Fortunately, today’s earthquake was only a large earthquake and not a great (M > 8.0) earthquake. In fact, today’s earthquake was less than 0.5% the size of the Japanese earthquake, and thankfully does not appear to have caused a significant tsunami or significant damage and loss of life in Mexico. There are currently no reported deaths from this earthquake, and the damage seems to be limited to on the order of 1000 collapsed houses near the epicenter. This is partly due to the fact that the earthquake was centered in a rather remote area, and partly due to the geometry of the faulting. In this region, the subduction fault is nearly horizontal, dipping very shallowly down to the northeast. Because the rupture propagated along this fault, sub-parallel to the surface of the earth, there was very little directivity effect on the surface from this earthquake. Instead, most of the shaking energy was absorbed in the crust, leading to relatively mild recorded ground motions even in the epicentral region (note, mild is a relative term: these regions still experience Mercalli Intensities up to VII, corresponding to very strong shaking).

The location of this earthquake is very interesting: it is just on the southern end of a portion of the Middle America Trench known as the Guerrero Gap. This region, extending from just south of Acapulco to around the city of Petatlán, has not experienced a large earthquake in over 150 years. It is considered by seismologists to be a region of high risk for a great earthquake. Because of this concern, the Mexican government has established an earthquake warning system for Mexico City, with seismometers arrayed along the Pacific coast and waiting to detect the Big One in the Guerrero Gap. The Sistema de Alerta Sísmica (SAS) detected this earthquake and issued a limited warning in Mexico City, giving schools, EMS agencies and transit agencies critical seconds to prepare for the imminent shaking. As it happened, the shaking in Mexico City was only around Mercalli Intensity VI, not enough to cause any significant damage, but if this had been a great earthquake in the Guerrero Gap the damage in Mexico City might have been greater.

SAS is a very good system and one of the first public warning systems ever deployed. However, EQW science has advanced significantly in the last few years and SAS has several limitations. First of all, it is based on S-wave measurements, so precious time is lost in waiting for the S-wave to arrive at the coastal stations before a warning can be issued. Second, due to the design of the system, it is really only capable of issuing meaningful warnings in Mexico City. The surrounding areas are left in the dark, so to speak. The SWS solution can perform better because it uses P-wave detection rather than S-waves, and because its distributed design and composite on-site and network-based capabilities are designed to maximize the warning times to all locations, including the epicentral region.

In this event, SWS could have provided about 3-4 seconds of warning time to Ometepec and Pinotepa Nacionál, two cities in the epicentral region totalling over 40,000 citizens. The city of Oaxaca, with over 260,000 people, and Acapulco with over 650,000, could have received 15-16 seconds of warning time from an on-site system. With a network-based warning, these warning times would have increased to 35-40 seconds. This is more than enough to get under cover and protect yourself in the event that your building starts to come down. Had this been a great earthquake of magnitude greater than 8, Mexico City would have been provided with more than one minute of warning.

Gilead Wurman
Chief Seismologist

This event has, to date, claimed almost 300 lives. The death toll will likely rise somewhat over the next few weeks, but this is a comparatively low death toll. While Turkey has had stringent building codes for some time, there is a significant compliance problem on the part of contractors. Consequently many buildings are not very earthquake-resistant, and collapse is common. This earthquake was only 10 miles from the city of Van, with a population of some 400,000. Under the circumstances one might expect death tolls in the thousands. For example, the M 7.6 Izmit earthquake in 1999 killed 17,000 in a city about three-quarters the size of Van.

One possible explanation is that the fault may have ruptured from south to north and from the bottom of the fault to the top. This would have directed the bulk of the seismic energy away from Van and toward the smaller town of Erciş, and this is consistent with the extent of damage in Erciş, which was some 30 miles from the epicenter.

This event highlights some of the major challenges involved in implementing earthquake warning. The event was in a relatively remote part of the country, and very close to the largest city in the region. In these circumstances there is very little time to issue a warning. Because of the remote location, the distance between stations is likely to be very large, meaning that one cannot rely on multiple stations right near the epicenter for all events. Consequently, the important thing is to implement a warning system like QuakeGuard which has robust single-station behavior, and to minimize the processing time to get to the warning. This is difficult to do if one tries to estimate the magnitude of the earthquake, but if the system estimates shaking intensity directly from the P-wave, the warning can be issued much faster.

Although this earthquake was indeed tragic, Turkey has to contend with the knowledge that the next major earthquake is likely to occur right underneath Istanbul, the capital and largest city. The Turkish government has been working hard to implement an earthquake warning system for Istanbul, and we can only hope that this system will be in place before that disastrous event.

