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Meeting Abstracts

2007 American Geophysical Union Meeting,
December 10-14, San Francisco, California
Upper Crustal Velocity Model of the Goldstream Valley, Central Alaska

by Ebel, J.E. (Weston Observatory, Department of Geology and Geophysics, Boston College, 381 Concord Rd., Weston, MA 02493, USA, ebel@bc.edu) and
Dougherty, S.L. (Boston College, Department of Geology and Geophysics, Devlin Hall 213, 140 Commonwealth Ave., Chestnut Hill, MA 02467, USA, doughesb@bc.edu) and
Leidig, M. (Weston Geophysical Corporation, 1004 Augusta Dr., Houston, TX 77057, USA, mleidig@westongeophysical.com)

A series of five explosions were detonated in the schist bedrock of the Goldstream Valley region of central Alaska and recorded on more than 120 local sensors to develop 1-D and 3-D models of the upper crust. Simple refraction analyses reveal that both P- and S-wave arrival times are azimuth dependent, with the fastest velocities seen in the north and northeast for P-waves and in the southeast and northeast for S-waves. In one layer over a half-space P models, average velocities of 3.46 ± 0.41 km\backslash\s for layer 1 and 4.79 ± 0.53 km\backslash\s for layer 2 are seen. Two layers over a half-space models indicate average P-wave velocities of 3.05 ± 0.19 km\backslash\s (layer 1), 3.90 ± 0.18 km\backslash\s (layer 2), and 5.12 ± 0.35 km\backslash\s (layer 3). Analysis of S-wave travel time data yields an average velocity for only one layer (2 or 3), with a value of 2.77 ± 0.47 km\backslash\s. The azimuthal dependence is modeled with velocity anisotropies of ± 1-10% and ± 2-12% for P and S, respectively, at a fast direction of ~40-70°, which is consistent with the northeast trending fault pattern of the region. Poisson's ratio calculations reveal a well constrained average value of 0.283 ± 0.031 at distances ≥ 7 km and a more scattered value of 0.277 ± 0.062 at 2-7 km distance from the source. These values are consistent with known Poisson's ratios for mica-quartz schist and greenschist or amphibolite facies pelitic schist, which are the major geological components of the region. The highly scattered Poisson values at close distances are likely due to the higher porosity of the bedrock at low pressures. In addition to the simple travel time analysis, the S-wave velocity structure of the upper crust is determined through multiple filter analysis and inversion of 0.4-2 sec period Rg waves. The resulting shear wave velocity model is incorporated into the refraction analysis results to help constrain the 1-D and 3-D models of the area.

2007 American Geophysical Union Meeting
December 10-14, San Francisco, California
Evaluating Thermoelastic Strain as an Earthquake Trigger

by J.E. Ebel (Weston Observatory, Department of Geology and Geophysics, Boston College, 381 Concord Rd., Weston, MA 02493, USA, ebel@bc.edu) and
Y. Ben-Zion(University of Southern California, Department of Earth Sciences, Los Angeles, CA 90089, United States, benzion@usc.edu)

Spatio-temporal variations of temperature at the Earth's surface cause strains and stresses below the surface in two ways. The diffusion of heat into the ground leads directly to thermal deformation that may have a significant amplitude several meters below the Earth's surface. More importantly, variations of surface temperatures produce surface tractions that propagate stress-strain variations many kilometers into the crust. Under the latter mechanism, seasonal temperature variations with spatial wavelengths of 10-30 km can produce thermoelastic strains with amplitudes of ~10-7-10-8over the seismogenic depth range of 1-10 km in the crust. Since the threshold for triggering seismicity is expected to decrease with increasing period of excitation, these strain levels and associated stresses (~7x102-7x103 Pa using a nominal rigidity of 70 GPa) may trigger seismicity. We suggest that thermoelastic strains may explain the growing evidence for seasonal variations in the earthquake activity rates at a number of places in the western U.S. Seasonal variations of seismicity with increased activity in the summer and autumn were observed in California for earthquakes less than M1.5 during the several years following the 1992 Landers earthquake, for earthquakes with M>=2.0 in California and Nevada from 1990-2005, and for earthquakes with M>=4.0 in California and Nevada from 1932-2005. A similar seasonality with increased seismicity during the autumn and winter has been documented at some of the Cascades volcanoes and in the Himalaya. Others have explained observed seasonal seismicity patterns by annual air pressure changes, rainfall effects or snow loads; however, some such explanations require unrealistic values of material properties. We argue that an annual thermoelastic effect appears to be the best explanation for the observed seasonal seismicity patterns in the dry, warm California climate.

