ShakeMap for the M 6.0 American Canyon quake of August 24 (Click to view larger.)
It was at 3:20 am early Sunday morning when tens of thousands of Bay Area residents woke up to an ominous rumbling under their beds. In the deep darkness of the night, the vibrations of the strongest earthquake in the Bay Area since the Loma Prieta Quake of 1989 made their way through our region. They were noticed as far south as Fresno and as far north as Chico. But nowhere else was it felt more strongly than in the City of Napa, and the southern parts of Napa and Sonoma Counties and westernmost Solano County. The epicenter of this magnitude 6.0 temblor lay halfway between Napa and Vallejo, almost 7 miles below the Napa County Airport. And strong it was indeed: The seismic station closest to the epicenter registered shaking of almost a half of a "g" at a distance of merely 2 miles, where "g" is the common unit for measuring ground acceleration in terms of the static gravitational g-pull of the Earth itself.
In Napa, just 6 miles northwest of the epicenter, the shaking let chimneys and brick facades collapse and glass windows burst. In wine cellars, stores and restaurants thousands of bottles of wine jumped off the shelves, spilling hundreds of gallons of the prized liquid on floors and carpets. Because the earthquake also ruptured gas lines, several fires broke out in Napa - the worst one consumed four trailers in a mobile home park on Orchard Avenue. As of early Sunday morning, almost 90 people were reported injured, mostly with cuts and bruises, but three people were admitted to hospitals in serious condition.
With its extensive network of seismic stations in the area, seismologists at the Berkeley Seismological Laboratory on Campus and at the USGS in Menlo Park were able to pinpoint the focus of the earthquake exactly to within one tenth of a mile. But despite such high precision, they were not able to determine on which fault the big temblor had occurred. Such uncertainty is not the scientists' fault. While the location of some faults in the Bay Area can easily be mapped, such as the Hayward fault along the foothills of the East Bay or the San Andreas Fault on the Peninsula, the faults north of San Pablo Bay are much more difficult to decipher. The reason why the location of Sunday's quake could not be pinned to a fault is the fact that faults are usually mapped by their expression on the surface. The curbs distorted by the creeping Hayward Fault in Berkeley and Hayward allow scientist to locate the fault to within inches. But faults do not leave such telltale traces in the loose soil of the epicentral area of Sunday's quake. There the sediments of the Napa River meet the mushy ground of the salt marshes of San Pablo Bay. In fact, experts from the California Geologic Survey - a State office - recently tried to map possible faults in the area, but found no conclusive evidence. For researchers, however, this quake was a blessing in disguise. Mapping the exact location of the aftershocks in three dimensions will yield a picture of the actual fault plane underground. Once these shocks are properly recorded and analyzed, it will be seen if the USGS was correct to initially assign the quake to the widely unknown West Napa Fault. (hra093)
This Good Friday started off as a lush, beautiful morning in Mexican's beach resort town of Acapulco. Thousands of tourists, mainly Mexicans, had gathered there to celebrate the Easter weekend, an important holiday in this mostly Catholic country. However, the peaceful atmosphere came to an abrupt end at 09:27 am local time, when an earthquake with a magnitude of 7.2 struck in the mountains about 80 miles northwest of the city. The seismic waves made the ground under Acapulco rattle and sway and frightened visitors and locals alike. Despite the strong shaking, no significant damage or injuries were reported.
The buildings in the popular resort town were not the only items shaken by the temblor. Within seconds of the earthquake's initiation about half a dozen seismic sensors in the Mexican state of Guerrero registered the initial seismic waves. These instruments are part of a dense network of accelerometers along Mexico's Pacific coast called Sasmex. This acronym stands for "Seismic Alert System in Mexico" and is comprised of almost 100 hundred sensors, which continuously transmit their recordings in realtime to a data center. There computers automatically sift through the data looking for suspicious signals. Once the computers recognized the shaking initially recorded by these six sensors in Guerrero, they went into action: First they calculated the location of the hypocenter, the magnitude of the earthquake and the time it would take for the shaking to reach major population centers in our neighboring country. Because the magnitude of quake near Acapulco easily exceeded a certain threshold, the computers issued a warning about the impending danger.
On Friday only 11 seconds passed between the origin time of the earthquake the issuance of the alert. A few seconds later this alert was braodcast by dedicated radio transmitters in Mexico's largest cities. Because Sasmex alerts are issued so quickly and seismic waves travel more slowly across the country, the alerts reached the population of Mexico City 71 seconds before the shaking started. Morelia, the largest city in Michoacan, had 58 seconds to prepare and the warning time for Chilpancingo, the capital of Guerroro, was still 35 seconds although the city is located closer to the quake's epicenter.
