Likely seismic. We made it through the first ~12 min of an initial jump in the 0.03 - 0.1 Hz seismic band but it spiked up to 2 - 3 um/s around the time of the lock loss.

From USGS: 6.0 magnitude 66km ESE of Hihifo, Tonga 2015-10-18 16:18:34 UTC 10.0 km deep

Terramon did not report the earthquake until after the lock loss.

SUS S_R_M saturating (Oct 18 16:40:56 UTC)
SUS S_R_M saturating (Oct 18 16:40:58 UTC)
SUS E_T_M_Y saturating (Oct 18 16:40:58 UTC)
SUS S_R_M saturating (Oct 18 16:41:02 UTC)
SUS S_R_M saturating (Oct 18 16:52:57 UTC)
SUS E_T_M_X saturating (Oct 18 16:52:57 UTC)
DRMI Unlocked (Oct 18 16:52:57 UTC)
Intention Bit: Commissioning (Oct 18 16:52:57 UTC)
ISC_LOCK state: DOWN (Oct 18 16:53:06 UTC)
SUS OMC SW watch dog tripped (Oct 18 16:53:11 UTC)
SUS OMC SW watch dog tripped (Oct 18 16:53:19 UTC)