Gert
29-12-2004, 08:19
Voor deze keer niet een ototje ...
Gevonden op het net .
Ongeveer 50 000 doden en we hadden het over oliebollen en vanaf morgen over vuurwerk ...
Alles is relatief kennelijk .
Of was het toch stiekem een atoombomproef ? :(
As the total devastation of low-lying coastal nations surrounding the Indian
Ocean becomes more and more apparent, I felt I should send a message to you
all and try to provide some context for this extreme event.
Many of you, I'm sure, have heard various reports of what happened, and what
caused the tsunamis to land at the various places they did. Although truly
tragic in nature, this is a case of a global event that actually does fall
within what I would consider my area of expertise: Marine Geology. So, I
figured, what good is it having a geologist as a friend/son/brother (in-law)
if you don't know anything more about big earthquakes than anybody else?
Plus, it seemed like a good opportunity to send a message to many of you I
don't talk to often enough.
The comments below are intended to provide some global and historical
context for the geological events of Dec 26, 2004. I am happy to continue
to answer questions if you care to email me, and I would strongly encourage
you all to research other large events (e.g., the magnitude 9.5 1960 Chile
earthquake, the magnitude 9.2 1964 Prince William Sound, Alaska, earthquake,
the magnitude 9.1 1957 Andreanof Islands, Alaska, earthquake, and the
magnitude 9.0 1952 Kamchatka earthquake) that produced tsunamis (although
none, to date, with the extreme human toll of the current event).
What exactly happened in northern Sumatra?
The earthquake in Sumatra was one of a class of earthquakes referred to as
"great" earthquakes or "megathrust" earthquakes.
The earthquake occurred on the interface of the India and Burma plates and
was caused by the release of stresses that develop as the India plate
subducts beneath the overriding Burma plate. The earthquake itself was 9.0
on the Richter scale, and caused reasonable damage. Due to its distance
from populated areas, and the overall depth of the earthquake (~10km) the
earthquake damage was largely confined to Indonesia.
In the region of the earthquake, the India plate moves toward the northeast
at a rate of about 6 cm/year relative to the Burma plate. Now, this is an
average rate of motion, and sections of the plates can become "frozen",
allowing stresses to build up until the energy is released over a short
period of time, resulting in a great earthquake.
The factors that are significant in creating an earthquake of this magnitude
are:
1 - The area of the zone of rupture.
In the case of the Sumatra earthquake, estimates are that the slip was
accomodated along ~1200km of the fault, and the width of the ruptured
section has been estimated at ~100km. This gives us a total ruptured area
of 1200km x 100km = 120,000km^2
2 - The average rate of motion.
The average slip along the fault line has been estimated at 15m - that is
the amount of motion that had to be accomodated by the "un-freezing" of this
zone of fault. We can back calculate to the annual average rate of motion
(6 cm/y) and estimate that this section of fault has been frozen for ~250
years. A good estimate of how frequently we can expect events like this.
3 - The strength of the rock.
This is referred to as the "Shear Modulus" and can be accurately estimated
for different types of rocks around the world. For those of you who are
curious, in oceanic crust it is ~ 6 x 10^10 Pa.
Using these three parameters, earthquake Moments are calculated, a measure
of energy in Newton-meters, and from that, the Magnitude is calculated.
Earthquake magnitudes are calculated on logarithmic scales, so an earthquake
of magnitude 3 is ~30x stronger than an earthquak of magnitude 2. The
Sumatra earthquake had a magnitude of 9.0.
Richter TNT for Seismic Example
Magnitude Energy Yield (approximate)
-1.5 6 ounces Breaking a rock on a lab table
1.0 30 pounds Large Blast at a Construction Site
1.5 320 pounds
2.0 1 ton Large Quarry or Mine Blast
2.5 4.6 tons
3.0 29 tons
3.5 73 tons
4.0 1,000 tons Small Nuclear Weapon
4.5 5,100 tons Average Tornado (total energy)
5.0 32,000 tons
5.5 80,000 tons Little Skull Mtn., NV Quake, 1992
6.0 1 million tons Double Spring Flat, NV Quake, 1994
6.5 5 million tons Northridge, CA Quake, 1994
7.0 32 million tons Hyogo-Ken Nanbu, Japan Quake, 1995; Largest
Thermonuclear Weapon
7.5 160 million tons Landers, CA Quake, 1992
8.0 1 billion tons San Francisco, CA Quake, 1906
8.5 5 billion tons Anchorage, AK Quake, 1964
9.0 32 billion tons Chilean Quake, 1960
10.0 1 trillion tons (San-Andreas type fault circling Earth)
12.0 160 trillion tons (Fault Earth in half through center,
OR Earth's daily receipt of solar energy)
160 trillion tons of dynamite is a frightening yield of energy. Consider,
however, that the Earth receives that amount in sunlight every day.
