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SUMMARY The ratio of seismogenic to aseismic deformation along active faults is needed to estimate their seismogenic potential and hazards. Seismologic and geodetic methods routinely capture coseismic displacements, but data acquisition requirements to fully document post-seismic deformation are not well known. Our study documents afterslip between about 18 months and 4 years after a mid-size earthquake and, based on remote structural mapping, we document fault rupture segments not previously associated with that earthquake. Persistent scatterer interferometric analysis of Sentinel-1A aperture radar data acquired between October 2014 and December 2018 reveals prolonged post-seismic deformation following the 11 May 2013 Mw 6.1 Minab earthquake and its aftershocks. The surface deformation data yield a sharp contrast across both the main seismogenic fault (here named the Irer fault) and its northeastern splay, and it is compatible with left-lateral motion along both faults. The PSI data helped us to identify and map the splay fault in the satellite imagery. We could then measure the geological offset along both faults, finding maximum displacements of about 1 km (main fault) and 350 m (splay). Our modelling of the observed post-seismic surface deformation pattern shows that post-seismic deformation was accommodated by left-lateral afterslip, not viscoelastic relaxation. This result is consistent with previous propositions that Mw 6 earthquakes do not measurably excite deeply seated viscoelastic relaxation mechanisms. Our afterslip modelling yields a slip pattern from the surface to a depth of 6 km to maximum 16 km, in agreement with the depth of the coseismic slip-distribution, and a maximum displacement of ∼7 cm along the fault, but located ∼8 km to the east of the coseismic slip maximum. Moment release during the observed afterslip in our study is Mw 5.7, or 12% of the coseismic moment released by main shock and aftershocks together. Combined with previously published results for the early post-seismic period (first 2 months), we estimate the aseismic moment to be at least ∼37% of the total, implying a high ratio of aseismic to seismic moment release for the Irer fault. Our results show that observation time windows well beyond 5 years are needed to record afterslip following mid-sized earthquakes. Thus, progress in understanding the transition from post-seismic to interseismic fault behaviour critically depends on the availability of data provided by satellite missions such as Copernicus Sentinel-1A. Similarly, robust comparison of the post-seismic rates with long-term geological rates requires palaeoseismic study and dating of related morphotectonic features.

The southern Baja California (Mexico) microplate has been rapidly moving away from the North America plate since ca. 12 Ma. This relative motion toward the northwest developed an oblique-divergent plate boundary that formed the Gulf of California. The rift-drift hypothesis postulates that when a continent ruptures and seafloor spreading commences, rifting on the plate margins ceases, and the margins start to drift, subside, and accumulate postrift sediments, eventually becoming a passive margin. In contrast to this hypothesis, the southern part of the Baja California microplate (BCM), and in particular its actively deforming eastern boundary zone, has continued significant rifting for millions of years after seafloor spreading initiated within the southern Gulf of California at 6–2.5 Ma. This is a process we call “rifting-while-drifting.” Global positioning system (GPS)–based data collected from 1998 to 2011 show relative motion across the eastern boundary zone up to ∼2–3.2 mm/yr with respect to a stable BCM. Furthermore, the velocity directions are compatible with normal faulting across the eastern boundary zone nearly perpendicular to the trend of the plate boundary at the latitude of La Paz and therefore a highly strain partitioned domain. North of 25°N latitude up to the Loreto area, there is a domain with no strain partitioning, and northwest-directed transtensional deformation dominates. From long-term geologic and paleoseismology studies, late Quaternary faulting rates are equal to or less than the GPS-derived rates, while geologic rates older than 1–2 Ma are commonly much higher. We suggest that the “rifting-while-drifting” process may be caused by the large topographic relief across the BCM margin, which created a significant gradient in gravitational potential energy that helps in driving continued relatively slow faulting. The relief was inherited from the much faster faulting of the BCM eastern boundary zone before plate motions largely localized along the modern transform–spreading centers in the axis of the Gulf of California. The low sediment flux from the small drainages and arid climate on the southern Baja California Peninsula result in the maintenance of underfilled to starved basins, and the relatively slow late Quaternary active faulting promotes continued topographic relief over millions of years.

The Gulf of California, Mexico, accommodates ~90% of North America-Pacific plate relative motion. While most of this motion occurs on marine transform faults and spreading centers, several fault segments in the central Gulf come close to peninsular Baja California. Here we present Global Positioning System and interferometric synthetic aperture radar data near the Ballenas transform fault, separating the peninsula from Angel de la Guarda Island. We observe interseismic motion between June 2004 and May 2009 and displacements associated with the 3 August 2009 Mw 6.9 earthquake. From the interseismic data we estimate a locking depth of 9–12.5 km and a slip rate of 44.9–48.1 mm/yr, indicating that faults east of Angel de la Guarda deform at negligible rates and that the Ballenas Transform accommodates virtually all of the relative motion between the North American plate and the Baja California microplate. Our preferred model for coseismic slip on a finite rectangular fault plane suggests 1.3 m of strike-slip displacement along a vertical rupture plane that is 60 km long and extends from the surface to a depth of 13 km in the eastern Ballenas Channel, striking parallel to Baja California-North America relative plate motion. These estimates agree with the seismic moment tensor and the location of the major foreshock and aftershocks and are compatible with the fault location identified from high-resolution bathymetric mapping. The geodetic moment is 33% higher than the seismic moment in part because some afterslip and viscous flow in the first month after the earthquake are included in the geodetic estimate. Coulomb stress changes for adjacent faults in the Gulf are consistent with the location of smaller aftershocks following the 2009 main shock and suggest potential triggering of the 12 April 2012 Mw 6.9 Guaymas earthquake.

