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Magnitude refers to the strength of an earthquake. Magnitude is reported on the Richter Magnitude Scale which ranges from 1 to 10 (M1.0–M10). Technically, today’s seismologists measure earthquakes with the Moment Magnitude Scale, but these values are converted to the Richter Scale for reporting to the public. The Richter Scale is logarithmic, meaning that each increase of one magnitude denotes a ten-fold increase in strength (ie. a M8.0 earthquake is 10 times stronger than a M7.0, or 1000 times stronger than a M5.0). Earthquakes M1.0–M5.0 are quite small, and rarely cause damage; M5.0 - M6.0 are moderate and may damage flimsy structures; M7.0–M9.0 are strong to catastrophic and can damage or entirely destroy structures.

The mainshock is the largest earthquake in a sequence. Earthquakes often occur in groups, with seismic activity over a short period of time. The mainshock is often proceeded be earthquakes (foreshocks), and almost always followed by earthquakes (aftershocks).

Aftershocks are smaller earthquakes that follow in the same vicinity of a mainshock. Generally, aftershocks are limited to within two rupture lengths of the mainshock (the aftershock zone). Large mainshocks can be followed by thousands of aftershocks, which can be very large, but decay in strength and number over time.

Remotely triggered earthquakes are earthquakes following a large mainshock that occur outside of the aftershock zone. As time and distance from the mainshock increase, it becomes more difficult to classify remotely triggered earthquakes.

A seismograph is an instrument to detect movements in the earth’s crust. It generally consists of a measurement device attached to a recording device. Earthquakes are detected using networks of seismographs that are arrayed worldwide. These devices can be quite sensitive, but their ability to detect earthquakes depend on the proximity of the quake and the seismic noise in the region.

Earthquakes occur at faults, which are zone of fracture between two blocks of rock. During an earthquake, the rock on one side slips relative to the other. The direction and type of slippage determines the type of earthquake. Faults where the rocks move upward or downward are called dip-slip. At strike-slip faults, blocks of rock move horizontally to each other. See these animations from the USGS.

When an earthquake strikes, it sends out energy in the form of seismic waves. Seismic waves come in two flavors: body waves, which move through the inside of the earth, and surface waves, which move along the surface of the earth. Within body waves, there are P waves, which move parallel to the wave direction, and S waves, which move perpendicular to the wave direction. Within surface waves there are Love waves, which cause the ground to move side-to-side, perpendicularly to the wave direction, and there are Rayleigh waves, which cause the ground to move up and down like rolling ocean waves. Of these classes, Love waves are especially destructive to structures.

The antipode refers to the other side of the globe from a reference location. Imagine you were standing at some location on the earth. If you drilled a hole straight through the earth from that point, you would emerge at the antipode to your original location.

This study sectioned the globe into discs of ten degrees in width and calculated the number of earthquakes in each slice as a rolling average. The values for the +/- five degree increments can then be tallied, which is what is represented by the bars in the circular plot. Degrees here refers to the angle around a circle (the globe). The study considers relative rates across 180 degrees on each side of a mainshock (adding up to 360 degrees, which encompases the whole globe).

The relative rate refers to the rate of earthquakes over the three days following a mainshock, compared to the historical average. A relative rate below one means fewer earthquakes occurred in this period than historically, whereas a relative rate above one indicates an increase in earthquakes.

Can one natural disaster influence the risk of another? It's well known that natural disasters can cause others in their immediate vicinity, for instance, hurricanes are often accompanied by flooding, and earthquakes are followed by aftershocks. But what about longer distance interactions? Could one earthquake trigger another on the other side of the world? hey click me for more info!

Our story begins in California in June of 1992. The dots below represent earthquakes recorded in the ten days from June 18th to June 28th. Their size and color correspond to their magnitude: larger earthquakes are bigger and more red. There were 44 quakes during these ten days, though most were small and wouldn’t have been felt by people. But in the early hours of June 28th, that changed.

At 5:00 on the morning of June 28th, a magnitude 7.3 earthquake struck, waking much of Southern California. The epicenter was located near Landers California, in the Mojave Desert. Thanks to the remote location, the damage and loss of life was minimal.

As expected, the mainshock was followed by a smattering of aftershocks which occurred in the immediate vicinity of the mainshock, but what played out over the next ten days was entirely unexpected.

