The Great San Francisco Earthquake(s)

On October 8th, 1865, the “Great San Francisco Earthquake” hit south of the city of San Francisco, magnitude 6.3.

On October 21st, 1868, the ‘Great San Francisco Earthquake” hit near Haywards, east of the city, across the bay, magnitude 6.8.

On April 18th, 1906, the “Great San Francisco Earthquake” hit the Bay Area, magnitude 7.6.

The death tolls were unknown (but small), 30, and about 3,000, respectively.

Eighteen significant earthquakes happened after that (and five or so had happened between the first “great quakes”) before February 9th, 1971, when the Sylmar earthquake (magnitude 6.7, death toll 65) occurred in the San Fernando Valley. So, about 25 major earthquakes happened in California, of varying degrees of significance with respect to property damage and loss of life, since the earliest influx of immigrants associated with the Gold Rush, which is how California got permanently and meaningfully populated by Europeans.

Right after the Sylmar earthquake, a law was passed that required that earthquake hazard be considered as part of the approval process for new development.

One hundred and six years of time during which a significant earthquake occurred about every four years, passed before the first meaningful response by the civilization living on top of these active faults. Civilization does, indeed, have its faults. As it were.

Will Seattle and Portland Suffer Cataclysmic Earthquakes Any Time Soon?

Meanwhile, to the north, in British Columbia, Washington State, Oregon and parts of northern California, earthquakes were not recognized as a problem. They hardly ever happened. Buildings, homes, bridges, gas-lines, and other infrastructure were deployed without consideration of earthquake hazard for decades.

However, the earthquake hazard in that region is probably much greater in some ways than the earthquake hazard around Los Angeles and San Francisco, which are regularly rocked by fault-line activity. Here, the great plates that make up our planet’s surface do something different than they do in the southern California.

In southern California, the plates are mainly grinding past each other. Fragments of the plates separated by fault lines are squishing past each other like an eraser rubbing against paper. It is not a smooth process, but rather one in which pressure builds up and is released at numerous locations, with each of those release events resulting in some sort of earthquake.

To the north, the main interaction between the plates is the subduction of one plate beneath the other. The subducting (going under) plate moves steadily under the continent, with little fanfare other than slowly elevating that part of the continent, tilting of the land upward to the west and downward to the east (relatively speaking). Then, every now and then, there is an adjustment. The top plate drops all at once, causing a major change in elevation that results in coastal areas being suddenly under the sea, and also resulting in a major earthquake, perhaps magnitude 9.

(Remember, each whole number on the scale used to measure earthquakes is one order of magnitude, so a magnitude 9 earthquake is 100 times stronger than a magnitude 7 earthquake).

It appears that the nearly 700 mile long zone of subduction has suffered 19 “subduction zone earthquakes” over the last 10,000 years, with many more affecting a smaller length of this zone. So, long term, a major earthquake affecting an area hundreds of miles long and who knows how wide, and by major earthquake I mean as never seen before by living humans in the region, and hardly ever observed in recent times anywhere on the planet, affects an area larger than many countries.

Can earthquakes be predicted?

Same with earthquakes. Sort of. The short term with earthquakes is, unfortunately very very short. We know when an earthquake starts that there will be an earthquake over the next several seconds or minutes. That is a little like predicting that it is going to be raining over the next little while when the first drops fall from the sky. You’ve heard of predicting earthquakes longer term, like over days. Every now and then someone observes something that seems to be associated with the geological processes that produce earthquakes, then there is an earthquake, and bingo, we’ve got a method of prediction. But so far every time this has happened, that method of prediction has been invalidated by reality, when it fails to predict subsequent quakes, or produces false positives.

(An interesting example of this happened just yesterday when a scientist — but not a geologist — happen to observe the presence of huge amounts of various gasses appearing along the coast of California, and thought this might be the indicator of an impending earthquake. This prediction was supported by a several years old research project that suggested that gas outflows might predict earthquakes. I’m pretty sure the gas outflow idea has not developed. And, it turns out that the scientist who observed the California gas was simply looking at a common meteorological phenomenon that involved normal human pollution combined with certain atmospheric conditions. Nothing to see here!)

