Larry J. Ruff
Shaking Down by the River
The ES-SSA (Eastern Section, Seismological Society of America) meeting was held in Memphis, Tennessee last October. While there was a diverse range of papers presented (see meeting information in this issue of SRL), there was a clear focus on New Madrid seismicity and hazards. Seismic hazard mapping is a major ongoing effort by various groups throughout the midcontinent region.
Observations in many different parts of the world have shown that ground shaking is stronger with longer duration at sites underlain by soft sediments compared to sites on hard rock. Of course, most examples of this amplification effect come from regions with high seismicity levels. Some of the better known earthquakes that provided evidence of the connection between sediments and damage include the 1985 Michoacán, México; 1989 Loma Prieta, California; and 1995 Kobe, Japan, events. It is possible to study this effect without the occurrence of large damaging earthquakes, even in regions of low seismicity in plate interiors. While not strictly true, soft sediments in the midcontinent region generally are found at the edges of rivers--exactly where many major cities are located. In addition to the Mississippi River, several other rivers flow through the states that surround the New Madrid Seismic Zone. Hence there is an abundance of soft sediments in the midcontinent region. "Soft" sediments can be characterized by a low shear-wave velocity in the uppermost layers.
While detailed mapping and zoning considerations are projects for professional seismologists and geologists, students can participate in the collection of shaking intensity data that establish the fundamental empirical connection between sediments and shaking amplification. Since accelerographs are not installed everywhere, we still rely upon detailed shaking intensity studies to identify areas of enhanced shaking. Whether you live in California, Tennessee, Austria, or Australia, anytime a widely felt earthquake occurs close to your town, there is a special educational opportunity for your students to perform a detailed intensity survey. In the USA, various official organizations will collect intensity data and publish maps of the results. But to identify small-scale variations in shaking that correlate with variations in sediments, it is necessary to do a more detailed canvassing of the felt area. These detailed surveys are a great student project.
One example of this type of student project comes from the small (M = 3.4) 2 September 1994 mid-Michigan earthquake (see Faust et al., Seism. Res. Lett. 68(3), 1997). It is easy to obtain the official forms for intensity studies (e.g., from the USGS Web site), and you can then send these forms home with students of all ages. GIS software can translate street addresses into geographic coordinates onto geologic maps. I could argue that this style of intensity survey represents a better scientific sample of shaking in the school district than the one or two shaking reports called into the local police department. This project also benefits the students and teachers. Student interest in earthquakes is always high after they feel an earthquake--so what can teachers do with this interest? Of course, one can talk about earthquakes and plate tectonics. In addition, a local intensity survey lets the students do something with "their" earthquake, and their interest will be further enhanced if they see their scientific study go into the seismological archive of detailed intensity surveys.
Until we have a more official archive of student-based intensity surveys, send any information about such cases to me at email@example.com, and I will post these examples on the EduQuakes Web pages. Students of all ages in all places can make a meaningful contribution toward the mitigation of earthquake hazards.
SRL encourages guest columnists to contribute to "EduQuakes." Please contact Larry Ruff with your ideas. His e-mail address is firstname.lastname@example.org.
Posted: 24 February 2000