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Interseismic deformation describes the gradual accumulation of crustal strain within the tectonic plate and along the plate boundaries before the sudden release as earthquakes. In this study, we use 5 years of high spatial and temporal geodetic measurements, including Global Navigation Satellite System and Interferometric Synthetic Aperture Radar to monitor 3‐dimension interseismic crustal deformation and horizontal strain rate in Taiwan. We find significant deformation (strain rate >8 × $\times $ 10−6 yr−1) along the plate boundary between the Philippine Sea and the Eurasian Plates in east Taiwan. The high strain rate in the southern part of the Western Foothills is distributed along a few major fault systems, which reveals the geometry of the deformation front in west Taiwan. Our results help identify active faults in southwest and north Taiwan that were not identified before. These findings can be insightful in informing future seismic hazard models. Plain Language Summary: An earthquake cycle includes three phases: interseismic, coseismic, and postseismic. Interseismic deformation refers to the continuous crustal deformation that is built up by active tectonics. Depending on the relative motion between tectonic plates, the earthquake recurrence interval could vary by a few orders between different locations. As a result, knowing the crustal deformation rate and deformation accumulated in different fault zones can be useful for investigating future earthquake hazards. Using space geodesy tools like Global Navigation Satellite System and Interferometric Synthetic Aperture Radar, we can monitor surface deformation during the interseismic period. In this study, we monitor interseismic deformation in Taiwan using geodesy. We find that east Taiwan where two tectonic plates collide has the highest rate of deformation. In southwest and north Taiwan where most of the population resides, there is also high‐level of deformation distributed across a few different faults, indicating that some of the faults have a higher risk generating future earthquakes. As a result, knowing the amount of faults slip and amount of deformation built up during this interseismic period may inform us of potential future earthquake hazards. Key Points: This study combines Interferometric Synthetic Aperture Radar and Global Navigation Satellite System and produces high‐resolution 3‐D interseismic crustal velocities and strain rate estimates in TaiwanStrain rate measurements show high surface strain cumulation along east and southwest TaiwanThe surface strain rates and the earthquake hazard models based on seismology and field study are in good agreement [ABSTRACT FROM AUTHOR]

We use seismic and geodetic measurements to optimize the fault geometry as well as coseismic slip distribution from the 2018 Mw 6.4 Hualien earthquake. We find that at least three faults were involved in the earthquake. The earthquake initiated from a south‐dipping fault in offshore Hualien, and slip transferred into the main west‐dipping oblique fault. The slip finally triggered movement of the east‐dipping Milun fault at shallower depth and caused surface rupture. Although there is no offshore data constraints, our inverted slip distribution shows that the majority of slip occurred between 5‐ and 10‐km depth on the main west‐dipping fault, and 1‐ to 3‐m slip on the shallower part of the Milun fault. Additionally, we process 5 months of postseismic deformation time series and find more than 5‐cm postseismic displacement occurred along the Milun fault but is insignificant near the Coastal Range located south of the Hualien City. Plain Language Summary: Taiwan is one of the most tectonically active regions in the world. In eastern Taiwan, the Longitudinal Valley is located between the Eurasian Plate and the Philippine Sea Plate, and the rapid plate collision has cause high surface deformation as well as earthquake hazard in Hualien, the biggest city in northern Longitudinal Valley. In this study, we use measurements during the earthquake to study the fault geometry and fault slip due to the Mw 6.4 Hualien earthquake occurred in February 2018. Our inversion results show that at least three faults are involved in the event, including a south‐dipping fault, a main west‐dipping fault, and the east‐dipping Milun fault located in the Hualien City. Our inverted fault slip at shallower depth is in agreement with field investigation after the earthquake. We also process 5 months of data to observe postearthquake surface deformation and find that there is more than 5 cm of displacement along the Milun fault, but insignificant displacement south of the Hualien City. Key Points: At least three faults were involved in the 2018 Mw 6.4 Hualien earthquakeMost of the slip occurred on the main west‐dipping fault close to the north Coastal RangeRapid postseismic transient along the Milun fault in the Hualien City, whereas insignificant displacement close to the Coastal Range [ABSTRACT FROM AUTHOR]