However, long term, earthquakes can be “predicted” using the term “predicted” in modern vernacular parlance. What I mean by that is that the earthquake hazard for a given region can be estimated over longish periods of time with reasonable certainty. We can say, for example, that there is a 63% probability of there being one or more earthquakes of 6.7 magnitude or greater between the years of 2007 and 2036 in certain clearly defined parts of California around San Francisco. This is based on a combination of empirical observation of earthquake frequency and an understanding of how earthquakes happen. According to one study, there is about a one in three chance of a Cascadia subduction zone earthquake (magnitude 8 or 9 or so) over the next fifty years.

So, when planning development or putting together emergency systems, it is possible to know two things. One, what kinds of earthquakes are going to happen (in terms of location, overall range, and magnitude, etc.) and what is the chance of something like this happening.

How do we adapt to earthquakes?

From this emerges something rather counter-intuitive. It turns out that the magnitude of the largest likely quake is more important than the likelihood that it will happen during any medium length time period. It does not matter if a magnitude 9 earthquake is 10% or 1% likely to happen over the next 20 years when you are building a major interstate highway bridge or a skyscraper. What matters is that you build the thing to handle a magnitude 9 earthquake (or, I suppose, prepare yourselves for total destruction of the thing, and have a backup plan of some kind). Development in southern California has to deal with magnitude 7-point-something quakes during the lifespan of a major long-lived structure, while development in Washington and Oregon has to deal with magnitude 8 or 9 quakes during the lifespan of a major long-lived structure. The truth is, your highway bridge near San Francisco has a good chance of being shaken by a magnitude 7 quake, while a highway bridge near Seattle may well outlive its usefulness and be replaced or retrofitted before the once in 500 year trans-Cascadia 9+ quake hits. But you still have to build it to handle the quake because you don’t want to be that guy. (Who didn’t, and then everyone died, and it was your fault.)

There is an interesting historical pattern in the recognition of, and in addressing, earthquakes both in the US an around the world. That century plus time period between what should have been a clue that San Francisco was a quake zone and the first meaningful safety conscious zoning regulation happened initially because developers covered up the first few quakes. They pretended they didn’t happen, downplayed, lied, etc. The 1906 quake was too big to really cover up, of course. Covering up switched to lobbying and lobbying kept regulations off the table for many more decades. Then several dozen suburbanites, voters, taxpayers, whatever got wiped out by a quake that really wasn’t all that bad compared to some of the earlier ones, and a law got passed. So this part of the pattern is denial, followed by different kinds of denial, then some more denial.

Denial of what? Science, of course.

The second part of the historical pattern is science progressing. While most early and mid 20th century construction went along blind to earthquake hazard in southern California because people were being willfully stupid, earthquake unsafe construction proceeded in the northern regions because science had not yet figured it out. Then the denial vs. science thing happened, and is still going on. Decisions have been made at various levels of government in the Cascade subduction zone area that will doom people of the future (one year from now, one century from now, we can’t say) to disaster.

A great new book on earthquakes: “Earthquake Time Bomb” by Robert Yeats

Do you find any of this interesting or important? Then you need to read Earthquake Time Bombs by Robert Yeats.

Yeats explains what earthquakes are. Then he discussed the development of earthquake science, and the politics, cultural response, and technological response to earthquakes, starting with the examples I gave above plus the Haiti earthquake. Then he goes around the world to most of the major earthquake zones and examines the same processes — the geology, the geological science, the engineering and political responses, etc. — in each area.

Yeats is an expert on this, and in fact, has been involved in what he refers to, I think correctly, as the “paradigm shift” in understanding earthquake hazard and risk. This is a shift that happens both within the science and the regulatory and social systems that necessarily address the hazards and risks. He also explains the difference between hazards and risks. Yeats is the go to guy when you want to find out about what to do about earthquakes.

How do we know about the 19 subduction zone earthquakes in the Pacific Northeast that happened over thousands of years? What went wrong at Fukushima, and how do the Japanese deal with earthquakes? What about that New Madrid fault in the middle of the US? What about the Rift Valleys of Africa (where I worked)? What are we doing to do next, what is undone, and how do we do it? These are all addressed in the book.

I came away from Yeats book feeling better about earthquakes. I already knew about the Cascadia quakes and a bunch of other stuff, having done research that required an understanding of tectonic processes myself (though this is not my area). What made me feel better is the simple fact that we can adapt to earthquake hazards by first understanding what they are locally, then applying the proper technology and other systems.

