 The previous under-estimate is believed to be due mostly to not modeling multifault ruptures, which limited the size of many ruptures.The largest probability decrease is on the San Jacinto fault, which went from 32% to 9%.Again this is due to multifault rupturing, but here the effect is fewer earthquakes, but they are more likely to be bigger (M ≥ 7.7) 

California earthquakes result from the Pacific Plate, heading approximately northwest, sliding past the North American continent.