Gilead Wurman
Chief Seismologist

Sept. 9 Vancouver BC, EQ ShakeMap (USGS)

Scotts Valley, California (September 9th, 2011) – A strong earthquake initially reported at a (magnitude 6.7) occurred today at 19:41:34 UTC off the western coast of Vancouver Island. Thankfully, the island is sparsely populated on its western shore. Reports of shaking have been received from as far away as Seattle, WA.

“Had a similar size event struck an urban area, significant shaking and potential damage may have occurred.

Regional Earthquake Warning networks, like the one we’re deploying in California, will provide users a level of protection and heads-up … automating system responses at businesses, fire stations and schools ,” states Scott Nebenzahl, Director of Government Affairs, Seismic Warning Systems, Inc.

A regional earthquake warning system would have provided approximately 19 seconds warning in Ucluelet, BC and over 60 seconds warning to central Vancouver, BC.

Short answer: no they are not. We do know of earthquakes triggering other earthquakes at regional distances: the Big Bear earthquake in 1992 was triggered by the nearby Landers Earthquake. This happens because a large earthquake can change the state of stress on the surrounding crust and nearby faults, sometimes causing them to rupture (and sometimes inhibiting them from doing so). But this stress-change effect dies off quickly with increasing distance from the fault, and at distances like those between Virginia, Colorado and California, the effect is long gone.

If someone should wonder in your presence whether these earthquakes are related, you now have a ready answer to show off.

And with that, good night!

Gilead Wurman
Chief Seismologist

Figure 1: Community Internet Intensity Map for the 23 August 2011 Mineral, VA Earthquake. Source: USGS

We here in California pride ourselves on being old hands at this sort of thing, but the fact is the last time we experienced a comparable earthquake here was in 2004, when the M 5.9 Parkfield earthquake occurred (though there have been more recent earthquakes in Baja California and off the Mendocino coast). Nevertheless, this is quite a rare event on the East Coast. It’s been more than a century since the last time this happened, and everyone (seismologists included!) is understandably excited.

As always, a bit of background first. The East Coast is referred to as an intra-plate region, since it is far away from any plate boundaries such as the San Andreas Fault here in California. This means it does not have many large faults, and those few faults that are capable of generating such earthquakes are very old and only break very rarely. The largest earthquake to hit Virginia in recorded history until today was the Giles County Earthquake of 1897. This earthquake is thought to be of comparable size to today’s event, though it was so long ago that there are no seismometer records of the event to calculate a magnitude. That earthquake appears to have been centered in western Virginia, along the New River in Giles County.

Going a little further back, another earthquake, possibly slightly smaller than today’s, occurred slightly south of today’s epicenter in Chesterfield County in 1875. That earthquake was felt strongly in Richmond and surrounding areas, and a reconstructed shaking map is shown in Figure 2. This map, from a paper published in 1971, was constructed from newspaper reports of the event and interviews with people who remembered it, often conducted years after the fact. Compare that with Figure 1, which shows the Community Internet Intensity Map of today’s earthquake. It is not as plain to see, but there does appear to be an area of heightened shaking to the northeast of the epicenter, just as in the 1875 event. There are at least two possible reasons for this: either the earthquake rupture was directed to the northeast, or the ground in that direction tends to promote the transmission and amplification of seismic waves. The same pattern was seen in the 1897 earthquake, and since that quake occurred in another part of the state it seems that the latter explanation is more likely.

Since we are discussing earthquakes in the eastern US, it’s worth mentioning two other places that have experienced large historical earthquakes. Charleston, SC experienced possibly the largest earthquake to hit the East Coast in recorded history in 1886. Again, there are no seismic records of the event but from people’s descriptions this earthquake was likely around M 7. Even larger than that event was the sequence of four earthquakes between M 7 and 8 that hit the area around New Madrid, MO over less than two months in 1811 and 1812. All these events are exceedingly rare, having recurrence intervals measured in centuries.

Figure 2: Shaking map for the 1875 Chesterfield County Earthquake. Source: Bollinger and Hopper, Bulletin of the Seismological Society of America, 1971

Back in the present, today’s earthquake highlighted a few important facts. First of all, there is no place in the US that is entirely free of earthquake hazards. Seismologists do their best to tease out where they are likely to occur, but they are limited by the fact that the historical record is often shorter than the recurrence times for major earthquakes. True, we are much more likely to experience an earthquake in California than in Virginia, but earthquakes can and do happen elsewhere.

