Laurie Dingler | Seismographs aren’t the only seismographs – Times Standard

For nearly a century, seismographs were the only instruments used to record and measure earthquakes. Seismographs use mechanical or electronic sensors to measure ground motion as a function of time. Modern seismographs, called wideband seismographs, can measure a wide range of frequencies, from rapid oscillations that oscillate several times per second to long rolling waves with periods of up to several minutes.

Why is it important to register such a domain? The earthquake is like a symphony. If all you can hear are the mid-frequencies of the viola and oboe, you get a very narrow view of Beethoven’s ninth. When faults slide, they produce a large number of vibrations that range from slow to very fast, all of which are important for gaining a complete understanding of the magnitude of an earthquake and how it occurs.

Early seismographs recorded a narrow range of frequencies. By the 1960s, seismic stations typically had two sets of instruments: short peaks of about one second and long ones designed to respond to surface waves with periods of about 20 seconds. I remember how exciting it was when the Berkeley Seismic Lab installed a seismograph “for too long” in 1973 that could pick up 100-second vibrations.

Modern broadband is not peaking around a single frequency. They can record equally fast vibrations and much longer vibrations. But they still have limits. They cannot record oscillations on the order of tens of minutes nor can they detect permanent oscillations—when the location where the seismograph sits is shifted inches or tens of feet by fault rupture.

Enter GPS. Today’s GPS satellite system is a network of about 30 satellites that can accurately track the locations of many points on Earth including your smartphone. Developed by the US military in the 1960s to aid in navigation and location, satellites constantly broadcast information about where they are. The signals are picked up by ground-based receivers and converted into a distance. The position of the receiver can be triangulated by knowing the distances of three or more satellites in the system.

In the 1980s, my colleague, Ken Hudnott, was a graduate student at Columbia University studying earthquakes in California. He created a number of survey points (monuments) and measured their locations very precisely using GPS satellites at that time. He was able to map minute changes in Earth’s motion near the faults. His thesis earned him a permanent position with the USGS.

The M6.9 Loma Prieta earthquake provided Keene and colleagues at the USGS the first opportunity to use GPS in post-quake investigations. They were able to compare GPS data before and after the earthquake to determine the slip pattern at the fault level.

Excitement about GPS soon grew in earthquake studies and new networks were set up in many parts of California. In 1989, a group of 12 GPS monuments was established in the Cape Mendocino region. The network helped decipher the surface deformation associated with the 1992 Cape Mendocino earthquake and, along with a survey of coastal elevation, provided the most detailed picture of how the thrust fault altered surface topography.

These early GPS seismic applications were not implemented in real time. Memorial sites were created and revisited at monthly, annual or longer intervals to record how the sites have changed. The GPS expeditions were labor-intensive because they required scholars to visit each monument and spend several hours carefully collecting data. With the appropriate protocol, monument sites can be located with a horizontal accuracy of less than one inch. The USGS team revisited the Cape Mendocino GPS network after the September 1, 1994, M7.0 Mendocino earthquake and found that the earthquake moved some stations more than one foot to the east.

By the 1990s, GPS demonstrated a valuable tool in studying plate motions and post-earthquake deformation. But they weren’t helpful in recording what was happening during the earthquake itself. To do this requires much more frequent sampling of the ground position. The 21st century heralded the era of continuous GPS, instruments that are constantly connected to the GPS satellite network and, at 15-second intervals, send data to a repository. These new systems are called GNSS (Global Navigation Satellite Systems) and they are able to provide earthquake information as they happen, just like a seismograph.

The most exciting application of GNSS is to estimate the magnitude of earthquakes as the Earth is still exploding. Large earthquakes of 8.5 to 9.5 on the Richter scale are a problem even for large-scale seismographs. The Japan Seismic Network, the most developed in the world, gave an initial intensity of 7.8 to the Great East Japan earthquake on March 11, 2011. Seismograph signals tend to saturate with magnitudes greater than 8.

The GNSS system can quickly capture the Great Seismometer by showing the magnitude and magnitude of Earth’s displacement. The Japan Meteorological Agency had a GPS network in 2011 but it has not been integrated into earthquake early warning systems or tsunami warning systems. It is now and JMA scientists will rely on both seismometers and continuous GPS sensors.

The JMA has now integrated GNSS into the Earthquake Early Warning System (EEW). EEW relies on detecting an earthquake’s rupture shortly after it begins and sending alerts farther that a strong shaking is coming soon. It allows a few critical seconds to slow/stop trains and other vital facilities and for people to drop, cover, and wait before the strongest seismic waves arrive. Over the next year, GNSS will be integrated into US earthquake early warning systems as well.

Just this week, a research paper by a team of scientists from University College London and Japanese universities proposed a new application of GNSS – satellite detection of atmospheric vibrations caused by a tsunami as it forms and travels across the ocean. STAY CONNECTED – I can’t wait for the next GPS chapter.

Laurie Dingler is Cal Poly Humboldt Professor Emeritus of Geology and an expert in tsunami and earthquake hazards. Questions or comments about this column, or want a free copy of Preparedness magazine “Living on Shaken Earth”? Leave a message at 707-826-6019 or email [email protected].

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