17:15 〜 19:15
[SCG45-P22] Spatiotemporal characteristics and focal mechanisms of tremor clusters along the mountain range of Taiwan
キーワード：tectonic tremors, focal mechanisms, tidal modulation, moment tensor inversion, Taiwan mountain belt
Tectonic tremors occur in various geological settings worldwide, with distinct characteristics reflecting their local tectonic environments. Here we analyzed five tremor clusters distributed along the mountain belt of Taiwan in the lower crust, using 2012-2022 tremor catalog proposed by Ide and Chen (2024, GRL). Different from subduction zone tremors, the mountain-wide tremor zones in Taiwan offer valuable insights into tremor generation in a collisional belt. We found that southernmost tremor cluster (Cluster 5) in the southern Central Range reveals highest occurrence rate (1.2 events/day, 36.6 seconds/day), the other tremor zones further north (Clusters 1 to 4 from the northernmost tip to central Central Range) exhibit lower rates (<0.2 events/day, <7 seconds/day). Their recurrence intervals range from 2-4 months, comparable to those observed in Mexico and Eastern Shikoku but notably, shorter than Cascadia's. Strong tidal modulation is evident in all clusters, as spectral analysis of tremor catalog showing that the prominent peaks coincide with constituents of Sa, O1, S1, M2, S2. When comparing tidal records with tremor activities, 47%, 84.7%, 92.9%, 97.2%, and 81.7% of tremors in Clusters 1 to 5 are found to coincide with higher than median water level. The weaker tidal correlation in Cluster 1 may reflect different physical environment and/or tectonic setting, which can be further discussed in moment tensor inversion.
To better understand the similarity and difference in deformation mechanisms, we next determine the focal mechanisms for five tremor clusters. Here we apply moment tensor inversion in the very low-frequency band (0.02-0.05 Hz) and employ broadband seismograms stacked relative to the hypocentral time of the tremor (Ide et al., 2015, GRL). The result reveals different deformation patterns between clusters. As a result, Cluster 5 exhibits a well-defined low-angle thrust mechanism with N-S striking, different from the E-W striking observed in Clusters 1-4. Note that Clusters 1-4 are less constrained due to smaller number of events. All five clusters reveal nearly horizontal P-axes and higher-angle T-axes, indicating a reverse-faulting regime under horizontal compression. The depth-dependent stress orientation is observed at tremor zones. At shallow depths of less than 20 km, the Cluster 1 area has experienced extensional deformation due to the combined effect of crustal thinning and Okinawa Trough extension, while Clusters 2-5 have experienced compressional stress consistent with the plate convergence direction. At deeper depth of 30 to 50 km, Cluster 1 area exhibits N-S striking while Clusters 2-5 show predominantly E-W striking as a result of compression following the continuous sinking of the Eurasian and Philippine Sea Plates driven by subduction processes. The observed stress orientation from mountain-wide tremors suggests a layered deformation, supportive of anisotropy study by Huang et al. (2015, Science). The orogenic processes may play a more important role in the upper crust deformation, whereas plate convergence and subduction contributes more to deformation in the lower crust deformation.
To better understand the similarity and difference in deformation mechanisms, we next determine the focal mechanisms for five tremor clusters. Here we apply moment tensor inversion in the very low-frequency band (0.02-0.05 Hz) and employ broadband seismograms stacked relative to the hypocentral time of the tremor (Ide et al., 2015, GRL). The result reveals different deformation patterns between clusters. As a result, Cluster 5 exhibits a well-defined low-angle thrust mechanism with N-S striking, different from the E-W striking observed in Clusters 1-4. Note that Clusters 1-4 are less constrained due to smaller number of events. All five clusters reveal nearly horizontal P-axes and higher-angle T-axes, indicating a reverse-faulting regime under horizontal compression. The depth-dependent stress orientation is observed at tremor zones. At shallow depths of less than 20 km, the Cluster 1 area has experienced extensional deformation due to the combined effect of crustal thinning and Okinawa Trough extension, while Clusters 2-5 have experienced compressional stress consistent with the plate convergence direction. At deeper depth of 30 to 50 km, Cluster 1 area exhibits N-S striking while Clusters 2-5 show predominantly E-W striking as a result of compression following the continuous sinking of the Eurasian and Philippine Sea Plates driven by subduction processes. The observed stress orientation from mountain-wide tremors suggests a layered deformation, supportive of anisotropy study by Huang et al. (2015, Science). The orogenic processes may play a more important role in the upper crust deformation, whereas plate convergence and subduction contributes more to deformation in the lower crust deformation.