Seismo Blog

Temblors where Three Plates Meet

2016-12-08 00:00:00 -0800

Categories: Cape MendocinoSan Andreas Fault

We have often said in this blog that California's most seismically active region lies around Cape Mendocino, halfway between San Francisco and the Oregon border. Thursday morning's off-shore temblor with a magnitude of 6.5 was a reminder of the seismic hazard in this region. People in all of Humboldt County and some places beyond were rudely awakened around 7 am by the swaying motion of the ground. Fortunately no serious damage or injuries were reported.

Map showing north coast of CA, faults, earthquake epicenters represented as stars, and so called beach ball focal mechanisms for two quakes.

This map shows the epicenters of some of the major earthquakes in the vicinity of the Mendocino Triple Junction during the last four decades. The large red arrow points to the location of Thursday's quake. The circles with blue and white quadrants depict the differences in orientation of the earthquakes on the Mendocino Transform Fault and those inside the Gorda Plate. (Modified after Rollins and Stein, Journal of Geophysical Research, 2010)

The reason for the strong seismic activity in the larger vicinity of California's westernmost point is a rather unusual constellation of plates: The Mendocino Triple Junction is the meeting point for three of the lithospheric plates, which make up the Earth's surface. Tectonically the area south of Cape Mendocino is dominated by the San Andreas Fault, the boundary line between the Pacific and the North American Plate. North of the cape, the Gorda Plate, a small cousin of the Pacific Plate, glides under North America in the Cascadia subduction zone. This zone extends from Northern California all the way into British Columbia.

The boundary line between the Pacific Plate and the Gorda Plate west of the cape is a fault very similar to the San Andreas. It is called the Mendocino Transform Fault. It points from Cape Mendocino for several hundred miles straight west into the Pacific Ocean. Along this fault, the Gorda Plate slides horizontally to the east with respect to the Pacific Plate with a speed of about 2 inches per year. This continuous motion is the cause for earthquakes along the fault line. The last significant quake there had a magnitude of 7.0. It happened in September 1994, about 60 miles west of Cape Medocino. The focus of Thursday's quake was almost 40 miles further west.

However, the Mendocino Transform Fault is not the only place in this region where strong earthquakes occur. While it is seismically quiet south of the line, there are many strong temblors within the Gorda Plate itself, north of the transform. On its way east, the relatively small Gorda Plate is battered by its collision with the giant North American Plate. At the same time, the Pacific Plate is pushing northwards from the south, adding further strain to the young oceanic crust of the Gorda Plate. The consequence of this double whammy by its big brothers is that the Gorda Plate gets squeezed. It has developed a system of cracks and faults. Over the decades, many strong quakes have occurred along these faults, like the one in 1994 and the one on Thursday.

Seismologists can distinguish between the quakes along the transform fault and those further north. One method is to compute their locations. While the quakes along the Mendocino Transform Fault are arrayed in a straight line like a string of pearls, the hypocenters inside the Gorda Plate appear to be more randomly distributed. Another way to distinguish between the two kinds of seismic events is to look at their focal mechanisms. Along the east-west trending transform, the movement is right-lateral. In contrast, the quakes within the Gorda Plate have a left-lateral motion along fault lines which trend southwest-northeast. Fortunately, both kinds of movement are mostly horizontal - hence these earthquakes do not pose a tsunami risk. (hra133)


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Temblors where Three Plates Meet

We have often said in this blog that California's most seismically active region lies around Cape Mendocino, halfway between San Francisco and the Oregon border. Thursday morning's off-shore temblor with a magnitude of 6.5 was a reminder of the seismic hazard in this region. People in all of Humboldt County and some places beyond were rudely awakened around 7 am by the swaying motion of the ground. Fortunately no serious damage or injuries were reported.

An Earthquake of Two Flavors

Not all earthquakes are created equal. Instead, there are certain distinct flavors of earthquakes. In temblors "California Style" the two flanks of an earthquake fault slide past each other horizontally with no or only very little vertical movement. This flavor is called "strike-slip". In other quakes, the movement of one of the blocks of a fault is mostly down - called "normal" faulting - or up, dubbed "thrust" faulting. And then there are earthquakes which combine some horizontal with some vertical movement. They are of a flavor called "oblique". But no matter what the flavor and no matter what the magnitude: The vast majority of earthquakes are of just one kind. When a quake starts as strike-slip, it stays the same flavor throughout the duration of the fault rupture. The same is true for the other flavors, i.e. what starts as a thrust ends as a thrust with no changes in the orientation of the fault movement in between.

The Missing Link

It has long been suspected, but scientists could not be sure. Are two of the most dangerous earthquake faults in Northern California, the Hayward Fault and the Rodgers Creek Fault, separate entities or are they connected in some way? The answer to this question lies literally in the mud. It is hidden in the invisible geology under the murky sediments of San Pablo Bay, the northern extension of the San Francisco Bay. San Pablo Bay, with its almost circular shape, separates the narrows between Point San Pablo and Point San Pedro just north of the Richmond Bridge from the Carquinez Strait, through which the Sacramento and the San Joaquin rivers bring their freshwater supplies from further east. Research just published in the journal Science Advances suggests very strongly that the missing link between these two faults has been found. The Hayward and the Rodgers Creek Faults have now to be considered as one more than 110 mile long fault.

Predicting Presidents and Not Earthquakes

Many years ago, when we at the Berkeley Seismology Lab decided to start publishing this Seismo Blog, we resolved not to do certain things: We were not going to trivialize earthquakes and the horrors they can cause as the most potent force of nature on our planet - and we were staying away from politics. And yet here we are, talking about politics less than a week after one of the most consequential upsets in American politics in decades. Well, the blogger will refrain from actually commenting on the results of the recent elections. But he can't bite his tongue any longer after having endured endless months of polls, predictions and forecasts, and after wasting countless hours in front of the TV listening to pundits, analysts and the infamous "talking heads." The bottom line is: All of them were absolutely wrong. In fact, never in modern times have their predictions about who will be our next president been worse.

Falling Like Dominoes

It seems that the trembling Earth under the Apennine mountains of Central Italy does not come to rest. The most recent earthquake on Sunday morning (local time) had a magnitude of 6.6. It destroyed dozens of historic buildings, among them the large Basilica of Saint Benedict in the town of Norcia, which is famous for its medieval wall. Many houses already weakened by the dozens of quakes occuring in the Italian regions of Umbria, Abruzzo and the Marches during the previous months collapsed. Several dozen people were hurt by the crumbling walls. However, as of this writing, nobody had perished in the latest temblor. Sunday's quake is by far the strongest in the latest sequence, which started last week with a magnitude 6.0 quake several miles further north. Looking at the behavior of the quakes in the central Apennine over the past few years, one can detect a clear pattern: Like a set of falling dominoes, the epicenters are steadily migrating northward.

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