31 AD Seismite from the Ein Gedi Trench. An Earthquake between 26 and 36 AD shook these sediments first folding them and then breaking them. The white aragonite layers to the right were thrust over the white aragonite layers to the left.
The Dead Sea is surrounded by some very active faults. Most of these are strike slip faults like the famous San Andreas Fault of California and, like the San Andreas Fault, they produce earthquakes.
The illustration below from Marco and Agnon, 1995, illustrates how we Geologists believe seismites are preserved in a column of sediments.
A. Sediments accumulate layer by layer as time passes.
B. When a sufficiently energetic earthquake is felt, the top layer right at the sediment-water interface is deformed.
C. After the earthquake, sediments continue accumulating layer by layer.
Next -> Dating the seismites
If that is the case, then this sediment core tells us that the Dead Sea must have covered the area where these sediments were observed at the time of the earthquake, since the seismites would not have formed if the lake had been at a level it is today or lower. In addition, the original research papers (refs 2 and 3) were able to identify almost every major written record of earthquakes in these layers of sediment.
You are correct. Dead Sea Level is -428 m below sea level in 2014 and is estimated to have been 33 m higher up at about -395 m below sea level in ~ 30 AD. The Dead Sea level has been dropping steadily since around 1960 AD thus exposing older lake bottom sediments.