Second, while much effort is expended in earthquake education in California, even we could stand a refresher once in a while on what to do in the event. As ample video coverage of today’s earthquake has shown, many people’s first reaction is to get up and try to run out of the building. This is a very dangerous response! In fact, that exact response led to the deaths of two people, who were hit by bricks falling off the side of a building in the San Simeon, CA earthquake of 2003. The appropriate response in virtually all situations is to shelter in place: find a sturdy table, chair or otherwise sheltered area, duck, cover your head and neck, and hold on to your shelter. Evacuate the building only after the shaking has stopped.

Finally, the East Coast is a very different place from California. Many structures there are much older than those in California, and the building codes there did not anticipate the levels of ground motion experienced today. What’s more, the Earth’s crust in the eastern US is much denser, colder and less fractured than that in California, which leads to the seismic energy from the earthquake being transmitted much farther than it is here. A M 6 earthquake in California is seldom felt far beyond the region of the epicenter, but today’s earthquake in Virginia was felt as far away as Canada. Anecdotally, the M 8 New Madrid earthquakes rang church bells in Boston. This means that, although the earthquakes are rare, the exposure of buildings and people to earthquake damage from these rare events is far greater than one would expect. Risk Management Solutions, a company that models major catastrophes, estimates that the damage from a large earthquake in New Madrid today could approach $500 billion!

Seismic Warning Systems is in the process of developing a regional Earthquake Warning System for deployment in California. The aim is to protect people and property by providing a few seconds’ warning before the shaking of a major earthquake is felt. The first deployment of this system will be in the Coachella Valley, as part of the CREWS project. Had such a system been in place in Virginia, for example, the three nearest urban centers (Richmond, Charlottesville, and Fredericksburg) could have had 10-15 seconds warning before feeling today’s earthquake. The North Anna nuclear facility, which was very close to the epicenter, could have received a bit less than 5 seconds warning, while Washington, D.C. and the Surry nuclear plant in Gravel Neck, VA could have received more than 30 seconds warning.

Gilead Wurman
Chief Seismologist

There are two major parts to the story when we think about geology, so let’s deal with them one at a time. The first part is the local soil conditions, sometimes referred to as the “site class.” The softer the soil the bigger the shaking. This effect can be severe. In 1999 there was a moderate (magnitude 5) earthquake at Bolinas, CA. This earthquake was recorded at seismometers on Treasure Island and Yerba Buena Island in the middle of the San Francisco Bay, 18 miles away. Yerba Buena Island is an outcrop of bedrock, meaning the rock goes all the way to the surface and there is no soil. In contrast, Treasure Island is man-made from mud, silt and sand deposited using a technique called “hydraulic fill.” This type of artificial land is very soft and dangerous in an earthquake. Even though the distance between the two seismometers was less than half a mile, the Treasure Island seismometer experienced eight times the shaking of nearby Yerba Buena Island. The Bolinas earthquake was not a large one, but imagine the effect if there were a major event on the Hayward or San Andreas faults.

Soil type and liquefaction hazard in the Bay Area. Orange areas denote high liquefaction hazard and soft Bay Mud deposits. Red denotes areas of very high liquefaction hazard and very soft Bay Mud or artificial landfill.

Unfortunately, this sort of artificial landfill is quite common on the margins of the San Francisco Bay, and even more common is so-called “Bay Mud” which occurs naturally and is almost as soft. During the 1989 Loma Prieta earthquake, two areas received significant damage in spite of their distance from the earthquake: the Marina District of San Francisco which suffered building collapses and fires; and the portion of the 880 freeway in Oakland known as the Cypress Stucture, which collapsed and caused 42 deaths, the majority of the casualties from that earthquake. Both of these structures were built on artificial fill. In fact, the Marina district was built on top of debris from the 1906 earthquake, which had devastated that part of the city 83 years earlier. In this earthquake, the local soil conditions were poor enough to overcome the distance (almost 50 miles) from the fault. Unfortunately, the situation will be much worse following a Hayward or San Andreas event, because the distance from the fault will be much less than 50 miles in most cases.

In addition to soil type, shaking is affected by the underground structure of the bedrock and soil. In certain places there are very deep sedimentary basins underground, and these basins will trap the energy from an earthquake and keep it bouncing around within the basin for a long time (several tens of seconds to minutes, depending on the magnitude of the earthquake). Unlike soil type, it is unfortunately difficult to judge where these basins are just by looking at the surface features. One enormous sedimentary basin is California’s Central Valley, but there are a number of smaller ones in the Bay Area. Santa Rosa is located on top of a deep basin, and as a result it was particularly hard-hit in the 1906 earthquake. There is also one beneath San Pablo Bay, and perhaps most worryingly, two beneath San Jose and the Silicon Valley. The Evergreen basin is on the east side of the Valley, and the Cupertino basin is on the west side. The sediments in these basins are as much as three miles deep in some places.

