Low-frequency earthquakes observed in close vicinity of repeating
earthquakes in the brittle upper crust of Hakodate, Hokkaido, northern
Japan
Abstract
We conducted a detailed investigation of an earthquake cluster
distributed from the lower crust to the upper crust beneath Hakodate,
Hokkaido, which included both low-frequency earthquakes (LFEs) and
regular earthquakes. Relocated hypocentres clearly show that both the
LFEs and regular earthquakes occurred close to each other in the brittle
upper crust of this non-volcanic area, while only LFEs occurred in the
lower crust. This indicates that LFEs can occur not only in the ductile
lower crust, but also in the brittle upper crust, which suggests that
LFEs can occur in an environment similar to that of regular earthquakes.
Regular earthquakes that occur in close vicinity of LFEs have very
similar waveforms and nearly overlapping source regions, which indicate
that they reflect the repeated rupture of the same asperity patch on a
fault. Temporally, the intervals between events in the repeating
earthquake sequence were very short, thus suggesting that they were
caused by a sudden increase in pore pressure. The cluster of LFEs and
repeating earthquakes, which has a rod-like distribution extending from
the bottom of the crust to the surface and tilted slightly eastward,
might represent a pathway of aqueous fluid movement sourced from the
subducting slab.