This week saw a very strong earthquake in an area of southern Mexico - a M7.4 event centered 25 km from Ometepec. It looks like it was a relatively shallow earthquake (~15 km) that was related to thrusting due to the subduction of the Cocos plate under Mexico. Although this earthquake was larger than we might expect in Mexico, it is in a zone of high earthquake hazard, so the earthquake itself was not surprising - remember, subduction zones and earthquakes go hand in hand. The effects of the earthquake on the area have been significant, with over 1000 building collapsed or damaged, but thankfully few lives were lost. After the earthquake, there has been a flurry of news in Mexico about a supposed "new volcano" that appeared between Huajintepec and Huixtepec municipality of Ometepec. The details are scant, but the mayor of the district claims the new volcano to be "tiny" (but in other articles, he says the "volcano" has been there "for many years"). As usual, there is a lot of misleading coverage, like this story that shows a picture of an undersea vent at NW Rota 1 in the Marianas Islands (with no caption to say so). There is also a lot of speculation in the Mexican media that the "new volcano" might somehow have caused the earthquake or all the aftershocks being felt in the region. Now, there are quite a few reasons why it is very unlikely that there is a "new volcano" formed by the Oaxaca earthquake or that the earthquake was somehow caused by this supposed volcano. First, why this earthquake didn't cause a "new volcano": Although volcanoes in subduction zones are common, the location of volcanoes is not arbitrary. This is why we get ranges of volcanoes. The Cascade volcanoes in the U.S. all occur well inland from the actual point where the Juan de Fuca plate subducts under North America (the "trench). If we look at the active volcanoes of Mexico (below), the volcanic arc is almost 300 km from Ometepec, so the likelihood a new volcano would occur there has a very low probability (next to zero) - you have to go a long way from Ometepec to find active volcanoes. Some articles have been comparing this to the emergence of Parícutin in 1943 - a case of an actual "new volcano", but Parícutin is smack in the middle of the active volcanic belt. To get magma to form, melting of the mantle has to occur and in subduction zones, to do that, you need water to come off the oceanic crust that is being pulled under the overriding plate. The water only comes off at a certain depth (which is related to temperature), so the location of the volcanic arc tends to be hundreds of kilometers from the trench (however, it varies from subduction zone to subduction zones based on the angle of the slab beneath the overriding plate).
Second, why any "new volcano" can't be the source of the seismicity in Oaxaca: Whenever geoscientists examine earthquakes, we look at how the earth moved along the fault. This produced the famed "beachball" diagrams of the focal mechanism of an earthquake. For earthquakes generated by magma rising under a volcano, we want dilation/extension as the crust moves out of the way to let the magma through. The Oaxaca earthquake had a reverse/thrust motion, meaning that the force was compressional, not dilation. This sort of motion is not consistent with magma movement and is very consistent with thrust faulting due to the subduction zone. The many aftershocks from the earthquake are highly common from any large earthquake, so a "new volcano" is just not necessary to explain the 80+ aftershocks felt so far. So, all this "news" about a new volcano in Mexico is likely the product of media hysteria after a disaster. The only actual "observations" I've seen is claims of "hot water gushing/bubbling" after the Oaxaca earthquake, but hot springs are common along faultlines in many locations around the world. This doesn't mean that magma near the surface is the source of heat, but hot water will follow faults to reach the surface. A new earthquake might open new pathways for this water. However, with such scant information, it is hard to say that even that is happening near Ometepec. What I can safely say is that a "new volcano" is not very likely at all, both in causing the earthquake or being produced by the earthquake.