The 11 December, 1995 earthquake (Mw=6.4): Implications for the present-day relative motion on the Rivera-Cocos plate boundary

The 11 December, 1995 earthquake is the largest and best constrained instru mentally recorded event which has occurred on the Rivera-Cocos plate bounda ry. The reported focal mechanism for this event indicates almost pure strik e-slip faulting with nodal planes oriented north-south and east-west. A vis ual inspection shows that the seismograms recorded worldwide strongly sugge st a directivity effect indicative of a rupture propagating eastward from t he epicenter. This observation is confirmed by a directivity analysis which shows a sharp reduction in the difference between observed and synthetic s eismograms when the rupture direction is at an azimuth of approximately 90 degrees. These results indicate that the east-west trending nodal plane of the earthquake of 11 December, 1995 is the actual fault plane. Considering that this is the largest instrumentally recorded earthquake in the region, it strongly suggests that the relative motion of the Rivera plate with resp ect to the Cocos plate takes place along east-west oriented faults, and tha t the sense of motion is right-lateral, strike-slip.