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The Phenomenon of Deep Tectonic Tremors

Prof. Greg Beroza and graduate students Justin Brown and Ana Aguiar are studying the newly discovered phenomenon of deep tectonic tremor. Tectonic tremor is a weak vibration of the ground. It was discovered in Japan, but has subsequently been observed in a variety of tectonically active areas around the world, including California. Stanford scientists, including recent graduate David Shelly, and their colleagues at the University of Tokyo have established that tectonic tremor is slip on the deep extension of the same faults that give rise to ordinary earthquakes; however, tremor occurs as swarms of many thousands of tiny, somewhat slow, slip episodes that individually are referred to as low frequency earthquakes.

Low frequency earthquakes are one of a family of recently discovered slow earthquakes (see figure) that differ from ordinary earthquakes in several respects. Ordinary earthquakes grow explosively with time; however, slow earthquakes grow at an approximately constant rate. Tiny, low frequency earthquakes take about one second to occur, but large slow earthquakes may take weeks to unfold. These slow earthquakes occur on parts of faults that are transitional between fast, unstable rupture and slow, steady rupture. Slow earthquakes have opened a new window into the deep roots of fault zones, and we would like to know what they might tell us about how faults work, and how slow earthquakes might be related to ordinary earthquakes.

Because they are located immediately adjacent to the source zone of dangerous, large earthquakes, slow earthquakes should increase the likelihood of the kind of earthquakes we are worried about; however, to date there have been a number of slow earthquakes and none have been followed by a large earthquake. We are in the early stages of understanding tremor, however, and it will continue to attract the intense scrutiny it deserves. Unlike ordinary earthquakes, tremor is strongly modulated by tides, and it can also be triggered by large waves from distant earthquakes. This sensitivity to small stresses suggests that tremor might also be sensitive to changes in the state of stress, such as faults might undergo in the lead-up to future earthquakes.