Some commentary:
There exists a similar plate boundary in the Cascadian subduction zone,
which marks the boundary between the Juan de Fuca and North American plates. It stretches from the Mendocino triple junction to the Nootka transform fault off the northwest coast of the United States. Here, the rate of plate convergence is ~4cm/y and the locked zone is ~1000km long and ~50km wide.
There is good evidence tha the last great subduction zone earthquake
occurred 300 years ago. If this plate moved similarly to the Indian plate,
it would generate an earthquake of similar proportions.
Does this mean that 50,000 Americans, and Japanese would die if we had such an earthquake? Thankfully, no.
These earthquakes happen infrequently on a human timescale, and in the Indian Ocean, there is pretty much only one fault that can generate an earthquake of this magnitude, so there has beenlittle attention on earthquake and tsunami related hazards in the Indian ocean. In contrast, there are many faults in the Pacific that can generate megathrust earthquakes, and as a consequence a rudimentary tsunami monitoring system exists in the Pacific.
Tsunamis are generated primarily in two fashions. One, the offset of the seafloor caused by an earthquake (such as the ~15m offset caused by the Sumatra earthquake). Two, offset of the seafloor created by submarine landsliding, often initiated by earthquakes - not necessarily large ones, either.
In the deep ocean, tsunamis are nearly undetectable - you would never notice one go by if you were in a boat, but precise instruments can identify the 1-2" increase in sea level that is associated with a tsunami in the deep ocean by measuring the pressure at the bottom of the water column.
The tsunami monitoring program uses these measurements to identify potential tsunamis, although the sparse data is currently insufficient to offer truly predictive capabilities concerning the ultimate path of tsunamis. Tsunami research is a relatively young field. While we understand fairly well the generative process, it is still difficult to predict the behavior of any
particular tsunami.
One last note, if you are at the beach and the water unexpectedly
disappears, take this event seriously and run to higher ground. As we haveall learned in this painful experience, these geological disasters do not
only exist in the realm of Hollywood and Geology 101 classes. They are
real, and natural, and recurring. In most cases, reaching an elevation of
25 ft above sea level will be sufficient to survive a tsunami (although you
may still get wet).
Most hazard managers suggest running, rather than
getting into your car as roads may be damaged by the generative earthquakes, or clogged by other traffic
Gevonden op het net .
Ongeveer 50 000 doden en we hadden het over oliebollen en vanaf morgen over vuurwerk ...
Alles is relatief kennelijk .
Of was het toch stiekem een atoombomproef ? :(
As the total devastation of low-lying coastal nations surrounding the Indian
Ocean becomes more and more apparent, I felt I should send a message to you
all and try to provide some context for this extreme event.
Many of you, I'm sure, have heard various reports of what happened, and what
caused the tsunamis to land at the various places they did. Although truly
tragic in nature, this is a case of a global event that actually does fall
within what I would consider my area of expertise: Marine Geology. So, I
figured, what good is it having a geologist as a friend/son/brother (in-law)
if you don't know anything more about big earthquakes than anybody else?
Plus, it seemed like a good opportunity to send a message to many of you I
don't talk to often enough.
The comments below are intended to provide some global and historical
context for the geological events of Dec 26, 2004. I am happy to continue
to answer questions if you care to email me, and I would strongly encourage
you all to research other large events (e.g., the magnitude 9.5 1960 Chile
earthquake, the magnitude 9.2 1964 Prince William Sound, Alaska, earthquake,
the magnitude 9.1 1957 Andreanof Islands, Alaska, earthquake, and the
magnitude 9.0 1952 Kamchatka earthquake) that produced tsunamis (although
none, to date, with the extreme human toll of the current event).
What exactly happened in northern Sumatra?
The earthquake in Sumatra was one of a class of earthquakes referred to as
"great" earthquakes or "megathrust" earthquakes.
The earthquake occurred on the interface of the India and Burma plates and
was caused by the release of stresses that develop as the India plate
subducts beneath the overriding Burma plate. The earthquake itself was 9.0
on the Richter scale, and caused reasonable damage. Due to its distance
from populated areas, and the overall depth of the earthquake (~10km) the
earthquake damage was largely confined to Indonesia.
In the region of the earthquake, the India plate moves toward the northeast
at a rate of about 6 cm/year relative to the Burma plate. Now, this is an
average rate of motion, and sections of the plates can become "frozen",
allowing stresses to build up until the energy is released over a short
period of time, resulting in a great earthquake.
The factors that are significant in creating an earthquake of this magnitude
are:
1 - The area of the zone of rupture.
