Cathryn, questions answered at the "**"s

Okay, so how do you seiso/geo guys map faults underwater? Ultrasound?
**Hmm, guess it is like ultrasound...if you want to image a fault at depth-several km below the seafloor, you use compressed air sound sources that are low frequency...mostly below what you can hear. This sound penetrates the seafloor, reflects off the various layers, some of which are offset by faults, and the reflections are received at pressure sensors in a long string...where they are processed to make an image...I do processing, but for California mostly use industry and USGS data. We also use high frequency sources that are audible, that image in high detail the upper 50 or 100 m if you have favorable soft sediment. If it is a seafloor strike-slip fault like the San Gregorio, or North Anatolia fault in Marmara Sea, it shows up clearly on detailed bathymetric maps...acquired using swath bathymetric techniques (multibeam...).

How do you know how they dip?
**The are sometime imaged as described above. If a fault is not too steep, there is often a reflection directly off the faults surface. If it is seismicly-active, the relocated earthquakes can give the orientation directly if not too far offshore (Islands help....).

How do you know how deep they run?
**Sometimes imaged on deep crustal acoustic experiments, but usually from the locations in 3D of earthquakes.

How do you know how often they are prone to slip, or thrust, etc.?
The first motions from earthquakes give what type of earthquake. If a fault slips, the first arrivals in 2 quadrants will be pushing, and the 2 from the other quadrants will be pulling. If you have a distribution of recorders in all directions around the quake, you can draw a "focal mechanism" that shows the slip, and the fault and the plane perpendicular to both the fault and the slip. If interested in earthquakes, have someone teach you how to read these, or look up "focal mechanism".....

How do you even know if they are strike slip faults or thrust faults, or something in between? If strike slip, how do you know the direction
**See above. But, you can also tell from how the layers are offset in 3D...which can be imaged in the subsurface. Or, how streams, ravines, volcanic features, sides of buildings...are offset.
Chris