The depth of wells varies from 31m to as deep as 4000m. The aquifers are mainly sandstone, limestone, and igneous rocks. Wells are maintained at different administra-tive levels: 1) basic wells and the regional wells are supported by the State Seismological Bureau, managed by the State Seismo-logical Bureau and provincial seismological bureaus, respectively; 2) local wells are supported by local governments and managed by the local government seismological bureaus; 3) the enterprise wells (springs) are supported and managed by businesses such as petroleum companies. After digital upgrades that began in 2000, most of the basic and regional monitoring wells sample with periods of 1hor 1min. Water level sensors of type LN-3A were manufactured by the Institute of Earthquake Science, which belongs to the China Earthquake Administration (CEA). The digital piezometer readings are converted into groundwater level with a resolution of 1 mm over the range between 0 and 10 m; the ab-solute accuracy is 0.2% full scale.