  Coulomb-stress theory has been used for years in seismology to understand how
earthquakes trigger each other. Whenever an earthquake occurs, the stress field
changes, and places with positive increases are brought closer to failure.
Earthquake models that relate earthquake rates and Coulomb stress after a main
event, such as the rate-and-state model, assume that the magnitude distribution
of earthquakes is not affected by the change in the Coulomb stress. By using
different slip models, we calculate the change in Coulomb stress in the fault
plane for every aftershock after the Landers event (California, USA, 1992,
moment magnitude 7.3). Applying several statistical analyses to test whether
the distribution of magnitudes is sensitive to the sign of the Coulomb-stress
increase we conclude that no significant effect is observable. Further, whereas
the events with a positive increase of the stress are characterized by a much
larger proportion of strike-slip events in comparison with the seismicity
previous to the mainshock, the events happening despite a decrease in Coulomb
stress show no relevant differences in focal-mechanism distribution with
respect to previous seismicity.