Survey stations across the Western United States recorded a flurry of seismic activity in the ten days following the Landers earthquake. Seismologists recorded 192 earthquakes outside of the aftershock zone during this time—four times the amount in the ten days proceeding the Landers quake. Even more surprising was that these earthquakes occurred at great distances from the aftershock zone, with many recorded hundreds or even thousands of miles from the mainshock epicenter.

Placing each earthquake as a dot on a timeline before and after the mainshock reveals the striking pattern. There is an especially large burst of activity in the three days following the mainshock, after which things taper off. This event was one of the first reports of remotely triggered earthquakes to gain wide recognition. Most scholars were convinced that this increase in seismic activity was triggered by energy sent out from the Landers quake, but many questioned how relevant it was.

Most earthquakes are small (M2.5–M4.0). These quakes are not typically felt by people, and are only recorded by sensitive seismographs. Earthquakes of magnitude 4.0–5.0 can cause shaking that might knock things off your shelf, but they’re unlikely to cause structural damage. What we’re really concerned about are quakes greater than M5.5; only one of the 192 earthquakes following the Landers mainshock met this criterion.

Over the next 15 years there were numerous papers published on remote triggering of earthquakes. It became well accepted that large earthquakes often trigger small earthquakes at long distances, and are usually followed by increases in seismicity. However, the question of whether source events could remotely trigger large earthquakes (M5.5+) remained contentious. Several researchers argued that the largest earthquakes (M8.0+) could remotely trigger M5.5+ quakes, but not all were convinced. Scientists squabbled over statistical details and seismic mechanisms until 2012, when we got a unique glimpse into the possibilities of remote triggering.

The spring of 2012 was an unusually quiet time for large earthquakes. In the ten days from April 1st to April 11th there were only five M5.5+ earthquakes. Note that from here on we will only consider earthquakes larger than M5.5.

On the afternoon of April 11th 2012, a massive M8.6 earthquake struck off the coast of Sumatra. The temblor triggered many aftershocks (not shown here) . This quake was unique for its size and mechanism. To date, this is the largest ever recorded strike-slip earthquake.

In the ten days following the Sumatra mainshock, 44 earthquakes M5.5+ were recorded. The statistics were clear: this nine-fold increase was highly significant, and most researchers agreed the likely explanation was remote triggering by the M8.6 Sumatra earthquake. This massive triggering of large earthquakes was unprecedented, and many wondered if the strike-slip mechanism at such a large magnitude was a unique combination that led to this outsized response.

When earthquakes strike, they send out seismic waves that travel through the earth’s crust. These waves often propagate out from the mainshock in a ‘+’ pattern meeting at the opposite side of the earth (the antipode). Analysis of the Sumatra quake showed that nearly all of the triggered earthquakes fell within the seismic waves from the mainshock.

The 2012 Sumatra quake showed that it was possible for a large earthquake to trigger other large earthquakes on the other side of the world, but the question remained: how common was this phenomenon? Many argued that the Sumatra quake was an outlier, and rigorous statistical analysis was needed to prove otherwise. Recently, a small group of researchers at Oregon State University carried out an analysis of 47 years of earthquake data to look for statistical evidence of remote earthquake triggering. Their method took each large mainshock (M7.0+) and then divided the globe into slices of ten degrees each emanating outward from the mainshock (the 30 degrees surrounding the mainshock was disregarded as it was already known to have elevated earthquake levels). For each slice, they compared the rate of M5.0+ earthquakes in the three days following the mainshock to a historical average to determine a relative rate of earthquakes.

The researchers found that in the three days following the largest earthquakes (M8.0+), the rate of M5.5+ earthquakes at certain locations was elevated as much as twice the average. Two main regions—around 80 degrees and in the 30 degrees surrounding the antipode—have significantly elevated rates. The area around the antipode is particularly intriguing, as that is typically where seismic waves from the mainshock converge. Although suggestive, skeptics may argue that these results merely show a correlation—without causation—between large mainshocks and elevated rates of M5.5+ quakes. However, there have been a handful of documented cases of remote triggering of large quakes with a demonstrated seismic mechanism that more convincingly show causation.

On September 11th 2008, a M6.6 earthquake struck off the coast of Halmahera, Indonesia. The quake sent seismic waves out towards Japan...