By David Jacobson, Temblor
Yesterday, at 10:27 p.m. local time, a M=5.4 earthquake struck the island of Luzon in the Philippines and was felt in the capital city of Manila, which is home to nearly 2 million people. On the USGS website, nearly 500 people reported feeling the quake, though we know many more actually felt it. According to the Philippine Institute of Volcanology and Seismology (PHIVOLCS), there are no reports of damage. Damage is also unlikely as the quake only caused light to moderate shaking. Despite the fact that the quake occurred at a depth of approximately 100 km, and was only a moderate magnitude, reports say that some people felt the quake for nearly 20 seconds in tall buildings.
The island of Luzon is the largest and most populated island in the Philippines, and is also very seismically active. To the west is the Manila trench, which is where the Sunda Plate subducts beneath the Philippine Sea Plate. To the east are both the East Luzon Trench and the Philippine Sea Trench, which are separated by a left-lateral transform boundary. The two subduction zones which bound the island mean that all of Luzon is susceptible to large earthquakes.
Yesterday’s earthquake occurred at a depth suggesting it was on or near the western subducting slab. However, based on the USGS focal mechanism the quake had extensional and strike-slip motion rather than compressional. The strike-slip component can be explained by understanding that because of the subduction zones on either side of the island, the southern part of Luzon is being sheared. Furthermore, from the Temblor image above, one can also see that the southern portion of the Manila Trench begins trending to the southeast. This means that due to prevailing plate motion vectors, this part of the subduction zone is likely a transform or oblique boundary. The extensional component is a bit more tricky given the compressional regime that the Philippines sits in. However, when the dip of a subducting slab changes, tensional cracks can form and extensional faulting can result. Therefore, based on the depth of this quake, this is a likely explanation.
In addition to large subduction zones flanking the island of Luzon, there are also a series of surface faults, including the Valley Fault System, which runs straight through Manila. While this fault has not ruptured recently, 4 times in the past 1,400 years, it represents a great seismic hazard to Manila since it is capable of M=7+ earthquakes.
Based on the Global Earthquake Activity Rate (GEAR) model, we can see what types of earthquakes the island of Luzon could expect. This model uses global strain rates and seismicity since 1977 to forecast the likely earthquake magnitude in your lifetime anywhere on earth. In the Temblor map below, one can see that in the area around yesterday’s earthquake, the likely magnitude is 7.0+, while for Manila, it is 6.75+. Therefore, while yesterday’s quake should not be considered surprising, this map does give an indication that large earthquakes in the Philippines are possible, and likely.