Low-frequency earthquakes (LFEs) are detected within tremor, as small, repetitive, impulsive low-frequency (1-8 Hz) signals. While the mechanism causing this depletion of the high frequency content of their signal is still debated, this feature may indicate that the source processes at the origin of LFEs are different from those for regular earthquakes. Key constraints on the LFE-generating physical mechanisms can be obtained by establishing scaling laws between their seismic moment and source durations. Here we apply a simple spectral analysis method to the S-waveforms of LFEs from Guerrero, Mexico to measure their seismic moments and corner frequencies, a proxy to source duration. We find characteristic values of seismic moment around 3e12 N.m (Mw 2.3) and of corner frequencies around 3 Hz with the corner frequency very weakly dependent on the seismic moment. This moment-duration scaling observed for Mexican LFE is similar to one previously reported in Cascadia and is very different from the established one for regular earthquakes. This suggests that they could be generated by sources of nearly constant size with strongly varying intensities. LFEs do not exhibit the self-similarity characteristic of regular earthquakes, suggesting that the physical mechanisms at their origin could be intrinsically different.