One final hazard should be mentioned. Soft soils, especially Bay Mud and artificial fill, have a lot of water in them. When the ground shakes, this water can come to the surface and cause liquefaction, a phenomenon in which the soil acts like a liquid for a short time and then solidifies again. This behavior has been known to cause houses to partially sink into the ground and roads to break up like icebergs on an ocean. In a few particularly unpleasant cases, liquefaction has caused underground septic tanks to float to the surface and rupture causing toxic waste spills. Liquefaction can be caused by even moderate shaking, so the effect is not limited to the immediate region of the fault. In a Hayward or San Andreas earthquake under the right conditions, the levees in the Sacramento River Delta could liquefy in large numbers, causing widespread flooding and contaminating drinking water supplies for all of California.

I hope it is clear now that the worst shaking in an earthquake can occur quite a distance from the epicenter. If an earthquake on the Hayward Fault has an epicenter in Richmond and ruptures southward to Fremont, the people in the eastern Silicon Valley will experience the full effect of the Three D’s: they will be close to the fault (though far from the epicenter); the earthquake will be rupturing toward them; and the soils and structure of the Evergreen basin will amplify and prolong the ground shaking from the earthquake. The good news is that, with an earthquake warning system in place, the Silicon Valley could be afforded 20-30 seconds of warning before they felt the shaking from that earthquake begin.

Gilead Wurman
Chief Seismologist
Seismic Warning Systems, Inc.

CRA SitRep #5: Japan Earthquake – NRC Info

March 16, 2011 15:00 PDT

Information, situation report, and donation links for businesses:
US Chamber BCLC http://bclc.uschamber.com/Programs/disaster/japan-earthquake-and-tsunami Help Desk for businesses 1-888-MY-BIZ-HELP (888-692-4943)
Calif. Dept. of Public Health (CDPH) www.cdph.ca.gov, www.bepreparedcalifornia.ca.gov
Google Person Finder link http://japan.person-finder.appspot.com/?lang=en
Relief organizations via Interaction http://www.interaction.org/crisis-list/interaction-members-support-japan-earthquake-response
U.S. Military Pacific Command (PACOM) www.pacom.mil
UN OCHA (Office for Coordination of Humanitarian Affairs) from www.cidi.org
The Aidmatrix Network is working with 2nd Harvest Japan http://2ndharvestjapan.blob.core.windows.net/index.html and NetHope Japan http://www.nethope.org/japan/
PLEASE NOTE THAT THESE REPORTS MAY CONTAIN INFORMATION THAT IS PRIVILEGED OR CONFIDENTIAL, AND MAY CONTAIN INFORMATION THAT IS NOT VERIFIED.

NRC Guidelines to US residents WITHIN JAPAN 3-16-11 [Note: Domestic text highlighted below]
NRC PROVIDES PROTECTIVE ACTION RECOMMENDATIONS BASED ON U.S. GUIDELINES

Under the guidelines for public safety that would be used in the United States under similar circumstances, the NRC believes it is appropriate for U.S. residents within 50 miles of the Fukushima reactors to evacuate.

Among other things, in the United States protective actions recommendations are implemented when projected doses could exceed 1 rem to the body or 5 rem to the thyroid. A rem is a measure of radiation dose. The average American is exposed to approximately 620 millirems, or 0.62 rem, of radiation each year from natural and manmade sources.

In making protective action recommendations, the NRC takes into account a variety of factors that include weather, wind direction and speed, and the status of the problem at the reactors.

Attached are the results of two sets of computer calculations used to support the NRC recommendations.

In response to nuclear emergencies, the NRC works with other U.S. agencies to monitor radioactive releases and predict their path. All the available information continues to indicate Hawaii, Alaska, the U.S. Territories and the U.S. West Coast are not expected to experience any harmful levels of radioactivity.

CDPH/CalEMA Press Release 3-15-11
Statement from California’s Department of Public Health and
Emergency Management Agency on Risk of Radiation Exposure

SACRAMENTO – Today the interim director of the California Department of Public Health, Dr. Howard Backer, and acting secretary of the California Emergency Management Agency, Mike Dayton, issued the following statement emphasizing Californians’ safety from radiation exposure and the risks of taking potassium iodide as a precautionary measure.

