Aseismic slip loading has recently been proposed as a complementary mechanism for moderate earthquakes (M3+) induced over the short operational period of hydraulic fracturing stimulations but located several kilometers away from the wellbore. However, aseismic/slow slip signals linked to fluid injection-induced earthquakes remain largely undocumented to date. Here we report a new type of induced seismic signal consisting of an impulsive broadband onset followed by protracted low-frequency ringing. Earthquakes characterized by hybrid-frequency waveforms (EHWs) differ from ordinary induced earthquakes by having broader P and S-pulses (implying longer source durations) and lower corner frequencies (implying either slower rupture speeds, lower stress drop values, or a combination of both). The characteristics described above are identical to low-frequency earthquakes found in plate boundary zones. EHWs could thus manifest the source process that bridges the slow (aseismic) slip inferred by recent modeling and observations near the wellbore to seismic slip at greater distances.