2007 American Geophysical Union Meeting
December 10-14, San Francisco, California
Analysis of Aftershock Activity in Stable Continental Regions: Implications for Aftershock Forecasting After Strong Earthquakes in the CEUS

by John E. Ebel and Anastasia M. Moulis(Weston Observatory, Department of Geology and Geophysics, Boston College, 381 Concord Rd., Weston, MA 02493, USA, email: ebel@bc.edu, macherid@bc.edu)

Statistical forecasts of the probabilities of future aftershocks after a large earthquake have become routine in California, an area where the average Omori-law properties of aftershock sequences have been determined from a large number of aftershock sequences. Unfortunately, there are not enough strong earthquakes with aftershock sequences in eastern North America for robust determinations of the average aftershock properties for that region. Thus, for the application forecasting in eastern North America, the aftershock properties from earthquakes in stable continental regions (SCRs) globally have been determined under the assumption that the aftershock behavior in all SCRs is comparable. The aftershock activity from 28 SCR earthquakes of Mw 6.0 and greater has been compiled, and the Omori-law parameters of this data set have been determined. Some SCR mainshocks have been followed by more active aftershock sequences, while some have been followed by relatively inactive aftershock sequences. Although the variation in SCR aftershock activity from one event to another is quite large, the average Omori-law aftershock properties are similar to those determined for California. These properties can be used as a basis for statistical forecasts of aftershock activity after strong earthquakes in eastern North America.

2004 American Geophysical Union Meeting, San Francisco, California
Evaluating Short-Term M4+ Earthquake Probability Forecasts for California and Western Nevada Since 2001

by John E. Ebel and Alan L. Kafka (Weston Observatory, Department of Geology and Geophysics, Boston College, 381 Concord Rd., Weston, MA 02493, USA, email: ebel@bc.edu, kafka@bc.edu)

Since 2001 we have made a number of forecasts of 10-day time periods of increased probability of earthquakes of M4 or greater (M4+) in California and Nevada. The forecasts are based on observations of non-Poissonian short-term clustering in the M4+ seismicity from 1968 to 2000 for Northern California/Western Nevada and from 1983 to 2000 for Southern California, and they assume that the M4+ earthquake statistics for these earlier time periods can be applied without change to the current and future seismicity. Since 2001, the observed versus expected outcome of the forecasts has been: Northern California/Western Nevada, 24 forecasts, 7 correct forecasts, 10 forecasts expected correct; Central California/Western Nevada, 33 forecasts, 8 correct forecasts, 11 forecasts expected correct; Southern California, 19 forecasts, 0 correct forecasts, 7 forecasts expected correct. For Northern and Central California and Western Nevada, the difference between the observed and expected number of correct forecasts is statistically insignificant. However, for Southern California the probability of 0 correct forecasts in 19 chances is only 0."2%". Since 2001, the M4+ seismicity in Southern California shows a strongly regular component with a period between 25-30 days, suggesting a connection to the monthly earth tidal cycle. Our analysis indicates that the temporal pattern of Southern California M4+ seismicity underwent a major change sometime in 2000-2001, even though the average rate of M4+ earthquakes remained constant.

76th Annual Meeting of the Eastern Section of the Seismological Society of America, Blacksburg, Virginia (2004)
The Effect of Long Period Energy on Expected Ground Motions of Future Earthquakes in Boston Massachusetts

Ebel, J.E., and Urzua, A., Weston Observatory, Department of Geology and Geophysics, Boston College, 381 Concord Rd., Weston, MA 02493, ebel@bc.edu; and Britton, J., Weston Geophysical Corporation, 57 Bedford St., Suite 102, Lexington, MA 02420

Boston, Massachusetts lies where a river estuary and harbor area have been modified by manmade land, and the unconsolidated soils in these landfill areas will modify earthquake ground shaking. In an earlier study, a GIS database incorporating surficial geology, borehole geotechnical data, and geologic profiles from previous studies was used to create microzonation maps for downtown Boston. The microzonation analysis showed that bedrock peak ground accelerations will be somewhat amplified in many parts of the study area, while those areas of Boston with more than 100 feet of soil could experience a factor of 3 ground motion amplification at 1.0 s period. Peak earthquake ground motions would fluctuate noticeably across the city. Inputting earthquakes with different source spectra into the microzonation analysis give different amounts of ground-motion amplification in the city. The design spectra from recent building codes in Boston are conservative when compared to the response spectra calculated using a high-frequency input earthquake with a source spectrum similar to that of the 1988 Saguenay earthquake. However, surface ground motion calculations for Boston show that the building codes may provide inadequate protection if the input earthquake has significant low-frequency energy. Thus, the frequency content of eastern North American earthquakes must be understood for proper microzonation analyses in the region.

