Fig. 2

Examples of the relative clock error estimation and estimated temporal changes in clock errors. a (top) Waveforms of the stacked cross-correlation functions (SCCFs) on days without clock errors (blue line) and on the day when the clock error occurred (orange line) for the station pair of N.AMKV–V.SAME at the Asama volcano. (bottom) Estimated delay times between the two SCCFs for each lag time. The solid and dashed red lines represent fitted straight lines using LAD and OLS regression, respectively. b Similar to a, but for the SCCFs when no clock error occurred. c Temporal changes in clock errors for 23 seismic stations at 13 volcanoes (blue circles). A positive clock error means that the clock of the station is delayed. The red crosses represent the clock errors estimated using SCCFs that were calculated after correcting timestamps of seismic ambient noise records