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At least 415 records · Page 23Linked to original sources
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↗
USGS2008

Multidecadal climate-induced variability in microseisms

Microseisms are the most ubiquitous continuous seismic signals on Earth at periods between approximately 5 and 25 s (Peterson 1993; Kedar and Webb 2005). They arise from atmospheric energy converted to (primarily) Rayleigh waves via the intermediary of wind-driven oceanic swell and occupy a period band that is uninfluenced by common anthropogenic and wind-coupled noise processes on land (Wilson et al. 2002; de la Torre et al. 2005). "Primary" microseisms (near 8-s period) are generated in shallow water by breaking waves near the shore and/or the nonlinear interaction of the ocean wave pressure signal with the sloping sea floor (Hasselmann 1963). Secondary microseisms occur at half of the primary period and are especially strongly radiated in source regions where opposing wave components interfere (Longuett-Higgins 1950; Tanimoto 2007), which principally occurs due to the interaction of incident swell and reflected/scattered wave energy from coasts (Bromirski and Duennebier 2002; Bromirski, Duennebier, and Stephen 2005). Coastal regions having a narrow shelf with irregular and rocky coastlines are known to be especially efficient at radiating secondary microseisms (Bromirski, Duennebier, and Stephen 2005; Shulte-Pelkum et al. 2004). The secondary microseism is globally dominant, and its amplitudes proportional to the square of the standing wave height (Longuett-Higgins 1950), which amplifies its sensitivity to large swell events (Astiz and Creager 1994; Webb 2006).

Seismological Research Letters↗
USGS2008

The April 18, 2008 Illinois earthquake: an ANSS monitoring success

The largest-magnitude earthquake in the past 20 years struck near Mt. Carmel in southeastern Illinois on Friday morning, 18 April 2008 at 09:36:59 UTC (04:37 CDT). The Mw 5.2 earthquake was felt over an area that spanned Chicago and Atlanta, with about 40,000 reports submitted to the U.S. Geological Survey (USGS) “Did You Feel It?” system. There were at least six felt aftershocks greater than magnitude 3 and 20 aftershocks with magnitudes greater than 2 located by regional and national seismic networks. Portable instrumentation was deployed by researchers of the University of Memphis and Indiana University (the first portable station was installed at about 23:00 UTC on 18 April). The portable seismographs were deployed both to capture near-source, high-frequency ground motions for significant aftershocks and to better understand structure along the active fault. The previous similar-size earthquake within the Wabash Valley seismic zone (WVSZ) of southeastern Illinois and southwestern Indiana was a magnitude 5.0 in June 1987. The seismicity associated with the WVSZ is thought to occur in a complex horst and graben system of Precambrian igneous and metamorphic units at depths between 12 and 20 km. Paleoliquefaction evidence suggests several major shaking events have occurred within the past 12,000 years (Munson et al. 1997).

Illinois↗
USGS2008

Preliminary report on the 29 July 2008 Mw 5.4 Chino Hills, Eastern Los Angeles Basin, California, earthquake sequence

The 29 July 2008 Mw 5.4 Chino Hills earthquake was the largest event to occur within the greater Los Angeles metropolitan region since the Mw 6.7 1994 Northridge earthquake. The earthquake was widely felt in a metropolitan region with a population of more than 10 million people and was recorded by hundreds of broadband and strong-motion instruments. In this report we present preliminary analysis of the event and discuss its significance within the seismotectonic framework of the northern Los Angeles basin as revealed by previous moderate earthquakes.

California↗
USGS2009

The SCEC/USGS dynamic earthquake rupture code verification exercise

Numerical simulations of earthquake rupture dynamics are now common, yet it has been difficult to test the validity of these simulations because there have been few field observations and no analytic solutions with which to compare the results. This paper describes the Southern California Earthquake Center/U.S. Geological Survey (SCEC/USGS) Dynamic Earthquake Rupture Code Verification Exercise, where codes that simulate spontaneous rupture dynamics in three dimensions are evaluated and the results produced by these codes are compared using Web-based tools. This is the first time that a broad and rigorous examination of numerous spontaneous rupture codes has been performed—a significant advance in this science. The automated process developed to attain this achievement provides for a future where testing of codes is easily accomplished. Scientists who use computer simulations to understand earthquakes utilize a range of techniques. Most of these assume that earthquakes are caused by slip at depth on faults in the Earth, but hereafter the strategies vary. Among the methods used in earthquake mechanics studies are kinematic approaches and dynamic approaches. The kinematic approach uses a computer code that prescribes the spatial and temporal evolution of slip on the causative fault (or faults). These types of simulations are very helpful, especially since they can be used in seismic data inversions to relate the ground motions recorded in the field to slip on the fault(s) at depth. However, these kinematic solutions generally provide no insight into the physics driving the fault slip or information about why the involved fault(s) slipped that much (or that little). In other words, these kinematic solutions may lack information about the physical dynamics of earthquake rupture that will be most helpful in forecasting future events. To help address this issue, some researchers use computer codes to numerically simulate earthquakes and construct dynamic, spontaneous rupture (hereafter called “spontaneous rupture”) solutions. For these types of numerical simulations, rather than prescribing the slip function at each location on the fault(s), just the friction constitutive properties and initial stress conditions are prescribed. The subsequent stresses and fault slip spontaneously evolve over time as part of the elasto-dynamic solution. Therefore, spontaneous rupture computer simulations of earthquakes allow us to include everything that we know, or think that we know, about earthquake dynamics and to test these ideas against earthquake observations.

