Acts 4:31

‘And when they had prayed, the place was shaken in which they were gathered together, and they were all filled with the Holy Spirit, and they spoke the word of God with boldness.'

Then once they had finished praying the place where they were gathered was shaken, regularly seen as a sign of God's presence (compare Exodus 19:18; Isaiah 6:4). Here it was intended to be linked with the filling of the Holy Spirit, and with the certainty that God was with them and had heard their prayer. It was a physical assurance of His presence. It may have been a local earth tremor, but it demonstrated the presence of the Creator.

And they were all ‘filled with the Holy Spirit' so that they could go forth and proclaim the word of God with boldness. The mighty power of God was continually with them in the fulfilling of their ministry, and was here renewed. Those who ‘spoke the word of God' were still at this stage the Twelve, who had already received the Holy Spirit by being breathed on by Jesus (John 20:22) and had experienced the ‘breath', fire and other tongues at Pentecost. This is thus a further empowering so as to give them the boldness to witness powerfully.

‘Shaking' is regularly an evidence of God's powerful activity. See Judges 5:5; Habakkuk 3:6 LXX compare Haggai 2:6 where the shaking would be of heaven and earth and sea, as well as dry land. See above where God is Master of heaven and earth and sea. God was showing the disciples what He would yet do.