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Was this morning's Monterey quake caused by manmade activity?

Monterey's earthquake earlier today at a very noticeable magnitude of 4.3 sent shockwaves through the region's inhabitants, literally. The uncharacteristically very shallow quake of barely 4 miles below the surface caused a flurry of comments to flood our VolcanoDiscovery monitoring service. Messages came in from a wide range of neighborhoods, communities and towns around Monterrey including Aptos, Ben Lomond, Boulder, Carmel, Corralitos, Hollister, Marina, Newman, Salinas, Santa Cruz, Sand City, Soquel, Watsonville, and many more.

Users reported feelings of a "passing truck", cracks, rumbling, rattling and shaking lasting anywhere between 2 and 30 seconds, often in two different waves. Many felt a rolling movement rather than the classical left-right shaking. Luckily no significant damage or injuries were reported to us.

This quake only came two days after a notable San Francisco Bay area one where authorities scrambled to deny any connection to manmade activity. That said, a very recent article published on 13 December 2020 in The Mercury News (Bay Area News Group) points to a study that relates quakes in Monterey County to oil operations. The research tied earthquakes in the area to wastewater injection, citing an example of this in Oklahoma as well. Although, according to the article, Western States Petroleum did not want to comment on a connection, the study by seismology professor Thomas Goebel from the University of Memphis and geophysics professor Manoochehr Shirzaei from Virginia Tech found that earthquakes in the vicinity, specifically San Ardo, " tended to occur near wastewater disposal wells and that changes to the rates of wastewater injection were shadowed by changes in earthquake activity, with a delay of around 13 to 17 months."

We're particularly surprised at VolcanoDiscovery to see this phenomenon crop up more and more in relation to US earthquakes, particularly since in Europe it is generally unheard of. Our understanding of earthquakes is that they are a natural phenomenon. The Monterey and Oklahoma quakes seem to point to a manmade cause. We will certainly be on the lookout for more academic research emerging from the United States to confirm this alarming trend.

Earthquake data:
Date & time: 2 Jan 6:42 am (GMT -8) local time (2 Jan 2021 14:42 GMT)
Magnitude: 4.3
Depth: 6.4 km
Epicenter latitude / longitude: 36.61°N / 121.21°W (Monterey County, California, United States)
Primary data source: USGS