Fast, compact, and power-efficient silicon microcavity electro-optic modulators are expected to be critical components for chip-level optical interconnects. It is highly desirable that these modulators can be driven by voltage swings of $1\mathrm{V}$ or less to reduce power dissipation and make them compatible with voltage supply levels associated with current and future complementary metal-oxide-semiconductor technology nodes. Here, we present a silicon racetrack resonator modulator that achieves over $8\mathrm{dB}$ modulation depth at $12.5\mathrm{Gbps}$ with a $1\mathrm{V}$ swing. In addition, the use of a racetrack resonator geometry relaxes the tight lithography resolution requirements typically associ ated with microring resonators and enhances the ability to use common lithographic optical techniques for their fabrication.