The problem is bad, of course, in regions where earthquake hazard is high, and pre-adaptation is not done for any of a number of reasons, including political or economic ones. Yeats contrasts Japan, the most earthquake ready country in the world, with Haiti, one of the least.

Geology is fun. Earthquakes are one place where the rubber hits the road in geology. This book is a great overview and an important analysis of earthquake hazard and risk worldwide. I highly recommend Earthquake Time Bombs by Robert Yeats.

Another interesting theme that runs through the book is the relationship between leadership, or lack thereof, and the level of magnitude of the disaster’s impact on people, which reminds us of the difference between Katrina and Sandy. In the case of Mount Pelée, local officials had an interest in keeping everyone in town for an upcoming election, so the leadership assembled a commission of sycophants to “study” the volcano’s unrest and determine that it would not threaten the town. Almost every person who lived there was killed when the main eruption occurred, with the death toll being in the tens of thousands. Another theme is the vital importance of effective monitoring and planning for volcanoes, floods, earthquakes, and tsunamis.

This is a book without pictures so the Kindle edition is a good choice if you like eBooks. Also, note that the British edition has a slightly different title.

UPDATE: With a bit of time passing, it is starting to look like a lot of stuff got shook-up, but there was not a lot of significant damage anywhere.

Mount Rainier is a stratovolcano. This means that it is made up of strata of lava, tuff/tephra, ash, and so on that has been pushed out into a conical pattern. The lava from stratovolcanoes is usually viscous enough that it does not spread very far, which contributes to the build up of the cone. Many of the world’s famous volcanoes are of this type (Fuji is a good example). As this volcano grows over time, it also collapses here and there, with avalanches (some of which are called “lahars”) redistributing material outward from the cone. These lahars are fast moving rivers of mud and volcanic stuff that would destroy forest, buildings, whatever was in their path and that can go a considerable distance from the cone of the volcano.

One of the largest lahars recorded by geologists is actually the Osceola lahar from Mount Rainier, about 5,600 years ago. That event covered about 330 square kilometers of landcape. Historically, one of the deadliest lahars was in Columbia, which Nevado del Ruiz produced on in 1985. About 23,000 people died in that event.

There are about three million people who live in the vicinity of Mount Rainier, so if something really big happened there it could have a significant human impact. About 150,000 people currently live on top of old mudflow/lahar sediments. Various models of possible mudflow patterns from Mount Rainier indicate effects quite a long way from the mountain, including heavily settled areas of and near Tacoma.

If you want to monitor the seismic activity at Mt. Reinier, you can check out the USGS Cascade Range Current Update. At present we are getting these messages from the update

“All volcanoes in the Cascade Range are at normal levels of background seismicity. These include Mount Baker, Glacier Peak, Mount Rainier, Mount St. Helens, and Mount Adams in Washington State; Mount Hood, Mount Jefferson, Three Sisters, Newberry Volcano, and Crater Lake, in Oregon; and Medicine Lake volcano, Mount Shasta, and Lassen Peak in northern California.”

and

“A swarm of small, shallow earthquakes occurred beneath the summit of Mount Rainier on Sunday, September 20, 2009, starting at ~0920 PDT. According to the Pacific Northwest Seismic Network (PNSN), the largest event at 0945 PDT has a preliminary magnitude of M 2.2 with a depth of 1.8 km, located 1 km NNE of the summit. The swarm also featured many small events that were too small to be located, and lasted for several hours before gradually waning. Such swarms have happened several times in the past decade. One example is the swarm on November 7, 2004, that included a M 3.2 event. We consider the September 20, 2009, swarm to be within the realm of normal activity at Mount Rainier.”

The USGS also has a seismic network. This page has a map with Mount Rainier on it. Keep an eye on that and if anything happens let us know!

The USGS gives details of two earthquakes in the Baja region, Mexico.

At UTC 18:33 a 5.0 quake seems to have occurred at 28.931°N, 113.022°W, which is in the Gulf of California (a.k.a. Sea of Cortez) near and south of the island Angel de la Guarda. The second and larger quake, a 6.0 (preliminary) magnitude quake occurred at 29.568°N, 113.578°W is near the same island, but on the other side (north) of it.