“The safety of all Californians is our highest priority, and we are in constant contact with the federal agencies responsible for monitoring radiation levels across the West Coast.

We want to emphasize that the U.S. Environmental Protection Agency, the Nuclear Regulatory Commission and the U.S. Department of Health and Human Services have all stated that there is no risk expected to California or its residents as a result of the situation in Japan.

We are actively monitoring the situation in Japan and are ready to take all steps necessary to protect Californians should risks develop.

We urge Californians to not take potassium iodide as a precautionary measure. It is not necessary given the current circumstances in Japan, it can present a danger to people with allergies to iodine, shellfish or who have thyroid problems, and taken inappropriately it can have serious side effects including abnormal heart rhythms, nausea, vomiting, electrolyte abnormalities and bleeding.

Our thoughts are with the people of Japan at this tragic time.”

Californians with questions about radiation exposure can contact the California Department of Public Health’s Emergency Operations information line at 916 341-3947.

For updated information on monitoring, response and relief efforts in California and Japan, please visit the California Emergency Management Agency website at www.calema.ca.gov.

Frequently Asked Questions (FAQ) and other information about radiation can be found here.

US Chamber BCLC (Business & Civic Leadership Center) 3/16/11

People, Damages, Impacts

More than 15,000 are still missing, with 24,000 awaiting rescue. Just in Fukushima, 1,200 people are unaccounted for still. The official numbers confirmed that 3,100 people have died, 1,885 people are injured and 2,369 people remain missing (World Bank has missing at 15,000).

About 416,300 people have been evacuated from the earthquake/tsunami affected provinces, half of which are from Miyagi (203,953 people). Others are from Fukushima (131,665 people), Iwate (46,405 people), Ibaraki (22,595 people), Tochigi (9,530) and Aomori (2,143 people). More than 550,000 are in temporary shelters and in need of additional water, food, blankets, and sanitation facilities. The dropping temperatures, disruption of gas and electrical supplies, are exacerbating problems in the shelters.

Significant parts of the country remain without water, power, and fuel. Essential services (electricity, gas and water) remains interrupted, with more than 843,000 households experiencing power shortages. As of the 15th, 3,385 buildings are destroyed and over 55,000 damaged either by earthquakes, tsunami or fire.

1.6 million households are still without water (320,000 households in Fukushima Prefecture, 290,000 households in Miyagi, 110,000 households in Iwate, and 670,000 households in Ibaraki). Water shortages may increase, due to communication lines still being down in some devastated areas.

The government is making significant progress to restore key roads, bridges and railways, but transportation systems still remain paralyzed. At least 128 roads and 21 bridges that were damaged by the earthquake/tsunami have been repaired.

Nuclear Reactors

The Dai-ichi plant is the most severely affected of three nuclear complexes that were declared emergencies after suffering damage from Friday’s quake and tsunami. Another explosion has occurred in unit 4 and possible meltdowns are occurring at units 2 and 3. Japan reports that radiation levels have fallen at the Fukushima Daiichi nuclear plant, after levels had spiked to harmful levels following a fire and two more explosions at the site. Workers at the plant had been withdrawn following the sudden spike in radiation but have now returned. 50 employees had been working at the plant to try and cool its four reactors. Radiation levels have fallen from 1000 millisieverts to 600-800 but even at these levels workers cannot carry out nominal tasks.

These levels of radiation leaking from the plant forced the government to order 140,000 people to seal themselves indoors Tuesday after an explosion and fire. According to Prime Minister Naoto Kan, radiation had spread from the four stricken reactors of the Fukushima Dai-ichi nuclear plant along Japan’s northeastern coast. The fires & explosions have injured 15 workers/military personnel and exposed up to 190 people to increased radiation.

The evacuation zone has been increased to 20-kilometer zone around Fukushima Daiichi. The Japanese advise that people within a 30-km radius should take shelter indoors. A 30-kilometer no-fly zone has also been established around the Daiichi plant. Iodine tablets have been distributed to evacuation centers – but no decision has yet been taken on their administration.

A 6.1M earthquake was reported today in Eastern Honshu, Japan. (This is reportedly not an aftershock of Friday’s quake). The Hamaoka nuclear power plant is sited about 62 miles from the epicenter. The International Atomic Energy Agency (IAEA) Incident and Emergency Centre (IEC) confirmed the plant continues safe operations.

Experts say the radiation from Japan’s reactors pose little threat to the wider world. Immediate injury from the reactors at the melting Fukushima plant pose small health risks in the short term. It is the long-term risks of cancer that pose the most serious risks at the moment for residents within a 20km radius of the Daiichi reactors.

