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) change d significantly because of the occurrence of the 1992 M(W) 6.1 Joshua Tree and the M(W) 7.3 Landers earthquakes. To quantify this change, fo cal 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 (S2) remained vertical, and cha nges in the state of stress consisted of variations in the trend of ma ximum and minimum principal stress axes (S1 and S3) and small variatio ns in the value of the relative stress magnitudes (phi). In general, t he stress state in the ECSZ has S1 trending east of north and phi = 0. 43 to 0.65, suggesting that the ECSZ is a moderate stress refractor an d the style of faulting is transtensional. South of the Pinto Mountain fault, in the region of the 1992 Joshua Tree earthquake, the stress s tate determined from the 1981 to 1991 background seismicity changed on 23 April and again on 28 June 1992. In the central zone, S1 rotated f rom N14-degrees +/- 5-degrees-E to N28-degrees +/- 5-degrees-E on 23 A pril and back again to N16-degrees +/- 5-degrees-E on 28 June. Thus, t he Landers mainshock in effect recharged some of the shear stress in t he region of the M(W) 6.1 Joshua Tree earthquake. Comparison of the st ate of stress before and after 28 June 1992, along the Landers mainsho ck rupture zone, showed that the mainshock changed the stress orientat ion. The S1 trend rotated 7-degrees to 20-degrees clockwise and became progressively more fault normal from south to north. Along the Emerso n-Camp Rock faults, the variation was so prominent that the focal mech anisms of aftershocks could not be fit by a single deviatoric stress t ensor. The complex distribution of P and T axes suggests that most of the uniform component of the applied shear stress along the northern p art of the rupture zone was released in the mainshock. The San Bernard ino Mountains region of the M(W) 6.2 Big Bear earthquake has a distinc tively different state of stress, as compared to the Landers region, w ith S1 trending N3-degrees +/- 5-degrees-W. This region did not show a ny significant change in the state of stress following the 1992 M(W) 6 .2 Big Bear sequence.