Geology topics

John F. Cassidy

Publications and source records attributed to John F. Cassidy.

3 recordsLinked to original sources

USGS2022

An assessment of uncertainties in VS profiles obtained from microtremor observations in the phased 2018 COSMOS blind trials

Site response is a critical consideration when assessing earthquake hazards. Site characterization is key to understanding site effects as influenced by seismic site conditions of the local geology. Thus, a number of geophysical site characterization methods were developed to meet the demand for accurate and cost-effective results. As a consequence, a number of studies have been administered periodically as blind trials to evaluate the state-of-practice on-site characterization. We present results from the Consortium of Organizations for Strong Motion Observation Systems (COSMOS) blind trials, which used data recorded from surface-based microtremor array methods (MAM) at four sites where geomorphic conditions vary from deep alluvial basins to an alpine valley. Thirty-four invited analysts participated. Data were incrementally released to 17 available analysts who participated in all four phases: (1) two-station arrays, (2) sparse triangular arrays, (3) complex nested triangular or circular arrays, and (4) all available geological control site information including drill hole data. Another set of 17 analysts provided results from two sites and two phases only. Although data from one site consisted of recordings from three-component sensors, the other three sites consisted of data recorded only by vertical-component sensors. The sites cover a range of noise source distributions, ranging from one site with a highly directional microtremor wave field to others with omni-directional (azimuthally distributed) wave fields. We review results from different processing techniques (e.g., beam-forming, spatial autocorrelation, cross-correlation, or seismic interferometry) applied by the analysts and compare the effectiveness between the differing wave field distributions. We define the M index as a quality index based on estimates of the time-averaged shear-wave velocity of the upper 10 ( V S10 ), 30 ( V S30 ), 100 ( V S100 ), and 300 ( V S300 ) meters and show its usefulness in quantitative comparisons of V S profiles from multiple analysts. Our findings are expected to aid in building an evidence-based consensus on preferred cost-effective arrays and processing methodology for future studies of seismic site effects.

Seismological Research Letters↗

USGS2017

Modified mercalli intensities for nine earthquakes in central and western Washington between 1989 and 1999

We determine Modified Mercalli (Seismic) Intensities (MMI) for nine onshore earthquakes of magnitude 4.5 and larger that occurred in central and western Washington between 1989 and 1999, on the basis of effects reported in postal questionnaires, the press, and professional collaborators. The earthquakes studied include four earthquakes of M 5 and larger: the M 5.0 Deming earthquake of April 13, 1990, the M 5.0 Point Robinson earthquake of January 29, 1995, the M 5.4 Duvall earthquake of May 3, 1996, and the M 5.8 Satsop earthquake of July 3, 1999. The MMI are assigned using data and procedures that evolved at the U.S. Geological Survey (USGS) and its Department of Commerce predecessors and that were used to assign MMI to felt earthquakes occurring in the United States between 1931 and 1986. We refer to the MMI assigned in this report as traditional MMI, because they are based on responses to postal questionnaires and on newspaper reports, and to distinguish them from MMI calculated from data contributed by the public by way of the internet. Maximum traditional MMI documented for the M 5 and larger earthquakes are VII for the 1990 Deming earthquake, V for the 1995 Point Robinson earthquake, VI for the 1996 Duvall earthquake, and VII for the 1999 Satsop earthquake; the five other earthquakes were variously assigned maximum intensities of IV, V, or VI. Starting in 1995, the Pacific Northwest Seismic Network (PNSN) published MMI maps for four of the studied earthquakes, based on macroseismic observations submitted by the public by way of the internet. With the availability now of the traditional USGS MMI interpreted for all the sites from which USGS postal questionnaires were returned, the four Washington earthquakes join a rather small group of earthquakes for which both traditional USGS MMI and some type of internet-based MMI have been assigned. The values and distributions of the traditional MMI are broadly similar to the internet-based PNSN intensities; we discuss some differences in detail that reflect differences in data-sampling procedure, differences in the procedure used to assign intensity numbers from macroseismic observations, and differences in how intensities are mapped.

Oregon, Washington↗

USGS2015

Introduction to the special issue on the 2012 Haida Gwaii and 2013 Craig earthquakes at the Pacific–North America plate boundary (British Columbia and Alaska)

The 27 October 2012 M w 7.8 Haida Gwaii thrust earthquake and the 5 January 2013 M w 7.5 Craig strike‐slip earthquake are the focus of this special issue. They occurred along the transform boundary between the Pacific and North American plates (Fig. 1). The most identifiable feature of the plate boundary, the strike‐slip Queen Charlotte fault, might be viewed as typical of continent–ocean transform faults because it separates the continental crust of the North American plate from oceanic crust of the Pacific plate for most of its length. However, the current relative plate motion of about 5 cm/yr is highly oblique to the Queen Charlotte fault, causing a transpressive plate boundary in the region.

Alaska, British Columbia↗