To better understand the physical mechanisms of slow slip events (SSEs) detected worldwide, we explore the scaling relationships of various source parameters and compare them with similar scaling laws for earthquakes. These scaling relationships highlight differences and similarities between slow slip events and earthquakes and hold implications for the degree of heterogeneity and fault-healing characteristics. The static stress drop remains constant for different-sized events as is observed for earthquakes. However, the static stress drop of slow slip events is within a range of 0.01-1.0 MPa, 1-2 orders of magnitude lower than that found for earthquakes, which could be related to the low stress state on the fault. The average rupture velocity, ranging from kilometers per second to kilometers per day, decreases linearly with increasing seismic moment in log-log space, unlike earthquakes that are nearly constant. This inverse relationship of rupture velocity with seismic moment could be related to the heterogeneity of fault properties. Slow slip events typically have ratios of event duration over dislocation rise time less than 3, while earthquakes have ratios greater than 3. This indicates that slow slip events are less pulselike than earthquakes in their mode of propagation and suggests that the healing behind the rupture front is delayed. The recurrence statistics of slow slip events on the northern Cascadia subduction zone are weakly time predictable and moderately antislip predictable (that is, the event size and preevent recurrence interval are anticorrelated), which may indicate that healing between events strengthens the fault with time.

We invert for the time‐dependent slip history of slow slip events on the Cascadia subduction zone using GPS data from 1998 to 2008. The 16 slip transients have sufficient station coverage to solve for the slip distribution on the plate interface. GPS time series are inverted for fault slip using the Extended Network Inversion Filter. Limited station coverage south of Portland (45.5°N latitude) restricts our analysis to events on the northern half of the subduction zone. Slip is resolved at the base of the seismogenic zone and the slip distributions suggest a potential segment boundary near Seattle (47.6°N) that correlates roughly with geologic and tectonic boundaries. Events that initiate to the north and south tend to overlap at about this latitude. We compile statistics on source parameters, such as propagation rate, recurrence interval, and stress drop, which can be used to constrain proposed models of the source mechanics. Over a 10 year period, total strain release from slow slip events is nonuniform along strike with the greatest cumulative slip (27 cm) centered beneath Port Angeles (48.1°N). This slip patch also exhibits the most regular recurrence of Mw ∼6 events relative to other locations along strike. The spatial extent of the slip patch beneath Port Angeles correlates with the along‐strike bend of the Cascadia subduction zone in northwestern Washington, suggesting that plate geometry plays an important role in controlling the along‐strike characteristics of slow slip.

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The along-strike variations of the velocity, thickness, and dip of subducting slabs and the volcano distribution have been observed globally. It is, however, unclear what controls the distribution of volcanoes and the associated magma generation. With the presence of nonuniform volcanism, the Aleutian-Alaska subduction zone (AASZ) is an ideal place to investigate subduction segmentation and its relationship with volcanism. Using full-wave ambient noise tomography, we present a high-resolution 3-D shear wave velocity model of the AASZ for the depths of 15-110 km. The velocity model reveals the distinct high-velocity Pacific slab, the thicker, flatter, and more heterogeneous Yakutat slab, and the northeasterly dipping Wrangell slab. We observe low velocities within the uppermost mantle (at depth <60 km) below the Aleutian arc volcanoes, representing partial melt accumulation. The large crustal low-velocity anomaly beneath the Wrangell volcanic field suggests a large magma reservoir, likely responsible for the clustering of volcanoes. The Denali volcanic gap is above an average-velocity crust but an extremely fast mantle wedge, suggesting the lack of subsurface melt. This is in contrast with the lower-velocity back-arc mantle beneath the adjacent Buzzard Creek-Jumbo Dome volcanoes to the east. The back-arc low velocities associated with the Pacific, the eastern Yakutat, and the Wrangell slabs may reflect subduction-driven mantle upwelling. The structural variation of the downgoing slabs and the overriding plate explains the change of volcanic activity along the AASZ. Our findings demonstrate the combined role of the subducting slab and the overriding plate in controlling the characteristics of arc magmatism. Plain Language Summary The subduction of oceanic plates underneath the continent plate is a complicated, three-dimensional process. The geometry of the downgoing plate and the associated volcanic activity vary along the subduction margin. It is not clearly understood what controls the distribution of arc volcanoes. We utilize an advanced seismic imaging technique to construct a detailed seismic velocity model of the Aleutian-Alaska margin, from crust to the uppermost mantle. The velocity model reveals multiple downgoing slabs, with various seismic velocities, thicknesses, and dip angles. The imaged Pacific slab, Yakutat slab, and Wrangell slab correlate spatially with the Aleutian arc volcanoes, the Buzzard Creek-Jumbo Dome volcanoes, and the Wrangell volcanoes, respectively. The mantle wedge, the wedge-shaped space below the overriding crust and above the downgoing slab, is characterized by different seismic velocities below these three volcanic areas. The Denali volcanic gap, a region with the absence of volcanic activity, is located above a seismically fast mantle wedge and an average crust. There is no melt below this volcanic gap based on our observations. Our findings explain the along-strike variation of volcanic activity along the Aleutian-Alaska subduction zone and demonstrate the combined role of the s...