Looking at where it is on the map, it couldn’t be the Edwards fault line. I believe that is in the Tertiary wedge of sediments.

Across Texas, the mighty Ouachita mountain range once stood high. Over time, this range eroded to a bunch of hills. Those hill, the roots of that mountain range, were covered with limestone when seas covered the area. After the seas retreated some of the limestone eroded, exposing the roots of these mountain (Llano and Marathon uplifts).

As these mountains eroded, they deposited sediments on the Eastern side, which became the Texas coast. These sediments go miles deep. It is in these sediments where I believe this Earthquake happened.