Strong strain and pore pressure changes are observed after three Mw 4.5+ local and one Mw 7.2 regional earthquake during 2010–2017 in borehole strainmeters near Anza, California. The strain change emerges immediately after the earthquakes and lasts 40–100 days with amplitudes up to 10−7, larger than the coseismic strain offsets. The pore pressure exhibits change immediately after the earthquakes at some boreholes and with a delay of 4–10 days at the others. A joint analysis of the observed postseismic strain and pore pressure change suggests that the postseismic strains could be explained by combined effects of poroelastic deformation due to earthquake‐induced pore pressure change and elastic deformation due to an earthquake‐triggered aseismic slip on a local fault. Our study indicates that, in addition to possible aseismic fault slips triggered by an earthquake, pore pressure changes after the earthquake could be even more important in producing postseismic deformation. Plain Language Summary Understanding the physical mechanisms producing postseismic underground deformation is important for assessing fault slip budget and seismic hazards. In this study, we seek to clarify the possible roles of aseismic fault slip and underground water in producing postseismic deformation, through a joint analysis of underground deformation and pressure change in the pores of underground water reservoirs observed in the boreholes in southern California following four middle/large‐magnitude earthquakes. We find that both the underground deformation and pore pressure in the underground water reservoir exhibit changes lasting 1–3 months after the earthquakes, with changing amplitudes larger than the coseismic changes. These observations are well explained by a mechanism in which the mainshock earthquakes instantly trigger aseismic slips on the local faults and alter the hydrological conditions in the region; the change of hydrological condition results in a postseismic change of pore pressure in the underground water reservoir and produces poroelastic deformation in the region, while the aseismic fault slips produce elastic deformation. This study indicates that, in addition to possible aseismic fault slips triggered by an earthquake, changes of pore pressure after the earthquake in the underground water reservoir could play an even more important role in producing postseismic deformation. Key Points We observe the strong months‐long change of strain and pore pressure after four Mw 4.5+ earthquakes in borehole strainmeters at Anza, California The postseismic strains last 40–100 days, and exhibit different trends and larger amplitudes up to 10−7 compared to coseismic strains Postseismic strain = poroelastic strain by earthquake‐induced pore pressure change + elastic strain by an earthquake‐triggered aseismic slip