Larry J. Ruff
OhioSeis & PS
OhioSeis is a new network of digital seismographs based on the MichSeis concept of "independent yet cooperative stations." The first OhioSeis session was held in Columbus, Ohio on Dec. 18, 1998 where the software, seismometers, and associated hardware were distributed to the station hosts. While it is convenient to choose the above date as the official start of OhioSeis, it is worth noting that the first OhioSeis-type station went into operation at the College of Wooster more than a year ago, and that there is a long history of independent seismograph operation in Ohio at various sites.
In the present configuration, there are fourteen stations in the OhioSeis network, with a few more in the planning stages. The stations are mostly hosted by universities and colleges but also include a museum, government agencies, and one high school. Most of the stations are now running and on the Web--testimony to the enthusiasm and dedication of the individual station operators. The genesis of OhioSeis is due largely to the interest and efforts of Dr. Michael Hansen and colleagues at the State of Ohio Geological Survey, as they were able to raise funds from both state and federal sources to subsidize the initial costs of OhioSeis.
Earthquakes do happen in Ohio--as millions of people were reminded by the September 25, 1998 event that occurred along the Ohio-Pennsylvania border. Hence one important mission for OhioSeis is earthquake monitoring. With its statewide distribution and broad-band seismometers, OhioSeis represents a different mode of monitoring than the traditional short-period networks. The high-quality digital data produced by OhioSeis should prompt new and different regional-scale research into seismicity and earth structure. There will be an article submitted to SRL in the future that will discuss the details of the hardware and software components of OhioSeis. Here I would like to mention briefly a couple of the aspects of the MichSeis/OhioSeis system that are relevant to data access and educational seismology.
One hardware feature of the system is the use of inexpensive "off the shelf" hand-held GPS units with a serial data port to provide absolute time. Perhaps the most interesting aspect relates to data distribution: The Macintosh that runs the seismograph is also on a full-time Internet connection and runs its own Web server to distribute the seismograms in real time. We exploit the advances in "user friendly" Internet services, as the Mac OS8 system includes a Web server that is controlled and started with just a few mouse clicks. The latest version of the SeismoGraf software resides in a private directory on the Macintosh, but it automatically writes duplicate data files into the Web space, thereby making the seismograms available to outside users in real time. Furthermore, the data files in the Web space are written in ASCII text-to be specific, they follow the AH ASCII format. To get these seismograms, you just need a Web browser--click on the file icons to display the ASCII text, then "Save As ..." to save the files on your computer. While the latest version of SeismoView will read these ASCII files, you could also download the files directly into a spreadsheet program, which is what many teachers prefer to do. In addition, I just received e-mail from Dr. Charles Ammon at St. Louis University. He has modified his "Wiggles" program to read these ASCII AH files directly. Thus, there are already several options available to researchers, teachers, and students to use the OhioSeis data. Although the OhioSeis network has been operational for just a few months, we have already recorded and displayed several teleseismic events and a couple of small regional events. You can look at these events and find links to the OhioSeis seismographs from the MichSeis web page at http://www.geo.lsa.umich.edu/MichSeis/.
EduPhase is a new subsection of EduQuakes that will feature a particular seismic phase. Any time we look at whole seismograms, we see many bumps and wiggles. In particular, when I show seismograms to students and I point to the P or S wave or to the Rayleigh wave, they always point to some other wiggle and ask, "What's that?" EduPhase is a place where we can ask that question and provide some seismograms, pictures, stories, and other educational insights. I eagerly solicit your contributions to the EduPhase subsection.
To get things started, I have chosen the PS/SP phase for this column; I frequently notice this phase on seismograms, but I don't know very much about it. Do any readers know of research studies and/or novel uses of this phase? If so, send e-mail to me via firstname.lastname@example.org and I will post any new information on the PS/SP Web page. Seismographs in North America are at a favorable epicentral distance to record PS/SP from seismicity in the southwestern Pacific subduction zones. The new OhioSeis seismographic stations recorded a nice PS/SP phase from some recent large earthquakes in the Vanuatu (formerly the New Hebrides) subduction zone. I have constructed some Web pages that display these seismograms and related information; they can be accessed through the EduQuakes link at the SSA Web pages or directly through the MichSeis Web page (address given above, then look for "PS"). Here, I'll just mention a few characteristics of this curious phase. As we can tell from its name, PS leaves the earthquake as a P wave that then turns in the mantle and hits the Earth's surface, to be converted into a S wave that turns in the mantle and arrives at the station (see ray tracing plots in the PS Web pages). The travel itinerary for SP is the same, except that it leaves the earthquake as an S wave and converts to P at the surface reflection. For a shallow source depth, the travel times of PS and SP are identical--if the Earth is spherically symmetric. However, deeper earthquakes produce a separation in arrival times for PS and SP. Go to some of the seismograms in the Web pages to look for this phase separation. The situation is complicated by other phases that arrive, such as PPS, but that is another story!
From the classic phase identifications of Gutenberg and Richter (or from the IRIS poster of wave arrivals by Astiz et al.), it seems that PS/SP is observed as a distinct phase with large amplitude in the distance range from 90 to 140 degrees. Why is that? We can easily come up with a reason for why we might not see it at greater ranges. If you look up the ray parameter for the PS phase at 140 degrees, notice that it is the same as for an S wave that travels out to epicentral distance of about 100 degrees. Thus, at distances greater than 140 degrees, the S wave part of the PS ray path would be diffracted around the core, which reduces its amplitude. That would seem to offer a good excuse as to why we should not see PS beyond 140 degrees--but is it correct? For distances closer than 90 degrees, the P wave part of the PS raypath is now turning in the uppermost mantle. Would that in itself lead to a diminished amplitude? Could it be that the ScS arrival obscures the PS phase? Or could it be that the reflection coefficient of P-to-S and S-to-P is the main control on PS amplitude? All good questions, and good projects for students and lab exercises. I eagerly await your answers!
SRL encourages guest columnists to contribute to "EduQuakes." Please contact Larry Ruff with your ideas. His e-mail address is email@example.com.
Posted: 20 May 1999