Civil Engineering Reference
In-Depth Information
FIGURE 3.44
View almost the same as in Fig. 3.43. A debris avalanche triggered by the Peru earthquake
on May 31, 1970, caused the devastation. The same palm tree is visible in both figures. The cross marks the
location of the former cathedral. The massive cathedral partially diverted the avalanche and protected the
palm trees. (
Photograph from the Steinbrugge Collection, EERC, University of California, Berkeley.
)
March 27, 1964, Prince William Sound Earthquake in Alaska.
As indicated in Sec.
2.4.5, this earthquake was the largest earthquake in North America and the second-largest
in the 20th century (the largest occurred in Chile in 1960). Some details concerning this
earthquake are as follows (Pflaker 1972, Christensen 2000, Sokolowski 2000):
●
The epicenter was in the northern Prince William Sound about 75 mi (120 km) east of
Anchorage and about 55 mi (90 km) west of Valdez. The local magnitude
M
L
for this
earthquake is estimated to be from 8.4 to 8.6. The moment magnitude
M
w
is reported
as 9.2.
●
The depth of the main shock was approximately 15 mi (25 km).
●
The duration of shaking as reported in the Anchorage area lasted about 4 to 5 min.
●
In terms of plate tectonics, the northwestward motion of the Pacific plate at about 2 to 3 in.
(5 to 7 cm) per year causes the crust of southern Alaska to be compressed and warped,
with some areas along the coast being depressed and other areas inland being uplifted.
After periods of tens to hundreds of years, the sudden southeastward motion of portions
of coastal Alaska relieves this compression as they move back over the subducting
Pacific plate.
●
There was both uplifting and regional subsidence. For example, some areas east of
Kodiak were raised about 30 ft (9 m), and areas near Portage experienced regional sub-
sidence of about 8 ft (2.4 m).
●
The maximum intensity per the modified Mercalli intensity scale was XI.
