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Claim: After the magnitude 7.0 earthquake in Northern Luzon on Wednesday, July 27, an 8.1 magnitude earthquake was expected to “return in the PM,” according to a volcanologist.
Why we fact-checked this: Several copy-pasted posts on Facebook claimed that a much stronger earthquake would occur, inciting panic in the wake of the magnitude 7.0 earthquake.
What the experts say: Both foreign and local seismologists have repeatedly reminded the public that an earthquake’s event parameters cannot be predicted. (READ: EXPLAINER: Is it possible to predict earthquakes?)
- Event parameters, as defined by the Philippine Institute of Volcanology and Seismology (Phivolcs), are the location and time of an earthquake’s occurrence as well as its magnitude. Phivolcs’ Seismological Observation and Earthquake Prediction Division records the event parameters of the latest seismic events in the Philippines.
Cannot be claimed as an aftershock: A magnitude 8.1 earthquake is also unlikely to happen as an aftershock. According to the US Geological Survey (USGS), aftershocks are “smaller earthquakes” that happen in the same vicinity as the large earthquake, which the USGS calls a “mainshock.”
- Why do aftershocks happen? The USGS said that aftershocks are “minor readjustments” along the portion of the fault where the mainshock occurred. Aftershocks also become less frequent as time passes.
- As of 7 am, on August 1, Phivolcs has recorded 2,010 aftershocks that followed the Luzon earthquake.
However, in the event that a magnitude 8.1 earthquake or any quake stronger than the Luzon earthquake occurs in the same area, the magnitude 7 earthquake that happened on July 27 will be classified as a foreshock and the stronger earthquake will be classified as the mainshock. Foreshocks precede larger earthquakes in the same location.
Failed prediction: The claim’s prediction also did not happen in the afternoon or evening of July 27 after the Luzon earthquake happened that day. According to Phivolcs’ records, the strongest aftershock that happened on July 27 was a magnitude 5 earthquake that struck Dinapigue, Isabela at exactly 5:04 pm.
Since 1950, the 1976 Moro Gulf Earthquake in Mindanao is the only magnitude 8.1 earthquake that has occurred in the Philippines, according to the National Geophysical Data Center of the National Oceanic and Atmospheric Administration. – Rochel Ellen Bernido and Lorenz Pasion/Rappler.com
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