Meilano Irwan

Measuring ground deformations with 1-Hz GPS data: the 2003 Tokachi-oki earthquake (preliminary report)

We analyzed 1-Hz GPS data observed at 14 stations of the GPS Earth Observation Network (GEONET) of the Geographical Survey Institute, Japan, associated with the 2003 MJMA8.1 Tokachi-oki earthquake, which occurred at the Kurile Trench. The GPS stations are located 70–240 km away from the epicenter. GPS data clearly captured rapid co-seismic ground displacements. At a GPS station 70 km away from the epicenter, coseismic displacements started 15 seconds after the origin time, and after 40 seconds at the stations 240 km away. Observed displacement amplitude exceeded 20 cm at GPS sites 240 km away from the epicenter. Displacement amplitudes attenuate with time and distance from the epicenter, oscillating with periods of 40–60 seconds. We compared the 1-Hz GPS data and displacement seismogram integrated from strong ground motion data, which showed fairly good agreements. In spite of careful screening of 1-Hz GPS data during 30 minutes preceding the main shock, no significant preseismic deformation over 1 cm in the horizontal components was recorded. 30 second sampling GPS data at 14 sites during 20 hours preceding the main shock did not show any significant pre-seismic deformation, either. These results indicate that pre-seismic strain change, if any, was smaller than 0.5–1.0×10−7 before the 2003 Tokachi-oki earthquake.

Rapid ground deformation of the Miyakejima volcano on 26-27 June 2000 detected by kinematic GPS analysis

A kinematic GPS analysis of data from the Miyakejima volcano captured a fast developing deformation event on 26–27 June 2000 in unprecedented spatial and temporal detail. Initial ground deformation toward east and upward was observed in the southeastern part of the volcano at 18:00 on 26 June 2000, almost simultaneous with earthquake swarms. Some time after 21:30 on 26 June 2000 the displacements at these sites turned from eastward to westward. Three hours later the displacement rates increased gradually at GPS sites in the western part of Miyakejima as the seismicity migrated and approached the west coast, and reached a climax with submarine eruption at 09:00 on 27 June 2000. A Genetic Algorithm was used to explore the parameter space and to find the best fitting source geometry. This analysis leads to an interpretation that the 18:00 26 June earthquake swarm was caused by a dike intrusion near the Oyama crater. Starting from 21:30 this dike deflated and a new dike intruded near the west coast. Following the propagation of this dike to the offshore, a spherical source began deflating in the southwest of Oyama crater.

Ground uplift detected by precise leveling in the Ontake earthquake swarm area, central Japan in 2002-2004

Repeated precise leveling in the earthquake swarm area of Ontake, central Japan has revealed uplift of 3–6 mm in proximity to the epicentral region of the most active earthquake cluster in 2002–2004. Although the uplift is small, the vertical displacement is significant even considering leveling error. This uplift is associated with increases in 3He/4He ratios and CO2 δ13C values at a mineral spring in the region, indicating an upper mantle contribution. A region of low resistivity at a depth of 2 km beneath the uplift area has also been inferred, suggesting that the observed uplift is related to changes in a shallow seismogenic layer due to increased hydrothermal input from the earthquake swarm area.