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The earthquake that lurk Jakarta

The earthquake that lurk JakartaStill attached to the memory Safaruddin, when Jakarta was rocked by an earthquake last year. Wednesday, September 2, 2009, at 14:55 pm, a motorcycle 27 years who usually hung near Wisma Nusantara was shocked, when the earth shook his feet hard.

"Kreeek ... kreeek," the sound was heard from above, so loud in the middle of the last roar of vehicles ply the surrounding Roundabout Hotel Indonesia, which began to condense. Safar look to the sky, tall buildings around it look swayed as if about to collapse on him.

He has not finished digesting what was happening, suddenly the people from the building of Wisma Nusantara ran-ass jump out of the building. "... The earthquake .. Earthquake" Without thinking anymore, Safar jump into the bike. He gas spur top of my voice along Jl Sutan Syahrir, away from the jungle of skyscrapers in the city center.

Not far from there, Sianto Wongjoyo, one manager at Dell Indonesia still 'trapped' in his office located upstairs Menara BCA, Grand Indonesia, Jakarta. Dell's new office a year moved into the building, is situated quite high, namely on Floor 48 of the 57 existing floors.

When the office started to shake, Sianto middle of a meeting. Usually he is not too sensitive to earthquakes. But once that earthquake shocks are big enough to resuscitate. The floor shook, glass-glass office shook, the walls creak. "This time I have to admit, really great guncangannya," Sianto describe.

With alacrity, the security officer to guide the employees gathered in the hall elevator. In my heart, not separated Sianto pray. Waiting anxiously, until the shaking stops. A moment later, all were evacuated out of the building, down the stairs one by one. Distance of 48 floors is enough to make the knee a little pains. "Pretty tired though." Below, thousands of visitors and employees with offices in Grand Indonesia, Plaza Indonesia, Wisma Nusantara, Hotel Nikko, had seen a small huddle.

Do not forget, the Jakarta also still have about 1400 other high buildings. Practical, office activities in many places in Jakarta was paralyzed for a moment. In fact, the epicenter was located in the waters south of Java between Sukabumi and Bandung, or rather in the coordinates of 7.809 degrees south latitude and 107.259 degrees east longitude.

In West Java earthquake measuring 7.3 Richter had claimed at least 79 lives, 21 victims lost, 63,717 houses were severely damaged, with estimated losses of more than Rp 300 billion. While in Jakarta, no casualties and damage. However, several buildings suffered cracks here and there. At least the incident reminds all that Jakarta is not the place safe from the threat of earthquakes.

According to the Deputy of Earth Sciences LIPI Research Professor Hery Harjono, the general area of Jakarta has a younger geological formations. The top layer is generally in the form of soft soil consisting of clay and sandy clay derived from beach sediment and sediment from floods originating from the late Holocene period (about 12 thousand years old.)

Then, below that there is volcanic sediment originating from the late Pleistocene (older than 12 thousand years). Beneath it there marine sediment and non-marine Early Pleistocene age (about 2588 million years). At the very bottom of the rocks of tertiary age (1.8 million - 6.5 million years).

Ir K Engkon Kertapati, researchers at the Center for Geological Survey - Geological Agency, said that Jakarta is above the ground is very weak and vulnerable to earthquake shocks. In geology, Jakarta is divided into two regions: the northern part of Jakarta where the soil is soft soil surface Holocene age, and Jakarta, the southern part of the soil layer is relatively more dense and older (Pleistocene).

When a strong earthquake occurred, north of Jakarta's most vulnerable regions undergo a process of liquefaction aka endless ground due to changes in soil properties of solids into the water because of the earthquake. In addition, the nature of the soil in the northern region that also would make tremor vines that have amplification or magnification of shocks to the buildings on it.

According Engkon, this is what makes Jakarta also felt the earthquake shocks Tasikmalaya whose center is almost two hundred kilometers from Jakarta. At that time, North Jakarta area experienced an earthquake amplification of up to 2 times, while the region south of Jakarta experienced an earthquake of 1.5 times amplification.

Therefore, LIPI Earthquake expert Dr Danny Hilman Tasik Natawidjaya say when the earthquake magnitude larger, ie more than 8SR, then it could be devastating earthquake in Jakarta. "It can be deadly, such as earthquakes in Mexico in 1985," said Danny. At that time, he explained, the source of the earthquake is more than 300 km. However, with the strength of the quake at 8.1 magnitude, the quake leveled the city of Mexico City.

Agency U.S. Geological Survey, USGS, states, at least 9500 people dead, 30 thousand people were injured, more than 100 thousand people vagrancy because their homes were destroyed, 412 buildings collapsed and other buildings damaged in the 3124 Mexico City, with total losses reaching U.S. $ 3 - 4 billion. 60 percent of buildings in other areas such as Ciudad Guzman, Jalisco also destroyed.

