The Sapporo cluster is named for the city of Sapporo on the island of
Hokkaido, Japan. The cluster covers the southeastern part of the island,
including events to a depth of ~80 km. The cluster lies in the “Hokkaido
Corner” at the intersection of the Japanese and Kurile trench systems.
It includes three major events, the 6.4 Mw Hikada earthquake on January
20, 1970, the 6.7 Mb Urakawa-oki earthquake on March 21, 1982 and the
6.6 Mw Eastern Iburi earthquake on September 5, 2018. The distribution
of seismograph stations is very good and the location calibration is
very robust. All events have depth control from near-source and
local-distance arrival time data and many events also have teleseismic
depth phases that are usually in close agreement. The distribution of
depths is bi-modal with a peaks at 32 and 60 km.
The arrival time dataset of the 2018 Eastern Iburi earthquake is quite
complex and the normal procedure of using data at all distances to
estimate the relative location resulted in significant location bias;
the mainshock epicenter was shifted several tens of km away from its
aftershock cloud. From a detailed analysis of the aftershock sequence,
Katsumata et al.
that the main rupture consisted of reverse faulting on two fault segments
separated by a short step-over on which the rupture initiated with
strike-slip motion and magnitude ~5. There is an unspecified time lag
(likely several seconds) between the initial rupture and the two
subsequent larger subevents. The arrival time data at short epicentral
distances consistently appears to be from this triggering subevent, but
at greater distances the arrival times display significant scatter,
apparently as the arrivals from one or the other of the two larger
reverse faulting events dominate. In the calibrated relocation all
arrival times beyond 8.0° for this event were skipped, so the calibrated
epicenter represents the triggering subevent near the middle of the
overall aftershock pattern.