Conflicing research on state of stress in Chile

The New York Times carries an article today about the state of stress in the vicinity of the epicenter of last February's magnitude 8.8 Chile earthquake. Different sources of information  - seismic waves, gps, tsunami waves - have yielded somewhat different pictures. The epicenter occurred outside of the 200-mile-long Darwin gap which had not seen a large quake since one that Charles Darwin observed in 1835.  Did the occurrence of the 2010 quake lead to a stress increase in this gap, and a higher risk of an upcoming large quake?  We have the usual conclusion: more data are needed.

Chile Earthquake: NSF Research Grants

From the National Science Foundation, May 3, 2010:

In response to the magnitude 8.8 earthquake that occurred in Chile on February 27, 2010, the National Science Foundation (NSF) has awarded three major Rapid Response Research (RAPID) grants to study how the quake happened, and learn from those findings for the future.

The Chile quake is the fifth largest earthquake ever recorded, and the second largest that has occurred since a modern suite of instruments was developed to collect data related to earthquakes.

"The new deployments will provide valuable data to help scientists understand earthquakes not only in Chile, but around the world," says Russ Kelz, program director in NSF's Division of Earth Sciences. "NSF is stimulating important advances in basic earthquake science, rapid-response geophysical and data communications technology, and international collaboration and data sharing."

Two NSF RAPID awards--made to a consortium of institutions including Ohio State University, California Institute of Technology, University of Hawaii, University of Memphis, and UNAVCO Inc., in Boulder, Colo.--provide for installation of 25 continuously operating global positioning system (CGPS) stations, as well as state-of-the-art satellite communications for data delivery.

Through another Chile quake RAPID award, 60 seismic stations arrived in Santiago, Chile, from the NSF-supported Incorporated Research Institutions for Seismology (IRIS) Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) Instrument Center in Socorro, New Mexico.

With data from the instruments, scientists will obtain high-resolution locations of aftershocks, carry out research on Earth structure in and around the Chilean subduction zone, and study major cities for a detailed analysis of local response.

All data collected as part of the U.S. deployments will be open and freely available immediately after collection.

More information about these RAPID projects may be found in IRIS Community Instrument Deployment in Chile and Science Highlights 2010 - UNAVCO Event Response.

Earthquakes, urbanization, and fatalities

Roger Bilham is a fellow of CIRES,the Cooperative Institute for Research in Environmental Sciences, at the University of Colorado, Boulder. His research includes archival research into historical earthquakes, the statistics of urban earthquakes, and global seismic hazards. His web page on the Haiti earthquake includes a link to a news piece he wrote for Nature, where he argues that the destructiveness of the Haiti quake was due not only to its large (although not enormous) magnitude and energy release, but also the amplification of ground shaking by softer sediments, the proximity of Port-au-Prince to the epicenter, and especially to poor building construction practices in a large urban center.

He presents the rather fascinating diagram below, plotting fatalities against earthquake magnitudes:

Note that the vertical scale is logarithmic. There is a general correlation that larger magnitudes cause more deaths, but this is far from a linear plot. Notice that may of the most deadly quakes have been in fairly recent years, and don't highly correlate with magnitude. Why? Urbanization and poor construction. Better earthquake engineering is one of the reasons that the damage in Chile, although bad enough, was less than that in Haiti, although the Chilean quake was much larger.

Dr. Bilham comments:

With a Richter magnitude of M=7 the Haiti earthquake is classed as a major earthquake, however, no previous M=7 earthquake has resulted in this many fatalities (the Messina, Italy, earthquake of 1908 killed 82,000). At 230,000 the death toll is close to being the most lethal earthquake since 1900, and the second most lethal earthquake ever. The large death-toll was caused by the almost complete absence of earthquake resistance in most of the structures in Port-au-Prince and the surrounding towns and villages.

A recent article (2009) discussing the seismic future of cities may be downloaded here. It forecasts a rising death toll from earthquakes, especially in the developing nations, where earthquake resistance building codes are either absent or not enforced.

Chile aftershock report

The U.S. Geological Survey presents more interesting information on the Chile magnitude 8.8 quake here. The aftershock report notes

In the time period since the earthquake's origin at 2010-02-27 06:34 to 2010-03-15 15:00 UTC, the USGS NEIC has located 229 aftershocks of magnitude 5.0 or greater. Eighteen of these aftershocks have magnitudes of 6.0 or greater.

