Confused by the existence of a charted displaced threshold in addition to a second; NOTAMed; displaced threshold; the pilot of a C-152 landed short of the second of two active displaced thresholds.

Narrative

Before the flight I checked the NOTAMs and observed a NOTAM for a displaced threshold on Runway 31 at SGJ. I knew the flight was VFR and during daylight hours so I foresaw no issue with recognizing and abiding with a displaced threshold. After contacting St. Augustine Tower I heard a radio discourse between the Tower and an aircraft that had landed short of the displaced threshold on Runway 31. Once Runway 31 was in visual contact I made sure to identify what I believed to be the location of the displaced threshold. This location was marked with threshold markings. After landing past the threshold markings I was informed by the Tower that I had landed short of the displaced threshold. I realized afterwards that there were actually two sets of threshold markings and I determined that I had in fact landed short of the intended area for landing. Later observation revealed faded chevrons leading up the second threshold. These markings are conflicting and reflect an unclear representation of the usable runway available for landing. I determined that corrective actions in my situation would have been to better verify the actual threshold position with tower after noting the previous pilots mistake. In addition; I should have taken a more careful note of all markings on the runway before landing. Also an ideal fix would be clearer markings on the runway to prevent other aircraft from landing short in the future.