The M_W6.9 thrust-faulting earthquake in Hawai'i on May 4, 2018, occurred near the Kīlauea shield volcano. We use 57-day-long continuous Global Positioning System (cGPS) data and 6-day-long semi-continuous GPS data to develop a detailed coseismic slip model, and investigate its impact on volcanic activity. Combining these data with published models, we reconstruct a planar, very shallow dipping geometry model for the 2018 Hawai'i earthquake. Our results show the released moment is about 4.05×10¹⁹ N·m (M_W7.0), with a peak slip of approximately 2.4 m at 4.0 km depth. Comparative analysis confirms that the coseismic model is sensitive to low-dip fault geometry rather than high-dip angle splay faults. Based on the Coulomb failure-stress model, we find the Hawai'i event exerts more positive stress on Kīlauea than on Mauna Loa, and the mainshock decompresses the magma chambers of both volcanoes, potentially facilitating magma pathways and subsequent eruptions near the east rift zone (ERZ). The mainshock also adjusts the static stress field accumulated by interseismic creep, dike intrusion, and volcanic activity before the event. This study illustrates the physical correlation between earthquake sequences and volcanic events, providing insights into the seismogenic structure and stress perturbation patterns along the Hawai'i basal décollement fault.