Of note Prof. Masyhur Irsyam, civil engineering expert who also heads the team ITB revision Indonesian Earthquake Maps 2010, central Mexico earthquake occurred beneath the Pacific coastline of Mexico. Epicenter is 380 km from Mexico City.

Then why is the epicenter distance is so far still can rip buildings in Mexico City? Apparently the city was standing on a volcanic clay deposits of less than 2,500 years old. It causes tremors in the surface soil can have between 4-5 times amplification, and amplification of earthquakes on buildings could reach 21 times the vibration of the bedrock.

In Jakarta alone, the buildings are built, must meet seismic standards to 8 on the Richter Scale. According to Herman Sarwono, President Director of PT Insani general contractor firm Creative Resources, in Jakarta buildings built after 1989 must meet the requirements of the building structure and performance of building structures in accordance with the Indonesian National Standard 1989.

"In fact, the standardization of building construction in 2002, increased again through SNI 03-1726-2002 far more stringent than the standard ISO 1989," said Herman anymore. However, Masyhur said, there are several steps that need to be passed in a building in Jakarta planning for earthquake resistance.

First, it must be known oscillation or acceleration of the bedrock. This number can be obtained from the Indonesian Earthquake Maps 2010, in which the acceleration of the bedrock (Peak Base Acceleration / PBA), Jakarta is 12:19 g (g = gravity = 981 cm per second squared) to 10 percent likelihood in 50 years and for the period over 475 earthquakes yearly.

After that, keep in mind also the acceleration at the soil surface by calculating the effects of local soil conditions, such as whether the soft soil or hard soil. For Jakarta, the wobble in the rocks could have essentially the same, but the oscillation in the surface soil in North Jakarta and South Jakarta is different, because of differences in the soil.

The latter, need to be accounted sway in the building itself, which is based on the behavior of the building. "By knowing the sway of the building, then it can be calculated magnitude earthquake forces in buildings," said Masyhur.

In fact, until now still do not have a map of Jakarta microzonation earthquake, which can completely provide the information kelabilan soil maps, including a number of acceleration / wobble in the soil surface in each area of Jakarta. "Unfortunately in Jakarta, we do not have," said Masyhur.

In fact, Jakarta staked out by several active fault that is ready to 'supply' tremor which can be up to the Jakarta area. Among these are Cimandiri fault with a magnitude 7.2 earthquake ground motion magnitude and speed of 4 mm per year, Lembang fault with a magnitude 6.5 earthquake ground motion magnitude and speed 1.5 mm per year, and the Sunda Fault with a magnitude 7.2 earthquake SR and velocity of ground motion of 5 mm per year.

Not to mention the rumors of an ancient fault named Fault Ciputat from Ciputat allegedly lying to the City area. Danny Hilman suspect the existence of this fault from the existence of hot springs in the vicinity of the National Archives Building. Although Jakarta is not an active fault is detected, said Danny, history has recorded a large earthquake which devastated Jakarta earthquake that occurred in 1699 and 1852.

However, not all agree with the presence of a fault in Jakarta. "Personally I say no fault Ciputat," said Engkon. Because, Jakarta has no shallow seismic source which is an indication of fault activity. However, Engkon agree with Danny about the events that had rocked the 1699 earthquake in Jakarta.

Earthquake in 1699, said Engkon centered in the south of Mount Gede, who caused the damage to buildings and severe damage around Hanjawar, Peak. Sir Thomas Stamford Raffles was also noted in his History of Java, "1699 Earthquake mud spewing from the bowels of the earth. Lumpur was closed the river flow, causing an unhealthy environment, the more severe."

According to the book Encyclopedy of World Geography, this quake also caused Ciliwung closed by mud slides and trees falling, causing floods in many places. Not until a century later, an earthquake again hit Jakarta in 1780.

A book titled transits of Venus: New Views of the Solar System and Galaxy Mohr noted that the Observatory is located in Batavia, is a successful observatory reported several incidents Transit of Venus (the condition when the Sun Venus and Earth in one line.) However, the observatory was destroyed by an earthquake in 1780.

On August 27, 1883, Jakarta earthquake rocked a large return caused by the eruption of Krakatoa which triggered a tsunami 35 meters and killed 36 thousand people in western Java and south Sumatra. From the historical records before, Jakarta indeed had several times experienced a great earthquake.

What is clear, said Engkon, a threat to the population of Jakarta is a shallow earthquakes originating from South Java ie from the direction of subduction zones (megathrust), such as earthquakes Tasik. The vulnerability of Jakarta will be more severe if these areas densely populated and the buildings no or less attention to aspects of earthquake resistant buildings.

Therefore, Engkon suggested that Jakarta ready before disaster comes, especially in North Jakarta. The reason, in this region stand various critical infrastructure, from ports, import export activities, transportation, tourism areas, trading centers are also relics of history. "Because, however, the earthquake will not kill humans. But, the buildings collapsed was the one who can kill humans, "said Engkon.


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