Photo by David F. Walter, from Royal Arches, of the Happy Isles rockfall. Glacier Point is at the top right.
|
Almost anything that makes noise will make seismic waves that can be seen on a seismometer. Trees blow in the wind, cars drive, hikers walk. These die out over a short distance and so are not too interesting to seismologists, but some events at the Earth's surface do make seismic waves that can be studied. On July 10, 1996, about 80,000 tons of rock fell from the cliffs above Happy Isles and landed at a speed of over 240 miles per hour. One hiker was killed and about 1000 trees were toppled. The impact of this rock generated seismic waves measured up to 200 km (120 miles) away (shown at left). From these recordings, earthquake scientists calculated that the energy released was equivalent to a magnitude 2.1 earthquake. Figure from U.C. Berkeley website by R. A, Uhrhammer. Seismogram shows up-and-down motion (Z, top), toward-and-away from the rockfall (R, middle), and side-to-side (T, bottom). Much information from Wieczorek and others, GSA Bull, 2000. |
