Lab 3 - Calculation of Moment Magnitude

Download the spreadsheet file: moment-mag.xls and save it on the desktop of your computer.  Then go into Excel and use the moment-mag spreadsheet to carry out the following.

From the distribution of subsidence locations it has been estimated that the rupture zone for the January 1700 earthquake on the Juan de Fuca subduction zone was between 600 and 700 km long. The width of the current locked zone is estimated to be between 45 and 75  km. The plate is being subducted at a rate of 4.5 cm/y. The average time between earthquakes has been around 500 years (~200 to 1000 y), and hence, depending on the elapsed time the displacement on past (and future) earthquakes could be between 10 and 30 m.

Estimate the moment magnitude of the 1700 earthquake by entering the following values into the blank spaces of the spread-sheet:

a) length=650 km , width=50 km, displacement=20 m
b) same, but width=75 km
c) same, but width=45 km
d) length=650 km , width=50 km, displacement=10 m
e) same, but displacement=30 m

1) Based on these parameters, what are the likely Mw limits for the 1700 earthquake?

 
For the 1989 Loma Prieta earthquake (south of San Francisco) the rupture zone length is around 50 km and the width around 15 km. (The rupture zone seismic distribution for this earthquake is given in the notes.) The published Mw is 7.1.

2) Use the spreadsheet to help you estimate the displacement based on the parameters above.  Estimate the displacement if the length=45 km and the width=10 km.

 
The June 1946 Vancouver Island earthquake has a published ML of 7.3.

3) Make some reasonable estimates of the dimensions of the rupture zone and displacement of the 1946 earthquake.

 
The largest historical earthquake (Chile 1960) had an Mw of 9.5. (This is consistent with a rupture zone around 1200 km long and 200 km wide.) 

Find out what type of rupture zone characteristics might be associated with Mw values of 10, 10.5 and 11.

4) Speculate whether it is likely that there would ever be earthquakes of M 10 or more.  If not, try to explain why not on the basis of what we know about the earth.

Hand in a print-out of the completed spread-sheet, along with your answers to the questions numbered 1 to 4 above, by February 4th.