You've been hearing about many of these not insignificant aftershocks in the news.

Below is a current aftershock map, which can be found with updates here. Note the scale - these have been occurring along the plate boundary over a distance of about 1000 km, and over an area of about 50,000 square kilometers!

Chile poster - fault plane solution

With all the significant aftershocks after the 8.8 Chile aftershock, the accumulation of more data, and more analysis, the USGS poster has already changed.

But in my desire to present the different pieces of the earlier version of the poster, below is the fault plane solution (aka focal mechanism solution):

The fault plane solution, based on waves generated by the quake recorded at different seismic stations around the world, allows the inference of the type of faulting and the orientation of the fault plane.

The solution above is that of a nearly north-south striking fault. If the fault plane is dipping to the east, it is shallow; if it is dipping to the west, it is steep. Either would be consistent with the first motion analysis,but only the former makes more sense for the tectonic setting: the subduction of the Nazca Plate beneath the South Americn plate.

Arild Andresen of the University of Oslo has a very nice online tutorial on fault plane solutions.

Chile poster - South American earthquake depths

Going back to my Shaking Earth entry of March 1, note the epicenters within the box using different colored symbols.

Compare this to the figure below from the Chile earthquake poster. This shows the depth of the quakes in the box of the earlier post, projected onto the central line.  The colors show the depths of the earthquake hypocenters  (or foci, plural of focus): red for depths of 0-69 km, green for 70-299 km, and blue for depths of 300-700 km. (The figure below doesn't show any blue dots.) But you can see the downward dipping slab of seismicity, also known as a Benioff zone, along the Nazca plate as it subducts beneath the South American plate.  There are also some shallow (red) quakes to the right,occurring in the overriding South American plate.  And, you will also note the volcanoes on the South American plate, represented by the yellow triangles.  These occur when the downgoing slab partially melts, and the magma rises to the surface.  This cross-section view (from the side) complements the earlier map or plan view (bird's-eye view) to give a three-dimensional representation of seismicity.

Chile earthquake moves South America 10 feet

Sunday I was in the Deutsches Museum in Munich, which has halls and halls full of scientific instruments and explanations, some more interesting than others.  I liked the historical instruments the most, such as the Magdeburg spheres.  There was one hall dedicated to geodesy, measuring the size and shape of the earth, and the surveying of its surface, from the use of chains and compasses to total stations to gps.

The Berkeley Seismo blog has a nice entry on the movement in South American due to the Chile earthquake. Concepción, near the epicenter, moved over 10 feet!  This is because earthquakes are caused by displacement on a fault.

I plan to discuss the westerly motion and its relation to the balloon diagram of the fault plane solution in a later post.

Figure from the Berkeley Seismo blog

Chile earthquake intensities

Below is another clip from the USGS Chile earthquake poster showing the Modified Mercalli intensities from the recent 8.8 magnitude quake. Recall that magnitude is a seismograph-based estimate of the energy released by a quake and will be more or less the same for all seismic stations that record the waves. Intensities are evaluated according to human reactions and effects on buildings, are highest near the epicenter, and fall off with distance. I was surprised to see that Santiago has almost 5 million residents, so lots of people that are at risk.  The intensity in Santiago was VII ("Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken") on a scale of I-XII.

 

This map of the intensity data is a little easier to read:

No doubt about it, this was an intense event.

Chile acceleration map

I spoke too soon. The USGS National Earthquake Information Center has posted a PGA map (peak ground accleration) for the Chile earthquake.

This is given as a contour map. Maximum values at the epicenter are about 30% of the acceleration due to gravity, which approaches the 50% shown on the seismic hazard projections of the last two blog entries.

Presumably, the blue dots represent data points from seismographs/accelerographs.

Slowing down on acceleration - Chile seismic hazard

Thank you to the anonymous commentator on my last post. Although it was not explicit in the USGS poster, it makes sense that the figure on acceleration would not represent actual data from the Chile quake.

As best I can tell, the source of the information is probably Figure 6 from an article by Shedlock and Tanner: Seismic Hazard Map of the Western Hemisphere, part of the Global Seismic Hazard Assessment Program.

