Report: Earthquake and tsunami in the Indian Ocean on 11th April 2012

[Toni Cruzado]

> Report: Earthquake and tsunami in the Indian Ocean on 11th April 2012
>
> On April 11th, 2012, at 02:38 PM local time (April 11th, 2012, at
> 08:38 AM UTC) and 04:43 PM UTC (April 11th, 2012, at 10:43 AM UTC)
> two earthquakes occurred approximately 100 km southwest of the
> southwestern coast of Sumatra, Indonesia, with magnitudes Mw 8.6 and
> Mw 8.2 at depths of approximately 23 km and 16 km, respectively.
> Because of the earthquakes' magnitudes and their relatively shallow
> depths, there was a significant danger of a destructive tsunami.
> Therefore, the subscribers of the Tsunami Alarm System were informed
> of the event immediately after the first earthquake. Approximately
> two hours later, the second (smaller) earthquake caused further
> uncertainty and the situation continued to be monitored.
>
> At around 02:45 PM UTC, when additional data and information on the
> tectonic mechanisms of both earthquakes was available (indicating
> that there was no continuing danger of a destructive tsunami, see
> below) an All-Clear Signal was sent to the subscribers of the
> Tsunami Alarm System.
>
> While a tsunami was caused by the stronger of the two earthquakes,
> according to media reports and sea level readings, the tsunami wave
> height was rarely higher than one meter. Thus the tsunami caused
> relatively little damage.
>
> In many cases given earthquakes of this magnitude and depth much
> bigger tsunamis would be generated. However the specific tectonic
> mechanism that caused both these earthquakes involved the plates
> moving mainly horizontally to each other. Strong tsunamis are
> typically only generated when the sea floor and the water above it
> move vertically. While horizontal plate movement is generally
> nothing unusual, it normally causes much weaker earthquakes. This
> earthquake was in fact the strongest earthquake ever recorded that
> didn't cause any major damage, either by itself or by secondary
> effects like a tsunami.
>
>
> Geological backround:
> The M8.6 April 11, 2012 earthquake off the west coast of northern
> Sumatra, Indonesia, occurred as a result of strike-slip faulting
> within the oceanic lithosphere of the Indo-Australia plate. The
> quake was located approximately 100 km to the southwest of the major
> subduction zone that defines the plate boundary between the Indo-
> Australia and Sunda plates offshore Sumatra. At this location, the
> Indo-Australia plate moves north-northeast with respect to the Sunda
> plate at a velocity of approximately 52 mm/yr.
> Large strike-slip earthquakes, while rare, are not unprecedented in
> this region of the Indo-Australian plate. Since the massive M 9.1
> earthquake that ruptured a 1300 km long segment of the Sumatran
> megathrust plate boundary in December of 2004, three large strike-
> slip events have occurred within 50 km of the April 11, 2012 even.
> These earthquakes occurred on April 19 2006 (Mw6.2), October 4 2007
> (Mw6.2) and January 10, 2012 (Mw7.2). In all three cases, the style
> of faulting was similar. These events align approximately with
> fabric of the sea floor in the diffuse boundary zone between the
> Indian and Australian plates.
>
> Best Regards
> Your Tsunami Institute Team
>
> Sources:
> Tsunami Institute Tuebingen, Germany
> United States Geological Service (2012): Earthquake Summary. 2012
> April 12th
>

[S Cruzado]

"This earthquake was in fact the strongest earthquake ever recorded that didn't cause any major damage, either by itself or by secondary effects like a tsunami."

Sandra Cruzado

Sent to the addressee from my WHITE iPhone.

El 13/04/2012, a las 22:40, Toni Cruzado <toni.c...@gmail.com> escribió:

Report: Earthquake and tsunami in the Indian Ocean on 11th April 2012

On April 11th, 2012, at 02:38 PM local time (April 11th, 2012, at 08:38 AM UTC) and 04:43 PM UTC (April 11th, 2012, at 10:43 AM UTC) two earthquakes occurred approximately 100 km southwest of the southwestern coast of Sumatra, Indonesia, with magnitudes Mw 8.6 and Mw 8.2 at  depths of approximately 23 km and 16 km, respectively. Because of the earthquakes' magnitudes and their relatively shallow depths, there was a significant danger of a destructive tsunami. Therefore, the subscribers of the Tsunami Alarm System were informed of the event immediately after the first earthquake. Approximately two hours later, the second (smaller) earthquake caused further uncertainty and the situation continued to be monitored.

At around 02:45 PM UTC, when additional data and information on the tectonic mechanisms of both earthquakes was available (indicating that there was no continuing danger of a destructive tsunami, see below) an All-Clear Signal was sent to the subscribers of the Tsunami Alarm System.

While a tsunami was caused by the stronger of the two earthquakes, according to media reports and sea level readings, the tsunami wave height was rarely higher than one meter. Thus the tsunami caused relatively little damage.

In many cases given earthquakes of this magnitude and depth much bigger tsunamis would be generated. However the specific tectonic mechanism that caused both these earthquakes involved the plates moving mainly horizontally to each other. Strong tsunamis are typically only generated when the sea floor and the water above it move vertically. While horizontal plate movement is generally nothing unusual, it normally causes much weaker earthquakes. This earthquake was in fact the strongest earthquake ever recorded that didn't cause any major damage, either by itself or by secondary effects like a tsunami.

Geological backround:

The M8.6 April 11, 2012 earthquake off the west coast of northern Sumatra, Indonesia, occurred as a result of strike-slip faulting within the oceanic lithosphere of the Indo-Australia plate. The quake was located approximately 100 km to the southwest of the major subduction zone that defines the plate boundary between the Indo-Australia and Sunda plates offshore Sumatra. At this location, the Indo-Australia plate moves north-northeast with respect to the Sunda plate at a velocity of approximately 52 mm/yr.

Large strike-slip earthquakes, while rare, are not unprecedented in this region of the Indo-Australian plate. Since the massive M 9.1 earthquake that ruptured a 1300 km long segment of the Sumatran megathrust plate boundary in December of 2004, three large strike-slip events have occurred within 50 km of the April 11, 2012 even. These earthquakes occurred on April 19 2006 (Mw6.2), October 4 2007 (Mw6.2) and January 10, 2012 (Mw7.2). In all three cases, the style of faulting was similar. These events align approximately with fabric of the sea floor in the diffuse boundary zone between the Indian and Australian plates.

Best Regards

Your Tsunami Institute Team

Sources:

Tsunami Institute Tuebingen, Germany

United States Geological Service (2012): Earthquake Summary. 2012 April 12th

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