Lori Dengler | All about earthquake swarms – Times-Standard

It’s been a rollercoaster week in the eastern San Francisco Bay Area. The US Geological Survey recorded 120 earthquakes, all centered within a few miles of the city of San Ramon. Most were too small to be felt, but six reached the M3 range and the largest, a magnitude 4.2 on Monday, prompted more than 5,000 “Did you feel that?” questions. Reporting to the USGS website, some are as far away as Fresno and South Lake Tahoe.

This week’s seismic activity continued the swarm of earthquakes that began last November. On November 9, an area two to four miles from San Ramon was lit up by small earthquakes. Starting at 3.8, 88 small earthquakes followed over the next 24 hours. None of them were big enough to cause any damage, but many of them felt light in the East Bay area. This flurry of activity faded over the next few days, and by mid-December, it appeared to have ended. But on December 19, activity increased by 3.8 and 4.0. Activity declined again but picked up this week, resulting in the greatest amount of strength (4.2) and the most powerful portion of the swarm to date.

Earthquake swarms behave very differently from typical earthquake sequences. Most strongly felt earthquake sequences consist of a large earthquake (mainshock), followed by smaller earthquakes (aftershocks). Some may be preceded in the preceding hours/days/weeks by smaller earthquakes (foreshocks) and some may be characterized by a doublet, which are large earthquakes of roughly the same size close together in time and space. But the general pattern of larger earthquakes followed by weeks/months/years of decreased seismic activity is the norm.

The M7 Mendocino fault earthquake that occurred on December 5, 2024 is a good example of a typical earthquake sequence. It was centered about 40 miles west of Cape Mendocino and was immediately followed by smaller earthquakes. Nearly 200 aftershocks were recorded in the first 24 hours. The number decreased rapidly, to 80 on the second day and 35 on the third day. The largest aftershock was M5.3 in size, and occurred nine days after the main tremor.

All of the aftershocks are located in a 40-mile zone between M7.0 and Cape Mendocino, clearly defining the area that exploded in the main shock. By December 20, the rate had dropped to a few per day. The number continued to slowly decline, recording a few per week by August. There are still occasional earthquakes in the aftershock zone, but we have largely returned to the level of activity that characterized the region before M7.0.

This pattern of major aftershocks applies to a wide range of earthquake sizes. The largest aftershock is approximately one unit smaller in size than the mainshock. The duration of the aftershock sequence depends on the size of the main shock. Very large earthquakes have very long windows of aftershocks; The 2011 M9.1 in Japan has continued to produce aftershocks for nearly a decade.

Aftershocks are caused by slip irregularities during the main earthquake. M7 in December 2024 caused rocks to the north on the Gorda Plate to move about six feet compared to rocks on the Pacific Plate to the south. This slide was by no means smooth or continuous. There is a tremendous amount of pressure holding the two sides of the fault together; The fault zones are not smooth, and some spots have moved a little more than others, causing uneven slip distribution and stress irregularities. Stress is concentrated near the ends of the rupture in the transition between the part of the fault that has moved and the part that has not moved, and many aftershocks have been in these areas. Aftershocks are nature’s way of leveling out this uneven distribution of pressure and will continue until the area finds a new stable configuration.

Swarms behave very differently. They consist of tens to thousands of earthquakes of similar size that are closely clustered in time and space with no consistent decrease in magnitude. It may last for days, weeks, and in unusual cases, years. It may start, stop, and start again at irregular intervals. The San Ramon swarm, as I write, has produced 600 earthquakes, including seven of magnitude 3.8 and 4.2 with three periods of intense activity so far. The earthquakes are closely clustered, creating a spot a few miles in diameter on the map.

Swarms are not uncommon and San Ramon is not the only California swarm occurring right now. On January 19, a 4.9 magnitude earthquake occurred in Southern California’s Coachella Valley near Indio. Since then, 680 earthquakes have struck a two-mile-wide area centered eight miles north of the San Andreas Fault, including 12 with a magnitude of M3 or larger. While near San Andreas, these were not created due to this fault. The larger earthquakes in the swarm show an extensional fault nearly perpendicular to the San Andreas trend.

The most common location for swarms in California is Imperial County near the Salton Sea. Every year this region experiences one or more small earthquakes, sometimes lasting only a few days and other times lasting several weeks. Very small earthquakes always occur in this transition zone between the southern tip of the San Andreas transform fault and the spreading center in the Gulf of California, but much of the activity occurs in bursts. In June 2021, a particularly strong swarm struck the region triggering 2,400 earthquakes in a one-month period, including one M5.3 and seven in the M4 range.

Swarms of earthquakes are also common in volcanic regions, sometimes heralding the arrival of an eruption and other times, the movement of magma at depth. One of the most interesting of these flocks is located near the town of Pahala on the southern coast of Hawaii. More than 400 M3 and larger earthquakes and thousands of smaller earthquakes have occurred in this region since 2015. I wrote about this swarm (May 31, 2021) because of its longevity and depth. The Pahala swarm earthquakes are 20 to 30 miles deep and show no relationship to volcanic activity at the surface. It is still unclear what causes these unusual earthquakes, but all explanations relate to fluids and magma movement at depth.

Fluids are key to understanding earthquake swarms. There is no magma movement to explain the California swarms, but there are other fluids. Earthquakes occur below the water table and there are multiple sources of fluid in areas where earthquakes occur. Some fluids are released during compression of deeply buried sediments, others through crystallization and metamorphism. Fluids play an important role in enhancing or dampening earthquake activity. High water pressure in the pores pushes mineral grains apart from each other, reducing friction and allowing fault movement at stresses where dry rock is firmly stuck. Continuing fluid supply will produce a continuous stream of earthquakes. We see this every day in the geothermal geysers north of Santa Rosa. Injecting water into hot rock provides a constant source of energy but also produces more small earthquakes year after year than any other part of the state.

California is full of countless drawbacks. Fault movement over millions of years has placed very different types of rocks next to each other. This can impede or enhance deep fluid flow. I believe the location of the San Ramon swarm between the Calaveras and Pleasanton faults is no coincidence. Fluid is blocked by lateral discontinuities, promoting swarm formation. Earthquake swarms are no stranger to San Ramon, with at least seven having been identified since 1976.

The good news about earthquake swarms is that they don’t seem to lead to larger earthquakes. No earthquake has ever been linked to a major earthquake. They live and die with the fluid source that feeds them, and the volumes are not large enough to significantly change the pressure on the surrounding faults. But there is always a warning. Nature has a way of surprising us, and there are plenty of other earthquakes capable of producing large, damaging earthquakes at any time, regardless of whether there is a quake swarm occurring nearby.

Lori Dengler is professor emeritus of geology at Cal Poly Humboldt and an expert in tsunami and earthquake hazards. Questions or comments about this column, or want a free copy of the “Living on Shaky Ground” preparedness magazine? Leave a message at 707-826-6019 or email [email protected].

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