“We’ve had swarms ten times in the last 50, 55 years in the (San Ramon) area and none of those were followed by a bigger earthquake,” according to seismologist Lucy Jones.
The observation executes itself somewhat differently in the East Bay, where a series of early-morning tremors have become so habitual, as to have a tendency to produce an effect even where nothing is trembling. The most powerful shock was in one burst south of San Ramon, and the magnitude (4.2) was felt by at least a dozen other smaller shocks all in a burst of over an hour. The earthquake was felt traveling tens of miles distant in San Francisco and in the cities of East Bay such as Oakland and Richmond.
To commuters, the most direct impacts seem to come in the form of every day safety measures, and not the infrastructure that has been destroyed. To minimize derailment risk and ensure that conditions are sound in the tracks, Bay Area Rapid Transit slowed down trains to carry out inspections, a process that would restore normal speeds in service once these inspections are complete. The interruptions are brief in nature, but they provide a feasible level to the psychological level already in place to people; the constant reminder that the earth can shake anytime.
The point in these clusters that is unique is that they do not act as the most prevalent earthquake script in the community. Numerous sequences of earthquakes are of the type of a mainshock-aftershock-type, with one large event and a sequence of smaller ones, which decrease in intensity. The swarms are not alike and even experts do not make a distinct boundary. The U.S. Geological Survey defines swarms as chains that contain a relatively large count of earthquakes in a comparatively tiny region that can not be categorized in the standard way of aftershock disintegration. The biggest quake in swarms may be placed in the middle and the activity may be steady or may vary on a daily, weekly, or monthly basis.
This has been happening in the East Bay of the Bay Area over decades. Jones has termed the Monday cluster as the 10th reported sequence since 1970 in the San Ramon region. The same conclusion is drawn by other scientists that monitor the area: swarms do not necessarily sound destructive, and they can dissipate in the manner they started – which is, without a single characteristic main earthquake.
There is, however, an inherent question that most Californians will have in the back of their heads via repeated small shocks, or whether the constant action gives some kind of clue of a bigger quake. The tension has been made plain by USGS seismologist Annemarie Baltay: “There’s gonna be a big earthquake in the Bay Area. We just can’t say exactly when and where. So you should be prepared for that.” Long-term predictions are not definitive, but help provide a background, the USGS has determined that there is a 72 percent probability of an earthquake of magnitude 6.7 or more in the San Francisco Bay Area in a 30-year time frame calculated in 2014.
Scientists also warn against interpreting swarms as a count down clock. Jones has noted that a swarm is not a necessary precursor though sometimes major quakes were predetermined by minor ones. Among various guidelines provided by the researchers is the fact that following any earthquake, there is approximately 1 in 20 probability of another one occurring that will be of a larger magnitude, an increase in odds, but not at a particular location or time.
Why do swarms happen at all? In various parts of the world, scientists tend to relate swarms to an additional component to existing in the ground. USGS seismologist David Shelly has explained that how fluids interacting with faults can be used to aid swarm-like behavior, and may develop as feedback whereby small slips open pathways that allow fluids to move which in turn may promote more slipping. The experts have also observed in the East bay a web of faults in and around the Calaveras system and Mount Diablo which could aid in explaining the phenomenon of the same small area, being lighted up many times with cluster.
To the locals, the most helpful difference is factual: swarms are disturbing even without escalation. The unceremonious and recurrent movement of the earth is a source of stress in itself, particularly during the still hours, and at the same time a reminder of a truth of life in the Bay Area: readiness is everything when there can be no certainty.
