Dover, DE earthquake
At shortly before 5 pm on Thursday, November 30, 2017, there was a magnitude 4.1 earthquake east of Dover, DE. This event has caused little damage and no casualties. It is of significant scientific interest because earthquakes of this size on the east coast of the United States are rare in general, and earthquakes in Delaware, even more so.
Here's a map of all of the earthquakes (orange dots) bigger than a M=2 that have happened in the eastern US since Jan 1, 2011. They're scaled by magnitude. Notice that the circle in Delaware (the one with the red arrow pointing at it) is kind of off on its own. That's the earthquake that just happened. There are other clear clusters of earthquakes. There's one in eastern Tennessee/western North Carolina. There's one around the big 2011 M=5.8 earthquake in northern Virginia. But there really hasn't been much going on seismically in Delaware. So why did this one happen, and why was it so big?
The answer, I'm afraid, is that we have no satisfying answer (at least not that I, your humble narrator, am aware of). We understand much less about earthquakes in plate interiors (like the eastern United States) than we do earthquakes along plate boundaries (like California, Alaska, or Mexico). We do know that there are a lot of faults along the eastern margin of the US. Some of them date back to the days when Pangea broke up (about 180 million years ago).
When you break up a tectonic plate, it's a little like breaking a snickers bar. It doesn't just break neatly into two pieces. The crunchy outer bit (in our case, the Earth's crust) breaks in a few different places before the whole bar settles on one final place to come apart. These other cracks in the chocolate... I mean, crust... are known as "failed rifts".
The east coast has lots of failed rifts. These failed rifts tend to fill with sediments and become what we call "rift basins" (grey shaded regions on the map here). Notice that one of these basins ends pretty close to the location of our Delaware earthquake. Also note that in general, the earthquake locations above don't really consistently line up with the locations of these basins.
So, did a failed rift from the breakup of Pangea play a role in our most recent earthquake? Maybe. But there's clearly a whole lot more to the story than we currently understand.
One way we can study these types of earthquakes is to map out the locations of the aftershocks that follow them. Aftershocks tend to be very small, so if we want to find and locate them accurately, we need seismic stations pretty much right on top of them. To that end, a group of scientists (yours truly included) went out to Delaware the very next morning to install seismometers across the study area. The team included researchers from Carnegie, but also scientists and students from the United States Geological Survey (USGS), the University of Maryland, Columbia University, Lehigh University, and the Defense Nuclear Facilities Safety Board. We got a lot of help from the Delaware Geologic Survey, and the Delaware Division of Fish and Wildlife in finding good spots for our stations. We're also very grateful to the generous land owners who opened their doors to us and allowed us to install our equipment on their property.
All told, we got 14 stations up and running on the Delaware side, and another four on the New Jersey side within 24 hours of the earthquake (shown in red). We'll be looking at these data over the next weeks and months to try to understand the structures and forces responsible. We won't be able to completely answer the question "what causes these earthquakes" but we will have a new piece to add to the puzzle. One of these days, we'll see what the big picture looks like.
