Distinct Rupture Processes on a Fault where M6-class Earthquakes Reoccurred in a Short Period: Analysis of the 2011 and 2016 Northern Ibaraki, Japan, Earthquakes using Near-field Waveforms

Abstract

Abstract Crustal earthquakes are generally recognized as having recurrence periods of more than 1000 years. However, in northern Ibaraki Prefecture, Japan, two moderate-sized earthquakes occurred on almost the same fault plane in 2011 and 2016. Moreover, it has been reported that the slip distributions derived from crustal deformation data from these two events are almost identical. We performed near-field waveform inversion analysis to estimate the details of the spatiotemporal distribution of the co-seismic rupture. Our results reveal large slip areas during each earthquake at different locations across the bend of the fault plane. These results indicate that the shear stress of the fault area where the main slip occurred during the 2011 event was not rebuilt in such a short period, a phenomenon that had not previously been discussed for this sequence. Our results also suggest that the fault geometry affected such complex fault ruptures.