This seismic hazard map of South America depicts the median peak ground acceleration (PGA) with a 10% probability of exceedance in 50 year. "PGA, a short-period ground motion parameter that is proportional to force, is the most commonly mapped ground motion parameter because current building codes that include seismic provisions specify the horizontal force a building should be able to withstand during an earthquake. This seismic hazard map of the Americas depicts the likely level of short-period ground motion from earthquakes in a fifty-year window. Short-period ground motions effect short-period structures (e.g. one-to-two story buildings)."

"There are three major elements comprising the method used to calculate the PGAs: "1) the characterization of seismic sources; 2) the characterization of attenuation of ground motion; and 3) the actual calculation of probabilities."

Thank you, Anonymous.

Fig. 6 from Shedlock and Tanner:

Acceleration due to seismic waves - Chile poster

One of the primary hazards intrinsically associated with an earthquake is groundshaking.  This is a vector, with both horizontal ad vertical components.  The second derivative of the displacement is the acceleration.   Buildings are made to withstand vertical acceleration - after all, that is what gravity is.  However, by default, buildings are not engineered to withstand horizontal accelerations, although lateral bracing can be used, and should be within seismic zones.

The figure below, from our USGS poster on the Chile quake, plots ground motion accelerations from the Chile quake, and shows a couple of very interesting features. Note that the maximum acceleration caused by the Chilean quake is around 4.8 m/s², which is half the acceleration of gravity (9.8 m/s²). That's pretty impressive. Also, note that the contours of equal acceleration run in a north-south direction, parallel to the plate boundary between the Nazca and South American plates.

Chile earthquake Facebook groups

 Social networking and aggregation of rss feeds can provide us with plenty (too much?) of information on breaking events.  I haven't really relied on these sites too much in the past, and have even been somewhat skeptical, but here are two Facebook grounps related to the Chile earthquake that you might want to look at:

Chile Earthquake

and

Chile Quake Recovery - NY Times .


I try to answer questions posed by readers. One asked about recovery from the even larger 1960 event. For now, see what the NY Times site above has to offer. I plan to talk more about damages and comparison of the Haiti and Chile quakes in an upcoming post.

Chile earthquake - tectonics in text

Does that make it the textonic? Text tonic?

We are working our way through the USGS poster on the Chile quake.

The text on the tectonic setting:

This earthquake occurred at the boundary between the Nazca and South American tectonic plates. The two plates are converging at a rate of 80 mm [3 inches] per year. The earthquake occurred as thrust-faulting on the interface between the two plates, with the Nazca plate moving down and landward below the South American plate. 

Coastal Chile has a history of very large earthquakes. Since 1973, there have been 13 events of magnitude 7.0 or greater. The February 27 shock originated about 230 km north of the source region of the magnitude 9.5 earthquake of May, 1960 the largest instrumentally recorded earthquake in the world. This magnitude 9.5 earthquake killed 1655 people in southern Chile and unleashed a tsunami that crossed the Pacific, killing 61 people in Hawaii, Japan, and the Philippines. Approximately 870 km to the north of the February 27 earthquake is the source region of the magnitude 8.5 earthquake of November, 1922. This great quake significantly impacted central Chile, killing several hundred people and causing severe property damage. The 1922 quake generated a 9-meter local tsunami that inundated the Chile coast near the town of Coquimbo; the tsunami also crossed the Pacific, washing away boats in Hilo harbor, Hawaii. The magnitude 8.8 earthquake of February 27, 2010 ruptured the portion of the South American subduction zone separating these two massive historical earthquakes. 

A large vigorous aftershock sequence can be expected from this earthquake. [As we will discuss, this has definitely been the case.]

Educational resources for the Chile earthquake

There is so much material out there on these devastating events ... hard to be selective, but in these times of supposed information overload, that's our challenge. I do want to work through more details of the USGS poster. And there are many fascinating news stories.

But today, especially for teachers looking for more educational resources on seismology, an excellent web site is that of IRIS, Incorporated Research Institutions for Seismology.

In particular, there is a page dedicated to resources on the Chile quake.

But there are slide shows, animations, fliers. Look around, and use what serves your purpose. That is what the educational mission of IRIS is about.

A closer look at the epicentral region, Chile quake

The figure below is another piece of the poster on the Chile quake available from the U.S. Geological Survey.

This figure includes quakes with magnitudes > 5.5, whereas the last figure only showed quakes with magnitudes > 7.

