Geology topics

Karen L. Meagher

Publications and source records attributed to Karen L. Meagher.

4 recordsLinked to original sources

USGS2008

Finding concealed active faults: Extending the southern Whidbey Island fault across the Puget Lowland, Washington

The southern Whidbey Island fault zone (SWIF), as previously mapped using borehole data, potential field anomalies, and marine seismic reflection surveys, consists of three subparallel, northwest trending strands extending ∼100 km from near Vancouver Island to the northern Puget Lowland. East of Puget Sound, the SWIF makes landfall between the cities of Seattle and Everett but is concealed beneath a thick mantle of young glacial deposits and vegetation. A ∼20‐km‐wide, northwest trending swath of subparallel, low‐amplitude aeromagnetic anomalies crosses this region of the Puget Lowland and is on strike with the SWIF. The most prominent aeromagnetic anomaly, the Cottage Lake lineament, extends at least 18 km and lies approximately on strike with the SWIF on Whidbey Island. Subtle scarps and topographic lineaments on Pleistocene surfaces, visible on high‐resolution lidar topography at a number of locations along the SWIF, lie on or near these magnetic anomalies. In the field, scarps exhibit northeast‐side‐up and vertical relief of 1 to 5 m. Excavations across several lidar scarps lying on or near magnetic anomalies show evidence for multiple folding and faulting events since deglaciation, most likely above buried reverse/oblique faults. Excavations in areas away from magnetic anomalies do not show evidence of tectonic deformation. In total, paleoseismological evidence suggests that the SWIF produced at least four earthquakes since deglaciation about 16,400 years ago, the most recent less than 2700 years ago.

Washington↗

USGS2008

The 8 October 2006 Md 4.5 Cowlitz chimneys earthquake in Mount Rainier National Park

An M d 4.5 earthquake located ∼12 km east of Mount Rainier occurred on 8 October 2006 at 02:48 UTC ( figure 1 ). Although not large enough to be damaging or of major tectonic significance, a summary description of the earthquake is warranted because of its proximity to Mount Rainier, and because earthquakes of M d ≥ 4.5 are relatively rare in this region. Previous events of M d ≥ 4.5 have occurred approximately once a decade within a radius of ∼60 km from this mainshock, with the closest and most recent prior earthquake being an M d 4.9 event located 46 km to the southwest in 1989. Magnitudes in this paper refer to the coda duration magnitude determined by the Pacific Northwest Seismic Network (PNSN) ( Crosson 1972 ). We refer to the 2006 event as the “Cowlitz Chimneys” earthquake because it occurred beneath the Cowlitz Chimneys, a prominent ridge in Mount Rainier National Park. This paper describes the mainshock's focal mechanism and its aftershock distribution. The inferred source mechanism, its ordinary aftershock sequence, and the lack of significant triggered seismicity near the volcanic edifice lead us to conclude that this was a regular tectonic crustal earthquake rather than one related to volcanic processes.

Seismological Research Letters↗

USGS2003

Earthquake recordings from the 2002 Seattle Seismic Hazard Investigation of Puget Sound (SHIPS), Washington state

This report describes seismic data obtained during the fourth Seismic Hazard Investigation of Puget Sound (SHIPS) experiment, termed Seattle SHIPS . The experiment was designed to study the influence of the Seattle sedimentary basin on ground shaking during earthquakes. To accomplish this, we deployed seismometers over the basin to record local earthquakes, quarry blasts, and teleseisms during the period of January 26 to May 27, 2002. We plan to analyze the recordings to compute spectral amplitudes at each site, to determine the variability of ground motions over the basin. During the Seattle SHIPS experiment, seismometers were deployed at 87 sites in a 110-km-long east-west line, three north-south lines, and a grid throughout the Seattle urban area (Figure 1). At each of these sites, an L-22, 2-Hz velocity transducer was installed and connected to a REF TEK Digital Acquisition System (DAS), both provided by the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) of the Incorporated Research Institutes for Seismology (IRIS). The instruments were installed on January 26 and 27, and were retrieved gradually between April 18 and May 27. All instruments continuously sampled all three components of motion (velocity) at a sample rate of 50 samples/sec. To ensure accurate computations of amplitude, we calibrated the geophones in situ to obtain the instrument responses. In this report, we discuss the acquisition of these data, we describe the processing and merging of these data into 1-hour long traces and into windowed events, we discuss the geophone calibration process and its results, and we display some of the earthquake recordings.

Washington↗

USGS1999

Geological map of Washington - Southwest quadrant (digital edition)

This report comprises digital spatial data that constitute a partial transcription of the 1:250,000-scale Geologic map of Washington - southwest quadrant (Walsh and others, 1987); digital base material, symbolsets, and ARC Macro Language (AML) procedures to create a geologic map on a shaded-relief base from the digital spatial data; and Postscript and RTL plotfiles for such a geologic map. The digital transcription is incomplete: offshore folds and faults, the southern limit of the continental ice sheet in the Puget Lowland, the published base map (Washington Division of Geology and Earth Resources map TM-1), geologic unit correlation diagrams, and most of the explanatory material that accompanies Walsh and others (1987) are not present here.

Washington↗