Simple Tools Measure the Earth’s Major Movements

Seismographs, the instruments that measure earthquakes, are essentially simple tools. In contrast, earthquakes and the Earth’s crust are very complex. Using the former to measure the latter can result in some conflicting figures.

“It’s essentially a weight on a spring,” said Tom McEvilly, assistant director of the University of California Seismology Station in Berkeley, in describing the heart of the seismograph. “The weight serves as an inertial reference point--that is, it tends to stand still as the earth around it shakes.”

Measuring the relative movement of the moving earth to the weighted mass is done in several ways, from the simple electromechanical devices used by pioneering seismologist Charles F. Richter to the sensitive electronic sensors and high-speed computers used today.

Weight on a Spring

The results are usually expressed graphically by wavy lines indicating the size and frequency of shock waves produced by earthquakes. Earthquakes are catalogued and compared using the well-known Richter scale, a logarithmic numerical expression of that weight on a spring. The scale was devised in the early 1930s by Caltech professor Richter.

With the information recorded by seismographs, scientists can estimate the relative strength of the earthquake and use a scale to assign it a Richter magnitude. With additional information about local subsurface geology, they can estimate the depth at which the quake occurred, and by measuring the time lag between the arrival of fast-moving, high-frequency shock waves and slower, low-frequency shock waves, scientists can tell how far away the event occurred and estimate its epicenter.

Initial readings at the Berkeley lab are done by computer. But they measure the size and intensity of shock waves slightly differently than Richter did before computers were invented, so the lab also records temblors on rotating drums of photosensitive paper. The difference between these methods accounts for many of the conflicting measurements that come from a single seismograph station.

Defects in Earth’s Crust

Conflicting Richter readings from different seismographic stations--such as the 6.1 reading Thursday from the National Earthquake Center in Colorado and the 6.0 in Berkeley--probably are the result of defects in the Earth’s crust and the unsymmetrical energy output of earthquakes, said Karen McNally, director of the Charles F. Richter Laboratory at UC Santa Cruz.

“In general, there is always a certain little uncertainty--plus or minus a quarter of a magnitude,” McNally said. “There are local faults and defects--crustal structures and mountains and valleys--that tend to get in the way and distort the shock waves.”

Each whole number on the Richter scale represents a tenfold increase in the size of the shock wave created by an earthquake and a 32-fold increase in the earthquake’s energy, said Bob Uhrhammer, manager of the UC Berkeley seismology station. Thus, the 1971 San Fernando temblor, at Magnitude 6.4, produced three times the shock of Thursday’s 6.1 tremor.

The largest earthquake ever recorded on the scale was the 8.4 Easter Day earthquake in Alaska in 1964. However, records of old temblors have allowed researchers to estimate that a 1906 earthquake in South America and a 1933 tremor in Japan probably would have registered at 8.9. The 1906 San Francisco earthquake is estimated at about 8.3.