Seismological Research Letters↗
USGS2009

PAGER-CAT: A composite earthquake catalog for calibrating global fatality models

We have described the compilation and contents of PAGER-CAT, an earthquake catalog developed principally for calibrating earthquake fatality models. It brings together information from a range of sources in a comprehensive, easy to use digital format. Earthquake source information (e.g., origin time, hypocenter, and magnitude) contained in PAGER-CAT has been used to develop an Atlas of Shake Maps of historical earthquakes (Allen et al. 2008) that can subsequently be used to estimate the population exposed to various levels of ground shaking (Wald et al. 2008). These measures will ultimately yield improved earthquake loss models employing the uniform hazard mapping methods of ShakeMap. Currently PAGER-CAT does not consistently contain indicators of landslide and liquefaction occurrence prior to 1973. In future PAGER-CAT releases we plan to better document the incidence of these secondary hazards. This information is contained in some existing global catalogs but is far from complete and often difficult to parse. Landslide and liquefaction hazards can be important factors contributing to earthquake losses (e.g., Marano et al. unpublished). Consequently, the absence of secondary hazard indicators in PAGER-CAT, particularly for events prior to 1973, could be misleading to sorne users concerned with ground-shaking-related losses. We have applied our best judgment in the selection of PAGER-CAT's preferred source parameters and earthquake effects. We acknowledge the creation of a composite catalog always requires subjective decisions, but we believe PAGER-CAT represents a significant step forward in bringing together the best available estimates of earthquake source parameters and reports of earthquake effects. All information considered in PAGER-CAT is stored as provided in its native catalog so that other users can modify PAGER preferred parameters based on their specific needs or opinions. As with all catalogs, the values of some parameters listed in PAGER-CAT are highly uncertain, particularly the casualty numbers, which must be regarded as estimates rather than firm numbers for many earthquakes. Consequently, we encourage contributions from the seismology and earthquake engineering communities to further improve this resource via the Wikipedia page and personal communications, for the benefit of the whole community.

Seismological Research Letters↗
USGS2009

BSSA: Worth thinking about

The Bulletin of the Seismological Society of America ( BSSA ) is a powerful community project that has helped us share the information necessary to keep our field moving forward since 1911. In some ways, BSSA is much like it has always been, and each issue provides us with a collection of research that has been improved by the peer review process and copyedited, typeset, and printed to make it easily readable. But BSSA has also constantly evolved, and the online edition with full-text searching of all content back to volume 1 and linked references that speed our navigation through the related literature is unlike anything J. C. Branner could have imagined when he edited the first issue.

Seismological Research Letters↗
USGS2009

OxCal: Versatile tool for developing paleoearthquake chronologies: A primer

Ages of paleoearthquakes (events), i.e., evidence of earthquakes inferred from the geologic record, provide a critical constraint on estimation of the seismic hazard posed by an active fault. The radiocarbon calibration program OxCal (4.0.3 and above; Bronk Ramsey 2007, 2001) provides paleoseismologists with a straightforward but rigorous means of estimating these event ages and their uncertainties. Although initially developed for the chronologic modeling of archaeological data from diverse sources (e.g., radiocarbon, historical knowledge, etc.), OxCal is readily adaptable to other disciplines requiring chronological modeling, such as paleoseismology (Fumal et al. 2002; Lindvall et al. 2002; Kelson et al. 2006; Noriega et al. 2006; Lienkaemper and Williams 2007; Yen et al. 2008).

Seismological Research Letters↗
USGS2009

Recent advances in rotational seismology

Rotational seismology is an emerging field of study concerned with all aspects of rotational motions induced by earthquakes, explosions, and ambient vibrations. Two recent monographs (Teisseyre et al. 2006; Teisseyre et al. 2008) and a Bulletin of the Seismological Society of America special issue on Rotational Seismology and Engineering Applications (Lee, Celebi et al. 2009) are useful starting points. Rotational seismology is of interest to a wide range of disciplines, including various branches of seismology, earthquake engineering, and geodesy, as well as to physicists using Earth-based observatories for detecting gravitational waves generated by astronomical sources.

Seismological Research Letters↗
USGS2009

A method to establish seismic noise baselines for automated station assessment

We present a method for quantifying station noise baselines and characterizing the spectral shape of out-of-nominal noise sources. Our intent is to automate this method in order to ensure that only the highest-quality data are used in rapid earthquake products at NEIC. In addition, the station noise baselines provide a valuable tool to support the quality control of GSN and ANSS backbone data and metadata. The procedures addressed here are currently in development at the NEIC, and work is underway to understand how quickly changes from nominal can be observed and used within the NEIC processing framework. The spectral methods and software used to compute station baselines and described herein (PQLX) can be useful to both permanent and portable seismic stations operators. Applications include: general seismic station and data quality control (QC), evaluation of instrument responses, assessment of near real-time communication system performance, characterization of site cultural noise conditions, and evaluation of sensor vault design, as well as assessment of gross network capabilities (McNamara et al. 2005). Future PQLX development plans include incorporating station baselines for automated QC methods and automating station status report generation and notification based on user-defined QC parameters. The PQLX software is available through the USGS (http://earthquake. usgs.gov/research/software/pqlx.php) and IRIS (http://www.iris.edu/software/ pqlx/).

Seismological Research Letters↗
USGS2012

The crowbar chronicles and other tales

The analysis of historical earthquakes often relies heavily on archival accounts describing the effects of shaking on structures and people. Newspaper articles are among the most common, useful, and easily found sources of information. Dramatic earthquake effects are almost certain to have made the news during historic times; the challenge for modern seismologists is not to be overly swayed by articles that focus on the most dramatic rather than the representative effects in a region. At the other end of the spectrum, rarely does a historical newspaper explicitly note that an earthquake was not felt in a certain area: it is not news when nothing happens. When earthquake effects are subtle, the vexing question is often, did they go unreported entirely?

Seismological Research Letters↗