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Powerful Bohol Earthquake October 15 2013: Way to Explain It
« on: October 23, 2013, 06:15:50 AM »
by Jes Tirol
Bohol Chronicle

Proem
October 15, 2013 was declared a National Holiday to commemorate the Eid al Adha or The Feast of Sacrifice of the Muslims.  As it turned out, it became a real “Day of Sacrifice” for Boholanos, Cebuanos, and other people of the Central Visayas because at 8:12 A.M. a destructive earthquake of magnitude 7.2 struck.
As an Earthquake Engineer (I’m listed as one of the earthquake engineers of the world by the Earthquake Engineering Research Institute (EERI) of California, U.S.A.), a certified Structural Engineering and Geotechnical Engineering expert by the Phil Institute of Civil Engineers (PICE), it behooves upon me to share my analysis of the earthquake that occurred.
 
Initial Announcement
Initially the PHIVOLCS National Office said that the earthquake was Magnitude 7.2 with epicenter at a place 2 km and southeast 16o from Carmen, Bohol.  The depth of focus (source) was 33 km below the ground.

An untrained person would accept the announcement and it was immediately picked up by media.

Actually the announcement was just an approximation.  The location could not be plotted.  Two (2) kilometers from Carmen, Bohol is still within Carmen, Bohol.  So where was it?  Where will we start the measure?  The depth of focus or origin of 33 km below the ground is what is called as “average earth depth”.  It actually means “I do not know”.

I went to the Manga, Tagbilaran PHIVOLCS Station and Engr. Nolan Evangelista, the personnel in-charge gave me the coordinates: Latitude 9.8o north and Longitude 124.2o east as the initial location.  No actual depth of focus was yet available.

The Intensity Scale (measure of damage at the place) was given as Intensity 7 for Tagbilaran City and Cebu City.  The other places were not yet reported.  Now, it can be estimated.  In the Rossi-Forel Intensity Scale from zero (0) to ten (10) Dauis, Baclayon, Loboc and Carmen would be Intensity 7.  Maribojoc, Antequera, and Tubigon would be Intensity 8.  Loon, Bohol would be a higher part of Intensity 8 and could already qualify as Intensity 9.  All other towns to the north could be between Intensity 6 or 7.

As of 6:00 P.M. Oct. 17, 2013 the website of Phivolcs revised the epicenter as 6 km southwest of Sagbayan, Bohol.  The depth of focus is now given as 12 km.  I think this would still be revised because the given coordinates (latitude & longitude) would not fit the cited location.  (Note: The USGS gives it as Lat = 9.86o north and Long = 124o east.)

Magnitude Scale is the measure of the energy released by the earthquake.  Magnitude M7.2 is already very strong.  The national newspapers said that it was 32 times stronger than the Hirosima Atomic Bomb.  I made a computation using the Gutenber Richter Formula and my result was 63 times stronger than the Hirosima A-bomb.  The magnitude scale is used by engineers because it can be used to design buildings.

Intensity Scale is the measure of the damage at the site.  Intensity 7 is already described as “destructive”.  Well-built or “engineered” structures are already destroyed.

This scale is preferred by our social service agencies because they can gage how to respond to the scale of damage.

Different Behavior
The depth from the ground surface down to 60 km is classified as shallow earthquake.  The depth of the Oct. 15 earthquake is officially listed as 12 km down.

In shallow earthquakes the vertical forces is usually stronger than the lateral or side to side sway of the building.  It means that the building or structure will be lifted upward and slammed down (Bis: ibúndak).  It is the reason why you can hardly see an “X” from damage of the walls caused by the lateral forces.  Instead, you can see that buildings and hills are “crumpled” to the ground.  Vertical objects, like columns, post, and the like will exhibit an “elephant’s foot” or bulging at the base.

The effect of the Feb. 8, 1990 earthquake in Bohol (Magnitude 6.8) also had a strong vertical force than the lateral force.  I presented a scientific paper about that earthquake in an international conference of earthquake engineers in Taipei, Taiwan on Aug. 12, 1998.  It was the first time that someone proved that the vertical forces can be stronger than the lateral forces.  Since there was no English term for the vertical effects I used the Sugboanon word “witik”. It was my paper that caused the Americans to change their building codes to include the vertical forces by year 2000.  It is now copied by the Philippines Building Code.  So far as I know, it is my American friend Dr. Sami Masra who is lecturing about the witik.

Since 1998 I have been writing and lecturing about the need to strengthen the Church buildings in Bohol.  Unfortunately the funding was used only to beatify and “restore” the churches.  All that money is now lost because the Baclayon Church and Loboc Church are now destroyed.

It is just unfortunate that while the Americans believed in my theory and changed their building code, nobody in Bohol would believe that a whole church building could be lifted and slammed down to destruction.  What I have been warning since 1998 has now come to pass.

Source of the Earthquake
The PHIVOLCS say that the source of the earthquake was the East-Bohol Fault.  Sometime in year 2001 or 2002, this particular fault exhibited a long crack in the town of Loboc, Bohol.  My warning was just dismissed by the government agencies because according to them it was just “land subsidence”.  I disagreed to this observation because a land subsidence would not cross the Loboc River, which the crack did.  Unfortunately again I was just too far ahead by ten (10) years.

As observed during that time I said that the fault line is “trending” or sloping towards the northwest or generally towards Cebu.  The “earth block” is pushing or compressing the rocks towards the northwest.

