The Tangshan aftershock sequence

Aftershocks of the Tangshan, China, earthquake sequence occurred on strike‐slip, normal, and thrust faults in an intensely faulted region. The generally northeast trending aftershock zone outlines an area 150 km long and 30 km wide, except for a northwest‐southeast trend of aftershocks which is about 60 km wide. The main shock fault plane inferred from the locations of the aftershocks varies in strike from N30°E at the southern end to N50°E at the northeastern end, with the change in strike occurring near the epicenter of the main shock. Fault plane solutions of the aftershocks indicate that strike‐slip faulting occurred along the Tangshan fault and thrust faulting occurred on smaller, transverse faults south and west of the Tangshan fault. Although faulting during the main shock included right‐lateral, strike‐slip, and thrust movement, perturbations to the prevailing stress field due to slip during the main shock and the configuration of existing faults encouraged normal faulting in the northeast part of the zone. The largest aftershock occurred about 45 km northeast of the main shock and was associated with normal faulting at shallow depths. Two other large aftershocks, both associated with large strike‐slip components, occurred southwest of the main shock. Fault plane solutions and P wave and S wave forms of these two aftershocks indicate primarily left‐lateral motion along a northwest striking fault plane. The aftershocks range in depth from near the surface to about 20 km. Fault plane solutions of 61 of the aftershocks indicate that the overall deformation consists of north‐south extension accommodated by both normal and strike‐slip faults such that some east‐west crustal shortening and possibly some crustal thinning took place.