Major and great earthquakes are usually followed by hundreds or even thousands of smaller earthquakes,
called aftershocks, that occur along and adjacent to the fault displaced during the
large earthquake. In some earthquake sequences, a smaller earthquake called a “foreshock”,
precedes the mainshock, like the one that occurred two days before the 2011 Tohoku-oki
magnitude 9.1 earthquake. With rare exceptions, all magnitude 7 to magnitude
9 earthquakes have a collection of smaller-magnitude aftershocks that can occur for weeks, years,
or even centuries after the mainshock. On the other hand, many major and great earthquakes
do not have a foreshock preceding the mainshock. Important observations about foreshocks and
aftershocks are provided by the 2002 magnitude 7.9 earthquake in the Alaska Range. First
some background. The Alaska range is a narrow 650-km-long mountain range far from the subduction
zone plate boundary. The Yakutat terrane was an oceanic plateau that traveled northwest
on the Pacific Plate and began colliding with the continental margin about 25 million years
ago. It now acts like a geologic snowplow driving immense blocks of continental crust
in a counterclockwise rotation. Right-lateral strike-slip faulting along the Castle Mountain
and Denali faults accommodate this crustal block rotation. At a magnitude 7.9, the November
3, 2002 Denali mainshock was one of the largest strike-slip earthquakes in North America.
The earthquake rupture started at a depth of 5 km with 4 meters of displacement on
the previously unknown Su-sit-na Glacier fault. It continued with up to 8 meters of right-lateral
displacement on the Denali Fault, and then branched onto the Totschunda Fault, for a
total rupture length of 340 km. Let’s now examine the entire sequence of earthquakes
that occurred along the Denali Fault from late 2002 into 2003. The sequence started
on October 23 with the magnitude 6.7 Nenana Mountain earthquake that was followed by aftershocks
spread over a 45-km-long segment of the Denali Fault. The resulting fault displacement decreased
shear stress on that part of the fault but increased stress on the adjacent segment to
the east. On November 3, the magnitude 7.9 mainshock occurred with epicenter located
22 km east of the magnitude 6.7 epicenter. The October 23 earthquake is now called a
“foreshock” because it triggered the magnitude 7.9 mainshock eleven days later. However,
we did not know it was a foreshock until November 3 when the larger mainshock occurred. That
magnitude 7.9 mainshock was destined to eventually occur but was hastened by the change in stress
caused by the foreshock. Seismologists have analyzed countless foreshock – mainshock
pairs of earthquakes, and earthquake triggering in general. Unfortunately for earthquake prediction,
there are no special characteristics of an earthquake that let us know it is a foreshock
of a larger mainshock that is yet to come. The major redistribution of stress during
the mainshock triggered an aftershock sequence of smaller earthquakes along the mainshock
rupture zone. Aftershocks are smaller earthquakes on patches of the rupture zone and adjacent
smaller faults that occur as rocks along the rupture adjust to the new state of stress.
Seven hundred and sixty aftershocks of magnitude 3 or larger occurred during the 30 days following
the November 3 mainshock. The entire earthquake sequence reduced a large amount of shear stress
that had built on the Denali Fault over the preceding decades and centuries.
Fourteen aftershocks had magnitudes greater than 5.0 and the largest occurred ten minutes
after the mainshock with magnitude 5.6. In general, larger aftershocks are concentrated
in the hours to days following the mainshock. Remember that the magnitude scale is logarithmic
and elastic energy released by an earthquake increases by a factor of 33 with every unit
increase in magnitude. So, at magnitude 5.6, the largest aftershock released less than
1/1000th of the energy released by the magnitude 7.9 mainshock. On this graph, we can see that
the rate of aftershocks dies off with time after the mainshock. This rapid decay in rate
of aftershocks was first studied in detail by Fusakichi Omori, one of the most important
figures in the history of seismology. Omori examined aftershocks of the magnitude
8 Nobi earthquake that occurred in central Japan in 1891. The Nobi earthquake is the
largest instrumentally recorded inland earthquake in Japanese history. Omori found that the
decay of the aftershock rate was best analyzed using a logarithmic graph of aftershocks per
day versus days after the mainshock. He showed that the rate of Nobi aftershocks decreased
with the reciprocal of time since the mainshock. This is a general observation now referred
to as Omori’s Law. Plotting data for the 2002 Denali earthquake sequence on a logarithmic
graph shows that the decay in the aftershock rate follows Omori’s Law.
In a typical mainshock – aftershock sequence, 97% of energy is released by the mainshock
while the hundreds to thousands of aftershocks release the remaining 3%. Although only a
small portion of the energy of an earthquake sequence is released by aftershocks, the largest
aftershocks and the highest rate of aftershocks occur during the critical first hours and
days immediately following the major earthquake. This is exactly the time when emergency workers
are locating and rescuing injured people from collapsed or damaged buildings. Because those
buildings can shift or even collapse during aftershocks, rescuers are at greatest risk
when their expertise and service is most important.