Mexico's government started to finance and build seismic alert systems shortly after the huge 8.1 quake, which devastated Mexico City in September 1985. Several years ago the various earthquake early warning initiatives in the country were integrated into Sasmex, which currently is able to detect and warn of earthquakes along the Pacific coast and in the central volcanic belt. Soon the alert system will be expanded into the states of Veracruz and Chiapas.
Mexico is not the only seismically active country in the world with such a warning system. Japan, Taiwan, Romania, Turkey and several other nations operate similar systems - but not the United States. Although the technology is available along the west coast (see blog on October 2, 2012) and a demonstration system is operating, a public alert system does not exist. California even has a law mandating Earthquake Early Warning, but this legislation has hardly any teeth, as it does not appropriate funds to build such a system. Mexico is clearly ahead of California when it comes to warning the populace of seismic shaking. (hra092)
|Figure 1. This animation shows the earthquakes, which occurred offshore of Northern Chile in the two weeks preceeding the M8.2. The sequence culminated in Monday's event. (animation courtesy of GFZ Potsdam)|
The great magnitude 8.2 earthquake, which struck Northern Chile in the early evening hours on Monday, April 1, 2014, was the strongest temblor in the world since late May last year, when a magnitude 8.3 event shook the Sea of Okhotsk in the Russian far east. Although the magnitudes of the two events are almost the same, there are hardly any similarities between these earthquakes. Last year the earthquake occurred more than 380 miles below the surface and belonged to a class of very deep earthquakes. Although such quakes have been known and recorded for almost a century, the actual physical processes which lead to rupture at this enormous depth are not yet fully understood. However, because they occur so deep underground, such quakes usually do not cause much damage on the surface.
Monday's quake however originated less than 20 miles below the surface, about 60 miles offshore of the Chilean harbor town of Iquique close to the border with Peru. It belongs to a class of shallow depth, thrust earthquakes in which one flank of the rupture plane rapidly slides upward with respect to the opposite flank. Most of the strongest earthquakes on the planet belong to this class of events, among them the Good Friday earthquake, which struck Alaska fifty years ago last week. Because of this rapid upward movement, such thrust earthquakes can generate tsunamis, when they occur offshore. Monday's temblor was no exception.
|Figure 2. Strong earthquakes along the coast of Chile line up on this map like a string of chain links. The location of Monday's quake is marked in red. The last big quake in Chile had a magnitude of 8.8 and occurred in 2010 farther south (Read more from past blogs on March 1st and March 9th 2010). The largest earthquake ever recorded, a magnitude 9.5, can be seen at the bottom of the map. (figure courtesy of GFZ Potsdam)|
However with a maximum height of about seven feet along the the sparsely populated north Chilean coast, its tsunami did not cause any significant damage. Although at least six people died as a result of the shaking by the seismic waves originating from this quake, the damage in the coastal towns close to the epicenter was very limited.
Seismologist had long assumed, that a strong earthquake would strike this region of Chile. The first indication that something was brewing offshore of Iquique came on March 16, when a magnitude 6.7 event struck in exactly the same region. Since then hundreds of quakes with magnitudes larger than 4 were recorded, culminating in the strong event on Monday. While the National Earthquake Information Center operated by the USGS in Golden, Co., determined its magnitude to be 8.2, other world-wide quake monitoring organizations like Geofon, run by the Deutsches Geoforschungszentrum (GFZ) in Potsdam, Germany, computed a slightly lower magnitude of 8.1 The location of the epicenter is marked as red dot on figure 2.
The last time a significant earthquake occurred in this region was in 1877, when a magnitude 8.8 event struck. The extent of its rupture is indicated by the orange oval in figure 2. In the time since that event, the tectonic stress generated by the subduction of the Nazca Plate under the huge South American plate has been accumulating in this area without being released by earthquakes. Seismologists call this build-up of stress along a tectonic boundary the development of a "seismic gap". On Monday, after accumulating for more than 130 years, this stress has been released and the last seismic gap along the coast of Chile has been filled. (hra091)
|The epicenters of dozens of earthquakes with magnitudes greater than 6 dot this map of the tectonic situation near Cape Mendocino. The location of Sunday night's temblor is shown as an orange star. The earthquake labelled with a blue '2010' was described in a previous blog. (Map courtesy of Ross Stein, USGS) (Click to view larger image.)|
Another strong earthquake occurred late Sunday night in the far northern regions of the State. The offshore magnitude 6.9 temblor was widely felt from Roseburg and Medford in southern Oregon all the way to the Bay Area and beyond. Although no significant damage was reported on land within the first several hours of the quake, residents in Humboldt County felt the shaking for dozens of seconds. The quake struck at 10:18 pm. Its hypocenter was almost exactly 50 miles west of Eureka on what is known as the Gorda Plate.
