Crustal structure and relocated earthquakes in the Puget Lowland, Washington, from high‐resolution seismic tomography

Abstract

The availability of regional earthquake data from the Pacific Northwest Seismograph Network (PNSN), together with active source data from the Seismic Hazards Investigation in Puget Sound (SHIPS) seismic experiments, has allowed us to construct a new high‐resolution 3‐D, P wave velocity model of the crust to a depth of about 30 km in the central Puget Lowland. In our method, earthquake hypocenters and velocity model are jointly coupled in a fully nonlinear tomographic inversion. Active source data constrain the upper 10–15 km of the model, and earthquakes constrain the deepest portion of the model. A number of sedimentary basins are imaged, including the previously unrecognized Muckleshoot basin, and the previously incompletely defined Possession and Sequim basins. Various features of the shallow crust are imaged in detail and their structural transitions to the mid and lower crust are revealed. These include the Tacoma basin and fault zone, the Seattle basin and fault zone, the Seattle and Port Ludlow velocity highs, the Port Townsend basin, the Kingston Arch, and the Crescent basement, which is arched beneath the Lowland from its surface exposure in the eastern Olympics. Strong lateral velocity gradients, consistent with the existence of previously inferred faults, are observed, bounding the southern Port Townsend basin, the western edge of the Seattle basin beneath Dabob Bay, and portions of the Port Ludlow velocity high and the Tacoma basin. Significant velocity gradients are not observed across the southern Whidbey Island fault, the Lofall fault, or along most of the inferred location of the Hood Canal fault. Using improved earthquake locations resulting from our inversion, we determined focal mechanisms for a number of the best recorded earthquakes in the data set, revealing a complex pattern of deformation dominated by general arc‐parallel regional tectonic compression. Most earthquakes occur in the basement rocks inferred to be the lower Tertiary Crescent formation. The sedimentary basins and the eastern part of the Olympic subduction complex are largely devoid of earthquakes. Clear association of hypocenters and focal mechanisms with previously mapped or proposed faults is difficult; however, seismicity, structure, and focal mechanisms associated with the Seattle fault zone suggest a possible high‐angle mode of deformation with the north side up. We suggest that this deformation may be driven by isostatic readjustment of the Seattle basin.

A Double Difference Tomography Study of the Washington Forearc: Does Siletzia Control Crustal Seismicity?

“…Figure 7c illustrates that seismicity increases at depths greater than 15 km, below the interpreted extent of Siletzia as noted earlier. This differs from most previous studies which locate seismicity within the deepest portion of the Crescent Formation (e.g., Stanley et al, 1999; Van Wagoner et al, 2002). Lees and Crosson (1990), however, determined that the Crescent Formation, as defined by high‐velocity anomalies, correlated with low levels of seismicity, consistent with our new seismological observations.…”

“…An earthquake focal mechanism for one of the larger events located at 17.6 km depth (M1.7, 9 November 2007-02:17:40 UTC) determined using 44 P wave first motions and the method of Hardebeck and Shearer (2002) is consistent with reverse motion near the Seattle fault zone, with the preferred nodal plane (strike/dip of 85°/55°) coinciding with the strike of the surface trace of the SFZ. This interpretation for the Seattle fault deviates from that of Pratt et al (1997) and Johnson et al (2004) as it maintains a high dip (>50°) to ~20 km depth, in accordance with interpretations of , Van Wagoner et al (2002), and ten Brink et al (2006).…”

A Double Difference Tomography Study of the Washington Forearc: Does Siletzia Control Crustal Seismicity?

“…Figure 7c illustrates that seismicity increases at depths greater than 15 km, below the interpreted extent of Siletzia as noted earlier. This differs from most previous studies which locate seismicity within the deepest portion of the Crescent Formation (e.g., Stanley et al, 1999; Van Wagoner et al, 2002). Lees and Crosson (1990), however, determined that the Crescent Formation, as defined by high‐velocity anomalies, correlated with low levels of seismicity, consistent with our new seismological observations.…”

“…An earthquake focal mechanism for one of the larger events located at 17.6 km depth (M1.7, 9 November 2007-02:17:40 UTC) determined using 44 P wave first motions and the method of Hardebeck and Shearer (2002) is consistent with reverse motion near the Seattle fault zone, with the preferred nodal plane (strike/dip of 85°/55°) coinciding with the strike of the surface trace of the SFZ. This interpretation for the Seattle fault deviates from that of Pratt et al (1997) and Johnson et al (2004) as it maintains a high dip (>50°) to ~20 km depth, in accordance with interpretations of , Van Wagoner et al (2002), and ten Brink et al (2006).…”

The CRESCENT Generation 0 Cascadia Community Velocity Model: Constraints From the Joint Inversion of Teleseismic Receiver Functions and Ambient Noise Data

