The earth's silent shift
Beneath the bustling city of Kobe, the ground is moving in a way that generates no shaking and makes no sound. This is aseismic creep, a slow, steady slip along a geologic fault. The Sannomiya Fault, a segment of the active Arima-Takatsuki Tectonic Line, shows this silent behavior. Instead of locking up and building strain that is violently released in an earthquake, this portion of the fault glides almost imperceptibly.
This subtle motion is tracked by one of the densest geodetic monitoring networks on the planet. While traditional creep meters involve rods or wires physically stretched across a fault trace, monitoring in Japan is dominated by the GPS Earth Observation Network System (GEONET). Maintained by the Geospatial Information Authority of Japan, GEONET consists of over 1,300 permanent GPS stations. These stations, spaced about 20 kilometers apart, can detect crustal movements as small as a few millimeters. By analyzing the data from these stations over months and years, seismologists can map the slow-motion creep of faults like the Sannomiya. For comparison, other well-studied creeping faults, such as the Hayward Fault in California, move at rates of 3 to 9 millimeters per year.