A study on non-volcanic tremor in an active fault zone

Numerous studies about non-volcanic tremor around the plate boundaries have revealed that the tremors are located on deep extension of a fault zone. They are sensitive to small stress change caused by various reasons such as the dynamic stress change induced by surface waves with large amplitudes from teleseismic events. Therefore, its occurrence is related to the stress accumulation on the seismogenic zone. Chao and Obara (2012) found that non-volcanic tremors were excited by surface waves with large amplitudes from teleseismic events near the active fault zone. However, a relation of the triggering phenomena in an active fault zone with the stress changes remains unclear relative to plate boundaries because we have little information about the configurations of fault zones. In order to elucidate a relationship between shallow crustal earthquakes and tremors in the inland active fault zone, it is necessary to determine precisely the hypocenter and its mechanisms. We deployed temporal seismic stations to establish a dense seismic station network in the Hinagu fault zone, Kyushu Island, Japan, which is one of the source regions of the nonvolcanic tremor pointed out by Chao and Obara (2012). We obtained hypocenters with highresolution relative to the previous study by using waveform data from the network. The tremors are located along the strike direction of the fault zone and beneath the seismogenic zone. This indicates that the tremors occurred on deep extension of the fault zone. According to the previous studies, the non-volcanic tremor is superposition of overlapping shear faulting of many small low frequency earthquakes. Based on this characteristic, we i determined the focal mechanisms of the tremor by applying S-wave polarization analysis to entire tremor waveforms. The mechanisms have similarities to those of the shallow earthquake in this region. In addition, one of the nodal planes of the focal mechanisms was almost parallel to the strike direction of the active fault. This suggested that the stress condition near source region is same as the shallower seismogenic region. The relations between dynamic stresses and strains are unclear, however, the large normal stress relative to the shear stress may play a role in triggering. It is also important to investigate the activities of non-volcanic tremor to understand the status of stress near source region. By using matched-filter technique, we attempt to detect the activities of non-volcanic tremor in this region. We obtain three events with relatively large amplitude. However, amplitude of most detected events is comparable with ambient noise level at the stations. These events seem to occur without external stress perturbation acting on the region. From the synthetic test about the detection capability, our results indicate state of stress condition on deep extension of a fault zone is close to critical condition to failure and the high possibilities of the slip occurred in deep extension of a fault zone. In the seismogenic zone, the region that is characterized by high P-wave velocity (Vp) exists parallel to the strike direction of the Hinagu fault zone. The low Vp region is seen below the tremor zone and northeast part of the fault zone. Comparing the results with the velocity anomaly in this region, we interpret that the non-volcanic tremor in this region occurs in the transient zone from shallow seismogenic zone to the deep lower crust. These results are consistent with the model about the plate boundaries. ii