(Due
at
4
pm
Friday,
Jan
29th,
2016)

Read
Chapter
6
and
Chapter
8
in
Earthquakes
(Bolt)
and
the
online

presentations.
You
also
need
a
ruler
to
do
some
measurements

1. On the figure below, the 10 km scale bar (assume the same scale both horizontally
and vertically) is given. What is the approximate moment magnitude of this
earthquake if the average fault slip (D) is 5 meters during the event? Using the
information below to solve the problem.

Seismic moment Mo, defined as Mo= µ x A x D, where A= L x W. L the length of
the fault, W the width of the fault, D is the average slip on the fault, and µ is shear
modulus. Mo measured in Nm, is related to magnitude by the following relation:
Log(Mo) = 1.5Mw+ 9.0
or solving for Mw we find Mw=2/3log M0 − 6
Solving for Mo we find Mo = 101.5Mw +9 (Nm)
In the equations above, Mw is the moment magnitude, and Mo is the seismic
moment. Use µ = 3.0X1010 N/m2 for the value of shear modulus.
2. If the moment magnitude increases by 2, how many times will seismic moment
increase?

3. What
is
the
energy
in
Joules
of
this
earthquake?

4. The
1964
Alaska
earthquake
was
rated
by
measuring
seismograms
as
body
wave

magnitude
mb=6.5,
surface
wave
magnitude
Ms=8.6,
and
the
moment
magnitude

of
9.2
Explain
the
difference
and
why
Mw
is
the
best
description
for
the
size
of

this
earthquake.

5. Using
the
formula
in
problem
1
to
estimate
the
seismic
moment
of
the
above

1964
Alaska
earthquake.

6. According
to
Bath
law
(we
will
learn
later),
on
average
the
largest
aftershock
has

a
magnitude
about
1.0
smaller
than
the
mainshock.
What
is
the
ratio
of

mainshock
and
the
aftershock
in
seismic
moment?
What
is
the
ratio
in
peak

ground
motion
at
the
same
station?
Here
we
assume
the
mainshock
and

aftershock
at
roughly
same
location.

7. In
2004,
a
movie
named
“10.5”
aired
as
a
television
miniseries
in
the
United

States
on
May
2,
2004
and
May
3,
2004.
It
is
a
disaster
movie.
The
plot
focuses
on

a
series
of
catastrophic
earthquakes
along
the
United
States’
west
coast.
The
last

one
has
a
magnitude
of
10.5
in
Richter
scale.
It
is
actually
a
fun
movie,
you
can

see
a
trailer
in
youtube
(https://www.youtube.com/watch?v=KJmCFfNWAE4).

The
largest
one
occurred
on
the
San
Andreas
fault.
As
we
discussed
in
class,
it

has
a
length
of
1300
km.
The
width
of
this
fault
is
about
15
km.
Let’s
suppose

that
the
moment
magnitude
of
this
event
is
10.5
as
well.
Please
estimate
its

average
slip?

(using
the
rigidity
value
given
in
problem
1).