It is fun to watch the news agencies get all confused. The only on the scene report we have is from about four hundred miles north-northeast, in San Diego, where the city hall got a bit rattled and was evacuated. It is also confusing the reporters that this place Baja is called “California” yet seems to be in Mexico. It would help if they knew how to spell the town reference in the USGS report, which you can read here.

NASA Gives California’s San Andreas, Other Faults a 3-D Close-up

When a swarm of hundreds of small to moderate earthquakes erupted beneath California’s Salton Sea in March, sending spasms rumbling across the desert floor, it set off more than just seismometers. It also raised the eyebrows of quite a few concerned scientists. The reason: lurking underground, just a few kilometers to the northeast, lays a sleeping giant: the 160 kilometer (100 mile) long southern segment of the notorious 1,300- kilometer (800 mile) long San Andreas fault. Scientists were concerned that the recent earthquake swarm at the Salton Sea’s Bombay Beach could perhaps be the straw that broke the camel’s back, triggering “the big one,” a huge earthquake that could devastate Southern California.

The southern end of the San Andreas has remained silent, at least for now. But the earthquake swarm and more recent, widely felt earthquakes in the Los Angeles area have stirred renewed interest in earthquake research. A multi-year project currently under way at NASA’s Jet Propulsion Laboratory, Pasadena, Calif., is seeking to improve our understanding of these mysterious and sometimes deadly natural hazards by using a groundbreaking, JPL-developed airborne radar to study earthquake processes along the San Andreas and other California faults.

The ‘Mother of California Faults’

Formed 15 to 20 million years ago, the San Andreas has defined California’s seismic history and dramatically altered its landscape. It serves as the boundary between the two massive tectonic plates upon which the Golden State rides: the Pacific and North American plates.

Grinding horizontally past each other in a roughly north-south direction at up to 3.5 centimeters (1.4 inches) a year, the fault is a battle zone of pulverized rock, extending to depths of at least 16 kilometers (10 miles). In some places, the plates “creep” quietly past each other, producing small to moderate earthquakes, in a process known as aseismic creep.

But other parts of the fault get “stuck.” They lock in place for sometimes hundreds of years before eventually releasing their pent-up frustrations in epic lunges, such as those responsible for the large magnitude 7.9 earthquakes that struck a then sparsely populated Southern California near Fort Tejon in 1857, and San Francisco in 1906.

From San Luis Obispo south to the Cajon Pass near San Bernardino, the San Andreas forms a largely unbroken line that is often clearly visible from the ground and air. South of Cajon Pass, however, the fault zone becomes more complex. Here, several different faults share the “burden” of moving the tectonic plates, including the San Andreas and the parallel and intersecting San Jacinto and southern San Andreas faults, among others. North of San Luis Obispo, the fault zone similarly splits into nearly parallel faults, with the Hayward and Calaveras faults sharing the plate motion with the San Andreas in the San Francisco Bay area.

Paleoseismological studies dating back 1,500 years have shown that large earthquakes occur on the southern San Andreas about every 250 to 300 years, on average. Yet the extreme southern segment of the fault hasn’t budged for about 320 years. It is apparently overdue, primed for another large event.

Last year, the United States Geological Survey estimated that such a large earthquake, originating near the Salton Sea and rupturing the ground northward to near Lake Hughes in Los Angeles County, could devastate an eight-county region, killing up to 1,800, injuring 50,000, displacing a quarter million people, significantly damaging 300,000 buildings and causing an estimated $213 billion in damage.

Searching for Clues From Above and Below

Like doctors assessing the health of a patient, scientists use a broad array of tools to “listen” to the San Andreas and other faults, looking for clues about their past, present and future behavior. They dig trenches across faults, and place instruments, such as seismographs, creep meters and stress meters, into the ground to try to detect any changes that might be occurring above or below Earth’s surface.

Increasingly, they also rely on space-based technologies, such as those being developed at JPL. Space-based instruments can image minute Earth movements to within a few centimeters (fractions of an inch), measuring the slow buildup of deformation along faults and mapping ground deformation after earthquakes occur. Among these tools are the Global Positioning System and interferometric synthetic aperture radar, or InSAR.

Until recently, the only InSAR data available for the San Andreas and other California faults have come from European Space Agency, Canadian and Japanese radar satellites. But those satellites aren’t dedicated to or optimized for studying earthquakes, and the availability of their data is limited.

