Along‐strike variations in short‐term slow slip events in the southwest Japan subduction zone

Abstract

Short‐term slow slip events accompanied by nonvolcanic deep low‐frequency tremors and deep very low frequency earthquakes in southwest Japan were investigated systematically by means of ground tilting studies. The change in tilt usually lasts for several days. By using a genetic algorithm and a least squares method, we inverted the data for tilt steps that were caused by slow slip events and were detected by at least four stations situated near the source of the tremor. Fault parameters were estimated for 54 slow slip episodes that occurred mainly in the western Shikoku, northeastern Kii, and Tokai regions from 2001 to 2008. In eastern Shikoku, two slow slip episodes were detected quantitatively for the first time. The fault geometries of all the slow slip events were located within the belt‐like distribution of tremors in the transition zone between the locked and aseismic slip zones at the plate interface of the subducting Philippine Sea Plate. The spatial extent of the fault geometry corresponds roughly to the distribution of clusters of nonvolcanic tremors and very low frequency earthquakes. The moment magnitudes ranged from 5.4 to 6.2, and the slip was ∼1 cm for each slow slip event. The rate of moment release by the detected slow slip events was 40–60% of the moment accumulation expected from the relative plate motion, and it showed regional differences. They may reflect the along‐strike variations in plate convergence and/or the characteristic size of the slow slip fault plane on the plate interface.