Figure 1 - uploaded by Tom Brocher
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Map showing locations of Dry SHIP seismic shots and recorders in the Puget Lowland. Abbreviations: BI-Bainbridge Island, LS-Lake Sammamish, LW-Lake Washington, MI-Mercer Island, VI-Vashion Island.
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This report describes the acquisition, processing, and quality of seismic reflection and refraction data obtained in the Seattle basin, central Puget Lowland, western Washington, in September 1999 during the Seismic Hazards Investigation of Puget Sound (SHIPS). As a sequel to the 1998 SHIPS air gun experiment (also known as “Wet SHIPS”), the 1999 e...
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We present a focal mechanism catalogue of earthquakes that occurred in the southeastern Alps and surrounding areas from 1928 to 2019. The area involved in the process of convergence between the Adria microplate and Eurasia is one of the most seismically active regions in the Alpine Belt. The seismicity is minor, with the Ms =thinsp;6.5 Friuli earth...
Citations
... 14) and Dry SHIPS (ref. 15) as shown in Supplementary Fig. S1), and oceanic Moho (PmP) reflections from Wet SHIPS were used in the integrated 3D tomographic inversion for P-wave velocity in a model comprising cells of dimension 4 km horizontally and 2 km vertically. The active source surveys mostly constrain velocities in the upper and middle crust 19 , whereas the earthquakes and PmP reflections determine deeper velocities 17,22 ; the PmP reflections, which are assumed to arise from a smoothly varying interface, also locate the Moho of the subducting oceanic plate 17 . ...
Earth's largest earthquakes occur in subduction zones, along the
boundary between the subducting and overriding plates. Non-volcanic
tremor generated by slow slip between the plates is thought to originate
on, or near, this boundary. Earthquakes also occur in the down-going
plate as fluids are released, and zones of anomalously low seismic
velocities observed beneath several subduction zones are interpreted to
be the subducting oceanic crust. Yet, the exact location of the plate
boundary remains uncertain. Here we interpret a three-dimensional
seismic tomography model from the northern Cascadia subduction zone in
the northwest USA. We find that the low-velocity zone varies
considerably along the Cascadia margin. In places, we observe the
low-velocity zone to crop out at the surface and separate from the
descending plate at depths of 35-40km. We argue that the low-velocity
zone here cannot represent oceanic crust as previously suggested, and
instead the zone mostly represents sediments that have been subducted
and underplated beneath the North American continent. We also find that
tremor signals correlate with the position of the low-velocity zone,
implying that slow slip and tremor may be facilitated by trapped fluids
and high pore fluid pressures in subducted sedimentary rocks at, or
close to the plate boundary. Our results also imply that the plate
boundary beneath Cascadia is much deeper than previously thought.
... We use data from Seismic Hazard Investigations of Puget Sound (SHIPS) projects, a series of studies designed specifically to help characterize the seismic hazard in the region. There have been five SHIPS experiments to date: " Wet " SHIPS in 1998 (Fisher et al., 2000); " Dry " SHIPS in 1999 (Brocher et al., 2000a, b); " Kingdome " SHIPS in 2000 (Brocher et al., 2000aBrocher et al., , 2002); " Seattle " SHIPS in 2002 (Pratt et al., 2003b); and " Bellingham " SHIPS in 2002 (). The 1999 and 2000 experiments were designed to study the Seattle basin, and it is those data we analyze here. ...
... Four shorter and less densely instrumented crosslines provide constraints on the shallow, three-dimensional structure of the eastern side of the basin. During the 1999 SHIPS experiment (Brocher et al., 2000a), 1008 seismometers were installed along the lines with a nominal spacing of 100 m. To record shear waves, 239 of our instruments were three-component recorders distributed nominally at 400-m spacing. ...
Recent observations indicate that the Seattle sedimentary basin, underlying Seattle and other urban centers in the Puget Lowland, Washington, amplifies Iong-period (1-5 sec) weak ground motions by factors of 10 or more. We computed east-trending P- and S-wave velocity models across the Seattle basin from Seismic Hazard Investigations of Puget Sound (SHIPS) experiments to better characterize the seismic hazard the basin poses. The 3D tomographic models, which resolve features to a depth of 10 km, for the first time define the P- and S-wave velocity structure of the eastern end of the basin. The basin, which contains sedimentary rocks of Eocene to Holocene, is broadly symmetric in east-west section and reaches a maximum thickness of 6 km along our profile beneath north Seattle. A comparison of our velocity model with coincident amplification curves for weak ground motions produced by the 1999 Chi-Chi earthquake suggests that the distribution of Quaternary deposits and reduced velocity gradients in the upper part of the basement east of Seattle have significance in forecasting variations in seismic-wave amplification across the basin. Specifically, eastward increases in the amplification of 0.2- to 5-Hz energy correlate with locally thicker unconsolidated deposits and a change from Crescent Formation basement to pre-Tertiary Cascadia basement. These models define the extent of the Seattle basin, the Seattle fault, and the geometry of the basement contact, giving insight into the tectonic evolution of the Seattle basin and its influence on ground shaking.
