I wonder what isolation devices they use?
The Internet has answer for everything
. It says they use both passive and active isolation mechanisms:
https://www.ligo.caltech.edu/LA/page/vibration-isolation
"Vibration Isolation
For an instrument that needs to remain as still as possible, it is ironic that LIGO is so sensitive that it can feel the smallest vibrations from near and far. LIGO is essentially a giant seismometer capable of sensing vibrations from traffic on nearby roads, weather patterns on the other side of the continent, staff biking alongside detector arms, ocean waves crashing on shores thousands of miles away, and of course nearly every significant earthquake on the planet. Since gravitational waves will make themselves known through vibrations in LIGO's mirrors, the only way to make gravitational wave detection possible is to isolate LIGO's components from environmental vibrations to unprecedented levels. The change in distance between LIGO's mirrors (test masses) when a gravitational wave passes will be on the order of 10-19 m. To achieve this level of sensitivity, LIGO was constructed with multiple levels of active and passive vibration isolation systems. Many of LIGO's larger infrastructure systems that provide some additional levels of isolation are discussed in previous sections. But LIGO's most sensitive components (its optics) required even more complex and highly specialized mechanisms for isolating them from even the smallest imaginable vibrations.
Outside of its pre-stabilized laser, LIGO's vibration isolation systems are comprised of two basic elements: Optics Suspensions and Seismic Isolation.
Optics Suspensions (Passive Isolation)
LIGO's mirrors must be so well shielded from vibration that the random motion of the atoms within the mirrors and their housings can be detected. To achieve this level of shielding, each of LIGO's 40 kg test masses is suspended within a 360 kg quadruple pendulum system (or 'quad'). This system of multiple pendulums significantly reduces motion at the level of the test masses where it really counts. Suspensions make use of both passive and active vibration isolation components.
[...]
Seismic Isolation (Active Isolation)
The first line of defense against vibration is LIGO's "active" damping system. In contrast to Initial LIGO's instrument, where optics were located on top of seismic isolation platforms, the Advanced LIGO instrument suspensions are mounted below in- and out-of-vacuum active vibration/seismic isolation systems, giving them the quietest possible environment for operation.
Internal seismic isolation platforms (ISIs) employ position- and vibration-sensors tuned to different frequencies of environmental vibrations, along with permanent-magnet actuators that work together to counteract ground movements keeping the instrument virtually motion-free. This level of isolation can reduce the magnitude of vibrations introduced to the suspensions (at the point of their attachment to the ISI) to a level of at most 2x10-13 m. The suspensions do the rest, reducing this noise level nearly six more orders-of-magnitude to achieve LIGO's desired detection sensitivity of 10-19 m."
Wonder what our turntables would sound like when designed this way!!!