Dover, DE: Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba ya Dhoruba. Tukio hili limesababisha uharibifu mdogo na hakuna majeruhi. Kwa sababu ya hali ya hewa, tetemeko la ardhi la kiwango hiki katika pwani ya mashariki ya Marekani ni nadra sana, na tetemeko la ardhi katika Delaware ni zaidi. Picha hii inaonyesha matetemeko ya ardhi makubwa zaidi ya M2 yaliyotokea katika eneo la mashariki mwa Marekani tangu Januari 1, 2011. Zinaongezeka kwa ukubwa. Kwa mfano, mviringo wa Delaware una alama nyekundu, na mviringo wa Delaware una alama nyekundu, na mviringo wa Delaware una alama nyekundu. Hiyo ni tetemeko la ardhi ambalo limetokea tu. Kuna makundi mengine ya wazi ya matetemeko ya ardhi. Kuna moja katika mashariki ya Tennessee na magharibi ya North Carolina. Mlipuko huo ulisababisha tetemeko kubwa la ardhi la 2011 lenye ukubwa wa 5.8 katika jimbo la Virginia. Lakini kwa kweli hakukuwa na mengi yanayotokea kwa seismic katika Delaware. Kwa nini hii ilitokea, na kwa nini ilikuwa kubwa sana? Jibu, ninaogopa, ni kwamba hatuna jibu la kuridhisha (angalau si kwamba mimi, msimuliaji wako mnyenyekevu, ninajua). Kwa mfano, kuna uwezekano mkubwa kwamba tetemeko la ardhi katika eneo la ndani la bara (kama vile Amerika ya Mashariki) linaweza kusababisha tetemeko la ardhi katika eneo la ndani (kama vile California, Alaska, au Mexico). Kwa kweli, kuna makosa mengi katika eneo la mashariki la Marekani. Mifano hiyo ilitokana na kipindi cha kuanguka kwa Pangea, karibu miaka milioni 180 iliyopita. "Kutengeneza ""plate ya tectonic"" ni kama kuvunja bar ya Snickers." Haivunjiki kwa urahisi katika vipande viwili. Sehemu ya nje (katika kesi yetu, ganda la Dunia) huvunjika katika sehemu kadhaa tofauti kabla ya bar nzima kukaa mahali pa mwisho pa kutenganishwa. Mvunjiko huo mwingine katika chokoleti... Namaanisha, makapi... yanajulikana kama "mapengo yaliyoshindwa". Pwani ya mashariki ina maporomoko mengi yaliyoshindwa. "Makundi hayo ya mabonde yanaendelea kuenea na kuwa ""mabonde ya mabonde"" (mikoa yenye rangi ya kijivu kwenye ramani)." Angalia kwamba moja ya mabwawa haya inaisha pretty karibu na eneo la tetemeko letu la ardhi Delaware. Pia kumbuka kwamba kwa ujumla, maeneo ya tetemeko la ardhi hapo juu si kweli mfululizo mstari juu na maeneo ya mabwawa haya. Kwa hiyo, je, ufa ulioshindwa kutokana na kuvunjika kwa Pangea ulichangia tetemeko letu la hivi karibuni la ardhi? Labda. Lakini ni wazi kuna mengi zaidi ya hadithi kuliko tunavyoelewa kwa sasa. Njia moja ya kuchunguza aina hizo za matetemeko ya ardhi ni kuchora mahali ambapo matetemeko ya baadaye hufuata. Kwa sababu ya athari za shambulio la ardhi, ikiwa tunataka kugundua na kuamua eneo sahihi, tunahitaji vituo vya uchunguzi wa tetemeko la ardhi juu ya shambulio hilo. Kwa hiyo, kikundi cha wanasayansi (wenyewe pamoja) walienda Delaware asubuhi iliyofuata ili kuweka seismometers katika eneo la utafiti. Timu hiyo ilijumuisha watafiti kutoka Carnegie, lakini pia wanasayansi na wanafunzi kutoka Shirika la Uchunguzi wa Jiolojia la Marekani (USGS), Chuo Kikuu cha Maryland, Chuo Kikuu cha Columbia, Chuo Kikuu cha Lehigh, na Bodi ya Usalama wa Vifaa vya Nyuklia vya Ulinzi. Kwa mfano, katika jimbo la Delaware, tumepewa msaada wa shirika la Delaware Geological Survey na Delaware Division of Fish and Wildlife ili kupata maeneo mazuri ya kutengeneza vituo vya uchunguzi. Tunashukuru sana kwa wamiliki wa ardhi ambao wametufungulia milango yao na kuturuhusu kuweka vifaa vyetu kwenye mali yao. Kwa jumla, tulipata vituo 14 na kuendesha upande wa Delaware, na nyingine nne upande wa New Jersey ndani ya masaa 24 ya tetemeko la ardhi (kuonyeshwa kwa nyekundu). Tutaangalia data hizi katika wiki na miezi ijayo kujaribu kuelewa miundo na nguvu zinazohusika. "Hatuwezi kujibu kabisa swali ""ni nini husababisha matetemeko haya ya ardhi"" lakini tutakuwa na kipande kipya cha kuongeza kwenye fumbo." Siku moja tutaona jinsi picha hiyo kubwa inavyoonekana.