Cj. Wolfe et al., OCEANIC TRANSFORM EARTHQUAKES WITH UNUSUAL MECHANISMS OR LOCATIONS - RELATION TO FAULT GEOMETRY AND STATE OF STRESS IN THE ADJACENT LITHOSPHERE, J GEO R-SOL, 98(B9), 1993, pp. 16187-16211
On oceanic transforms, most earthquakes are expected to occur on the p
rincipal transform displacement zone (PTDZ) and to have strike-slip me
chanisms consistent with transform-parallel motion. We conducted a sea
rch for transform earthquakes departing from this pattern on the basis
of source mechanisms and locations taken from the Harvard centroid mo
ment tensor catalogue and the bulletin of the International Seismologi
cal Centre, respectively. Events with unusual mechanisms occur on seve
ral transforms. We have determined the source mechanisms and centroid
depths of 10 such earthquakes on the St. Paul's, Marathon, Owen, Heeze
n, Tharp, Menard, and Rivera transforms from inversions of long-period
body waveforms. Relative locations of earthquakes along these transfo
rms have been determined with a multiple-event relocation technique. M
uch of the anomalous earthquake activity on oceanic transforms is asso
ciated with complexities in the geometry of the PTDZ or the presence o
f large structural features that may influence slip on the fault. Reve
rse-faulting earthquakes occur at a compressional bend in the Owen tra
nsform in the area of Mount Error and at the St. Paul's transform near
St. Peter's and St. Paul's Rocks. A normal-faulting earthquake on the
Heezen transform is located at the edge of a pull-apart basin marking
an extensional offset of the fault. Normal-faulting earthquakes along
the Tharp, Menard, and Rivera transforms may also be related to exten
sional offsets. Some events with unusual mechanisms occur outside of t
he transform fault zone, however, and do not appear to be related to f
ault zone geometry. For instance, earthquakes with mechanisms indicati
ng reverse-faulting on ridge-parallel fault planes are located near th
e ridge-transform intersections of the St. Paul's and the Marathon tra
nsforms. Possible additional contributors to the occurrence of anomalo
us earthquakes include recent changes in plate motion, differential li
thospheric cooling, and the development of a zone of weakness along th
e fault zone, but we do not find strong evidence to confirm the influe
nce of these processes.