Tuesday, July 12, 2011

6.6 Quake Rocks PHL Negros Islands

Negros Philippine Map
Early this morning, I was awakened by a shake which lasted for about 30 seconds as I noticed it. I elbowed my son, who slept beside me to wake him up and asked him if he felt the tremor, But he never moved an inch from his sound sleep. Anyway I grabbed my mobile phone and checked the time. It's 4:30 a.m. I went back to sleep again until I woke up at 6:33 a.m.

I went downstairs and checked the Internet. The report said, "Philippines was rocked by 6.6 quake intensity." It's was terrible! I quipped.

While updating this blog as of 4:57 p.m. this day, I felt a quake of 10-second duration with 5 seconds gap. I called up my grandson, Bryan and pointed to him my PC which is shaking. He held the PC's monitor screen to stop from rocking.

How does earthquake magnitude and earthquake intensity differ from each other?

According to Andrew Alden, the earthquake magnitude indicates the energy a quake expends, while intensity denotes how strongly an earthquake affects a specific place. Therefore the magnitude measures how big an earthquake is and intensity measures how bad it is.

He added that the modified Mercalli Intensity Scale of 1931 is the basis for the U. S. evaluation of seismic intensity.

I also visited the PHILIPPINE INSTITUTE OF VOLCANOLOGY AND SEISMOLOGY (PHIVOCS) to determine how strong this morning earthquake that hits the Philippines, particularly the Negros Island and some parts of Mindanao.

Using the intensity scale, the PHIVOCS came out with EARTHQUAKE INFORMATION NO. 2 that said: Intensity V - Sipalay City, Negros Occidental; Intensity IV - Oton, Iloilo; Dumaguete City; Binalbagan and Hinigaran; Sibulan; Intensity III - San Jose, Antique; Dapitan City; Talisay City, Negros Occ.; Intensity II - Cagayan de Oro City; Dipolog City; La Carlota City, Negros Occ.; and Intensity I - La Castellana, Negros Occ.

The origin of the earthquake is tectonic, the PHIVOCS reported.

Its epicenter was located 78 kilometers southwest of Cauayan, Negros Occidental. There was no damage reported.

What do these scales mean? The Mercalli scale has 12 divisions, using Roman numerals from I to XII:

I. Not felt except by a very few under especially favorable circumstances.

II. Felt only by a few persons at rest, especially upper floors of buildings. Delicately suspended objects may swing.

III. Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration like passing truck. Duration estimated.

IV. During the day felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, and doors disturbed. walls make creaking sound. Sensation like heavy truck striking building. Standing motorcars rock noticeably.

V. Felt by nearly everyone; many awakened. Some dishes, windows, broken; a few instances of cracked plaster; unstable objects overturned. Disturbance of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop.

VI. Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight.

VII. Everybody runs outdoors. Damage negligible in buildings of good design and construction slight to moderate in well built ordinary structures; considerable in poorly built or badly
designed structures. Some chimneys broken. Noticed by persons driving motor cars.

VIII. Damage slight in specially designed structures; considerable in ordinary substantial buildings, with partial collapse great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving motor cars disturbed.

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed over banks.

XI. Few, if any (masonry), structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.

XII. Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air.

In Richter's magnitude scale:

Magnitude Description Earthquake effects Frequency of occurrence
Less than 2.0 Micro Micro earthquakes, 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 kilometres across. 1 per year
9.0–9.9 Devastating in areas several thousand kilometres across. 1 per 20 years
10.0+ Massive Never recorded, widespread devastation across very large areas; see below for equivalent seismic energy yield. Extremely rare (Unknown)
(Based on U.S. Geological Survey documents.) Updates:

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