The NeMO Net Bottom Pressure Recorder (BPR) is located near the center of Axial caldera at the summit of the volcano. BPRs precisely measure the pressure from the overlying ocean, and by doing so they can detect vertical movements of the seafloor due to volcanic activity (for example, if the seafloor were uplifted the BPR would measure a pressure decrease). The NeMO Net BPR sends back four pressure measurements per hour from the seafloor (purple,in the plots below). After the predictable variations from ocean tides (black curve) are removed, the de-tided BPR data (red) is then evaluated to look for any sudden vertical movements of the seafloor that may signal the onset of a volcanic eruption.

Results from BPR's at Axial during the 1998 eruption
There were BPRs in two "rumbleometer" instruments that were in place at Axial volcano when the 1998 eruption occurred. The 1998 eruption caused a large volume of magma that had been stored in a reservoir beneath Axial's summit caldera to be removed and intruded into the south rift zone. The removal of this large volume of magma caused the whole summit of Axial seamount to subside, just like a balloon that had some of the air taken out of it. The BPR that was at the center of Axial caldera in 1998 eruption measured 3.2 meters (10.5 feet!) of subsidence. Another BPR was in the rumbleometer that was stuck in the 1998 lava flow until it was extracted in the summer of 1999. This second BPR showed a subsidence of 1.4 meters (4.6 feet) at the eruption site, which is 3.5 km (~2 miles) from the caldera center. These measurements can be used to model the depth of the magma reservoir beneath Axial caldera. For more information see the NeMO Explorer Rumbleometer virtual site.