Segmented trapdoor fault in Kita-Ioto Caldera, Japan: Insights from
milli-meter tsunami waves captured by an array network of ocean bottom
pressure gauges
Abstract
Submarine calderas with active magma supply have recently been
identified as potential sources of volcanic tsunamis due to sudden
meter-scale uplift by trapdoor faulting, occurring every few years to a
decade. These trapdoor uplifts are seismically recorded as
non-double-couple earthquakes with magnitudes M > 5.
Kita-Ioto Caldera, a submarine caldera in the Izu-Bonin arc, caused such
earthquakes every 2-5 years. Our previous study (Sandanbata and Saito,
2024) analyzed data from a single ocean bottom pressure (OBP) gauge in
the Philippine Sea, confirming trapdoor uplifts during the earthquakes
in 2008 and 2015. However, high temporal-resolution data for the
earthquakes in 2017 and 2019 were lost, preventing source mechanism
investigation. To address this, we examine OBP data of the two recent
earthquakes from the array network of DONET, deployed off the
southwestern coast of Japan. Despite the poor signal-to-noise ratio in
each record, we successfully detect clear tsunami signals associated
with the earthquakes using a waveform stacking method, with sea-surface
wave heights of only 1-2 mm. By analyzing the data, we propose source
models that represent trapdoor uplifts in the submarine caldera and
accurately reproduce the detected tsunami waveforms, confirming the
recurrence of trapdoor uplifts. Notably, differences in the tsunami
waveforms between the 2017 and 2019 earthquakes suggest that different
segments of the intra-caldera fault system were activated. This
segmentation likely influences the recurrence characteristics of the
inflation cycle in calderas, which would be a key to understanding the
magma accumulation process and assessing the sizes and timings of future
trapdoor uplifts.