Petra, Read your archived note 14430 on the Parkfield fissure. I can't tell what it is without seeing it (and even then might not be able to tell). But, I have the following suggestions: it can not be a large silent (slow) earthquake-it certainly would have been detected.
It sounds like some sort of of land failure (landslide). Or, maybe a giant sandworm like in "Tremors"? :)
I can't read all the posts anymore-getting busy and was on (unpaid) vacation for the first week April. Chris