“…While the absolute velocities in the shallow subsurface (upper 5 km) may be less accurate due to the limited sensitivity of ambient noise surface waves at our recovered periods, the model reproduces key geological structures and provides a regional-scale framework that can complement higherresolution studies. For example, shear-wave velocities in the Seattle Basin reach values as low as ∼1.0 km/s in our model, with uncertainties <0.1 km/s, consistent with previous active-source studies (Brocher et al, 2001;Van Wagoner et al, 2002). Borehole data indicate even lower sediment velocities (Brocher, 2005;Brocher & Christensen, 2001), but the agreement suggests that our model captures, to first-order, structures that are key to hazard assessments.…”

“…Areas with relatively deep Z 2.5 correlate well with the spatial locations of sedimentary basins throughout the Cascadia forearc basin, including the Puget Lowlands, the Portland and Tualatin basins, and the Willamette Valley, although depths seem somewhat shallower than we might expect based on higher resolution studies (Johnson et al, 1999;Jones, 1996;Yount et al, 1985). In the Puget Lowlands, the maximum Z 2.5 depth is 2-4 km (compared to 6-7 km from Frankel & Stephenson, 2000), while in the Great Valley it reaches 3-5 km, which is more consistent with prior seismic studies (Aagaard et al, 2008;Ahdi et al, 2023;Brocher et al, 2001;Holbrook & Mooney, 1987;Lee et al, 2014;Stephenson, 2007;Stephenson et al, 2017;Van Wagoner et al, 2002) and the National Seismic Hazard Model (NSHM; Moschetti et al, 2024). The San Francisco Bay Area exhibits a deep Z 2.5 that merges with the Great Valley at our model resolution, though finer-scale models clearly separate the two (Aagaard et al, 2020).…”

Deep Structure of Siletzia in the Puget Lowland: Imaging an Obducted Plateau and Accretionary Thrust Belt With Potential Fields

“…These are likely flows extending outward from volcanic centers (Dragovich et al., 2016; MacDonald et al., 2013) which integrates well with our modeling as a partial basin fill. Tomographic data also support this interpretation (Van Wagoner et al., 2002), which shows a high seismic velocity region where we model the basalts, as well as distinctive offsets on the velocity contours outlining the basement/sediment contact, approximately matching our modeled geometries for the neotectonic offsets across this rock package.…”

A Double Difference Tomography Study of the Washington Forearc: Does Siletzia Control Crustal Seismicity?

“…Figure 7c illustrates that seismicity increases at depths greater than 15 km, below the interpreted extent of Siletzia as noted earlier. This differs from most previous studies which locate seismicity within the deepest portion of the Crescent Formation (e.g., Stanley et al, 1999; Van Wagoner et al, 2002). Lees and Crosson (1990), however, determined that the Crescent Formation, as defined by high‐velocity anomalies, correlated with low levels of seismicity, consistent with our new seismological observations.…”

“…An earthquake focal mechanism for one of the larger events located at 17.6 km depth (M1.7, 9 November 2007-02:17:40 UTC) determined using 44 P wave first motions and the method of Hardebeck and Shearer (2002) is consistent with reverse motion near the Seattle fault zone, with the preferred nodal plane (strike/dip of 85°/55°) coinciding with the strike of the surface trace of the SFZ. This interpretation for the Seattle fault deviates from that of Pratt et al (1997) and Johnson et al (2004) as it maintains a high dip (>50°) to ~20 km depth, in accordance with interpretations of , Van Wagoner et al (2002), and ten Brink et al (2006).…”

The CRESCENT Generation 0 Cascadia Community Velocity Model: Constraints From the Joint Inversion of Teleseismic Receiver Functions and Ambient Noise Data

“…While the absolute velocities in the shallow subsurface (upper 5 km) may be less accurate due to the limited sensitivity of ambient noise surface waves at our recovered periods, the model reproduces key geological structures and provides a regional-scale framework that can complement higherresolution studies. For example, shear-wave velocities in the Seattle Basin reach values as low as ∼1.0 km/s in our model, with uncertainties <0.1 km/s, consistent with previous active-source studies (Brocher et al, 2001;Van Wagoner et al, 2002). Borehole data indicate even lower sediment velocities (Brocher, 2005;Brocher & Christensen, 2001), but the agreement suggests that our model captures, to first-order, structures that are key to hazard assessments.…”

