intermediate principal stress axis (Sz) remained vertical, and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes ($1 and $3) and small variations in the value of the relative stress magnitudes (~b). In general, the stress state in the ECSZ has $1 trending east of north and 4) = 0.43 to 0.65, suggesting that the ECSZ is a moderate stress refractor and the style of faulting is transtensional. South of the Pinto Mountain fault, in the region of the 1992 Joshua Tree earthquake, the stress state determined from the 1981 to 1991 background seismicity changed on 23 April and again on 28 June 1992. In the central zone, $1 rotated from N14 ° --5°E to N28 ° -.+ 5°E on 23 April and back again to N16 ° + 5°E on 28 June. Thus, the Landers mainshock in effect recharged some of the shear stress in the region of the Mw 6.1 Joshua Tree earthquake. Comparison of the state of stress before and after 28 June 1992, along the Landers mainshock rupture zone, showed that the mainshock changed the stress orientation. The $1 trend rotated 7 ° to 20 ° clockwise and became progressively more fault normal from south to north. Along the Emerson-Camp Rock faults, the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor. The complex distribution of P and T axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock. The San Bernardino Mountains region of the Mw 6.2 Big Bear earthquake has a distinctively different state of stress, as compared to the Landers region, with S~ trending N3 ° --+ 5°W. This region did not show any significant change in the state of stress following the 1992 Mw 6.2 Big Bear sequence.