Update 14:03 UTC
Fire Department of Japan has reported in his third report that as far as 2 hours after the earthquake,  no damage has been detected in the villages or cities and that the Kashiwazaki-Kariwa Nuclear Power Station has not been damaged neither.

Update 13:44 UTC
The strongest earthquake we could trace at Sado Island was a M 6.6 earthquake which occurred at December 9, 1802.

Update 13:34 UTC
Japan seismological agency, JMA is reporting a 5+ intensity, considered by Earthquake-report.com as moderately dangerous for damage.
5+    Sado-shi Aikawa-sanchome and Sado-shi Kawaharada-hommachi*
5-    Sado-shi Ogimachi*, Sado-shi Hamochi-hongo* and Sado-shi Akadomari*
4    Sado-shi Iwayaguchi*, Sado-shi Ryotsu-minato*, Sado-shi Matsugasaki*, Sado-shi Chigusa*, Sado-shi Niibo-uryuya*, Sado-shi Mano-shimmachi*, Sado-shi Ryotsu-shisho* and Sado-shi Aikawa-sakaemachi*
Japan seismological intensity values are scaled from 0 to 7. 5+ JMA can be compared with 7 MMI (very strong shaking).

Sado Island earthquake – Intensity map courtesy JMA Japan

Update 13:32 UTC
Sado Province was a province of Japan until 1871; since then, it has been a part of Niigata Prefecture. It was sometimes called Sashū or Toshū. It lies on the eponymous Sado Island, off the coast of Niigata Prefecture (or in the past, Echigo Province).
Sado was famous for the silver and gold mined on the island. In the Kamakura Period, the province was granted to the Honma clan from Honshū, and they continued to dominate Sado until 1589, when Uesugi Kagekatsu of Echigo Province took over the island. The Tokugawa shoguns later made Sado a personal fief after Sekigahara, and assumed direct control of its mines.
Since 2004 Sado city has comprised the entire island.
Its rich history and relaxed rural atmosphere make Sado one of the major tourist destinations in Niigata Prefecture. The island has several temples and historical ruins, and offers possibilities for various outdoor activities, as well as fresh local food.
Sado is famous as the major breeding area for the Japanese Crested Ibis. The last known Japan-born Japanese Crested Ibis died in captivity in 2003 on the island. Currently, birds from China are being bred in a captive program in a facility in Niibo area. The Ibis, Toki in Japanese, is a major symbol of the Island and can be found on several tourist items. There are plans to release Ibis in the wild at the end of 2008. (source Wikipedia)

Update 13:31 UTC
The Island area does not seem to be particularly dangerous, although moderate earthquakes are fairly common.

The strong shallow and thus moderately dangerous earthquake occurred almost below Sado Island, an island approx. 50 km from the western Honshu coast.

Most important Earthquake Data:

Magnitude : 5.7
UTC Time : Wednesday, February 08, 2012 at 12:01:37 UTC
Local time at epicenter :  Wednesday, February 08, 2012 at 09:01:37 PM at epicenter
Depth (Hypocenter) : 10 km
Geo-location(s) :
10 km from Sawazakihana Light (Sado Island)
38 km SW Ryotsu (pop 16,359)
72 km NW Nagaoka (pop 195,318)

for more information and updates, go to:    http://earthquake-report.com/2012/02/08/strong-shallow-earthquake-near-sado-island-honshu-japan/