Seismotectonics of the Blanco Transform Fault System, Northeast Pacific: Evidence for an Immature Plate Boundary

Abstract

At the Blanco transform fault system (BTFS) off Oregon, 138 local earthquakes and 84 double‐couple focal mechanisms from ocean‐bottom‐seismometer recordings jointly discussed with bathymetric features reveal a highly segmented transform system without any prominent fracture zone traces longer than 100 km. In the west, seismicity is focused at deep troughs (i.e., the West and East Blanco, and Surveyor Depressions). In the east, the BTFS lacks a characteristic transform valley and instead developed the Blanco Ridge, which is the most seismically active feature, showing strike‐slip and dip‐slip faulting. Sandwiched between the two main segments of the BTFS is the Cascadia Depression, representing a short intra‐transform spreading segment. Seismic slip vectors reveal that stresses at the eastern BTFS are roughly in line with plate motion. In contrast, stresses to the west are clockwise skewed, indicating ongoing reorganization of the OTF system. As we observed no prominent fracture zones at the BTFS, plate tectonic reconstructions suggest that the BTFS developed from non‐transform offsets rather than pre‐existing transform faults during a series of ridge propagation events. Our observations suggest that the BTFS can be divided into two oceanic transform systems. The eastern BTFS is suggested to be a mature transform plate boundary since ∼0.6 Ma. In contrast, the western BTFS is an immature transform system, which is still evolving to accommodate far‐field stress change. The BTFS acts as a natural laboratory to yield processes governing the development of oceanic transform faults.

A Reciprocity‐Based Efficient Method for Improved Source Parameter Estimation of Submarine Earthquakes With Hybrid 3‐D Teleseismic Green's Functions

“…They found that the seismic slip is well constrained by the 600°C cutoff depth. However, microseismicity constrained by the local OBS array indicates that the brittle seismogenic zone of the Blanco OTF can penetrate to depths below the 600°C isotherm (Kuna et al., 2019; Ren et al., 2023). The microseismicities and petrological studies sampled from other OTFs have also reported semibrittle deformation at temperatures higher than 600°C (Kohli et al., 2021; Prigent et al., 2020; Yu et al., 2021).…”

Machine Learning‐Based New Earthquake Catalog Illuminates On‐Fault and Off‐Fault Seismicity Patterns at the Discovery Transform Fault, East Pacific Rise

“…For example, the seismicity gap between 23 and 25 km corresponds to the deepest section of the western valley and the dense seismicity patch between 34 and 40 km is located beneath an along‐transform ridge (Figure 1 and Figure S6 in Supporting Information ). These valleys and ridges might have been formed as the local tectonic system evolves, affecting regional stress field and causing fault segmentation into distinct patches (Dewey et al., 1998; Gregg et al., 2006; Grevemeyer et al., 2021; Ren et al., 2023).…”

“…Microseismicity provides valuable insights into the fault structure, slip mode, and tectonic processes at OTFs (e.g., Gong & Fan, 2022; Gong et al., 2022; Kuna et al., 2019; McGuire et al., 2012; Ren et al., 2023; Wolfson‐Schwehr et al., 2014; Yu et al., 2021). We introduce a new workflow for detecting and locating microearthquakes using machine learning techniques.…”

“…Moreover, large earthquakes repeatedly rupture the same fault patches, generating characteristic earthquakes (McGuire, 2008; Shi et al., 2021). Several mechanisms have been proposed to explain seismic and aseismic partitioning of OTFs, including along‐strike variations in frictional properties (Y. Liu et al., 2020), crust‐mantle interactions (Kuna et al., 2019), deep fluid circulation (Kohli et al., 2021; Roland et al., 2012; Yu et al., 2021), and variations in fault structure due to microplate formation (Wolfson‐Schwehr et al., 2014) or rotation (Gong et al., 2022; Ren et al., 2023). Collectively, these mechanisms reflect the feedback between fault slip, local plate motion, fault architecture, and fluid and magma fluxes within OTF systems.…”