... The high and low bounds result in interval Q s,int values ranging from 90 to greater than 1000 in the deeper basin strata. These values are broadly consistent with an inversion for Q s in the Seattle basin carried out by Li et al. (2006) (Fig. 11a) using data recorded in the 1999 SHIPS experiment (Brocher et al., 2000b). Li et al. (2006) computed values of 16 to 160 (1 and 8 Hz) at the top of the basin and 40 to 300 (1 and 8 Hz) at about 8 km depth. ...
Simple spectral ratio (ssr) and horizontal-to-vertical (h/v) site- response estimates at 47 sites in the Puget Lowland of Washington State document significant attenuation of 1.5- to 20-Hz shear waves within sedimentary basins there. Amplitudes of the horizontal components of shear-wave arrivals from three local earthquakes were used to compute ssrs with respect to the average of two bedrock sites and h/v spectral ratios with respect to the vertical component of the shear-wave arrivals at each site. ssr site-response curves at thick basin sites show peak amplifications of 2 to 6 at frequencies of 3 to 6 Hz, and decreasing spectral amplification with increasing frequency above 6 Hz. ssrs at nonbasin sites show a variety of shapes and larger resonance peaks. We attribute the spectral decay at frequencies above the amplification peak at basin sites to attenuation within the basin strata. Computing the frequency-independent, depth-dependent attenuation factor (Qs,int) from the ssr spectral decay between 2 and 20 Hz gives values of 5 to 40 for shallow sedimentary deposits and about 250 for the deepest sedimentary strata (7 km depth). h/v site responses show less spectral decay than the ssr responses but contain many of the same resonance peaks. We hypothesize that the h/v method yields a flatter response across the frequency spectrum than ssrs because the h/v reference signal (vertical component of the shear-wave arrivals) has undergone a degree of attenuation similar to the horizontal
component recordings. Correcting the ssr site responses for attenuation within the basins by removing the spectral decay improves agreement between ssr and h/v estimates.
In subduction zones, landward dipping regions of low shear wave velocity and elevated Poisson’s ratio, which can extend to at least 120 km depth, are interpreted to be all or part of the subducting igneous oceanic crust. This crust is considered to be overpressured, because fluids within it are trapped beneath an impermeable seal along the overlying inter-plate boundary. Here we show that during slow slip on the plate boundary beneath southern Vancouver Island, low frequency earthquakes occur immediately below both the landward dipping region of high Poisson’s ratio and a 6–10 km thick shear zone revealed by seismic reflections. The plate boundary here either corresponds to the low frequency earthquakes or to the anomalous elastic properties in the lower 3–5 km of the shear zone immediately above them. This zone of high Poisson’s ratio, which approximately coincides with an electrically conductive layer, can be explained by slab-derived fluids trapped at near-lithostatic pore pressures. Regions of the subducting oceanic crust are often considered to be overpressured, owing to fluid trapped beneath an impermeable seal along the overlying inter-plate boundary. Here, the authors show that slow slip earthquakes at the Cascadia subduction zone occur immediately below a 6-10 km-thick shear zone, in which slab-derived fluids are likely trapped at near-lithostatic pore pressures.
Ground shaking during earthquakes was measured over the Seattle sedimentary basin, western Washington State, using the 87 seismometers in the 2002 Seattle Seismic Hazard Investigation of Puget Sound (Seattle SHIPS, or SHIPS02) array. The array consisted of 2-Hz geophones recorded on Refraction Technology (Reftek) recording systems. The instruments were in place for 4 months, from the end of January through May, 2002. Horizontal-component recordings of shear waves from 5 teleseisms and 7 local earthquakes were used to compute spectral ratios with respect to the average of 5 bedrock sites located in the Olympic Mountains at the west side of the array and in the Cascade Range foothills on the east side of the array. Results show amplifications of 0.3 to 0.8 Hz seismic waves by factors of 6 to 8 at sites over the deep Seattle basin (>2.5 km of sedimentary strata), with the peak amplification being at about 0.3 Hz. All sites in the central Puget Lowland are amplified significantly, however, which indicates the deposits in the upper 2.5 km that extend beyond the deep basin are responsible for much of the amplification. The variance, the horizontal to vertical amplitude ratio, and the crosscorrelation of arrivals from nearby stations are consistent with the presence of scattered energy and surface waves from multiple directions in the later portions of the long-period signal. At frequencies above 1 Hz the amplification levels are greatest in areas of soft fill such as the Duwamish river valley.
In the past decade, Earth scientists have recognized the seismic hazards
that crustal faults and sedimentary basins pose to Seattle, Washington
(Figure 1). In 1998, the US. Geological Survey and its collaborators
initiated a series of urban seismic studies of the upper crust to better
map seismogenic structures and sedimentary basins in the Puget Lowland.
These studies are called the Seismic Hazard Investigations of Puget
Sound (SHIPS).In March 1998, we conducted our first SHIPS study, an
investigation of the upper crustal structure of the Puget Lowland, using
marine airgun sources and land recorders [Fisher et al., 1999].The study
was nicknamed Wet SHIPS. In September 1999, we obtained a seismic
refraction line to study the upper crustal structure in the Seattle area
in a land-based study nicknamed Dry SHIPS [Brocher et al., 2000] (Figure
1). In March 2000, we recorded the demolition of the Seattle Kingdome
sports stadium using a dense array of seismic recorders for a detailed
site response study; this study was nicknamed Kingdome SHIPS (Figure 1).
1] 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.
