Fig. 3
![Fig. 3](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2Fs40623-018-0941-2/MediaObjects/40623_2018_941_Fig3_HTML.png)
Examples of monthly CCFs for a the BB station pair (HDK–TKD) with 100-s lag time in the 1–10-s period band and b the SP station pair (MGR–SUN) with 50-s lag time in the 0.2–5-s period band. Color heatmap correlates to the normalized amplitude of CCFs. Black lines indicate the total and annual average of the monthly CCFs. The data length (in months) is displayed in parentheses