Recently an unofficial land spreading measurement was done using a GPS with one millimeter accuracy.  (Note: My source of information does not want to be revealed because it was an unauthorized job.)  The measurement was done between Kamambogan, Ubay, Bohol and Poblacion Alicia, Bohol.  The measurement from January 2012 to January 2013 yields a land spreading of 2.5 cm. So when the East Bohol fault cracked 10 years ago until the present we can assume a total spreading of 25 cm or 10 inches.  Unfortunately the vertical rise of the ground was not measured.  Nonetheless it can be shown that there was already land spreading that was already capable of pushing and activating the unknown fault line.

The New Crack
I have read the La Seismologia En Filipinas by Fr. Miguel Saderra Maso, S.J., published by the Observatorio De Manila (Manila Observatory) published in Manila in 1895.  This book has a record of earthquakes in the Philippines from 1599 to 1886.  This book has two records of earthquakes in Bohol that was similar to the Oct. 15 earthquake.  The June 25, 1876 earthquake is described only as “Ligero = Light” equivalent to Intensity 4.  The July 23, 1884 earthquake is described as “Violento = Violent”, equivalent to Intensity 8.

The Southeast Asia Association of Seismology and Earthquake Engineering publication has a record of a 1936 earthquake with epicenter at Carmen, Bohol.  This particular earthquake was described as “strange”.  It was a strong earthquake that exhibited a strong up and down movement which was strange during that time because everybody believed only in the lateral movement of earthquakes.  It was proposed that it was caused by the collapse of a large sinkhole.

The recent Oct. 15, 2013 earthquake was also strange because it was the first time that a fault line has exhibited itself from Loon, Bohol towards Central Bohol.  We can call it now as the “Central Bohol Fault Line”.  This fault line is a “thrust fault” or the East Bohol Fault dived under this fault line.  I have seen the Strong Motion Acceleration (SMA) graph of the earthquake for the primary and secondary waves and I agree that it is a thrust fault.

My theory is, this fault line is somehow connected to the Tayasan, Negros Oriental earthquake that occurred on Feb. 6, 2012.  The Tayasan earthquake epicenter has latitude 9.82 north and the new crack has also latitude of 9.8 to 9.84 north.  They are of the same level.  This is another statement that will put me in a controversy.

My analysis is based on the fact that if you will draw a line from the Tayasan Earthquake epicenter towards Carmen, Bohol, the location of the aftershocks occurred either to the right or left of the line and moving backward from Carmen to Tayasan.

The aftershock that occurred on Oct. 17, 2013 at 1:38 P.M. was listed as Magnitude 4.0 with coordinates Lat = 9.73o north and Long = 123.65o east.  It was described as 10 km deep and located 25 km Northwest of Tagbilaran City.  It was located near the town of Bolhoon, Cebu or a few kilometers from Lighthouse No. 1 of Tagbilaran.  On the opposite side of the town of Bolhoon in the Island of Cebu was the epicenter of the Tayasan Earthquake.
Herewith is the graph of the aftershocks as shown in the Earthquake-Report.com as updated on Oct. 17, 2013 at 11:18 A.M. by Armand Vervaeck and James Daniell:
 
Location of Aftershocks of 10/15/13 Bohol Eqk.
 
Observed Behaviors
In Earthquake Engineering, it is expected that the ground near the site (about 50 Km radius) will produce a stronger vertical force than the horizontal force.  The ground will practically rise and slam down bringing with it the buildings, houses, bridges, roads, mountains, etc.

When the strong primary or vertical wave reached the surface most of it will be converted to sound waves causing a low rumbling sound.  It will be followed soon by the Secondary waves that soon convert to the Love and Rayleigh surface waves.  The Love wave behaves similar to a river flowing along the ground surface that slams into an obstruction.  The Rayleigh wave is like the summersault of an acrobat.  If it hits a shaken structure it will cause the structure to “wiggle”.  It is the usual cause of “bridge collapse” because while the slab is lifted the columns will wiggle and when the slab “returns” the column may not be in the proper position and the slab will continue down for missing the column.

A Magnitude 7.2 earthquake is already capable of disturbing a large body of water.  For a long and sloping beach the effect will be a “tsunami”.  For a “short (towards the deep sea)” beach the effect will be only a “sea surge”.  In the shores from Tagbilaran City to Tubigon, there was actually a sea surge but not a tsunami.  (Note: The Facebook account of Dr. Mutya Kismet Tirol-Macuno has a picture of Caingget Beach during the top of the surge and after the surge.  The seawater rose to the level of the second step of the concrete stairs.)

The earthquake could disturb the methane and sulfur deposits of the sea that could kill fishes.  This effect is reported in Loon, Bohol.

The similar 1886 earthquake at the shorelines from Dimiao to Anda produced a “red-tide” effect causing the seawater to become red.  I just hope that the recent earthquake will not produce a similar “red-tide” effect.

Sinkholes and sand boils are common effects of strong earthquakes.  Drying up of riverbeds and water-wells can be also observed.  Land flows or actual transferring of huge tract of lands are also expected.  Rising and subsidence of the land surface is also expected.

Comment
I do not blame anyone for not listening to my warnings.  My warnings were too far ahead.  I begun my warnings in 1998 and keep on repeating until I became like a broken phonograph record.  When I made a warning of the return of the 1906 Tagbilaran earthquake for the 100 years cycle (for 2006), the  Late Vice Mayor Rizalino Israel challenged me to stand at the shorelines of Taloto to see the “tsunami”.  I could not explain why the earthquake actually returned seven (7) years late.

Be that as it may, I really commiserate to the victims of the present tragedy.  Why it actually happened on Eid Al Adha or The Feast of Sacrifice, only God can tell.  Millions of people in the Central Visayas have to sacrifice.
Romans 10:9-10
"If you declare with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved. For it is with your heart that you believe and are justified, and it is with your mouth that you profess your faith and are saved."

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