- 著者
- 土山 絢子 平 貴昭 中島 淳一 Roland Bürgmann
- 雑誌
- JpGU-AGU Joint Meeting 2020
- 巻号頁・発行日
- 2020-03-13
Low-frequency earthquakes generally have relatively stronger spectral components in the lower frequency range, compared to what is expected based on their magnitude. This type of earthquakes has been considered to be generated in some specific environments, such as volcanic regions and subduction zones that are also generating slow slip events and tremors. They usually occur in deeper regions below ~ 15km; however, some recent studies show that they can also be observed among aftershocks of large inland earthquakes in Japan (i.e., Niigata in 2004, Iwate-Miyagi in 2008, Kumamoto in 2016) with very shallow depth (~5km). Here, we attempt to detect such ‘low frequency aftershocks’ associated with a very recent earthquake sequence that occurred in Ridgecrest in California in 2019 on the source of waveform data from Plate Boundary Observatory Borehole Networks and nearby instruments archived at NCEDC, SCEDC and IRIS. We are considering The Frequency Index (FI) as a way to identify candidate events. FI is defined as: FI = log10(AH/AL), where AH and AL represent the mean spectral amplitude within the high-frequency band (e.g., 10-15 Hz) and low-frequency band (e.g., 2-5 Hz), respectively. We will discuss our results and the methods used to identify low frequency aftershocks from their spectra.
- 著者
- 土山 絢子 中島 淳一
- 雑誌
- JpGU-AGU Joint Meeting 2020
- 巻号頁・発行日
- 2020-03-13
Generation mechanism of deep earthquakes in subduction slabs is still enigmatic. Through cross-correlation analysis of earthquakes in central Japan, we identify five pairs and one cluster of deep earthquakes showing strong waveform similarities (coherence > 0.9) for a 40-s time window at more than three stations. For those clusters, we relocate hypocenters by hypoDD, determine the focal mechanisms solutions, and carry out spectral analysis to estimate physical parameters of seismic ruptures (i.e., stress drop, radiated energy and efficiency). The obtained results show that some similar deep earthquakes in each cluster occurred at a slightly different spot on a single fault and have almost the same stress drop. The scaled energy and radiation efficiency of the groups precisely estimated from high-quality similar waveforms show that seismic parameters are significantly different between faults. A wide range of source parameters that have been reported for deep-focus earthquakes may not represent the differences in generation mechanisms of earthquakes but reflect the diversity of inherent fault frictional properties.