Zhenming Wang

Shallow SH-wave seismic investigation of the Mt. Angel Fault, Northwest Oregon, USA

Abstract The Mt. Angel Fault is likely one of the most active faults near the Portland metropolitan area, and was probably associated with the 1993 Scotts Mills earthquake. SH-wave seismic techniques used to image the Mt. Angel Fault suggest that the fault offsets late Pleistocene gravel (∼22 to 34 ka) at several locations. Within the study area, displacement of the late Pleistocene gravel along the strike of the Mt. Angel Fault increases from no obvious displacement on the northwest to approximately 18 m on the southeast. This trend of increasing offset along the strike of the fault is paralleled by topographic and geomorphic trends. A reconnaissance geologic investigation at an anomalous bend in the Pudding River near the projected trace of the Mt. Angel Fault revealed potential tectonic deformation in sediments younger than the late Pleistocene gravel imaged by SH-wave data. The results of this study have contributed to the paleoseismic record of the Mt. Angel Fault, laid the groundwork for future geologic investigations along the Pudding River, and determined potential sites for future paleoseismic trenching investigations.

Seismic Evaluation of Bridges on and over the Parkways in Western Kentucky – Summary Report

This report (KTC-07-02/SPR246-02-1F) provides an overall summary on the seismic investigative study performed on bridges on/over the five parkways in Western Kentucky. The comprehensive study was further divided into the followings tasks, each reported separately as follows: (1) The first report of this study (KTC-07-03/SRP246-02-2F) involved data collection and field inspection of bridges on/over the parkways. The resulting inventory contains data of three hundred fifty-one (351) bridges on/over the parkways, detailing their construction type, soil profile, present condition, etc. (2) In KTC-07-04/SPR246-02-3F, a preliminary seismic evaluation and ranking was performed on all bridges within the inventory. Details of the evaluation and ranking procedure are outlined. In this task seventeen (17) bridges, that are deemed susceptible to major earthquakes, were identified. (3) Detailed seismic evaluations of the seventeen (17) bridges were subsequently carried out using time-history analysis for a projected 250-year seismic event. The results of the analysis are presented in KTC-07-05/SPR246-02-4F. (4) KTC-07-06/SPR246-02-5F presents the preliminary evaluation and ranking of bridge embankments along the parkways. (5) The last report, numbered KTC-07-07/SPR246-02-6F, provides the latest seismic hazard maps for the expected earthquake (EE), probable earthquake (PE), and maximum credible earthquake (MCE), which will be used in seismic analysis and design of highway infrastructures in Kentucky.

DYNAMIC SITE PERIODS FOR THE JACKSON PURCHASE REGION OF WESTERN KENTUCKY

For this report, conventional seismic refraction and reflection techniques were used to determine the shear-wave velocities of the more poorly consolidated, near-surface sediments for a matrix of sites in the region. Conventional seismic P-wave reflections along with existing drill hole and seismic reflection data in the region were then used to determine the depth to the top of the bedrock at the sites investigated. These data were used in SHAKE91 to calculate the fundamental period of the ground motion at the sites. This period, identified in the study as the dynamic site period, is the period at which ground motions in the sedimentary column are most apt to be amplified as a result of a seismic shear wave propagating from the top of the bedrock to the surface. Based on the results in this report, it is recommended that bridges, overpasses, and other engineered structures built in the region be designed so that their natural periods do not coincide with the fundamental period of the sedimentary column, thereby avoiding damage during an earthquake as a result of in-place resonance.

Site Characteristics, Instrumentation, and Recordings of the Central United States Seismic Observatory

The Central United States Seismic Observatory is a vertical seismic array in southwestern Kentucky within the New Madrid Seismic Zone. It is intended to record the effects of local geology, including thick sediment overburden, on seismic-wave propagation, particularly strong ground motion. The three-borehole array is composed of seismic sensors placed on the surface, in the bedrock, and at various depths within the 585-mthick sediment overburden. The array’s deep borehole also provides a unique opportunity to describe the geology and geophysically measure the complete Late Cretaceous through Quaternary stratigraphy in the northern Mississippi Embayment. Based on surface and borehole geophysical measurements, the thick sediment overburden and its complex heterogeneous stratigraphy have been partitioned into a seven-layer sediment velocity model overlying a bedrock half-space. The Sand P-wave sediment velocities range between 160 and 875 m/s, and 1,000 and 2,300 m/s, respectively, and bedrock velocities between 1,452 and 3,775 m/s, respectively. In addition, high-resolution seismic-reflection profiles acquired within a 1-km radius of the array have imaged a complex geologic model, including steeply dipping N30°E-striking faults that have uplifted and arched post-Paleozoic sediments in a manner consistent with a dextral transpression component of displacement. The subparallel fault strands have been traced 1.4 km between reflection profiles and are adjacent to the array. The fault deformation extends above Paleozoic bedrock, affecting the Late Cretaceous and Eocene Mississippi Embayment sediments, as well as the base of the Quaternary. The Paleozoic and Cretaceous horizons show as much as 75 and 50 m of relief, respectively, with the middle Eocene and basal Quaternary disrupted 25 and 15 m, respectively. The differential fault offsets suggest episodic activity during the post-Paleozoic, and represent the first indications of Quaternary neotectonics in this part of Kentucky. More important, these faults may be the first evidence for a hypothesized northeast 1Department of Earth and Environmental Sciences, University of Kentucky 2Kentucky Geological Survey, University of Kentucky

Seismic-Hazard Maps and Time Histories for the Commonwealth of Kentucky

The ground-motion hazard maps and time histories for three earthquake scenarios, expected earthquakes, probable earthquakes, and maximum credible earthquakes on the free surface in hard rock (shear-wave velocity >1,500 m/s), were derived using the deterministic seismic hazard analysis. The results are based on (1) historical observations, (2) instrumental records, and (3) current understanding of the earthquake source, recurrence, and ground-motion attenuation relationship in the central United States. It is well understood that there are uncertainties in the groundmotion hazard maps because of the uncertainties inherent in parameters such as earthquake location, magnitude, and frequency used in the study. This study emphasizes the earthquakes that would have maximum impacts on humans and structures. The ground-motion parameters, including time histories, are intended for use in the recommended zone (not site-specific) where the structure is assumed to be situated at the top of a bedrock foundation. For sites underlain by soils, and in particular for sites underlain by poorly consolidated soils, it is recommended that site-specific investigations be conducted by qualified professionals in order to determine the possibilities of amplification, liquefaction, slope failure, and other considerations when subjected to the ground motions.