Lateral velocity variations in the Orozco Transform Fault inferred from observed incident angles and azimuths of P ‐waves

Summary Body waves from earthquake and explosive sources which were recorded by a three‐component ocean bottom seismometer (OBS) in the Orozco transform fault show pronounced systematic deflections of incident angles and azimuths from those expected for a laterally homogeneous velocity structure. After considering several phenomena which could affect the observations, we conclude that the observed pattern of deflections in part reflects regional lateral velocity variations. Possible models explaining the observations are examined by tracing rays through three‐dimensional velocity structures. A model in which the Moho is elevated beneath a north‐south‐trending topographic high to the west of the OBS can explain observations from sources to the north, west and south and is compatible with seismic refraction results in the region. East of the OBS, a low‐velocity region simulating a magma chamber results in very steep incident angles and large azimuthal deflections which qualitatively match those observed from earthquakes in the central transform region. Although these data alone do not provide irrefutable evidence for a magma chamber, that the corrections to fault plane solutions and hypocentres implied by this model can explain major inconsistencies in the source parameters calculated relative to a laterally homogeneous velocity model supports its validity.