2010 October 25, Kepulauan Mentawai Region, Indonesia, Magnitude
(Mw) - 7.7, Depth = 20.6 km, Plate type: subduction interface plate
boundary between the Australia and Sunda plates, STRIKE=319;DIP= 7;
SLIP= 98, Energy released - 1.4 + 0.4 x 1015 Nm and Moment rate – 8.1 x
1025 dyne.cm/s in 5 seconds.
Since the magnitude of the earthquake is 7.7 on Richter scale, it is a
destructive earthquake (big earthquake). The earthquake occurred between the
Australian and Sunda plates. Since the magnitude is above 7.0 and shallow type of
earthquake, it has generated tsunami.
Shallow earthquakes are between 0 km and 70 km in deep. Intermediate
earthquakes, normally occur at the depth of 70 - 300 km and deep seated
earthquakes occur at a depth of 300 - 700 km. In general, the term "deep-focus
earthquakes" is applied to earthquakes deeper than 70 km.
1. Based on the above definition the earthquake is a Shallow earthquake.
2. Since it is a shallow earthquake there must be more surface wave, which has
devastating potential.
The energy released by this earthquake (1.4 + 0.4 x 1015 Nm) indicates that
amount of energy in the form of radiation should less than that of fracture energy,
since it is a tsunami generic earthquake, the involvement of oceanic plates requires
more energy for rupture process. Due to the involvement of oceanic plates the
earthquake should be a slow earthquake.
Strike
The strike is an angle used to specify the orientation of the fault and
measured clockwise from north. For example, a strike of 0° or 180° indicates a fault
that is oriented in a north-south direction. A strike of 90° or 270° indicates east-west
oriented structure.
For this earthquake from the nodal plane one it is inferred that the value of
strike is 319, which means that almost NE – SW direction.
N
319
E
Dip
Dip is the angle that describes the
measured from Earth's surface, or a
horizontal fault is 0°, and the dip of a
In this case the value of dip
nature.
W
steepness of the fault surface. This angle is
plane parallel to Earth's surface. The dip of a
vertical fault is 90°.
is 7o, which means the fault is shallower in
Slip
Slip is to describe the direction of motion across the fault.
1. If the hanging wall moves to the right, the slip direction is 0° (Left
Lateral Strike – Slip).
2. If it moves up, the slip angle is 90° (Reverse).
3. If it moves to the left, the slip angle is 180° (Right Lateral Strike –
Slip).
4. If it moves down, the slip angle is 270° or -90° (Normal).
In this case the value of slip 98o, indicates that it is thrust fault and very little
right lateral component is involved. Going by predominant factor the fault falls in
the category of thrust fault.
Beach Ball Mechanism
The Moment rate which is 8.1 x 1025 dyne.cm/s in 5 seconds clearly indicates
it is a slow earthquake. It means it should be typical tsunami generic slow
earthquake.
South-western Pakistan region, Magnitude: 7.2, Depth – 68 km, Plate
type: This major earthquake occurred as a result of normal faulting within
the lithosphere of the subducted Arabian plate. (i.e.) it is the collision
between Oceanic and continental plates. STRIKE=78; DIP= 31; SLIP= 60, Energy - 1.4+0.3*1016 Nm and Moment rate - 1.4e+26 dyne.cm/s.
Since the magnitude of the earthquake is 7.2 on Richter scale, it is a
destructive earthquake (big earthquake). The earthquake occurred between the
oceanic and continental plates.
Shallow earthquakes are between 0 km and 70 km in deep. Intermediate
earthquakes, normally occur at the depth of 70 - 300 km and deep seated
earthquakes occur at a depth of 300 - 700 km. In general, the term "deep-focus
earthquakes" is applied to earthquakes deeper than 70 km.
1. Based on the above definition the earthquake is a Shallow earthquake.
2. Since it is a shallow earthquake there must be more surface wave, which has
devastating potential.
The energy released by this earthquake (1.4 + 0.3 x 1016 Nm) indicates that
amount of energy in the form of radiation should less than that of fracture energy,
since due to the involvement of oceanic plates with continental plates requires more
energy for rupture process. Due to the involvement of oceanic plates the earthquake
should be a slow earthquake.
Strike
The strike is an angle used to specify the orientation of the fault and
measured clockwise from north. For example, a strike of 0° or 180° indicates a fault
that is oriented in a north-south direction. A strike of 90° or 270° indicates east-west
oriented structure.
For this earthquake from the nodal plane one it is inferred that the value of
strike is 78, which means that almost north – south direction.
Strike
The strike is an angle used to specify the orientation of the fault and
measured clockwise from north. For example, a strike of 0° or 180° indicates a fault
that is oriented in a north-south direction. A strike of 90° or 270° indicates east-west
oriented structure.
For this earthquake from the nodal plane one it is inferred that the value of
strike is 78, which means that almost W - E direction.
N
E
Dip
78
W
Dip is the angle that describes the steepness of the fault surface. This angle is
measured from Earth's surface, or a plane parallel to Earth's surface. The dip of a
horizontal fault is 0°, and the dip of a vertical fault is 90°.
In this case the value of dip is 31o, which means the fault is lesser steep in
nature.
Slip
1.
2.
3.
4.
Slip is to describe the direction of motion across the fault.
If the hanging wall moves to the right, the slip direction is 0° (Left Lateral
Strike – Slip).
If it moves up, the slip angle is 90° (Reverse).
If it moves to the left, the slip angle is 180° (Right Lateral Strike – Slip).
If it moves down, the slip angle is 270° or -90° (Normal).
In this case the value of slip -60o, means that it 2/3rd of the fault is normal
and 1/3rd of the fault is left lateral strike slip fault. Going by predominant factor
the fault falls in the category of normal fault. Even though the main plate is
subducting beneath the Eurasian plate, at the fault region it is showing divergent
nature.
Beach Ball Representation
The style of faulting is an indicator of rock deformation and reflects the type
of forces pushing or pulling on the region. Near Earth's surface, the orientations of
these forces are usually oriented such that one is vertical and the other two are
horizontal (NS and EW). If the vertical force is the largest, then normal faulting, here
in this case the vertical force is more.
It can be concluded that approximately 1.4e+26 dyne.cm/s was released in
approximately three seconds, which shows the earthquake is a fast earthquake.
Comparison
Kepulauan Mentawai Region
Destructive Earthquake
More shallower
Tsunami generic
More fracture energy released
Slow earthquake
Thrust fault
South Western Pakistan
Destructive earthquake
Less shallow
No tsunami generated
More radiated energy
Fast earthquake
Oblique fault with more
component.
normal