Header image for expedition: Cradle of the Earthquake: Exploring the Underwater San Andreas Fault 2010

Ask an Explorer

During the expedition, we invited the public to send questions to the team at sea; questions and replies are posted here.

I am currently a Geology undergraduate at the University of New Hampshire. Recently we have been studying different fault types and my understanding is the San Andreas Fault is currently a right lateral strike slip fault. Will the current research being undertaken be able to determine if this was the case in the distant geologic past perhaps at the time the fault was formed? Was the San Andreas fault always a right lateral strike slip or did it exist as another fault that evolved as the tectonic plates shifted over time? I am also interested in the data that will be used to help determine earthquake probability, if any, that your team hopes to uncover from this exciting study.

Question from: Michele, UNH geology student

Hi Michele,

Thanks for the great questions! We already know quite a bit about the San Andreas, for example the fault has always been a right-lateral strike slip fault. The transform system was formed about 27 Ma when the Pacific-Farallon ridge was subducted near what is now Southern California. Since then, the Mendocino Triple Junction has been migrating northward, and the transform system connecting the Juan de Fuca-Gorda Ridge system to the east Pacific Rise has lengthened. There are some great animations of this at: http://emvc.geol.ucsb.edu/1_DownloadPage/Download_Page.html 

The transform system is composed of a broad system of faults, the main part of which is presently the San Andreas, which carries about half the plate motion. The region the fault traverses was formerly a subduction zone, and as the triple junction moves north, there is some "conversion" between the former subduction zone (Cascadia) and the newly lengthening San Andreas. Nobody is sure at all how in the heck that works, but that was one of the goals of this project, to start getting at things like that, we have to first map what is there, and that really hadn't been done.

The earthquake probability is also a long term goal that requires many bricks be put in place. One of them is the long term paleoseismic history, which we are working on in a linked NSF funded project looking at turbidites as a proxy for earthquakes. Our papers on this so far are at my lab website: http://activetectonics.coas.oregonstate.edu  under "publications".

There are possible linkages to the Cascadia subduction zone through stress triggering, and the structural and mapping work from this Exploration will really help us to model the dynamics of what happens to one of these great faults when the other has a major earthquake. Before this, we really didn't even know where the fault was for sure, or how it interacts with the Triple Junction. Now we have it mapped, and have some strong clues about the interaction that we'll be working on this winter.

Answer from: ~ Dr. Chris Goldfinger - Marine Geologist/Geophysicist, OSU Expedition PI