First, some definitions and functionality. The red and black circles indicate seismic stations. They may be any type of station - national, regional, or urban. Red indicates ANSS supported or funded stations, black indicates non-ANSS supported. The red lines indicate a communications link.
Some sites have multiple links - with secondary links indicated with dotted lines.
Not all stations have multiple telemetry links but some number of critical stations do. These sites will produce time-series data, either triggered or a combination of triggered and continuous data. In some instances, data may be collected from several stations at a single site. These sites are denoted by yellow triangles and in Mitch's nomenclature, Tier 1 systems are called Data Concentrators. In general, these will be unstaffed facilities, but there may be a local contact.