Skala Gempa Bumi dan Salah Kaprah

2 09 2009
Earthquake

Earthquake

Gempa yang baru saja saya rasakan tadi sangat menghebohkan. Kejadian tersebut berlangsung cepat dan menyebabkan kepanikan massal. Para karyawan di gedung Mitra tempat saya bekerja spontan berhamburan menuju lokasi aman di luar gedung 12 lantai ini.

Lokasi Gempa Tasikmalaya Hari Ini

Lokasi Gempa Tasikmalaya Hari Ini

Saat kepanikan mereda, ada hal yang cukup mengganggu telinga ini. Pers melaporkan bahwa gempa yang terjadi telah diukur sebesar 7.4 skala Richter (SR). Padahal seingat saya yang dulu sempat mengambil mata kuliah SI 3511 – Pengantar Rekayasa Gempa, skala gempa terlalu sering disalah kaprahkan. Skala yang terukur dari gempa tersebut seharusnya dinyatakan dalam Momen magnitude (Mw), dan bukan dalam skala Richter.

Belum lagi pernyataan beberapa pihak yang mengklaim dapat meramalkan gempa susulan, sedangkan dalam sejarah belum pernah ada bidang ilmu yang dapat meramalkan kapan dan dimana terjadinya gempa.

Untuk itu saya mengajak anda untuk membuka mata dalam memahami skala gempa.

Mekanisme Gempa Tektonik

Mekanisme Gempa Tektonik

Terdapat skala ukuran umum dalam pengukuran gempa yaitu Local Magnitude dan Momen Magnitude.

1. Skala Mercalli (Skala Intensitas Kerusakan)

Merupakan skala gempa yang diukur berdasarkan analisis intensitas kerusakan atau dampak yang ditimbulkan oleh gempa itu.

I. Instrumental Not felt by many people unless in favourable conditions.
II. Feeble Felt only by a few people at best, especially on the upper floors of buildings. Delicately suspended objects may swing.
III. Slight Felt quite noticeably by people indoors, especially on the upper floors of buildings. Many do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated.
IV. Moderate Felt indoors by many people, outdoors by few people during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rock noticeably. Dishes and windows rattle alarmingly.
V. Rather Strong Felt outside by most, may not be felt by some outside in non-favourable conditions. Dishes and windows may break and large bells will ring. Vibrations like large train passing close to house.
VI. Strong Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books fall off shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Damage slight.
VII. Very Strong Difficult to stand; furniture broken; damage negligible in building of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. Noticed by people driving motor cars.
VIII. Destructive Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture moved.
IX. Ruinous General panic; damage considerable in specially designed structures, well designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X. Disastrous Some well built wooden structures destroyed; most masonry and frame structures destroyed with foundation. Rails bent.
XI. Very Disastrous Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.
XII. Catastrophic Total damage – Almost everything is destroyed. Lines of sight and level distorted. Objects thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move position.

2. Skala Richter (Skala Local Magnitude)

Merupakan skala logaritmik yang mengukur gempa secara lokal berdasarkan energi yang dilepaskan oleh gempa tersebut.

Richter Magnitudes Description Earthquake Effects Frequency of Occurrence
Less than 2.0 Micro Microearthquakes, not felt. About 8,000 per day
2.0-2.9 Minor Generally not felt, but recorded. About 1,000 per day
3.0-3.9 Often felt, but rarely causes damage. 49,000 per year (est.)
4.0-4.9 Light Noticeable shaking of indoor items, rattling noises. Significant damage unlikely. 6,200 per year (est.)
5.0-5.9 Moderate Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings. 800 per year
6.0-6.9 Strong Can be destructive in areas up to about 160 kilometres (100 mi) across in populated areas. 120 per year
7.0-7.9 Major Can cause serious damage over larger areas. 18 per year
8.0-8.9 Great Can cause serious damage in areas several hundred miles across. 1 per year
9.0-9.9 Devastating in areas several thousand miles across. 1 per 20 years
10.0+ Epic Never recorded; see below for equivalent seismic energy yield. Extremely rare (Unknown)

Nah, setelah membaca tabel intensitas kerusakan skala Richter (SR) di atas sekarang bayangkan jika gempa sebesar 7.4 SR benar-benar terjadi.

Semoga kita tidak salah kaprah lagi.


Aksi

Information

One response

5 10 2009
lusi

yang disiarkan di media adalah skala gempa di hiposentrum yang terletak persis di titik episentrum, dlm hal ini kota tasikmalaya. tentunya skala ini akan mengecil seiring dgn jarak, boleh dikatakan yang dirasakan di jakarta,bekasi hanya berkisar 5 SR.

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