Update December 8th:
Nearly five weeks, since we have published this report about the earthquake swarm near Darmstadt. And it is still continuous. Today the 38th earthquake was felt in Mühltal. Although its magnitude was probably far below 2.0, people from Ober-Ramstadt, Nieder-Ramstadt and Modau felt it.
The strongest quakes in the last 5 weeks, on November 29th, was felt as far as southern Darmstadt. It reached Magnitude 1.7 on Richter Scale. Some changes in swarm behaviour were observed recently. 10 days between November 28th quake and December 8th, a period where no other quake occurred. Same between November 29th and November 21st. But: these minor quakes, following after a calm period, have typical characteristics of normal earthquakes. Smaller forshocks (sometimes), a few aftershocks, end of activity.
However, there wasn't any bigger, damaging earthquake in the last weeks, but it can happen any time. And small quakes are still so frequent, that we can "promise" swinging Chistmas ornaments in Mühltal during the next two weeks.
It began with a minor earthquake of M 3.2 in March. A quake, which was not unusual for a tectonic active region like Darmstadt, which is located on the eastern rim of Upper Rhine Graben. Although it was the strongest quake for several years in the south of German Bundesland (federal state) Hesse and caused some minor damage, it was not concerning. No aftershocks were felt in the next days, and life went on.
Until May 17th ... football fans in Darmstadt (and everywhere else in Germany) were preparing for the final game of German Football Cup (DFB-Pokal), Bayern München vs. Borussia Dortmund, when the quake hit in the early evening. Magnitude 4.2, the strongest quake in Germany for three years, the strongest in Hesse for decades. The epicenter was located in Mühltal, a little municipality 8 km south of Darmstadt. More than 100 homes were damaged here and the economical loss exceeded 1 million US-$. Even people in Frankfurt / Main and Stuttgart felt the shocks.
Damage after M 4.2 earthquake in Nieder-Beerbach (Mühltal) - Copyright: U. Knappe
"Mainshock!", was said by seismologists and media. The March quake was called a foreshock, and "some minor aftershocks might follow in the coming days."
Indeed, minor aftershocks followed and were felt by the residents of Mühltal and nearby municipalities. But it was kind of strange: These quakes had Magnitude 1.2 (May 18th), 1.7 (May 27th), 1.4 (May 29th), 1.0 (May 30th) and so on. Magnitudes far below the usual benchmark (M 2.0), from where on quakes can usually be felt by humans. Far below the level, where media start to report. All this quakes happened in very shallow depths of 3 - 4 km.
One more week of minor quakes followed, some quakes, which were felt, were even too weak to be detected automatically, until the next "big one" happened.
June 8th, early afternoon, Magnitude 3.4, stronger than the initial quake in March. New minor damage was caused, people in Mühltal were scared and started to ask questions about the origin of what happened. "Normal aftershock activity!", was the official statement for the next months, where more minor quakes were felt. But these tremors did not get any attention in media, as most of this quakes were not felt in Darmstadt.
Area of shaking - M 4.2 earthquake on May 17th
August 18th: Magnitude 2.1, enough to be felt in Darmstadt, enough to generate even more concern.
August 25th: Magnitude 2.2, felt in Darmstadt again and more and more people started to question the "aftershock-theory". A time, where conspiracy theories were formed. From "secret fracking operations" to a "new forming volcano", the creativity was large :). But none of this was a real explanation, everything could be excluded.
A few more quakes were felt in the next weeks.
September 19th: Magnitude 2.6! Until now, a clear pattern of epicenters on an east-west striking line near Mühltal was visible. But this quake destroyed the rudimental mountain of information: The epicenter was in northern Darmstadt! The rest was similar: Very shallow depth and felt much too strong for its magnitude. Besides to one aftershock (in this case, the word "aftershock" can be used without doubt), it was the only quake in Northern Darmstadt so far.
October 28th: The swarm was born. After 5 weeks without any felt quakes, it started again with a minor tremor of M 1.1 in the very early morning, waking up some people. But the big one followed in the early evening: Magnitude 3.5, the second strongest quake of this swarm so far, the second strongest quake in Germany 2014. Felt as far as Frankfurt/Main, caused new minor damage in Mühltal.
"It's over!" can not be said anymore. Scientists and residents are sure that this earthquake swarm will continue, for months, for years maybe. More big quakes around M 3 may follow.
Today, on November 5th, residents of Mühltal were waken up by a Magnitude 1.5 quake. The 30th felt and confirmed (!!) earthquake since March. This makes 46% of all felt earthquakes in Germany 2014 (at least 65).
Strongest earthquakes in Darmstadt area 2014
What is the origin of this quakes? How will this swarm go on?
Questions, who can not be answered. Even seismologists have problems to give clear statements, as this is a unique situation for them. Earthquake swarms like this are known from the German-Czech border region, where the magma chamber of an extinct volcano causes regular swarms. (Usually! In 2014 the situation was different. Instead of a swarm, 3 "big" earthquakes of M 4.2, 4.5 and 3.6 hit the area during the summer months. Without significant aftershocks, without any foreshocks, without any signs of a swarm. Another puzzle for German seismologists) But there are no other areas, where swarms like this happened during historic times. Apart from one: Darmstadt.
From 1868 to 1871 Darmstadt was hit by a significant earthquake swarm with many hundred single events. It's called the "Groß-Gerau earthquake swarm". Groß-Gerau is a little town 12 km north west of Darmstadt, which suffered the heaviest damage by the swarm. There, there earthquakes were much stronger than any others happening in the next 14 decades. Some quakes might have exceeded Magnitude 5, they were felt from Basel (Switzerland) to Cologne (Western Germany).
Some people believe that the current situation might be the beginning of a new, similar swarm. The "Groß-Gerau Swarm" had its largest quakes 14 month after the first quakes were felt. However, this is nothing more than speculation. Nobody is able to predict how it will continue. Nevertheless, it should be important for all people in Mühltal, Darmstadt and Groß-Gerau to prepare for big quakes. They are living in an earthquake prone area and should know how to behave when and after it happens.
Seismologists are sure that all this quakes have a tectonic origin. Some characteristics (shallow depth, phased occurrence) are similar to fracking or mining induced quakes, but neither fracking nor mining is taking place in Mühltal.
It's just happening, no other explanation is possible. And people have to deal with it.