Susanna's maiden name was Foster, though she was adopted by Jonas Humphrey at a very young age.
From 9 page article in "The American Genealogist," Vol. 68, No 1, pp 14-22 of January 1993:
Susanna was the daughter of Thomas Foster. Her mother, Elizabeth Seamer, married (1) Thomas Foster, and they had four children before he died. Elizabeth then married Jonas Humphrey, and they had one son, Jonas, Jr. Jonas Humphrey was a stepfather to Susanna.