[S02P] PM-P

[S02P-03] A study of earthquake detection capacity of DONET

*Kazuyoshi NANJO^1,2,3,4, Yojiro Yamamoto⁴, Keisuke Ariyoshi⁴, Hiroki Horikawa⁴, Shuichiro Yada⁴, Narumi Takahashi^5,4 (1. University of Shizuoka, 2. Shizuoka University, 3. The Institute of Statistical Mathematics, 4. Japan Agency for Marine-Earth Science and Technology, 5. National Research Institute for Earth Science and Disaster Resilience)

A reliable estimate of the magnitude of completeness, above which all events are assumed to be detected by a seismic network, is critical for many seismicity-related studies, because they evaluate the statistical properties of seismicity that strongly depend on correct completeness estimates. Here the PMC (Probability-based Magnitude of Completeness) method was adopted with a case analysis of the Nankai Trough. A seismicity catalog that includes events in the Nankai Trough observed by DONET (Dense Oceanfloor Network system for Earthquakes and Tsunamis) was created by JAMSTEC (Japan Agency for Marine-Earth Science and Technology). Using seismicity data for 2015-2019, the spatiotemporal variability of the magnitude of completeness was found. The magnitude of completeness was lower than 1 in one of the areas where stations are densely deployed, whereas it was larger than 2 at the periphery and outside of the DONET area. Next, the temporal evolution of the magnitude of completeness was evaluated, highlighting how the failure of sets of observing stations influenced the magnitude of completeness if not repaired. Then, the possible use of PMC was explored as a tool with simulation computation of node malfunction, where stations are aggregated around the 12 nodes (hubs that connect the stations) and connected through the two oceanfloor backbone cables to JAMSTEC. A simulation showed that the magnitude of completeness in the area near failure nodes was 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. Next, an example of how to use information on the magnitude of completeness as prior knowledge to seismicity-related studies was demonstrated. The b value of the Gutenberg-Richter frequency-magnitude distribution was computed, taking the magnitude of completeness into consideration. It was 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. Based on this finding, the b value was interpreted as a proxy that can help to image stress heterogeneity when there is a slow slip event.

Acknowledgments: This study was partially supported by Chubu Electric Power’s research based on selected proposals (K.Z.N., K.A., Y.Y., N.T.), the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Earthquake and Volcano Hazards Observation and Research Program (K.Z.N.) and under the STAR-E (Seismology TowArd Research innovation with data of Earthquake) Program Grant Number JPJ010217 (K.Z.N.), and JSPS KAKENHI Grant Number 22K03766 (Y.Y.).