“…Areas with relatively deep Z 2.5 correlate well with the spatial locations of sedimentary basins throughout the Cascadia forearc basin, including the Puget Lowlands, the Portland and Tualatin basins, and the Willamette Valley, although depths seem somewhat shallower than we might expect based on higher resolution studies (Johnson et al, 1999;Jones, 1996;Yount et al, 1985). In the Puget Lowlands, the maximum Z 2.5 depth is 2-4 km (compared to 6-7 km from Frankel & Stephenson, 2000), while in the Great Valley it reaches 3-5 km, which is more consistent with prior seismic studies (Aagaard et al, 2008;Ahdi et al, 2023;Brocher et al, 2001;Holbrook & Mooney, 1987;Lee et al, 2014;Stephenson, 2007;Stephenson et al, 2017;Van Wagoner et al, 2002) and the National Seismic Hazard Model (NSHM; Moschetti et al, 2024). The San Francisco Bay Area exhibits a deep Z 2.5 that merges with the Great Valley at our model resolution, though finer-scale models clearly separate the two (Aagaard et al, 2020).…”

Deep Structure of Siletzia in the Puget Lowland: Imaging an Obducted Plateau and Accretionary Thrust Belt With Potential Fields

“…These are likely flows extending outward from volcanic centers (Dragovich et al., 2016; MacDonald et al., 2013) which integrates well with our modeling as a partial basin fill. Tomographic data also support this interpretation (Van Wagoner et al., 2002), which shows a high seismic velocity region where we model the basalts, as well as distinctive offsets on the velocity contours outlining the basement/sediment contact, approximately matching our modeled geometries for the neotectonic offsets across this rock package.…”

A Double Difference Tomography Study of the Washington Forearc: Does Siletzia Control Crustal Seismicity?

“…Figure 7c illustrates that seismicity increases at depths greater than 15 km, below the interpreted extent of Siletzia as noted earlier. This differs from most previous studies which locate seismicity within the deepest portion of the Crescent Formation (e.g., Stanley et al, 1999; Van Wagoner et al, 2002). Lees and Crosson (1990), however, determined that the Crescent Formation, as defined by high‐velocity anomalies, correlated with low levels of seismicity, consistent with our new seismological observations.…”

“…An earthquake focal mechanism for one of the larger events located at 17.6 km depth (M1.7, 9 November 2007-02:17:40 UTC) determined using 44 P wave first motions and the method of Hardebeck and Shearer (2002) is consistent with reverse motion near the Seattle fault zone, with the preferred nodal plane (strike/dip of 85°/55°) coinciding with the strike of the surface trace of the SFZ. This interpretation for the Seattle fault deviates from that of Pratt et al (1997) and Johnson et al (2004) as it maintains a high dip (>50°) to ~20 km depth, in accordance with interpretations of , Van Wagoner et al (2002), and ten Brink et al (2006).…”

The CRESCENT Generation 0 Cascadia Community Velocity Model: Constraints From the Joint Inversion of Teleseismic Receiver Functions and Ambient Noise Data

“…While the absolute velocities in the shallow subsurface (upper 5 km) may be less accurate due to the limited sensitivity of ambient noise surface waves at our recovered periods, the model reproduces key geological structures and provides a regional-scale framework that can complement higherresolution studies. For example, shear-wave velocities in the Seattle Basin reach values as low as ∼1.0 km/s in our model, with uncertainties <0.1 km/s, consistent with previous active-source studies (Brocher et al, 2001;Van Wagoner et al, 2002). Borehole data indicate even lower sediment velocities (Brocher, 2005;Brocher & Christensen, 2001), but the agreement suggests that our model captures, to first-order, structures that are key to hazard assessments.…”

“…Areas with relatively deep Z 2.5 correlate well with the spatial locations of sedimentary basins throughout the Cascadia forearc basin, including the Puget Lowlands, the Portland and Tualatin basins, and the Willamette Valley, although depths seem somewhat shallower than we might expect based on higher resolution studies (Johnson et al, 1999;Jones, 1996;Yount et al, 1985). In the Puget Lowlands, the maximum Z 2.5 depth is 2-4 km (compared to 6-7 km from Frankel & Stephenson, 2000), while in the Great Valley it reaches 3-5 km, which is more consistent with prior seismic studies (Aagaard et al, 2008;Ahdi et al, 2023;Brocher et al, 2001;Holbrook & Mooney, 1987;Lee et al, 2014;Stephenson, 2007;Stephenson et al, 2017;Van Wagoner et al, 2002) and the National Seismic Hazard Model (NSHM; Moschetti et al, 2024). The San Francisco Bay Area exhibits a deep Z 2.5 that merges with the Great Valley at our model resolution, though finer-scale models clearly separate the two (Aagaard et al, 2020).…”

Deep Structure of Siletzia in the Puget Lowland: Imaging an Obducted Plateau and Accretionary Thrust Belt With Potential Fields

“…These are likely flows extending outward from volcanic centers (Dragovich et al., 2016; MacDonald et al., 2013) which integrates well with our modeling as a partial basin fill. Tomographic data also support this interpretation (Van Wagoner et al., 2002), which shows a high seismic velocity region where we model the basalts, as well as distinctive offsets on the velocity contours outlining the basement/sediment contact, approximately matching our modeled geometries for the neotectonic offsets across this rock package.…”