Economic

The impact of the earthquake/tsunami continued to drag down stock markets. The benchmark Nikkei 225 stock plunged for a 2nd day, dropping more than 10% to close at 8,605.15 while the broader Topix lost more than 8%. To lessen the damage, Japan’s central bank made two cash injections totaling 8 trillion yen ($98B) on Tuesday into the money markets after pumping in $184B on Monday.

Japan’s crude steel production capacity could be cut by 20% because of damage to key steel mills. Operations in four steel mills in Iwaki, Miyagi, and Ibaraki Prefectures have been disrupted.

According to the World Bank, initial loss estimates range between $15 billion – $35 billion. Estimates put repair costs in the tens of billions of dollars – costs that would add to a massive public debt that is the biggest among industrialized nations, at 200 percent of gross domestic product. The minister of economic and fiscal policy said that the economic damage from the Great East Japan Earthquake could be far more serious than that of the 1995 Great Hanshin Earthquake near Kobe, which incurred economic losses worth 10 trillion yen ($122 billion) and not surprising as all the cities, towns and villages along the Sanriku coast were devastated.

Damage to the U.S. and world economies is expected to be relatively moderate and short-lived. Oil prices are falling, helping drivers around the world. The Dow Jones industrials were down more than 180 points in mid-afternoon trading. Stocks plunged almost 11% in Japan, 5% in Germany and 4% in France. Some U.S. auto parts makers could benefit if Japanese plants in the United States substitute U.S. parts for those they usually get from Japan.

The reconstruction along Japan’s NE coast could provide some economic growth. Japan’s contribution to the world’s economy fell from 18% in 1995 to 9% in 2010. The region hard-hit by the quake accounts for just 6 % – 7% of Japan’s output, about half as much as the area hit by the 1995 Kobe quake.

Response Management

100,000 soldiers have been mobilized by the government, and 90 medical teams were deployed by the Japanese Red Cross who are trying to provide basics care for 430,000 in remote towns spread along the coast. So far, emergency workers have rescued 15,000 people.

International Response

Due to the earthquake, the European Commission activated the European Civil Protection Mechanism, which facilitates cooperation in disaster response, and 31 countries participate in its operations. They pool their resources and make them available to disaster-stricken countries world-wide. 20 of them have offered personnel and equipment through this mechanism so far.

Eight experts of The U.S. Nuclear Regulatory Commission (NRC) are scheduled to arrive in Japan on Wednesday. These experts will provide technical advice on managing the situation in Fukushima Dai-ichi Nuclear Power Plant.

In response to a first request by Japan, Canada, working with the Canadian Red Cross, will provide approximately 25,000 woven thermal wool blankets from its emergency relief supply stockpile to support the urgent efforts for the people of Japan. The Adventist Development and Relief Agency (ADRA) Japan continues coordinating with the Japanese Department of Social Services (DSS) and anticipates involvement in managing and coordinating evacuation centers in the affected area. ADRA is preparing to accommodate 1,000 evacuees, coordinating the procurement of food, non-food items and equipment, and transportation. G8 countries are ready to provide Japan with help to overcome the consequences of the disaster. World Vision International confirmed a $10M appeal across their partnership to finance a massive relief response. A disaster relief team from Operation Blessing International (OBI), the 6th largest international humanitarian organization, has arrived in Sendai, Japan and is distributing clean drinking water and emergency food rations today.

As numbers of people missing continues to grow, Chinese, Korean, Portuguese and Spanish languages have been added to Japanese and English on the website set up by the International Committee of the Red Cross (ICRC) (together with worldwide Red Cross and Red Crescent partners) to assist people anxious for news about loved ones in the disaster area. An initial survey carried out by the Japanese Red Cross Society revealed that large numbers of foreigners – in particular Brazilians, Chileans and Peruvians – were living in the areas affected by the disaster.

Earthquake/Tsunami Context

The eastern Shizuoka Prefecture in central Japan was hit by a strong earthquake (not an aftershock) hit on Tuesday night measuring 6 – 6.4M at a depth of about 8½ miles. There may be slight changes in sea levels but there’s no need to worry about tidal damage.

According to the Global Disaster Alert and Coordinating System (GDACS) waves up to 5 meters high starting hitting shores in Japan almost immediately after the 9.0M earthquake on Friday. Larger waves of around 10 meters started rushing ashore around 20 minutes following the quake. The state of the art warning systems in Japan did all they could to inform people on time they were in harm’s way. This sheds light on current land use and re-development may need to take into serious consideration where and how to re-build.

