Coseismic and Postseismic Deformation Associated with the 2018 Mw 7.9 Kodiak, Alaska, Earthquake from Low-Rate and High-Rate GPS Observations

Low-rate (15 s) and high-rate (1 s) Global Positioning System (GPS) solutions were employed to investigate the coseismic deformation associated with the 23 January 2018 M(w )7.9 Kodiak, Alaska, offshore earthquake. The coseismic displacements of kinematic (high-rate observations) and quasi-kinematic (2 hr of low-rate observations after the earthquake) GPS solutions are consistent with those derived from static GPS daily solutions, and all three sets conform to the characteristics of a strike-slip earthquake. A comparison between the static and quasi-kinematic results suggests that postseismic deformation within the first four days was negligible. We inverted the static displacement field for the slip in a five-segment model and inferred that the largest slip occurred on right-lateral south-southeast-striking (2.27 m) and left-lateral northeast-striking (2.42 m) fault segments. The inverted geodetic moment is M(0 )9.66 x 10(20) N . m (M(w )7.92). The nine-month postseismic horizontal displacements at 12 nearby GPS sites we studied are < 1 cm, and the maximum postseismic deformation is 7.4 mm at site AC26. The postseismic transient was fit slightly better by a logarithmic model with a decay time of 6.4 days than by an exponential model with a decay time of 75.0 days. The postseismic deformation can be explained well by afterslip and/or viscoelastic relaxation.