Seismological aspects of the Guatemala Earthquake of February 4, 1976

Detailed analyses of teleseismic surface waves and body waves from the Guatemala e rthquake of February 4, 1976, show the following: (1) Left lateral displacement along a vertical fault with a strike varying from N66øE to N98øE is consistent with the teleseismic data. (2) The seismic moment was 2.6 X 102? dyn cm. The directivity ofthe surface wave radiation i dicates an asymmetric (1:2.3) bilateral faulting with a total ength of 250 km. In modeling the displacement a rupture velocity of 3 km/s was used, and the fault curvature was included. (3) If a fault width of 15 km is assumed, the average offset is estimated to be about 2m. This value is about twice as large as the average surface offset. (4) Although t e observed directivity suggests a uniform overall displacement along the fault, the body wave analysis suggests that the earthquake consists of as many as 10 independent vents, each aving a seismic moment of 1.3-5.3 X 1026 dyn cm and a fault length of about 10 km. The spatial separation f these vents varies from 14 to 40 km. This multiple-shock sequence suggests that the rupture propagation is jagged and partially incoherent with an average velocity of 2 km/s. (5) The average stress drop estimated from surface waves i about 30 bars, but the local stress drop for the individual events may be significantly higher than this. (6) The complex multiple event is a manifestation of aheterogeneous distribution f the mechanical properties along the fault, which may be caused by either asperities, differences in trength, differences in pore pressure, differences in lip characteristics (stable sliding versus stick slip), or combinations of these factors. (7) This complexity has important bearing on the state of stress along transform faults and is important inassessing theeffect of large arthquakes along other transform faults like the San Andreas.