Waveform Signatures of Earthquakes Located Close to the Subducted Gorda Plate Interface

Abstract

Complex seismic velocity structure near the earthquake source can affect rupture dynamics and strongly modify the seismic waveforms recorded near the fault. Fault-zone waves are commonly observed in continental crustal settings but are less clear in subduction zones due to the spatial separation between seismic stations and the plate boundary fault. We observed anomalously long duration S waves from earthquake clusters located near the interface of the subducted Gorda plate north of the Mendocino triple junction. In contrast, earthquakes located just a few kilometers below each cluster show impulsive S waves. A nodal array experiment was conducted around the Northern California Seismic Network station KCT for two months to investigate the origin of the complex S waves. Beamforming analysis shows that the S waves contain three arrivals that have different horizontal slownesses, which we term S1, S2, and S coda. Similar analysis on P waves also show two arrivals with different horizontal slownesses, which we term P1 and P2. P1 and S1 have larger horizontal slowness than P2 and S2, respectively, indicating that the phase pairs are body waves with different ray paths. Building upon a seismic refraction profile, we construct 1D velocity models and test different thicknesses and VP/VS ratios for the subducted oceanic crust. The arrival times and relative slownesses of P1/P2 and S1/S2 phases indicate that they are the direct and the Moho reflected phases, respectively. Their properties are consistent with a crustal thickness of ∼6 km and a moderate VP/VS ratio (∼1.8). The S coda is more difficult to characterize but has a clear dominant frequency that likely reflects the near-source velocity and attenuation structure. Our study indicates that waveforms from earthquakes near the interface of the subducted slab can be used to infer detailed structural information about the plate-boundary zone at seismogenic depths.

Subduction intraslab-interface fault interactions in the 2022 M w 6.4 Ferndale, California, earthquake sequence

“…Figure 2 shows map view and cross-sectional views of aftershock seismicity, along with focal mechanism solutions obtained using correlation-based relative polarities and S/P amplitude ratios. On the basis of the correspondence with existing plate interface models (10,13), converted seismic phases (14,15), and tomography studies (9), which approximately track the shallow eastward dip of the upper extent of seismicity, we infer that seismicity was likely concentrated within the upper ~2 to 4 km of the Gorda slab. The strike orientations of the focal mechanism nodal planes are mostly consistent with the trends defined by the earthquake locations.…”

“…Because of limitations in focal sphere coverage, the nodal plane dips of some solutions are poorly constrained (fig. S3), and unmodeled complex ray paths [e.g., ( 15 )] may increase misfit. Despite these caveats, focal mechanisms are broadly consistent with three categories, as labeled in Fig.…”

“…White dashed line shows the Slab2 ( 13 ) model, while the solid white line shows the model of McCrory et al ( 10 ) for the subduction interface. Black boxes show the area where the Fortuna ( 23 ), and equivalent C2 ( 15 ) clusters were observed with long-term activity before the 2022 earthquake. White boxes in (A) and (B) show the extents of zoomed views in parts (C) and (D), respectively.…”

“…Previously faulted and hydrated areas may also form localized zones of weakness with long-lived clusters of seismicity over a period of at least several years ( 15 , 23 ). Although these clusters locate near the subduction interface, evidence from focal mechanisms, locations, and waveform characteristics indicates that they likely occur within the Gorda slab rather than on the interface itself.…”

“…Although these clusters locate near the subduction interface, evidence from focal mechanisms, locations, and waveform characteristics indicates that they likely occur within the Gorda slab rather than on the interface itself. Gong and McGuire ( 15 ) identified five seismicity clusters preceding the 2022 Ferndale sequence by many years. They inferred these clusters occurred mostly in the uppermost Gorda slab based on phases reflected from the slab Moho.…”