A New 3-D Radar Tool

Now, JPL scientists have added a new airborne radar tool to their arsenal. Called the Uninhabited Aerial Vehicle Synthetic Aperture Radar, or UAVSAR, this L-band wavelength radar flies aboard a modified NASA Gulfstream III aircraft from NASA’s Dryden Flight Research Center, Edwards, Calif. The compact, reconfigurable radar, housed in a pod under the aircraft’s fuselage, uses pulses of microwave energy to detect and measure very subtle deformations in Earth’s surface, such as those caused by earthquakes, volcanoes, landslides and glacier movements.

UAVSAR works like this: flying at a nominal altitude of 13,800 meters (45,000 feet), the radar collects data over a selected region. It then flies over the same region again, minutes to months later, using the aircraft’s advanced navigation system to precisely fly over the same path to an accuracy of within 4.6 meters (15 feet). By comparing these camera-like images, called interferograms, over time, scientists can measure the slow surface deformations involved with the buildup and release of strain along earthquake faults.

(UAVSAR is currently wrapping up a two-month expedition in Greenland and Iceland to study the flow of glaciers and ice streams. See this and this).

‘Mowing the Lawn’

Last November, JPL scientists began conducting a series of UAVSAR flights over regions of Northern and Southern California that are actively deforming and are marked by frequent earthquakes. About every six months for the next several years, the scientists will precisely repeat the same flight paths to produce interferograms. From these data, 3-D maps will be created for regions of interest, including the mighty San Andreas and other California faults, extending from the Mexican border to Santa Rosa in the northern San Francisco Bay. Last month, the scientists completed their first full map of the San Andreas. Some regions, such as Parkfield on the central San Andreas, and the Hayward fault, have already had more than one flyover.

“We’ll be ‘mowing the lawn,’ so to speak, mapping the San Andreas and adjacent faults, segment by segment, and then periodically repeating the same radar observations,” said Andrea Donnellan, one of three JPL principal investigators on the UAVSAR fault mapping project, and program area lead for Natural Disasters in NASA Headquarters’ Science Mission Directorate, Washington.

“By comparing these repeat-pass radar observations, we hope to measure any crustal deformations that may occur between observations, allowing us to ‘see’ the amount of strain building up in the San Andreas and adjoining faults,” Donnellan said. “This will give us a much clearer picture of which faults are active and at what rates they’re moving, both before earthquakes and after them.”

Donnellan said the UAVSAR fault mapping data will substantially improve our knowledge of regional earthquake hazards in California. “The 3-D UAVSAR data will allow scientists to bring entire faults into focus, allowing them to see the faults not just at their surfaces, but also at depth,” she said. “When integrated into computer models, the data should give scientists a much clearer picture of California’s complex fault systems, such as those in the Los Angeles basin and in the area around the Salton Sea.”

The scientists will estimate the total displacement occurring in each region. As more observations are collected, they expect to be able to determine how strain is partitioned between individual faults. They’ll also be able to measure ground signals caused by human activities, such as pumping water into or out of the ground or drilling for oil.

The UAVSAR flights will serve as a baseline for pre-earthquake activity. Should earthquakes occur during the course of this project, the team will measure the deformation at the time of the earthquakes to determine the distribution of slip on the faults, and then monitor longer-term motions after the earthquakes to learn more about fault zone properties. “Airborne UAVSAR mapping can allow a rapid response after an earthquake to determine what fault was the source and which parts of the fault slipped during the earthquake,” said Eric Fielding, another JPL principal investigator on the UAVSAR project. “Information about the earthquake source can be used to estimate what areas were most affected by the earthquake shaking to guide rescue and damage assessment response.”

The UAVSAR data will also be used to test the earthquake forecasting methodology developed by UC Davis scientist John Rundle under NASA’s QuakeSim project (see http://www.jpl.nasa.gov/news/news.cfm?release=2003/-074 ). The experiment identifies regions that have a high probability for earthquakes in the near future.

Mapping Faults from the Salton Sea to Santa Rosa

Donnellan’s research will focus on Southern California between the Salton Sea and the Pacific coast, along with the Los Angeles basin, the seismically active Transverse Ranges (the east-west-oriented mountain ranges located between San Diego and Santa Barbara), and the San Francisco Bay area up through Santa Rosa.

