1

EPS 122: Lecture 12 – Moment tensors

Moment tensors

Reading: Fowler p111-140

EPS 122: Lecture 12 – Moment tensors

Fault planes

Rock ruptures and slides releasing stress

Stress builds deforming the rock

2

EPS 122: Lecture 12 – Moment tensors

First motion

time

time

time

time

time

time

de

pth

What does the waveform look like?

The first motion direction observed at many stations around an earthquake can tell us the type of fault and orientation of the fault planes.

Explosive source

EPS 122: Lecture 12 – Moment tensors

San Andrea Fault

What does the waveform look like?

map view

time

time

time

time

Consider a strike-slip earthquake on the San Andreas Fault:

3

EPS 122: Lecture 12 – Moment tensors

First motion

The first motion direction observed at many stations around an earthquake can tell us the type of fault and orientation of the fault planes.

Consider a strike-slip earthquake on the San Andreas Fault:

Fault plane solutions

EPS 122: Lecture 12 – Moment tensors

Landers earthquake

InSAR image

• Radar interferogram of land surface before and after Landers

• Shows surface motion in direction of satellite viewing angle

• One fringe represents a change in distance of one wavelength (28 mm)

• 28 June, 1992

• magnitude 7.3

• strike-slip fault

• rupture 85 km long

Massonnett et al 1993

4

EPS 122: Lecture 12 – Moment tensors

Landers earthquake

InSAR image

• One fringe represents a change in distance of one wavelength (28 mm)

20 fringes

NW corner moved 56 cm

Massonnett et al 1993

EPS 122: Lecture 12 – Moment tensors

Landers earthquake

Massonnett et al 1993

5

EPS 122: Lecture 12 – Moment tensors

Landers earthquake

What would surface deformation look like?

And the slip on the fault…

mag 7.3 3-6 m slip

Massonnett et al 1993

EPS 122: Lecture 12 – Moment tensors

First motion within a sphere

In spherical Earth we plot the lower hemisphere

Fault plane solutions

Surface view

6

EPS 122: Lecture 12 – Moment tensors

Indicate fault type Fault plane solutions

…and stress regime

Transform or strike-slip

e.g. San Andreas

Normal

e.g. Mid-Ocean Ridges

Thrust or reverse

e.g. Subduction

EPS 122: Lecture 12 – Moment tensors

Mid-ocean ridges Fault plane solutions

7

EPS 122: Lecture 12 – Moment tensors

Subduction zones Fault plane solutions

ridge

EPS 122: Lecture 12 – Moment tensors

Transform Fault plane solutions

ridge subduction

8

EPS 122: Lecture 12 – Moment tensors

California Fault plane solutions

EPS 122: Lecture 12 – Moment tensors

Forces – double couple

9

EPS 122: Lecture 12 – Moment tensors

Forces

double couple compensated linear vector dipole

isotropic

EPS 122: Lecture 12 – Moment tensors

Long Valley Earthquake Moment tensor inversion

– double-couple solution

10

EPS 122: Lecture 12 – Moment tensors

Moment tensor inversion

Long Valley Earthquake – full solution

EPS 122: Lecture 12 – Moment tensors

JUNCTION explosion – Nevada test site Moment tensor inversion

11

EPS 122: Lecture 12 – Moment tensors

Bardarbunga earthquakes Volcanic earthquakes

EPS 122: Lecture 12 – Moment tensors

Bardarbunga earthquakes

Nettles and Ekström, 1998

Volcanic earthquakes