The definition USGS uses
USGS says liquefaction takes place when loosely packed, water-logged sediments at or near the ground surface lose their strength in response to strong ground shaking. Sediment under a building can then fail. The FAQ says damage does not usually occur until the magnitude is somewhere above 4 or 5, and that the threshold is not a single number: distance, the ground, and the building also matter. That sentence is a description of past damage, not a magnitude at which a particular city is safe.
Two earthquakes the FAQ names
The USGS liquefaction FAQ gives two examples. The 1964 Niigata, Japan earthquake caused widespread liquefaction in Niigata that destroyed many buildings. During the 1989 Loma Prieta, California earthquake, liquefaction of the soils and debris used to fill a lagoon caused major subsidence, fracturing, and horizontal sliding of the ground surface in the Marina District of San Francisco. Niigata is not the Japan Trench page’s 2011 earthquake. Loma Prieta is not a row on the USGS list of the 20 largest earthquakes since 1900.
Sand boils
USGS defines a sand boil as sand and water that come out onto the ground surface during an earthquake because of liquefaction at shallow depth. A sand boil is evidence that liquefaction happened. It is not a measurement of magnitude.
Where else this site mentions it
The New Madrid page records that USGS includes liquefaction among the landscape effects of the 1811–1812 earthquakes. That is a historical account of one intraplate sequence, not a map of who will see sand boils. The 1964 Alaska earthquake is a different event from Niigata 1964. This page does not draw a susceptibility map.