Could it be an underwater geysir? 26 seconds is right in the period length for a geysir. Maybe due to the higher pressure due to greater depth it has more of an "explosion" characteristic than traditional geysirs so that it causes a seismic event?
I don't think geysers really exist under the sea. The pressure would keep steam from forming and therefore stop a pressure build up. Every geothermal feature I have seen under the ocean has been a steady stream of superheated fluid. As an example, I took a temperature reading on one black smoker and it was 320°C and still fluid.