[S06P-04] Relation between deep crustal fluids, crustal seismicity and structure in North Ibaraki revealed by receiver functions?
The northern Ibaraki region in northeastern Japan is one of the regions that have seen an increased upper-crustal seismic activity following the 2011 Mw9.0 Tohoku earthquake. This seismic activity has been attributed to fluids in the lower crust (e.g. Shiina et al., 2024), but the structural relation between the lower crustal fluids and upper crustal seismic swarm remains unclear. To investigate possible tectonic structures and their association with fluid intrusion and earthquake generation in this region, we carry out a receiver function analysis. This method can resolve seismic velocity contrasts associated with tectonic structures (e.g. faults and shear zones) and fluid presence at depth, enabling the elucidation of the earthquake generation mechanism in this region. We analyze teleseismic waveforms of over 600 events recorded between 2016 and 2023 by stations located within and around the region of increased seismicity: 7 stations of the Geological Survey of Japan, AIST, and nearby Hi-net stations. To suppress low-frequency noise and noise amplification associated with the removal of instrument response during preprocessing, we use a deep neural network model for denoising teleseismic waveforms recorded by short-period seismometers (Feng et al., 2024). The denoising model is based on a multitask encoder-decoder denoising technique (Yin et al., 2022) for 3-components seismograms in the time domain. The resulting receiver functions calculated from denoised waveforms show enhanced and sharper coherent crustal phases at higher frequencies (~2Hz), enabling easier identification and further analysis of intra-crustal structures. At stations located within the upper-crustal seismic swarm area, we observe a strong negative phase in receiver functions with delay times of about 2.7 – 3.5s after direct P arrival, seemingly increasing towards the NW. Additionally, we observe a phase with delay times within the first 2s exhibiting polarity reversal in the transverse component. The observed negative phase likely originates from the top of a low velocity zone in the lower crust, representing the upper boundary of a fluid rich layer reported by previous studies (e.g. Usuda et al., 2022; Shiina et al., 2024). Meanwhile, the phase observed within 2s suggests a sub-horizontal layer with possible anisotropy at preliminary depths no more than ~15km, near the bottom of the upper-crustal seismogenic zone. Based on our results, we discuss the possible presence of tectonic structures such as faults and shear zones which allow fluid migration from the lower crust to the upper crust, leading to by brittle deformation and subsequent triggering of earthquakes in this region.
Acknowledgement: This study is supported by MEXT Project for Seismology toward Research Innovation with Data of Earthquake (STAR-E) Grant Number JPJ010217. We used some data from the nationwide High-Sensitivity Seismograph Network, Hi-net.
Acknowledgement: This study is supported by MEXT Project for Seismology toward Research Innovation with Data of Earthquake (STAR-E) Grant Number JPJ010217. We used some data from the nationwide High-Sensitivity Seismograph Network, Hi-net.