We adopted the Probability-based Magnitude of Completeness (PMC) method and
performed a case analysis of the Nankai Trough, a target region monitored for
future megathrust earthquakes. JAMSTEC (Japan Agency for Marine-Earth Science
and Technology) has created a seismicity catalog that includes events in this
region observed by DONET. Using seismicity data for 2015-2019, we found
spatiotemporal variability of completeness magnitude Mp. Mp was lower than 1 in
one of the areas where stations are densely deployed, whereas Mp was larger
than 2 at the periphery and outside of the DONET area. We then evaluated the
temporal evolution of Mp, highlighting how the failure of sets of observing
stations influenced Mp if not repaired. Stations are aggregated around the 12
science nodes (hubs that connect the stations) and connected through the two
oceanfloor backbone cables to JAMSTEC. We explored the possible use of PMC as a
tool with simulation computation of node malfunction. A simulation showed that
completeness estimates in the area near failure nodes were about 1 magnitude
larger. If such failure occurred for nodes near the region which straddles the
rupture zones of the previous Tonankai and Nankai earthquakes in 1940's, it
would most pronouncedly affect earthquake monitoring among nodes' failures. It
is desirable to repair these nodes or replace with new ones when their
malfunction occurs. We then demonstrated an example of how to use Mp
information as prior knowledge to seismicity-related studies. We used the b
value of the Gutenberg-Richter distribution, and computed it taking Mp into
consideration. We found that the spatial and temporal changes in b were
strongly correlated to the magnitude-6 class slow slip that grew over two years
on the Nankai Trough plate boundary, indicating the b value as a proxy that can
help to image stress heterogeneity when there is a slow slip event.Comment: 6 figure