Fig. 4

Results of the envelope correlation method. a Velocity vertical waveform at the N.ASGH station during 03:00–04:00 May 26, 2019. The orange broken rectangles indicate the 1-min time window used for the envelope correlation analysis. b and c show the results of source estimation for the earlier and later time windows in a. The red stars in b and c show the best locations of volcanic tremors for each time window. The color contours represent the normalized residual projected on the horizontal plane at a depth of 37 km, N–S, and E–W sections. The white circles show the result of the bootstrap resampling method. The blue stars indicate the hypocenters of DLF earthquakes based on the JMA unified hypocenter catalog