[1] Multiple mechanisms have been suggested to explain seaward motion of the south flank of Kīlauea Volcano, Hawai‘i. The consistency of flank motion during both waxing and waning magmatic activity at Kīlauea suggests that a continuously acting force, like gravity body force, plays a substantial role. Using finite element models, we test whether gravity is the principal driver of long-term motion of Kīlauea's flank. We compare our model results to geodetic data from Global Positioning System and interferometric synthetic aperture radar during a time period with few magmatic and tectonic events (2000–2003), when deformation of Kīlauea was dominated by summit subsidence and seaward motion of the south flank. We find that gravity-only models can reproduce the horizontal surface velocities if we incorporate a regional decollement fault and a deep, low-viscosity magma mush zone. To obtain quasi steady state horizontal surface velocities that explain the long-term seaward motion of the flank, we find that an additional weak zone is needed, which is an extensional rift zone above the magma mush. The spreading rate in our model is mainly controlled by the magma mush viscosity, while its density plays a less significant role. We find that a viscosity of 2.5 × 1017–2.5 × 1019 Pa s for the magma mush provides an acceptable fit to the observed horizontal surface deformation. Using high magma mush viscosities, such as 2.5 × 1019 Pa s, the deformation rates remain more steady state over longer time scales. These models explain a significant amount of the observed subsidence at Kīlauea's summit. Some of the remaining subsidence is probably a result of magma withdrawal from subsurface reservoirs.

SUMMARY The Crandall Canyon Mine, Utah, collapse in 2007 August resulted in a total of nine fatalities. We processed data from the ALOS satellite acquired before and after the collapse to quantify surface subsidence associated with the collapse to better understand the collapse process. The deformationshowsasteepV-shapedpatternofsubsidencewithslightasymmetry.Wecompare the fit of four different models that simulate the subsidence pattern. The first two models use elastic half-space rheology. We find that collapse alone cannot explain the observations, and a component of normal faulting is required to fit the data. The second set of models simulates collapseinelasto-plasticmedia.Onlyasmallcomponentofnormalfaultingisrequiredinthese models. We suggest that considering elasto-plastic material behaviour is particularly important for shallow deformation modelling, where microfractures and other non-elastic rheology are common. Disregarding this material behaviour can lead to biased model parameter estimates.

The role that the motion of microplates has played in the development of the Eastern California Shear Zone in western North America is unclear. It was previously proposed that shear related to Baja California motion has propagated from the Gulf of California northward into Nevada. However, there is evidence that the formation of the Eastern California Shear Zone predates the inception of Baja California motion. Thus, either Baja California began to move northwestward prior to 6 Ma, or the Eastern California Shear Zone formed by shear strain resulting from different microplate motion than Baja California. The role of Sierra Nevada motion on the development of an adjacent shear zone remains untested. We present a numerical modeling study that examines the deformational response of western North America to Baja California and/or Sierra Nevada microplate motion. In particular we study if, and under what condition of microplate motion, shear strain can localize in the northern and southern part of the Eastern California Shear Zone (separated by the Garlock fault). We find that given the pre-existing weakness from Miocene Basin and Range extension (a lithospheric strength contrast or normal faults) shear strain from either the Baja California or Sierra Nevada microplates can lead to the formation of the Eastern California Shear Zone. Furthermore, the presence of a pre-existing weakness explains the earlier initiation of strike-slip faulting north of the Garlock fault. Our models suggest that today, Baja California microplate motion is the major player for present-day deformation rates along the Eastern California Shear Zone and was in the past 6 Myrs also a major driver of strain localization in southern Eastern California Shear Zone within the Mojave block.

The driving forces of microplate transport remain one of the major unknowns in plate tectonics. Our hypothesis postulates that the Baja California microplate is transported along the North America–Pacific plate boundary by partial coupling to the Pacific plate and low coupling to the North America plate. To test this idea, we use numerical modeling to examine the interplate coupling on a multiple-earthquake-cycle time scale along the Baja California–Pacific plate boundary and compare the modeled velocity field with the observed geodetic motion of the Baja California microplate. We find that when the strain can localize along a weak structure surrounding microplate (faults), high interplate coupling, produced by frictional tectonic stresses, can reproduce the observed kinematics of the Baja California microplate as seen from geodetic rigid-plate motions. We also find that the northward motion of Baja California can influence the fault slip partitioning of the major faults in the North America–Pacific plate boundary region north of Baja California.

SUMMARY We present a new surface velocity field for Baja California using GPS data to test the rigidity of this microplate, calculate its motion in a global reference frame, determine its relative motion with respect to the North American and the Pacific plates, and compare those results to our estimate for Pacific‐North America motion. Determination of Pacific Plate motion is improved by the inclusion of four sites from the South Pacific Sea Level and Climate Monitoring Project. These analyses reveal that Baja California moves as a quasi-rigid block but at a slower rate in the same direction, as the Pacific Plate relative to North America. This is consistent with seismic activity along the western edge of Baja California (the Baja California shear zone), and may reflect resistance to motion of the eastern edge of the Pacific Plate caused by the ‘big bend’ of the San Andreas fault and the Transverse Ranges in southern California.