In the case of the Sumatra earthquake, estimates are that the slip was
accomodated along ~1200km of the fault, and the width of the ruptured
section has been estimated at ~100km. This gives us a total ruptured area
of 1200km x 100km = 120,000km^2
2 - The average rate of motion.
The average slip along the fault line has been estimated at 15m - that is
the amount of motion that had to be accomodated by the "un-freezing" of this
zone of fault. We can back calculate to the annual average rate of motion
(6 cm/y) and estimate that this section of fault has been frozen for ~250
years. A good estimate of how frequently we can expect events like this.
3 - The strength of the rock.
This is referred to as the "Shear Modulus" and can be accurately estimated
for different types of rocks around the world. For those of you who are
curious, in oceanic crust it is ~ 6 x 10^10 Pa.
Using these three parameters, earthquake Moments are calculated, a measure
of energy in Newton-meters, and from that, the Magnitude is calculated.
Earthquake magnitudes are calculated on logarithmic scales, so an earthquake
of magnitude 3 is ~30x stronger than an earthquak of magnitude 2. The
Sumatra earthquake had a magnitude of 9.0.
Richter TNT for Seismic Example
Magnitude Energy Yield (approximate)
-1.5 6 ounces Breaking a rock on a lab table
1.0 30 pounds Large Blast at a Construction Site
1.5 320 pounds
2.0 1 ton Large Quarry or Mine Blast
2.5 4.6 tons
3.0 29 tons
3.5 73 tons
4.0 1,000 tons Small Nuclear Weapon
4.5 5,100 tons Average Tornado (total energy)
5.0 32,000 tons
5.5 80,000 tons Little Skull Mtn., NV Quake, 1992
6.0 1 million tons Double Spring Flat, NV Quake, 1994
6.5 5 million tons Northridge, CA Quake, 1994
7.0 32 million tons Hyogo-Ken Nanbu, Japan Quake, 1995; Largest
Thermonuclear Weapon
7.5 160 million tons Landers, CA Quake, 1992
8.0 1 billion tons San Francisco, CA Quake, 1906
8.5 5 billion tons Anchorage, AK Quake, 1964
9.0 32 billion tons Chilean Quake, 1960
10.0 1 trillion tons (San-Andreas type fault circling Earth)
12.0 160 trillion tons (Fault Earth in half through center,
OR Earth's daily receipt of solar energy)
160 trillion tons of dynamite is a frightening yield of energy. Consider,
however, that the Earth receives that amount in sunlight every day.
Some commentary:
There exists a similar plate boundary in the Cascadian subduction zone,
which marks the boundary between the Juan de Fuca and North American plates. It stretches from the Mendocino triple junction to the Nootka transform fault off the northwest coast of the United States. Here, the rate of plate convergence is ~4cm/y and the locked zone is ~1000km long and ~50km wide.
There is good evidence tha the last great subduction zone earthquake
occurred 300 years ago. If this plate moved similarly to the Indian plate,
it would generate an earthquake of similar proportions.
Does this mean that 50,000 Americans, and Japanese would die if we had such an earthquake? Thankfully, no.
These earthquakes happen infrequently on a human timescale, and in the Indian Ocean, there is pretty much only one fault that can generate an earthquake of this magnitude, so there has beenlittle attention on earthquake and tsunami related hazards in the Indian ocean. In contrast, there are many faults in the Pacific that can generate megathrust earthquakes, and as a consequence a rudimentary tsunami monitoring system exists in the Pacific.
Tsunamis are generated primarily in two fashions. One, the offset of the seafloor caused by an earthquake (such as the ~15m offset caused by the Sumatra earthquake). Two, offset of the seafloor created by submarine landsliding, often initiated by earthquakes - not necessarily large ones, either.
In the deep ocean, tsunamis are nearly undetectable - you would never notice one go by if you were in a boat, but precise instruments can identify the 1-2" increase in sea level that is associated with a tsunami in the deep ocean by measuring the pressure at the bottom of the water column.
The tsunami monitoring program uses these measurements to identify potential tsunamis, although the sparse data is currently insufficient to offer truly predictive capabilities concerning the ultimate path of tsunamis. Tsunami research is a relatively young field. While we understand fairly well the generative process, it is still difficult to predict the behavior of any
particular tsunami.
One last note, if you are at the beach and the water unexpectedly
disappears, take this event seriously and run to higher ground. As we haveall learned in this painful experience, these geological disasters do not
only exist in the realm of Hollywood and Geology 101 classes. They are
real, and natural, and recurring. In most cases, reaching an elevation of
25 ft above sea level will be sufficient to survive a tsunami (although you
may still get wet).
Most hazard managers suggest running, rather than
getting into your car as roads may be damaged by the generative earthquakes, or clogged by other traffic