Sincerely,

Peter Ohtaki
California Resiliency Alliance

Japan sits on four tectonic plates: Pacific, Philippine Sea, Amurian and Okhotsk

In the last few days the scientific analysis of this earthquake has advanced significantly. We now know that this is the largest earthquake in Japan’s recorded history. This is saying something for a country that sits on the boundaries of no fewer than four tectonic plates (Pacific, Philippine Sea, Amurian and Okhotsk). Those last two plates may sound unfamiliar, because scientific research is only now indicating that these plates exist. They were previously thought to be part of the larger plates of Eurasia and North America, respectively (the North American plate extends through Alaska and Kamchatka down to the north end of Japan). The largest earthquake recorded in Japan happened in the year 869, in the Jogan earthquake which had an estimated magnitude between 8.4 and 9.0 (there are no seismic records dating to that time, but geologists analyzed buried tsunami deposits to estimate the magnitude). As it happens, the Jogan Earthquake occurred along the same stretch of the Japan Trench, suggesting that last week’s earthquake may be a recurrence of the Jogan Earthquake (such large earthquakes recur only every 1000 years on average). As I mentioned in my last blog post, this region was not considered to be at the highest risk for an earthquake: the Nankai Trough south of Tokyo holds that dubious honor. However, this part of the Japan Trench has been observed using GPS for several years, and had been found to have a “slip deficit.” This means that this fault had not relieved built-up pressure from the motion of the plates as much as surrounding portions of the fault had. This relief comes most often, unfortunately, in the form of major earthquakes.

Japan experiences about 20 times more earthquakes than California because of its complex tectonic setting. The most devastating earthquake recorded here was the Great Kanto Earthquake of 1923, a M7.9 event that killed about 140,000 people in the Tokyo area. Last week’s earthquake measured 8.9 on the Moment Magnitude Scale, about 30 times the size of the Kanto Earthquake. It may even have been bigger: one solution puts the magnitude at 9.1, on par with the 2004 Sumatra Earthquake and about 60 times the size of the Kanto Earthquake (magnitude estimates vary because the true size of an earthquake is very difficult to measure).

Estimated slip on the fault plane

By carefully examining the seismic waves emanating from this earthquake and doing some very intensive calculations using computer clusters, seismologists can estimate the distribution of slip on the fault that ruptured. The figure on the right shows this slip. This figure shows a plan view of the fault, so keep in mind that the actual fault plane dips down to the left, into the Earth. The right edge of the fault plane is where the fault emerges on the seafloor off the coast of Sendai, and the black asterisk is the epicenter where the rupture started. This rupture was very large, measuring over 350 km in length and over 150 km in width. The maximum slip on the fault is more than 15 meters, meaning the Pacific plate moved more than 50 feet with respect to Japan! In some places, this led to the coastline of Japan itself being shifted by more than 8 feet (remember, the coast is more than 100 km from the fault itself). And though one tends to think of earthquakes as very sudden events, they do take some amount of time to rupture. In very small earthquakes, that time can be a second or less. A moderate earthquake like Loma Prieta in 1989 (M 6.9) takes about 15 seconds to rupture. This earthquake took almost 3 minutes from start to finish, and that’s just how long it took for the fault to move. The perceived shaking from the earthquake lasted much longer.

How a megathrust earthquake generates tsunami

Of course, a great deal of the damage from this earthquake was due to the tsunami it generated. It was this tsunami, for example, that seems to have dealt the critical blow to the nuclear reactors in Fukushima, which are now in danger of releasing radioactive material. This type of earthquake, a so-called “megathrust” event, often generates tsunami. This was the case in the 2004 Sumatra Earthquake, and last year’s 8.8 earthquake in Chile to a lesser degree. These earthquakes are technically simply “thrust” or “reverse” earthquakes. We call them megathrusts because of their size, and to differentiate them from smaller thrust faults that occur at all scales (the 1994 Northridge Earthquake and the 1999 Chi-Chi, Taiwan earthquake were both thrust earthquakes, but not megathrusts). Megathrusts occur almost exclusively in the oceans (the Himalayan megathrust in India is a notable exception), and when they rupture they cause a displacement in the seafloor as shown in the figure to the left. The plates continually press against one another, and in between earthquakes as the pressure builds up the upper plate (Japan and the Okhotsk plate in this case) bends and flexes to accommodate the pressure. In the figure, the dashed line shows the shape of the upper plate before the earthquake. When the earthquake happens, the upper plate relaxes back to its natural shape (the brown solid line in the figure), and the upward motion of the seafloor at the location of the fault pushes up the water column above it, generating the tsunami. The severity of the tsunami depends on how deep the earthquake is: a shallow earthquake will generate a large offset at the seafloor and a large tsunami, while a deeper earthquake may not even breach the surface and its tsunami will be much smaller.

