A checkout line is not where a majority of people expect to have an earthquake lesson, but that is where some of the Bay Area residents found themselves as they watched the shelves shake and faces turn towards each other to read the signs.
In the last one month, small groups of earthquakes shook the San Francisco Bay Area with recurring earthquakes taking center stage beneath San Ramon in the East Bay of the Calaveras fault. During a day in a single day, there were at least 19 earthquakes with a magnitude of 2 or more, one of which was a magnitude 3.6 that was felt widely. Patients also complained of experiencing an electric shock when being awakened by an early morning shaking, and another 2.9 in the morning had an epicenter in Montclair area of Oakland near the Hayward fault.
The term “swarm” may seem threatening, but it is a description applied by scientists to refer to a pattern: quakes in a comparatively small region that do not fit the common mainshock-aftershock sequence. In a standard aftershock sequence, the largest quake is likely to occur first, with the ones that follow it of decreasing magnitude, and in any case, the largest aftershock is usually approximately one magnitude smaller than the mainshock. In a swarm the biggest event may be in the center, magnitudes may cluster, and activity may not dissipate but increase.
It is important that difference since it alters the meaning of the shaking, and the meaning of the shaking not. The swarms occur in numerous environments and these situations are encountered in tectonically active areas in the western U.S. and the swarms tend to indicate complicated ways faults slide as opposed to one easy trigger. Studies that have been cited by Yellowstone scientists give descriptions of swarms as often associated with fluids (dominantly water) interacting with faults, such that variations in pressure may promote increased small failures, and that small failures may open up new routes to fluids. This is because other swarms can be experienced without a strong fluid signal, as it is how some faults will slide in fits and starts due to stretching of regional tectonic forces.
The trend that is observed in the San Ramon area is not new, as it comes with history. USGS seismologist Annemarie Baltay has indicated that similar swarms have taken place there in 2002, 2003 and 2015, and around nearby Danville in 2018, and have not led to a damaging Calaveras fault earthquake. Past swarms in that pocket of the East Bay have extended over the period of a couple of days to 42 days, and that is long indeed to any one who attempts to sleep through the recurrence of rumbles.
Nevertheless, swarms may have a weak predictive power. Seismologists usually warn that an earthquake can happen bigger after a smaller one, but these chances are low in any given week; one of the rules of thumb that scientists have given is that the probability of a specific earthquake being followed by a larger earthquake in the immediate future is 5%. Reflectively, California is characterized by small quakes which are precursors to major ones but the vast majority of small series do not develop to become destructive shaking.
The greater point of many residents is less of prediction and more of preparedness. The Bay area is located on a net of active faults and the USGS has estimated that the area is likely to experience an earthquake of magnitude 6.7 and above before the year 2043. The reason why early warning and household plans are highlighted during such weeks is in part that long-range risk.
The public early-warning network which can offer a head start before the onslaught of the strong shaking is one of the tools in the preparedness kit. The ShakeAlert® Earthquake Early Warning (EEW) System of earthquakes can transmit alerts, but also activate automated responses (slowing trains or closing valves) depending on the environment.
The swarms have served as a loud wake-up call in neighborhoods around San Ramon and throughout the Bay: the ground does not have to have the magnitude of a headline to make people look at supplies, their family contact plan, and the fastest way to Get To “Drop, Cover, and Hold On.”
