Phivolcs explains aftershocks-A A +A
Thursday, October 17, 2013
DO AFTERSHOCKS cause tsunamis? Does an earthquake cause movement of faults in Cebu?
These are among the questions that scared residents of Cebu, Bohol and other affected areas in the Visayas are thinking.
Philippine Institute of Volcanology and Seismology (Phivolcs) Deputy Director Bart Bautista says that while Bohol, Cebu and other affected areas in the Visayas may continue to feel aftershocks for about a week or more, these tremors will not be as strong as the 7.2-magnitude earthquake last Tuesday.
In their experience, the aftershocks are always weaker,” he said, adding that this was the case in the February 1990 quake, which was caused by the East Bohol Fault.
The Phivolcs website said that the 1990 tremor had a magnitude of 6.8 and its epicenter was 17 kms. east of Tagbilaran.
The 1990 and last Tuesday’s quakes were tectonic in origin.
The earth’s crust is made of tectonic plates that move or sometimes slide and brush against each other. The area between two plates is called a transform boundary.
Movement in this area creates a fault line, or a break in the earth’s crust.
Most, but not all, earthquakes occur along the transform boundary fault lines, according to the US Geological Survey (USGS).
Aftershocks occur after earthquakes, while the plates are still adjusting to new positions. The USGS states that the number and duration of aftershocks depend on the magnitude of the earthquake or mainshock.
Bautista said the aftershocks of last Tuesday’s earthquake are not strong enough to cause tsunamis.
“Normally, it would take a 7.5 (magnitude) and higher earthquake to produce a tsunami,” he told Sun.Star Cebu. “Here, it is nearly impossible. But even if there will be (a tsunami), it would not be destructive.”
A tsunami, according to USGS, is "a sea wave of local or distant origin that results from large-scale seafloor displacements associated with large earthquakes, major submarine slides, or exploding volcanic islands."
Phivolcs Director Renato Solidum Jr. earlier said that the even quake on Tuesday could not cause a tsunami since its origin is inland.
Asked if last Tuesday’s earthquake and its aftershocks, which were still felt in Cebu last night, would cause movement in Cebu’s faults, Bautista said Phivolcs still could not say.
A Phivolcs map on active faults in the country, shows our faults in Cebu.
Bautista said it is still unknown if these faults are active.
A fault is considered active if there is movement in the area in the last 10,000 years.
While it remains impossible to predict them, earthquakes alone do not kill people. Deaths are caused by buildings and other structures that collapse, falling debris and, as recently experienced in Pinamungajan, by people who panic and cause a stampede.
Bautista said building owners should assess their properties for damage. If there are many cracks or fractures on the walls, beams and foundations of the building, the structure is no longer safe. (Sunnex)
Published in the Sun.Star Cebu newspaper on October 17, 2013.