A couple of interesting patterns according to my eye:

1. Note that the epicenter of the main quake (yellow star) seems to fill a gap in the pattern of epicenter locations parallel to the Peru-Chile Trench, as if this part of the plate boundary was accumulating stress and needed some release. This, in fact, does happen (as I have previously reported in my Thanksgiving post for the North Anatolian Fault in Turkey), although I have not searched the scientific literature to see if that is actually the case here.

2. Note the aftershocks (orange circles) in the general vicinity of the main shock (star). They are not randomly distributed about the epicenter of the main quake, but are more linearly aligned, again parallel to the trench. So, the aftershocks also seem to be releasing some of the stress along the fault.

Stay tuned for more.

A teachable moment - the Chile earthquake

The U.S Geological Survey has already issued an informative poster on the Chile earthquake, which can be downloaded here.

Here is the first part of the poster:

You can see the past earthquakes along the subduction zone (hatched line), where the Nazca plate on the left dives beneath the South American plate on the east. The epicenter of yesterday's quake (star) is not far from the largest earthquake ever measured, from 1960. The colors denote the depth of the hypocenters.

We'll look at other parts of the poster in upcoming posts.

Tsunami warning in Japan from Chile quake

Tsunamis can travel across the ocean at 1000 km/hr (600 mph), although they are barely felt until the wave amplitude builds up when the wave feels ocean bottom near the coast.  But given the 10,000 miles across the ocean from Chile to Japan (here is a nice Java applet to calculate geographic distances), that's 20 hours!

Here was the warning put out by the Japan Meteorological Agency:

Tsunami Warning/Advisory
Issued at 09:33 JST 28 Feb 2010

******************Headline******************
Major Tsunami and Tsunami have been issued.
  PACIFIC COAST OF TOHOKU
  PACIFIC COAST OF HOKKAIDO
  JAPAN SEA COAST OF AOMORI PREF.
  KANTO
  IZU/OGASAWARA ISLANDS
  TOKAI
  PACIFIC COAST OF KINKI/SHIKOKU
  OKAYAMA PREF.
  ARIAKE SEA AND YATSUSHIRO SEA
  EASTERN PART OF KYUSHU
  KAGOSHIMA PREF.

Evacuate from the seashore immediately to the safe places near the above coasts.
Tsunami attentions are in effect at some of the other coasts now.

***********About Tsunami Forecast************

Tsunami height is expected to be 3 meters or more, Keep careful watch on tsunamis.

Tsunami height is expected to be up to 2 meters, Keep watch on tsunamis.

Tsunami height is expected to be about 0.5 meters, Pay attention to tsunamis.

El Sismo en Chile

Here is the event report from the Servicio Sismologico, Department of Geophysics, University of Chile:

DEPARTAMENTO DE GEOFISICA UNIVERSIDAD DE CHILE Blanco Encalada 2002 - Casilla 2777 Teléfonos: 9784298 - Fax 56-2-6873508 Dirección web : http://www.sismologia.cl E-ma il: sismoguc@dgf.uchile.cl SANTIAGO - CHILE SERVICIO SISMOLOGICO INFORME DE SISMO Fecha: 27 de Febrero del 2010 - Hora Local: 03:34

---

HIPOCENTRO

Hora UTC:	06:34:12 27/02/2010
Latitud:	-36 12' 28''
Longitud:	-72 57' 46''
Profundidad:	47.4 km
Magnitud:	8.3 (Mw) GUC
Fuente:	Servicio Sismológico (U. de Chile)

REFERENCIA GEOGRAFICA:  63 km al SO de Cauquenes

Intensidades Teóricas Simuladas

Intensidades  (Escala de Mercalli)

Fuente: ONEMI Y DIREMER

Concepcion IX Temuco VIII Talca VIII Rancagua VIII Santiago VIII

Valdivia VI Valparaiso VI Puerto Montt V Vicuna IV La Serena III

Copiapo III Antofagasta II Calama II

Note the maximum intensity of IX - "Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations"

Huge Chilean earthquake, magnitude 8.8

A huge earthquake struck offshore Chile Saturday 3:34 am local time.  The magnitude of 8.8 makes it one of the 5 or 6 largest earthquakes ever, according to the list from the NEIC:

	Location	Date UTC	Magnitude	Lat.	Long.	Reference
1.	Chile	1960 05 22	9.5	-38.29	-73.05	Kanamori, 1977
2.	Prince William Sound, Alaska	1964 03 28	9.2	61.02	-147.65	Kanamori, 1977
3.	Off the West Coast of Northern Sumatra	2004 12 26	9.1	3.30	95.78	Park et al., 2005
4.	Kamchatka	1952 11 04	9.0	52.76	160.06	Kanamori, 1977
5.	Off the Coast of Ecuador	1906 01 31	8.8	1.0	-81.5	Kanamori, 1977
6.	Rat Islands, Alaska	1965 02 04	8.7	51.21	178.50	Kanamori, 1977
7.	Northern Sumatra, Indonesia	2005 03 28	8.6	2.08	97.01	PDE
8.	Assam - Tibet	1950 08 15	8.6	28.5	96.5	Kanamori, 1977
9.	Andreanof Islands, Alaska	1957 03 09	8.6	51.56	-175.39	Johnson et al., 1994
10.	Southern Sumatra, Indonesia	2007 09 12	8.5	-4.438	101.367	PDE

There is a tsunami warning issued by the NOAA Pacific Tsunami Warning Center.

Below is the seismogram from my home institution of Franklin & Marshall College:

Click on the figure for a larger image.  The waves arrive a few minutes after 6:30, coordinated universal time (time scale on the right).

Largest earthquake ever recorded, Chile, 1960

That would be Chile, 1960. Same area as Darwin's quake. This helps put our recent small quakes in the Northeast in perspective.

From the USGS:

Chile
1960 May 22 19:11:14 UTC
Magnitude 9.5

The Largest Earthquake in the World

"Approximately 1,655 killed, 3,000 injured, 2,000,000 homeless, and $550 million damage in southern Chile; tsunami caused 61 deaths, $75 million damage in Hawaii; 138 deaths and $50 million damage in Japan; 32 dead and missing in the Philippines; and $500,000 damage to the west coast of the United States." (USGS)

"The series of earthquakes ... ravaged southern Chile and ruptured over a period of days a 1,000 km section of the fault, one of the longest ruptures ever reported. The number of fatalities associated with both the tsunami and the earthquake has been estimated to be between 490 and 5,700. Reportedly there were 3,000 injured, and initially there were 717 missing in Chile. The Chilean government estimated 2,000,000 people were left homeless and 58,622 houses were completely destroyed. Damage (including tsunami damage) was more than $500 million U.S. dollars. The main shock setup a series of seismic sea waves (tsunami) that not only was destructive along the coast of Chile, but which also caused numerous casualties and extensive property damage in Hawaii and Japan, and which was noticeable along shorelines throughout the Pacific Ocean area. There were several other geologic phenomena besides tsunamis associated with this event. Subsidence caused by the earthquake produced local flooding and permanently altered the shorelines of much of the area in Chile impacted by the earthquake. Landslides were common on Chilean hillsides. Cordón Caulle erupted forty-seven hours after the main shock. It is only a matter of time until Chile once again has a "world-class" earthquake whose impact, like the 1960 Chile event, will be felt around the world." (NOAA)

Valdivia, Chile (NOAA)

"Valdivia suffered catastrophic damage because of its proximity to the epicenter of the massive quake. Regional tectonic subsidence of five to seven feet occurred. There was extensive loss to agricultural lands from flooding. The horizontal ground motions, not the subsidence, caused the principal damage to structures away from shorelines and river channels. Older masonry structures were hard hit by the earthquake. However, many wood frame buildings performed well.

"The highest [tsunami] runup on the United States was at Crescent City, California. Here, the runup reached 1.7 m and the first wave arrived 15.5 hours after the tsunami was triggered. A total of $500,000 to $1,000,000 in damage was done by the tsunami to the United States west coast.

"Hilo was the hardest hit city in the Hawaiian Islands. The tsunami arrived at Hilo about 15 hours after it originated off the coast of south central Chile, 6,600 miles distant. The runup at Hilo was measured at 10.7 m. The tsunami changed into a bore as it passed the harbor entrance and advanced on to the bay front. The business district along Kamehameha Avenue and the adjoining low-lying residential areas of Waiakea and Shimmache were destroyed. Damage to property included 229 dwellings and 308 business and public buildings. Between the Wailoa and Wailuku Rivers, the water washed inland as far as the 6 m(20 ft) contour above sea level." (NOAA)