Download 2 Tremors 2: Aftershocks
Assunta Gergely
At approximately 5:59 p.m. an aftershock of 4.0 magnitude occurred southwest of Gladstone, an unincorporated community within the borough of Peapack-Gladstone in Somerset County, according to the United States Geological Survey.
During the nearly eight hours time between the original 4.8 earthquake and the 4.0 aftershock, 10 aftershocks initially registered between the magnitudes 1.8 and 2.2. But as of 8:45 p.m. Friday, that number increased to at least 14 aftershocks. And an additional three (two at 1.9 magnitude and one at 1.5 magnitude) were experienced after the 4.0 aftershock.
At 4:30 p.m., the Bedminster area experienced an aftershock of 1.8 magnitude and about 20 minutes later, Whitehouse Station felt an aftershock of the same magnitude. An hour earlier, Long Valley experienced an aftershock that measured 2.0 magnitude.
The United States Geological Survey says smaller earthquakes called aftershocks can occur in the same general area during days or weeks following the mainshock, which is the largest earthquake in the sequence. On Friday, that was the 4.8 magnitude that occurred with an epicenter near Tewksbury.
Similar to earthquakes it is impossible to predict the time and place of an aftershock. The USGS Earthquake Hazards Program shared its predictions Friday evening, saying there is a 78% chance of at least one aftershock with a magnitude of more than 3.0 will occur within the next week, but only 16% of an aftershock occurring with a higher magnitude of 4.0 and two percent chance of having an aftershock with a magnitude larger than 5.0.
These additional quakes, called aftershocks, are common after a large earthquake; many aftershocks can be small relative to the main quake, but some have the potential to be severe and destructive, as was the case in Turkey.
A powerful 6.4 magnitude aftershock struck near Antakya city in southern Turkey on Feb. 20, two weeks after the main quake, killing at least six. Another 5.6 magnitude quake struck near Malatya on Feb. 27, killing one person, injuring 110 and causing 29 buildings to collapse.
Chart shows about ten thousand earthquakes that have been recorded in southern Turkey since a 7.8 magnitude earthquake occurred on February 6. The frequency and intensity of these aftershocks have slightly diminished over time.
More than 570 aftershocks were recorded within 24 hours of the main quake on Feb. 6, and more than 10,000 were recorded in the three weeks afterward. The frequency and intensity of the aftershocks has diminished since then, but the temblors continue to be a threat.
Seismologists define aftershocks as temblors triggered by a large earthquake, close in time and location. Because many of these aftershocks are themselves large, they start their own chain of additional aftershocks.
The map locates an area in southern Turkey and northeastern Syria that is plotted in the next graphic, which is a map of all earthquakes recorded in southern Turkey in 24 hours after the 7.8 magnitude earthquake struck on February 6.
It can be difficult to associate all aftershocks with their parent events because of how many there are - in this case, thousands. However, the pattern can be clearly visualised around Antakya, where the magnitude 6.4 aftershock on Feb. 20 caused dozens of smaller quakes.
The chart shows all the earthquakes that originated within 30 kilometers around the city of Antakya in southern Turkey before and after a magnitude 6.4 earthquake occurred on February 20. The frequency of earthquakes in the area increased after the 6.4 magnitude quake, indicating that this quake, which was itself an aftershock of the 7.8 magnitude earthquake that struck on February 6, possibly started its own sequence of aftershocks.
Two maps show all earthquakes that occurred near the city of Antakya in southern Turkey before and after a magnitude 6.4 earthquake occurred on February 20. Aftershocks after the 6.4 magnitude quake were more clustered around the 6.4 magnitude quake epicentre - another indication they were connected to that quake and not the earlier ones.
At least 14 more earthquakes of magnitudes 5.4 and above occurred within two days of the main quake, causing more strong shaking in parts of southern Turkey. Some areas witnessed strong shaking more than three times.
Huvva Tuncay, who was living in a tent in the center of Antakya when the Feb. 20 earthquake hit, said: I cannot sleep at night. Is the same thing going to happen, are we going to experience another earthquake? We are very scared."
The USGS forecasts a 90% probability that the area already affected by the main shock will see moderate to large aftershocks of between magnitude 5 and 6 until March 10.A scenario with about a 10% probability would see one or more additional aftershocks larger than magnitude 7. The least likely scenario, with about a 1% probability, would see an aftershock of magnitude 7.8 or larger.
The frequency and intensity of aftershocks varies with factors such as the intensity and depth of the main shock, and plate tectonics. The chart below shows aftershocks of magnitude 4 and above that originated within 250km of the main shock from some of the deadliest earthquakes since 2010. Foreshocks, small earthquakes that occur before a larger one, preceded some of these large quakes.
The actual number of aftershocks in some of these cases could have been many more than what is recorded because of sparse seismometer coverage in certain areas, especially in the sea, said Margarita Segou, a seismologist with the British Geological Survey.
Dawn Lehman, professor of civil and environmental engineering at the University of Washington, said aftershocks and their effects were crucial to consider - especially in terms of protecting victims from building collapses after the main danger appeared to have passed.
The aftershock of 4.0 magnitude near Gladstone in Somerset County, New Jersey, was recorded just before 6 p.m., and while it was weaker than Friday morning's earthquake near Whitehouse Station, New Jersey, residents across New Jersey and parts of Delaware, Pennsylvania and New York have reported feeling the tremors.
The United States Geological Survey said people living in the area would likely feel one or more aftershocks in the coming weeks, though many of these quakes will be too weak for anything other than a sensor to pick up. There have been at least 14 aftershocks recorded so far.
Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a larger event or "mainshock." They occur within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed. As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the mainshock. The frequency of these aftershocks decreases with time. Historically, deep earthquakes (>30 km) are much less likely to be followed by aftershocks than shallow earthquakes.
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