Crustal Shear Wave Velocity Structure of Central Idaho and Eastern Oregon From Ambient Seismic Noise: Results From the IDOR Project

Abstract

We developed 3‐D isotropic crustal seismic velocity models of central Idaho and eastern Oregon from the IDOR (western IDaho and eastern ORegon) Passive seismic data. Ambient noise tomography yielded crustal velocity structure from vertical component Rayleigh wave group and phase velocity measurements. Results include a strong shear wave velocity contrast—faster in accreted Blue Mountains terranes west of the western Idaho shear zone (WISZ), slower in the Idaho batholith, emplaced within the Archean Grouse Creek block east of the WISZ—restricted to the upper‐to‐middle crust. In deeper crust not affected by mafic underplating during Columbia River Flood Basalt magmatism, the shear wave velocity of the Mesozoic Olds Ferry continental arc terrane is indistinguishable from that of the Archean Grouse Creek block basement. Crustal columns of the Olds Ferry terrane and the Permian‐Jurassic Wallowa intraoceanic arc terrane are characterized by low seismic velocities, consistent with felsic lithologies down to ∼20 km. West of the WISZ, the Bourne and Greenhorn subterranes of the Baker terrane, an accretionary complex between the arc terranes, have distinct shallow crustal seismic velocities. The Greenhorn subterrane to midcrustal depths is in an overthrust geometry relative to the Bourne subterrane. Lack of mafic lower crust in our results of the Wallowa or Olds Ferry arcs may be due to imbrication of upper crustal felsic plutonic complexes of these arcs. Shortening and thickening of the Blue Mountains arc terranes crust to >30 km, and subduction or delamination of their mafic lower crustal sections is a viable mechanism for growth of a felsic continental crust.

Multifault Opposing‐Dip Strike‐Slip and Normal‐Fault Rupture During the 2020 Mw 6.5 Stanley, Idaho Earthquake

“…Although there are some hot springs near Stanley (Druschel & Rosenberg, 2001), the moment tensor of the 2020 Stanley earthquake has such a large overall nondouble couple that minor geothermal activity cannot account for it. In addition, strong anisotropy sufficient to affect the long‐period waves used in the moment tensor inversions is unlikely in the shallow crust of the Stanley earthquake region (Bremner et al., 2019; Christian Stanciu et al., 2016). Based on the back‐projection results, InSAR ground deformation and aftershock distribution, we favor a multifault rupture process to explain the significant nondouble‐couple in the moment tensors of the Stanley earthquake.…”

“…The teleseismic data include 72 P wave and 37 SH-wave ground displacement recordings that are filtered in the frequency band from 0.005 to 0.9 Hz (Figure S3). The structural model used in the inversion is the local model (Bremner et al, 2019;Christian Stanciu et al, 2016;Davenport et al, 2017;Laske et al, 2013 maximum rupture velocity is set to 4 km/s. The subfault durations and rupture velocity are both relatively large for the small subfault dimensions, but this is allowed in order to accommodate the uncertainty in the hypocentral position.…”

Data Analysis of the 2020 Central Idaho Mainshock-Aftershock Sequence

“…The Stanley earthquake initiated near the intersection of the Idaho Batholith and Basin and Range Province of central Idaho near the northern termination of the Quaternary NWtrending Sawtooth normal fault at its junction with Eocene NE-trending Trans-Challis fault system (Figure 2; . Although these two provinces contain different crustal rocks, regional models suggest similar velocities throughout the epicentral region (Stanciu et al, 2016;Davenport et al, 2017;Bremner et al, 2019).…”

“…This study also identified reflected returns in the middle to lower crust that may be relevant to this study (e.g., brittle-ductile transition). Bremner et al (2019) used the same station array as the Stanciu et al (2016) study, but utilized surface wave information to derive crustal Vs models. We extracted the closest velocity model to the Stanley earthquake location from each of the three studies to derive our local Vp and Vs models (Figure 3).…”

Mapping the Sources of Proximal Earthquake Infrasound

“…Although reasonable sound speeds in the atmosphere vary by only a few percent, for example, 322 m/ s < c < 332 m/s (for temperatures −15°C to 0°C), the speeds of the seismic phases that may excite ground displacement are far more variable and less well constrained. Our analysis does not attempt to determine which seismic wave type is most "infrasonogenic," but instead locates sources for a range of potential seismic wave speeds, from P wave velocities of 5.6 km/s to S wave velocities of 3.2 km/s (see Supporting Information S1 analysis as well as the inferred velocity structure below 2 km from Bremner et al, 2019, andDavenport et al, 2017). Modeled surface wave speeds used here are limited to 1.4 km/s or faster given the prevalence of exposed crystalline rock in the region.…”