Complex sentence: The Mendocino Fault is seismically active with mostly small and moderate earthquakes, but the largest event that was unequivocally associated with the fault was the M 6.9 earthquake on September 1,1994 at 125.8 W longitude.
Split into simpler sentences: The Mendocino Fault is seismically active with mostly small and moderate earthquakes. The largest event that was unequivocally associated with the fault was the M 6.9 earthquake on September 1,1994 at 125.8 W longitude.