Question answered by Suzette Payne, Seismologist, Idaho National Laboratory
The response to your question of smaller earthquakes giving a prediction of larger ones does not have straight forward answer. Sometimes prior to large magnitude earthquakes, one or more foreshocks occur, but most often not. In the case of swarms, at the beginning earthquakes typically have smaller magnitudes and the largest magnitude earthquake occurs sometime later within the sequence.
Small to moderate size earthquakes occurring near Challis, Idaho is not new. Swarms of earthquakes have been detected near Challis since the 1970’s during special seismic surveys conducted by the University of Utah (Payne, unpublished work). The most notable sequences of earthquakes occurred following the 1983 moment-magnitude 6.9 Borah Peak, Idaho earthquake, which ruptured the central segment of the Lost River fault (Richins et al., 1987). Regional and local seismic monitoring increased in the area following this earthquake. Within three weeks of the 1983 mainshock, hundreds of aftershocks migrated northward from north of Mackay to Challis and finally to Twin Peaks (Zollweg and Richins, 1985). Borah Peak aftershocks continued for over a decade. Larger magnitude aftershocks occurred in 1984 (M5.8, M5.0), 1992 (M4.6) and 1994 (M5.2) south of Challis along the northern part of the Lost River fault within the aftershock zone (Payne et al., 2004; Zollweg, unpublished work). The recent swarm of earthquakes in 2014 (M4.8) near Challis is located near the end of the Lost River fault. The 2014 sequence of earthquakes may be a continuation of aftershocks related to the 1983 mainshock or could be a swarm related to regional tectonic stresses.
Payne, S. J., J. E. Zollweg, and D. W. Rodgers (2004) Stress triggering of conjugate normal faulting: Late aftershocks of the 1983 Borah Peak, Idaho earthquake, Bulletin of the Seismological Society of America, v. 94, No. 3, p. 828-844.
Richins, W. D., J. C. Pechmann, R. B. Smith, C. J. Langer, S. K. Goter, J.E. Zollweg, and J. J. King (1987) The 1983 Borah Peak, Idaho earthquake and its aftershocks, Bulletin of the Seismological Society of America, v. 77, no. 3, p. 694‑723.
Zollweg, J.E., and Richins, W.D., 1985, Later aftershocks of the 1983 Borah Peak, Idaho earthquake and related activity in central Idaho, in Proc. Workshop XXVIII on the 1983 Borah Peak, Idaho, earthquake, U.S. Geological Survey Open-File Report, 85-290, p. 345-367.
Swarm – Many small magnitude earthquakes occurring in the same location with one larger magnitude event (typically <M5) usually occurring some time in the middle of the sequence.
Mainshock-aftershock – The largest magnitude earthquake occurs first and many smaller magnitude earthquakes follow and are located near the same location as the mainshock. Typically mainshock-aftershock sequences occur following large magnitude earthquakes such as M7+, but the sequences can also occur for mainshocks with smaller magnitudes such as M3.
Foreshock – An earthquake that occurs in the same location and prior to a mainshock-aftershock sequence. Foreshocks can be any magnitude.
An earthquake is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called a fault.
- Wash your hands and open your candy bar.
- Use the knife to make cracks in the chocolate coating. These cracks represent fault lines.
- Pull the ends of the candy bar apart. This represents a tension fault.
- Push the ends of the candy bar together. This represents a compression fault.
- Move the two halves of the candy bar opposite one another. This represents shearing.
- What changes did you see in the layers of the candy, which represent the layers of the Earth. The movement of the plates causes, rifts, mountains, and earthquakes.