This “unflexing” of the upper plate has a second, longer term effect in addition to the tsunami. Note that farther from the fault the upper plate was actually flexing upward (dashed line), and after the earthquake it relaxes down. This subsidence is permanent, meaning that even after the tsunami has come and gone, there will be parts of the Japanese coast that will remain underwater forever (well, not forever, just for the next few hundred years).

Although it seems as though we have had a lot of very large earthquakes recently, megathrust events are very rare, occurring on average only once or twice per decade around the world. When such events happen, we must make a great effort to study and learn from them in preparation for the next one. This is particularly true here on the West Coast, as one of the greatest threats to our region of the world is the Cascadia Megathrust, a fault extending from Cape Mendocino in California all the way to British Columbia and capable of a magnitude 9 earthquake. The last time such an earthquake occurred was in 1700.

Gilead Wurman
Chief Seismologist

This earthquake was preceded by a magnitude 7.2 earthquake two days ago near the epicenter. In light of today’s quake, that earthquake is considered a foreshock. It is possible that that earthquake triggered this larger event by increasing the stress on the fault at today’s epicenter. This is only conjecture at this point, and more detailed analysis will be done by researchers around the world in the coming months to investigate this possibility.

This earthquake did not occur in one of the closely-watched seismic gaps bordering Japan. The most hazardous of these is considered to be the Nankai Trough, about 800 km southwest of this earthquake. The region of this earthquake last ruptured in 1938 in a magnitude 7.8 earthquake somewhat south of the epicenter, while the epicentral region and the fault extending northward last ruptured in a series of earthquakes between 1897 and 1901 (the exact location and size of these events is unknown, as there were no seismometers at the time).

This earthquake was very shallow, only about 24 km from the surface at the hypocenter (the point at depth below the epicenter). It is quite likely that this earthquake ruptured all the way to the surface of the seabed, which is why it has generated such a significant tsunami.

At this time the full extent of the damage is unknown. The USGS PAGER system is estimating between 10 and 1000 deaths, and between $1 billion and $100 billion in damage, but these estimates are based on the shaking from the earthquake and do not include tsunami damage. The tsunami is forecast to reach Hawaii around 5am PST, and a tsunami warning is in effect there. A tsunami warning is additionally in effect for the Aleutian island chain in Alaska.

Had Seismic Warning Systems had regional earthquake warning system in place in Japan, it might have provided an estimated 10-20 seconds of warning in Sendai and the epicentral region, and as much as 45-60 seconds in Tokyo.

We will update this blog as information develops.

Gilead Wurman
Chief Seismologist

A map of the region of the earthquake, showing the different types of faulting. Red faults represent an extensional mid-ocean ridge, while green faults are strike-slip.

The tectonic environment in which this earthquake occurred is very complex and interesting. As you can see on the map, the area between the Salton Sea and the Gulf of California is an area of transition between the strike-slip motion of the San Andreas Fault (in green) and the mid-ocean ridge that is opening up in the middle of the Gulf of California (in red).  There are many secondary faults in this area that are not shown on this map, some of which are quite capable of generating an event of this size.  When an event of this magnitude occurs it can significantly change the state of stress in the area, possibly leading to more or less risk of earthquakes on nearby faults.  In this case, the stress change has relaxed some of the pressure on the San Andreas Fault near the Salton Sea, ever so slightly reducing the likelihood of an earthquake on that fault.  More importantly however, the earthquake actually increased the pressure on both the San Jacinto and Elsinore faults, two major faults to the west of the San Andreas that are each capable of generating another earthquake of this magnitude or even larger!  What’s more, both the San Jacinto and Elsinore faults pass much closer to the large population centers of the LA Basin and the Coachella Valley than does the Laguna Salada fault.

When this earthquake first occurred, many people thought they were feeling the Big One.  And indeed, M 7.2 is nothing to sneeze at.  But the real question is, was this earthquake merely a warm-up act for another major quake, perhaps in the next few years?  It is impossible to accurately predict earthquakes before they happen, so only time will tell if these concerns are founded, but in the meantime the best course of action is simple.  As the Boy Scouts say, Be Prepared!

Gilead Wurman
Chief Seismologist
Seismic Warning Systems, Inc.

Update: The LA Times ran a story last week that echoes what I mentioned: that the risk of earthquakes on the Elsinore and San Jacinto faults has  been increased.