Spatio-temporal variations of low-frequency aftershocks of the 2019 Ridgecrest earthquake sequence

Ayako Tsuchiyama, Taka'aki Taira, Junichi Nakajima, & Roland Bürgmann

Published August 10, 2020, SCEC Contribution #10323, 2020 SCEC Annual Meeting Poster #067

Low-frequency earthquakes (LFEs) generally have relatively stronger spectral components in the lower frequency range, compared to what is expected based on their magnitude. This type of earthquake has been considered to be generated in some specific environments, such as volcanic regions and subduction zones where slow slip events and tremors also occur. LFEs 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 in the 2016 Kumamoto, sequences the 2004 Niigata-Chuetsu earthquake (Mw 6.6), and the 2008 Iwate-Miyagi earthquake (Mw 6.8).
However, it remains unclear whether they are caused by the local attenuation in the immediate vicinity of the LFAs or by some specific factors proposed for the generation mechanism of LFAs such as fluid migration. It is important to identify LFAs and to characterize their spatiotemporal activities for improving our understanding of the features of general LFEs. In this study, we focus on detecting LFAs and exploring if it would be possible to explain their generation mechanism by previous models of LFEs. We characterize the 2019 Ridgecrest earthquake sequence, California and its aftershock activities relying on waveform data from Plate Boundary Observatory Borehole Networks archived at IRIS. We analyze 26518 earthquakes in 01/01/2015-03/03/2020 around the Ridgecrest M7.1 mainshock. We then measure 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 (10-15 Hz) and low-frequency band (1-4 Hz), respectively. Since this index exhibits magnitude dependency especially for larger earthquakes, we focus on small-sized earthquakes with Mw<3.0. 21 LFA candidates in total occur within several days immediately after the foreshock and mainshock, then they appear to be exponentially decreasing whereas there is no candidate LFAs identified before the 2019 Ridgecrest earthquake sequence Our working hypothesis is that near-fault attenuation associated with a fractured fault zone at high pore-fluid pressures could cause the bandlimited nature of LFEs through the depletion of high frequencies (e.g., Gomberg, 2012, Bostock, 2017), which might be able to explain the reason why these LFA candidates have stronger low-frequency components than other normal aftershocks.

Key Words
Ridgecrest, LFE

Tsuchiyama, A., Taira, T., Nakajima, J., & Bürgmann, R. (2020, 08). Spatio-temporal variations of low-frequency aftershocks of the 2019 Ridgecrest earthquake sequence. Poster Presentation at 2020 SCEC Annual Meeting.

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