Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 284

phenomenology of this earthquake type, rather than in the engineering aspects. The
data for intermediate intraslab earthquake characteristics mainly come from the
Western part of Canada (British Columbia, Cascadia zone), Northern part of Japan
and Carpathian zone (Vrancea).
For deep earthquakes (below to 300 km), where the thermal effects are
dominant, the knowledge about the characteristics of ground motions are
practically missing in the technical literature. It is not clear how these earthquakes
are generated. So, deep earthquakes remain mysterious! Deep earthquakes will not
give up their secrets easily (Houston, 2007).
However, some general conclusions regarding the ground motions produced by
intraslab may be drawn.
-The rate of occurrence of earthquakes deeper than 70 km is several times lower
than those of crustal earthquakes (Houston, 2007). Eighteen percent of the
earthquakes with the magnitude greater than M 5.8 have depth greater than 100 km
and only 6 percent occur at depths between 400 and 700 km. Only about 2 percent
occur within the seismically quiet interval from 300 to 500 km (Houston, 2007).
- In contrast with the crustal earthquakes, for which the location of sources is
scattered over a large surface, the deep earthquakes are located in very small
surfaces, showing the repeatability of rupture in the same fault surface (Houston,
2007).
-Contrary to crustal earthquakes, deep earthquakes are notable for producing
very few aftershocks. But large deep earthquakes can trigger, after some time,
other large deep earthquakes at great distance. The time delays range from 2 to 140
minutes and the spatial separations range from 70 to 320 km (Houston, 2007).
-Generally, all the recorded ground motions have shown that the magnitudes are
stronger than for crustal events of the same size. All professionals accept this as a
matter of fact. So, for the South-western British Columbia, where crustal and
intraslab earthquakes frequently occur at the same places, the magnitude
recurrence for both earthquake types shows that for magnitude M 6, the predicted
rate of intraslab earthquakes is three to ten times the rate of crustal earthquakes.
For a recurrence of 475 years, the expected magnitudes are M 5.5 for crustal
earthquakes and 7 M for intraslab earthquakes (Adams and Halchuk, 2000).
Therefore, the expected magnitudes of intraslab earthquakes are greater than for
both subduction and intraplate earthquakes.
-As the fault breaks so deeply, the radiated seismic wave energy spreads over a
much larger area than in crustal earthquakes and very large areas experience
significant shaking.
-Therefore, the near-source effects, very important for the crustal earthquakes,
are not significant in the case of intraplate earthquakes.
-It is very interesting to notice that the initial rupture time is shorter for the deep
earthquakes than for the shallow ones (Houston, 2007). However, the duration of
ground motions frames in the category of long duration ground motions, due to the
increased distance from the source to the site, traveling through different upper
mantle rocks and crust soil layers.