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Results for “Seismological Research Letteres”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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At least 379 records · Page 21Linked to original sources
USGS2007

The 17 July 2006 Tsunami earthquake in West Java, Indonesia

A tsunami earthquake (Mw = 7.7) occurred south of Java on 17 July 2006. The event produced relatively low levels of high-frequency radiation, and local felt reports indicated only weak shaking in Java. There was no ground motion damage from the earthquake, but there was extensive damage and loss of life from the tsunami along 250 km of the southern coasts of West Java and Central Java. An inspection of the area a few days after the earthquake showed extensive damage to wooden and unreinforced masonry buildings that were located within several hundred meters of the coast. Since there was no tsunami warning system in place, efforts to escape the large waves depended on how people reacted to the earthquake shaking, which was only weakly felt in the coastal areas. This experience emphasizes the need for adequate tsunami warning systems for the Indian Ocean region.

Seismological Research Letters↗
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↗
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

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↗
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↗
USGS2010

Three-dimensional long-period groundmotion simulations in the upper Mississippi embayment

We employed a 3D velocity model and 3D wave propagation code to simulate long-period ground motions in the upper Mississippi embayment. This region is at risk from large earthquakes in the New Madrid seismic zone (NMSZ) and observational data are sparse, making simulation a valuable tool for predicting the effects of large events. We undertook these simulations to estimate the magnitude of shaking likely to occur and to investigate the influence of the 3D embayment structure and finite-fault mechanics on ground motions. There exist three primary fault zones in the NMSZ, each of which was likely associated with one of the main shocks of the 1811-12 earthquake triplet. For this study, three simulations have been conducted on each major segment, exploring the impact of different epicentral locations and rupture directions on ground motions. The full wave field up to a frequency of 0.5 Hz is computed on a 200 ?? 200 ?? 50-km 3 volume using a staggered-grid finite-difference code. Peak horizontal velocity and bracketed durations were calculated at the free surface. The NMSZ simulations indicate that for the considered bandwidth, finite-fault mechanics such as fault proximity, directivity effect, and slip distribution exert the most control on ground motions. The 3D geologic structure of the upper Mississippi embayment also influences ground motion with indications that amplification is induced by the sharp velocity contrast at the basin edge.

Seismological Research Letters↗
USGS2010

Displaying seismic deaggregation: The importance of the various sources

Seismic hazard deaggregation has become a standard part of probabilistic seismic hazard assessment (PSHA). The first product of PSHA is calculation of the likely severity of ground motion at a given range of annual probability levels, and this is extremely important for seismic design of structures to be built at the site under examination. However, for full analysis of proposed structural designs, engineers also need to examine scenario events to produce detailed time histories. To select such scenarios, a deaggregation of the hazard is performed, whereby the details of sources that contribute to the annual frequency of exceeding specified levels of ground motion, or P exc , are identified. A common format for such a deaggregation is shown in Figure 1 . This relates to the 475-year peak ground acceleration (pga) at Wellington, New Zealand (41.28°S 174.77°E), and shows the distribution in magnitude and distance of sources that contribute to P exc . Return period is approximately the reciprocal of P exc . Stiff soil site conditions ( Standards New Zealand 2004 ) were assumed. The analysis in Figure 1 used the interim version of the updated seismic hazard model for New Zealand ( Stirling et al. 2007 ), with the attenuation function developed by McVerry et al. (2007). Based on a Poisson time dependence model, a return period of 475 years corresponds to a 10% probability of exceedance in 50 years. From Figure 1 , it is apparent that for this site the main contribution to ground motion of this severity is from earthquakes of magnitude about 7.6 less than 10 km from the site (blue), and there is another strong contribution from larger events in the distance range 10 to 20 km (red). These correspond to the Wellington and Wairarapa faults, respectively (see Table 1 ). There are other events less than 10 km from the site and small contributions from other sources. At this site the major contributions are from specific faults nearby, which are readily identified. At sites where there is significant background seismicity, however, the plot will be much more complicated and not so easy to interpret. Figure 1 deaggregates probabilistic pga at the site; other parameters are also commonly deaggregated in the same way, in particular response spectral acceleration at a variety of natural periods. But the figure has a major shortcoming in that it represents only one return period; to obtain a full appreciation of the various contributing sources it is necessary to perform a succession of analyses to cover the full range of return periods.

Seismological Research Letters↗
USGS2010

Field survey of the Samoa tsunami of 29 September 2009

On 29 September 2009, a strong earthquake took place south of the Samoa Islands in the southcentral Pacific. It triggered a local tsunami, which caused considerable damage and 189 fatalities on the Samoa Islands and in the northern Tonga archipelago. We present here the results of a tsunami survey conducted by an International Tsunami Survey Team in the Samoa Islands on 4-10 October 2009 and in northern Tonga on 25–27 November 2009.

Manono, Manu'a Islands, Motu O Manu, Niuafo'ou, Ni↗
USGS2010

Temporal variations in Global Seismic Stations ambient noise power levels

Recent concerns about time-dependent response changes in broadband seismometers have motivated the need for methods to monitor sensor health at Global Seismographic Network (GSN) stations. We present two new methods for monitoring temporal changes in data quality and instrument response transfer functions that are independent of Earth seismic velocity and attenuation models by comparing power levels against different baseline values. Our methods can resolve changes in both horizontal and vertical components in a broad range of periods (∼0.05 to 1,000 seconds) in near real time. In this report, we compare our methods with existing techniques and demonstrate how to resolve instrument response changes in long-period data (>100 seconds) as well as in the microseism bands (5 to 20 seconds). High quality broadband data recorded by the GSN are fundamental to characterizing a wide range of Earth science issues including: the size and rupture of large earthquakes ( e.g. , Tsai et al. 2005 ); imaging the interior of the Earth ( e.g. , Van der Hilst et al. 1997 ); tracking global climate variation ( Aster et al. 2008 ); and monitoring calving glaciers (Ekström et al. 2003 , 2006a ). Recent studies based on theoretical Earth models ( Ekström et al. 2006b ; Davis and Berger 2007 ) suggest that broadband seismometer gain levels can vary with time. This has also been confirmed, for the STS-1 sensor, experimentally ( Yuki and Ishihara 2002 ). It therefore has become necessary to systematically check for temporal changes in amplitude at GSN stations. Many of these changes are frequency-dependent in nature and not a priori predictable ( Ekström et al. 2006b ). Robust methods that can be applied to a large number of stations in a broad range of frequency bands are necessary.

Seismological Research Letters↗