Meanwhile, JPL colleagues Paul Lundgren and Zhen Liu will focus on the central San Andreas fault between the Bay Area and Los Angeles. This area is a transition zone between the creeping part of the fault north of the Parkfield segment, which has experienced fairly regular moderate earthquakes of around magnitude 6, and the Carrizo Plain segment, which ruptured in the 1857 Fort Tejon earthquake. They will also integrate UAVSAR with GPS and satellite InSAR data to form more complete models of how the fault slips over time.

JPL’s Eric Fielding will focus on the Hayward fault along the east side of San Francisco Bay, identified as having the highest risk of a damaging earthquake in the Bay Area. The Hayward fault creeps in some parts, but also ruptured in a magnitude 6.8 to 7.0 earthquake in 1868 that caused extensive damage due to its location in the heart of the Bay Area. Fielding will analyze this creep to determine how much of the fault’s overall motion is being released gradually, without large earthquakes, and estimate how much of the fault has accumulated stress since the 1868 quake that could rupture again. From these data, Fielding’s team hopes to develop models of how stress and strain is evolving on the fault system and infer properties of the fault zone.

“Previous studies of the Hayward fault using satellite InSAR were limited by fixed satellite orbits and shorter radar wavelengths that only provided useful measurements in the urbanized areas of the San Francisco Bay,” said Fielding. “UAVSAR will give us a complete picture of the 3-D deformation and map much finer details than are possible from space.”

Initial science results from the UAVSAR fault mapping project will be available some time after the second round of mapping flights are completed. In the meantime, the science team is busy constructing computer models to compare with the actual UAVSAR data once they become available.

What’s Next?

Donnellan said UAVSAR is also serving as a flying testbed to evaluate the tools and technologies for future space-based radars, such as those planned for a NASA mission currently in formulation called the Deformation, Ecosystem Structure and Dynamics of Ice, or DESDynI. That mission, which will study hazards such as earthquakes, volcanoes and landslides, as well as global environmental change, will use both a light detection and ranging sensor, or lidar, and an L-band radar that is very similar to UAVSAR’s but with a much wider ground swath. DESDynI will be capable of providing repeat-pass interferometric data every eight days.

Once DESDynI is in orbit, UAVSAR will be used to calibrate its data and will complement its measurements by filling in gaps in its coverage.

“The Earth science community is anxiously awaiting the launch of DESDynI in a few years,” Donnellan said. “In the meantime, UAVSAR data will give us a head start on better understanding California’s complex fault systems. Its data will also help state and local governments mitigate losses from future earthquakes, including the inevitable ‘big one’ we all know is in our future.”

Learn more about UAVSAR

Learn more about other ongoing JPL earthquake research programs

Mud volcanoes sometimes erupt within days after nearby earthquakes. The number of such nearly coincident events is larger than would be expected by chance and the eruptions are thus assumed to be triggered by earthquakes. Here we compile observations of the response of mud volcanoes and other geologic systems (earthquakes, volcanoes, liquefaction, ground water, and geysers) to earthquakes. The compilation shows a clear magnitude-distance threshold for triggering, suggesting that these seemingly disparate phenomena may share similar underlying triggering mechanisms. The compilation also shows that pre-existing geysers and already-erupting volcanoes and mud volcanoes are much more sensitive to earthquakes than quiescent systems.

M MANGA, M BRUMM, M RUDOLPH (2009). Earthquake Triggering of Mud Volcanoes Marine and Petroleum Geology DOI: 10.1016/j.marpetgeo.2009.01.019

According to the Island Park News:

A modest swarm of earthquakes began in the northeast corner of the Yellowstone Caldera Friday, the Yellowstone Volcano Observatory reports today.
The quakes are about 10 miles (16 km) NNE of the north end of the Yellowstone Lake swarm that was active in late December and early January. As of 7:30 p.m. MST, 10 earthquakes were noted by the University of Utah Seismograph Stations. The largest was magnitude 3.3 and two other events were greater than 2.0. Depths are between 2 and 4 km.

Also noted is the possibility that Earthquakes may change Yellowstone Caldera

Scientists will analyze this data for months to come. They’re already saying the swarm may have created changes in the 37- by 25-mile Yellowstone Caldera in the center of the park, and its associated thermal features. The caldera has a magma plume that extends around 400 miles under the Earth. The caldera’s last large eruption was 640,000 years ago.
The swarm’s tremors have ranged in depth from less than a mile to almost 6 miles.