Spain is not a country that has strong seismic activity. With the exception of a few limited cases, like the Lorca earthquake of 2011 , the country's seismic activity is very low, and tremors don't usually cause any damage.
For that reason, when frequent seismic movements were registered a few weeks ago–more than 350 tremors in the last month– in the gulf of Valencia, opposite the coast of the Castellón province, alarms went off especially when, in the early morning hours of October 1, an earthquake registered a 4.2 on the Richter scale.
This unusual seismic activity may be caused by the injection of natural gas into the area that surrounds an old oil reservoir, now empty, which is 1750 meters deep and lies 22 kilometers from the Vinaroz area within the Castor project , which hopes to convert that reservoir into a gas storage facility  [es]:
de acuerdo a la regulación europea, los comercializadores (…) tienen la obligación de mantener unas reservas equivalentes a 20 días de ventas en firme del ejercicio anterior.
According to European regulations, suppliers (…) are obligated to keep reserves equal to 20 days of stable sales from the previous year.
Even though other types of these deposits exist in Spain that have not caused problems until now, the Castor project appears to be located in an especially unstable area. The seismic activity may be caused by the injection of what is called “cushion gas,” which, according to the blog Sience, Information, and Emergencies  [es], deals with:
600 millones de m3 [de gas] (…) cuyas funciones básicas son rellenar espacios “muertos” de difícil drenaje del yacimiento, servir como aislamiento entre el Gas de Trabajo y el acuífero subyacente (para evitar la posible producción de agua libre) y, en menor medida, contribuir al mantenimiento de la presión del yacimiento.
Six hundred million m3 [of gas] (…) whose basic functions are to refill “dead” spaces of difficult deposit drainage, to serve as insulation between the Gas de Trabajo and the underlying aquifer (in order to avoid the possible production of free water), and in small part to contribute to the maintenance of deposit pressure.
Furthermore, the Castor project is located over the Amposta faultline, which runs through the eastern coast throughout 51 kilometers and acts as the reservoir's border  [es]. José Luis Simón, Geology professor from Zaragoza University and an expert in the tectonic study of the area, told the daily Levante  [es]:
se inyecta gas a una presión bastante alta en un subsuelo submarino del que se conocen pocas cosas, pero una de las que se conocen es que justamente bajo el emplazamiento del pozo pasa una falla que se considera activa. (…) Las tensiones hubieran tardado cientos o miles de años [en producir un terremoto], pero al inyectar gas hacemos que se produzca ahora.
Gas is injected into a below underground undersea area about which little is known, at rather high pressure, but one thing that is known is that a faultline passes just under the reservoir site and is considered active. (…) The tensions have taken hundreds or thousands of years [to produce an earthquake], but upon injecting gas we are causing it to produce one now.
The minister of industry ordered to stop work  [es] on the platform this past September 26 and the experts from said ministry are considering completely stopping the project if a connection is shown between the project and the earthquakes. The autonomous governments of Cataluña and Valencia have activated their earthquake risk plans.
Area neighbors’ worries have been reflected on social networks, which have also served to bring together demonstrations and protests in the affected towns. It has to be pointed out that in a radius of 100 kilometers from the Castor platform, there are tourist areas like Salou  and historic villas like Peniscola  and the ruins of Tarraco , but also installations of the Tarragona petrochemical industry [es] and nuclear centers of Ascó I and II and Vandellós [es].
Economically, the project–which many emphasize is unnecessary–has overshot its budget. At the end of 2011, 1195.5 million euros were already invested, 89.54% over the initial foreseen cost. According to the Confidencial  [es], the previously mentioned budget presently
se ha disparado hasta los 1.700 millones. Todo un ejemplo emblemático de los modificados que han hecho de oro a las constructoras al calor de las vacas gordas.
(…) Un dinero que será sufragado por los usuarios del gas a través de la tarifa. (…) [Castor recibirá] En total, 250 millones de euros por ejercicio. Unos fondos que, como costes del sistema, se incluyen en el recibo [al consumidor].
[Solo podría] evitarse el impacto de esa instalación en los recibos del gas si puede demostrarse que ha existido una negligencia grave en el proceso de puesta en marcha.
has skyrocketed to 1,700 million. A characteristic example of the changes that have benefited the construction companies.
(…) A cost which would be defrayed by gas users throughout the rate. (…) [Castor will receive ] In total, 250 million euros per year. Some funds which, as system costs, are included in the bill [to the consumer].
[The impact] of this installation could only be avoided in gas bills if it can be demonstrated that grave negligence existed in the start up process.
On Twitter, the topic has been commented on a lot. The Tarragona resident Andrea Lahoz  shows her worry about the effect of the earthquakes in her area:
Vivo en una ciudad rodeada de fábricas y petroquímicas. Ahora gracias a #castor  vivo acojonada esperando al próximo terremoto…
— Andrea Lahoz (@Andrea_Lahoz) October 2, 2013 
I live in a city surrounded by factories and petrochemical plants. Now thanks to #castor  I live freaked out, waiting for the next earthquake…
The construction of Castor is paid for by the ACS company, whose president is Florentino Pérez , also president of the soccer club Real Madrid  [es] C.F. Trapichaire [es] underscores the double standard that Pérez shows at the time of approving certain expenses:
— Trapichaire (@Trapichaire) October 3, 2013 
— Blanca Civit (@blancacivit) October 3, 2013 
El buen progre  also doesn't take the humor seriously:
No se pq tanto escándalo en castellón. Si llenas la tierra de gases es normal que se tire pedos. #Castor 
— El buen progre (@Elbuenprogre) October 3, 2013 
Don't worry so much about the scandal in castellón. If you fill the ground with gas its normal for it to fart. #Castor 
According to a report by El Periódico [es], “The District Attorney's Office for the Castellón Province has agreed to initiate proceedings for a criminal investigation” to clarify the origin of the earthquakes. On Friday, October 4,
Sobre las dos de la tarde, miembros del servicio aéreo de la Guardia Civil, junto a varios agentes del cuerpo en Castellón, llegaron a la planta, situada a unos 23 kilómetros de la costa de Vinaròs. Según las fuentes, los agentes se trasladaron para “comprobar cómo se encuentra la plataforma y su funcionamiento”.
At about 2:00 in the afternoon, members of the Civil Guard air service, together with several agents from the group in Castellón, arrived at the plant, located 23 kilometers from the coast of Vinaròs. According to sources, agents moved the platform in order to “check the platform and its functioning.”
Agents verified that the plant has stopped. Now its a matter of waiting to get the report from the experts to know the final destination for the Castor project.