Sieh's principal research interest is earthquake geology, which uses geological layers and landforms to understand the geometries of active faults, the earthquakes they generate, and the crustal structure their movements produce.
His early work on the San Andreas Fault led to the discovery of how often and how regularly it produces large earthquakes in southern California.
Soon after joining Caltech, Sieh was recognized by the Los Angeles Times as a "Rising Star" in Southern California in 1988.
[2] Sieh is a member of the United States National Academy of Sciences (since 1999).
[3] He is the first holder of the AXA-NTU Chair on Natural Hazards in Southeast Asia[4] established in 2012.