VOLCANO MADNESS: [Geology2] Aftershocks: Earthquakes or Afterthoughts?

Friday, February 20, 2015

[Geology2] Aftershocks: Earthquakes or Afterthoughts?

Aftershocks: Earthquakes or Afterthoughts?

(EOS, weekly transactions American Geophysical Union 11/11/97)

S. E. Hough and L. M. Jones: United States Geological Survey, 525 S. Wilson Ave, Pasadena CA, 91106

We tend to view aftershocks as afterthoughts, in both their scientific and societal impact and importance. Seismologists who communicate with the media are often asked, 'was it just an aftershock?', with the 'just' implying, even if unsaid, 'if you say yes, then we don't have to worry about it'. Seismologists themselves tend to dismiss events subsequent to a large mainshock as very much secondary in importance. Even when appreciable in their own right, they typically generate only a fraction of the scientific interest and investigation as the mainshock. However, a consideration of recent earthquake sequences suggests that aftershocks may have important consequences. Were a future San Andreas event to produce an aftershock comparable in magnitude and relative location as the M6.5 Big Bear aftershock to the M7.3 Landers earthquake of 1992, the result could be a M6.5+ event at the northern edge the Los Angeles basin.

By every criteria ever used, an individual aftershock does not differ from an individual earthquake. In particular, the rupture processes and ground motions are indistinguishable. Aftershocks differ from other earthquakes only in that we expect them. Unlike most earthquakes, aftershocks occur within predictable bounds of space, time, and magnitude. They are most common immediately after the mainshock and decay in time with approximately the reciprocal of time since the mainshock [Omori's law, (Utsu, 1961)]. The magnitudes follow a Gutenberg-Richter relation, with the number of aftershocks proportional to ten to the power of the magnitude times a negative constant b. b is close to one so each unit decrease in magnitude leads to an order of magnitude decrease in number [Gutenberg and Richter, 1954]. This leads to a distribution of the largest aftershock that peaks at approximately one unit of magnitude below the mainshock (sometimes referred to as Bath's law [Richter, 1958]).

The above properties of aftershock sequences allow for time-dependent prediction of aftershock probabilities [Reasenberg and Jones, 1989]. The standard calculation of probabilities distinguishes aftershocks from mainshocks in that probabilities of future aftershocks are not affected by an earthquake that is already itself an aftershock. However, aftershock sequences clearly do have subsequences that sometimes stand out as distinct clusters: in reality, even aftershocks can have aftershocks.

Spatially, aftershocks cluster around the mainshock rupture surface but can occur at significant distance. The 1992 M7.3 Landers, California, earthquake triggered events hundreds of kilometers away [Hill et al., 1993]. A rule of thumb has been suggested to use the term aftershock only when the event occurs within one fault length (i.e., the length of the fault that ruptured in the mainshock; 80 km in the case of the Landers earthquake) of the mainshock rupture.

Aftershocks on faults other than the one that produced the mainshock are the norm. Hauksson et al. (1994) estimated that at least 20 faults were involved in the aftershock sequence of the Landers earthquake. The most distant was the M6.5 Big Bear aftershock at a distance of roughly 36 km from the mainshock epicenter [Figure 1; Jones and Hough, 1995]. Large aftershocks (close to M6) have occurred on faults distinct from the mainshock rupture after the 1994 M6.7 Northridge earthquake (11 hours later), the 1952 M7.5 Kern County earthquake (37 hours later), and the 1979 M6.5 Imperial Valley earthquake (8 hours later).

http://pasadena.wr.usgs.gov/office/hough/eos.html
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