76th Annual Meeting of the Eastern Section of the Seismological Society of America, Blacksburg, Virginia (2004)
The Boston College Educational Seismology Project: Inviting Students into the World of Science Research

A. Kafka, M. Barnett, H. Bellegarde, J. Ebel, M. Gruber, A. Macherides Moulis, A. Pfitzner, and D. Smith
Weston Observatory
Department of Geology and Geophysics
Boston College, Weston, MA 02493

The Boston College Educational Seismology Project (BC-ESP) operates an educational seismic network consisting of AS1 seismographs located at one high school (Weston High School, Weston, MA), two middle schools (McDevitt Middle School, Waltham, MA; and Weston Middle School, Weston, MA), and one elementary school (Garfield Elementary School, Brighton, MA). AS1 seismographs are also located at the Boston College campus and Weston Observatory as part of the BC-ESP. This project uses seismology as a medium for inviting students into the world of science research by inquiry-based learning through investigation of earthquakes recorded by seismographs in K-12 classrooms. Since seismology is an interdisciplinary science that requires understanding a wide range of scientific concepts, the BC-ESP offers numerous possibilities for introducing students to the nature of scientific inquiry and to a wide range of important scientific concepts. Seismographs measure the pulse of the Earth, and provide direct information about earthquakes, plate tectonics, and the structure of the Earth's interior. Thus, having their own seismograph in the classroom gives students a way of collecting real-world data and making measurements that provide them with an understanding of the internal structure of the Earth and processes by which the Earth changes. The AS1 seismograph, which serves as the classroom seismograph for this work, is ideal for this purpose because it is affordable, records earthquakes quite well considering its low cost, and is relatively simple to install and operate. Based on these classroom seismographs, we provide curriculum resources and research experiences for students in the K-12 schools. Examples of earthquakes recorded by the BC-ESP schools are: The magnitude 6.4 earthquake in Morocco (February 24, 2004), the magnitude 6.3 earthquake in Iran (May 28, 2004), the magnitude 6.4 earthquake in the Vancouver Islands (July, 19, 2004), and the magnitude 6.0 earthquake in the Aleutian Islands (August 7, 2004). These and other seismograms recorded by the BC-ESP seismographs can be viewed on our web site: www2.bc.edu/~kafka/SeismoEd/BC_ESP_Home.html.

76th Annual Meeting of the Eastern Section of the Seismological Society of America, Blacksburg, Virginia (2004)
WEBSHAKE: An Internet Tool to Perform Frequency Domain Level Ground Amplification Studies

URZUA, A., and EBEL, J. E., and ROSE, D.

A. Urzua
J.E. Ebel
Weston Observatory
Department of Geology and Geophysics
Boston College, Weston, MA 02493

D. Rose
Prototype Engineering, Inc.
57 Westland Ave., Winchester, MA 01890

To develop a site-specific elastic free-field absolute acceleration design response spectrum, it is common engineering practice to perform one-dimensional response analyses. These analyses normally require a bedrock (rock outcrop) input ground motion. The effect of the local soil conditions is modeled using a dynamic "soil profile" that incorporates engineering and index soil and/or rock properties, such as shear wave velocities, degradation characteristics, soil unit weights, etc., and the analyses typically perform the response spectrum calculations by solving for the vertical propagation of shear waves. The objective of this presentation is to introduce WEBSHAKE (Prototype Engineering, 2004) — a user-friendly web implementation of SHAKE91 (Idriss & Sun, 1992). WEBSHAKE is a computer program that performs frequency domain level ground soil amplification studies in a linearly viscoelastic material. The program makes use of the latest Microsoft .NET technology to create an Internet-based application. Both the front end and the Internet back end are written entirely in C#, except for the Fortran core, which performs the computations. Data entry and validation are done as a Windows desktop application. Once the data entry is complete, WEBSHAKE is run by sending the data to the .NET WebService. The results are returned through the Internet and saved to the desktop of the local user of the program. A database on the WebService is used to perform authentication. The application can be modified to store any desired data, including user identification, billing information, and statistics of analysis runs.