大般涅槃經義記 1.43

1.43

自下第四動地催集。言「爾時」者，動地時也。下正明動，大地山海皆悉震動，彰大異小，故下文言獨地動者名為地動，山、河、海水一切動者名大地動，是中略無動地所由。若具應言，以佛神力故地震動。

From here below, the fourth section deals with the quaking of the earth urging them to assemble. The words “At that time” mark the time of the earth-quaking. Below, it directly explains the quaking: the great earth, mountains, and oceans all shook, highlighting that the great differs from the small. Therefore, the text below states that when only the earth shakes, it is called an “earth-quaking”; when mountains, rivers, and ocean waters all shake, it is called a “great earth-quaking.” In this passage, the reason for the shaking of the earth is briefly omitted. If stated in full, it should say that the earth shook through the Buddha's spiritual power.