'Doublet' earthquakes

Last week, earthquake activity made international headlines as two major earthquakes struck Venezuela on June 23. There was also one that measured 6.9 in eastern Japan and a 5.6 that was measured about 7 miles from Willits, in northwestern California on June 25. There were also seven other earthquakes last week that were at least a 5 magnitude that were reported in New Zealand, Russia, South Pacific, Brazil and New Guinea.

The one in Venezuela has received a lot of news coverage as the two earthquakes measured 7.5 and 7.2. According to the United States Geological Survey, the first one happened near San Felipe, which is the capital of the state of Yaracuy. The second occurred 39 seconds later near Yumare, about 3 to 6 miles away. The strong earthquakes led to the collapse of many buildings and generated landslides. Casualties are estimated to be in the thousands.

As of late last week, the USGS was calling these earthquakes a “doublet.” Doublet earthquakes are rare and they are among the most unusual and destructive seismic events because they involve two large earthquakes occurring close together in both time and location. A typical earthquake sequence often has a single large mainshock followed by many smaller aftershocks. A doublet consists of two major earthquakes of similar magnitude. The second earthquake is not simply an aftershock but a separate mainshock capable of causing additional widespread damage. Scientists have discovered that doublet earthquakes occur because of the complex interactions between faults and the transfer of stress within Earth's crust.

The primary cause of doublet earthquakes is likely to be stress transfer. As tectonic plates move, enormous amounts of stress gradually build along faults over many years or even centuries. When one fault finally ruptures, it releases a tremendous amount of energy in the form of seismic waves. However, the earthquake does not eliminate all the accumulated stress. Instead, it redistributes stress throughout the surrounding crust. Some nearby faults experience a decrease in stress, while others experience an increase. If a neighboring fault is already close to failure, the additional stress transferred by the first earthquake may be enough to trigger a second major rupture. This was likely the scenario for the recent Venezuelan earthquakes.

One of the best-known examples is the 1997 Umbria-Marche earthquake sequence in central Italy. On Sept. 26, two earthquakes of magnitude 5.7 and 6.0 struck just hours apart. The first earthquake damaged buildings and forced many people to evacuate. The second quake caused additional building collapses, including parts of the historic Basilica of Saint Francis in Assisi. Although neither earthquake was exceptionally large, together they produced widespread damage across the region.

Another notable example occurred in 2006 near the Kuril Islands in the northwest Pacific Ocean. On Nov. 15, a magnitude 8.3 earthquake struck along the subduction zone where the Pacific Plate dives beneath the Okhotsk Plate. The Okhotsk plate is a minor tectonic plate covering near the western edge of the North American plate. Just over two months later, Jan. 13, 2007, a magnitude 8.1 earthquake ruptured an adjacent section of the same plate boundary. These powerful earthquakes generated tsunamis that were detected across the Pacific Ocean.

The 2012 Indian Ocean earthquake sequence is another interesting case. On April 11, two massive strike-slip earthquakes measuring magnitude 8.6 and 8.2 occurred approximately two hours apart west of northern Sumatra. Unlike the 2004 Indian Ocean earthquake, these events occurred within the oceanic plate rather than along the main subduction boundary. They were among the largest strike-slip earthquakes ever recorded. A strike-slip fault is a break in the Earth's crust where rock masses slide horizontally past each other, parallel to the fault line. This event helped to provide scientists with valuable information about how complex fault systems behave deep beneath the ocean floor.

Here in the Northwest, the most definitive and well-documented doublet earthquake sequence occurred in Klamath Falls, Ore., in September 1993. On that date, a magnitude 5.9 quake was followed by a magnitude 6.0 quake roughly two hours later. After the event, there were at least 300 additional tremors.

As I mentioned in a previous article, geologists studying marine sediment history have uncovered evidence that the Southern Cascadia Subduction Zone and the Northern San Andreas Fault may be partially synchronized. Massive megathrust earthquakes on the Cascadia fault line have historically generated intense regional stress that triggers a subsequent major earthquake on the San Andreas fault shortly after, which would be a multi-fault doublet system.

In terms of our local weather, there have been some occasional rain showers across the Inland Northwest. However, despite the recent rainfall, our June precipitation will likely finish below normal levels across much of the region. With the strengthening El Niño in the waters of the south-central Pacific Ocean, precipitation in Coeur d’Alene and surrounding areas will likely continue to be below average levels for the next six weeks with high fire danger levels. The exception would be a strong thunderstorm that drops a lot of rain at one time. It’s possible that we could see an increase of moisture from south later in August and September due to a stronger sub-tropical jet stream. Stay tuned.

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Contact Randy Mann at [email protected].