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.
In the shadow of a quake
The intense focus on this fault system follows a catastrophic event. At 5:46 AM on January 17, 1995, the Great Hanshin-Awaji Earthquake struck the region. The magnitude 6.9 quake was not centered on the Sannomiya Fault but on the nearby Nojima Fault, part of the same tectonic system. The tremors lasted only 20 seconds but resulted in the deaths of more than 6,400 people and caused an estimated $132 billion in damage. During the quake, the ground on nearby Awaji Island shifted horizontally by as much as 1.7 meters.
In the aftermath, Japan massively expanded its seismic and geodetic observation capabilities to better understand and monitor crustal deformation. The slow creep on the Sannomiya Fault presents a complex puzzle for researchers. This aseismic slip relieves some tectonic stress, which might reduce the likelihood of a major earthquake on that specific segment. The movement could also transfer stress to adjacent, locked segments of the fault system. This process could increase the strain on those locked patches, bringing them closer to a future rupture. The data gathered from the quiet slip here in Kobe helps assess seismic hazards across the entire region.