The compensated linear vector dipole (CLVD) mechanism (P wave radiation with conical nodal surfaces) found for the Mammoth Lakes earthquakes of May 1980 has been attributed to dike intrusion by Julian and his colleagues. There is a concern about this interpretation because magma intrusion through a crack should radiate compressional first motion everywhere. Chouet's calculation on a magma intrusion model, however, showed that the main dynamic motion as well as the final static displacement have patterns similar to the CLVD mechanism. We found an example of observed radiation pattern similar to the CLVD mechanism from eruption earthquakes at Mount Aso obtained by Sassa, and we interpret the observation as misientification of the main motion as the initial motion. In searching for a similar misidentification for the Mammoth Lakes earthquakes we found that there are numerous reports of compression for short-period P waves at stations where long-period P waves show unequivocal dilation. Furthermore, we found that the compressional short-period P waves show unequivocal dilatation. Furthermore, we found that the compressional short-period P waves show unequivocal dilatation. Furthermore, we found that the compressional short-period first motion is systematically earlier than the dilatational one. These observations an be explained if the source of magma intrusion generated compressional first motions which were very weak and of high-frequency contents in the direction where long-period P waves are dilatational. The seismic moment of the Mammoth Lakes earthquakes suggests an involvement of magma volume of the order of 10-2 km3. However, we have not yet observed long-period events or volcanic (harmonic) tremors which are often observed at active volcanos. We review recent advances in theories and observations on volcanic tremor and conclude that for the volume of magma reservoir suggested for the Mammoth Lakes area a fluctuation of pressure of the order of 10-2 bar would gnerate measurable tremor. This conclusion is obtained under the assumption that both the viscosity of magma and the radiation loss of the magma reservoir are low enough for the excitation of a harmonic tremor. We found that the concept of ''reduced displacement'' introduced by Aki and Koyanagi for body waves and by Fehler for surface waves is very useful for estimating the physical parameters of a volcanic tremor source.