This small cousin of the Pacific Plate glides under the North American Plate in the Cascadia subduction zone. This very seismically active area reaches from Northern California all the way to the state of Washington. The boundary line between the Pacific Plate and the Gorda Plate west of Cape Mendocino is a fault similar to the San Andreas, variably called the Mendocino Transform Fault or the Mendocino Fracture Zone. 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.
Sunday night's quake was located just north of this fracture zone inside the Gorda Plate. This area has to absorb a lot of mechanical strain, which makes it one of the most seismically active regions in the state. Since 1980 there have been at least ten strong earthquakes with magnitudes of 6.5 and above, including the one which shook the region on Sunday night. Taking all of these larger earthquakes and the thousands of smaller ones together, the region around Cape Mendocino has a very clear distinction among seismologists. When measured by the release of seismic energy, it is in fact the most active region of California, beating by far the active faults of the Bay Area and the seismically notorious Los Angeles Basin and its surrounding mountains. (hra090)
Anybody who has ever felt an earthquake will remember distinct jolts during the shaking. In strong temblors these sudden jolts can be sharp enough to throw a person off balance. In seismological terms they represent the arrivals of unique wave trains, like P- or S-waves, which are generated by the rupture in the earthquake source. Measuring and interpreting these onsets, as scientists call the jolts in a rather benign fashion, has been the bread and butter for studying the physics of earthquake sources and the structure of the Earth's interior for a very long time (see blog from January 1st 2012). That is why a discovery just over a decade ago surprised many seismologists. Japanese researchers found signals in the recordings of their dense sensor networks which had no onsets at all. Instead of producing a series of sharp peaks - the jolts - these recordings showed a gentle beginning and then rumbled on for minutes or even hours. In the end, the signal tapered off as sneakily as it began. Nowhere in those recordings could the researchers identify clear arrivals of distinct wave trains.
|Local earthquakes and tremor recorded at Tremorscope station TCHL. The top trace shows two small earthquakes (M2.1 and M1.8) which occurred just north of Parkfield, about 33 km from the station. The bottom trace (bottom trace) shows a 40 minute interval of tremor closer to the station but at a depth of 31 km. The data processing for both traces is the same.|
Similar signals were observed almost 90 years ago on Japanese volcanoes. There they were called volcanic tremor. Their cause: The movement of a fluid within the volcanic edifice, be it magma, hot water or steam, generates a murmur similar to the gurgling of a creek. But the new discovery had nothing to do with volcanoes. The murmur came from deep within the subduction zone east of Japan, where the Pacific plates dives into the Earth's mantle. Hence this type of signal got the name "non-volcanic" or "seismic" tremor. In the years that followed similar signals were observed in the Cascadia subduction zone off the coasts of Oregon and Washington. In 2005 two researchers at the Berkeley Seismological Laboratory, Bob Nadeau and David Dolenc, were the first to discover such seismic tremor in a strike slip fault. They identified the strange signal coming from deep along the San Andreas Fault just south of the tiny hamlet of Cholame in Central California.
In contrast to its volcanic sibling, the origin of the seismic tremor is by no means understood. It is possible that fluid deep below the brittle zone of an earthquake fault may play a role. But it was also observed that tremor episodes can be triggered by passing waves from other earthquakes. Looking through the vast archive of seismic recordings at the BSL, researchers found tremor episodes under Cholame which coincided with passing waves from strong local earthquakes, like the M 6.5 San Simeon quake of 2003 or the Parkfield earthquake of 2004 (M=6). Similarly, such tremor has been triggered by the waves of the strong M 9.0 Tohoku Earthquake, which struck Japan in March 2011 (see blog March 11, 2011 and the following).
Using the various sensors which are being installed in the Tremorscope network - described in the last blog entry - researchers hope to shed more light on these elusive rumbles and murmurs of the deep sections of the San Andreas Fault. (hra089)