Low-frequency earthquakes accompany deep slow slip beneath the North
Island of New Zealand
Abstract
Slow slip events have previously been observed along the Hikurangi
subduction zone beneath the North Island of New Zealand. These slow slip
episodes occur both on the shallow plate interface (< 15km
depth) and at the deeper end of the seismogenic zone (>
30km depth). We present the first catalog of low-frequency earthquakes
(LFEs) in the Hikurangi subduction zone, located beneath the Kaimanawa
Range on the central Hikurangi margin, downdip of a region that
regularly (every 4-5 years) hosts M7 slow slip events. To systematically
detect LFEs using continuous seismic data recorded by GeoNet, we
developed a matched-filter technique with template waveforms derived
from previous observations of tectonic tremor. The workflow presented in
this work is composed of two iterations of a matched-filter search. In
each iteration, the detections were gathered into families and their
common waveforms postprocessed with machine-learning methods to extract
high-quality waveforms, allowing us to pick seismic phase arrivals with
which to locate the LFEs. We found that LFEs occur in episodes of
intense activity during the neighboring updip M7 slow slip events. We
also observe a recurrence time of 2 years between other large bursts of
LFEs, suggestive of a shorter cycle of slow slip. We hypothesize that
these and other frequent LFE episodes highlight smaller slow transients
that have not yet been geodetically observed.