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7.0 Magnitude Earthquake Rocks Anchorage, Alaska

The morning of November 30th, 2018, at 8:29 AM local time, a 7.0 magnitude earthquake shook the city of Anchorage, Alaska. The origin of the quake was 7 miles north of the city, resulting in the residents of Anchorage feeling the full intensity of this earthquake. Luckily, the epicenter was at a depth of 27 miles into the Earth’s crust. The depth of the origin allowed for the seismic energy of the earthquake to diminish slightly while making the 27-mile vertical journey before wreaking havoc on the surface.

The Shake Map shows the extent and magnitude in the surrounding areas during the 7.0 earthquake near Anchorage, Alaska,.

The Shake Map shows the extent and magnitude in the surrounding areas during the 7.0 earthquake near Anchorage, Alaska,.

Upon reaching the surface, the resulting damages included widespread power outages, severe damage to roadways and other transportation infrastructure, and internal damage to residential and commercial structures. Immediately after the quake hit, the USGS released figures that contained frightening numbers depicting the probability of economic losses. The figure below shows that, according to the USGS predicted losses, there is a 35 percent chance of damages ranging from $100 million – $1 billion. The data goes on to show that there is a 20 percent chance that the economic losses could very well total over one billion dollars!

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Immediately after the quake and ongoing through this week, the area continues to be inundated with relentless aftershocks that still hold immense power. As of this morning, the area has been the recipient of over 2,700 aftershocks and tremors, ranging in magnitude from 1 up to 5. There is still potential for an aftershock to be nearly as powerful as the original incident itself, which would cause even more damage during the recovery process.

Looking Back

In 1964, Anchorage fell victim to a 9.2 magnitude quake that caused damage to such an extent that certain parts of the city were unrecognizable. This earthquake killed 15 people during the event and another 124 from the resultant tsunami. Only one earthquake in recorded history has been more powerful (9.5 magnitude in Chile 1960). In the wake of this devastating event, the changes to the building codes may have resulted in massive economic saves in relation to building loss during this most recent quake. One of the key ideas that resulted from the research in the aftermath of the 9.2 magnitude event was the concept of integrating ductility into modern architecture and design. Ductility is the ability to bend without breaking, which helps absorb some of the seismic motion during an earthquake. One way this could be achieved in the case of concrete structures would be ensuring the right amount of steel reinforcement is located in the correct areas of the structure. This is just one example of the engineering constructs resulting from the Earthquake Hazard Reduction Act of 1977, which was sparked by the enormous 1964 earthquake.

Sources:

https://www.adn.com/alaska-news/2018/12/06/2788-and-counting-when-do-tremors-stop-being-aftershocks-and-start-being-new-earthquakes/

https://www.curbed.com/2018/12/3/18124154/alaska-earthquake-anchorage-building-codes

https://earthquake.alaska.edu/anchorage-m70-what-we-know-so-far

https://www.pe.com/2014/04/07/earthquakes-alaska-disaster-jolted-nation-into-making-changes/

Structural damage from the Magnitude 8.8 earthquake in Chile in 2016 (Source: Expansion - CNN)

What is the difference between an earthquake’s magnitude and intensity?

Think about sitting around a campfire. The fire emits a measurable level of heat, and the nearer you sit to it, the hotter the fire feels. If you are farther from the fire, the heat is less intense. This simple example can explain common earthquake measurements – magnitude and intensity – and what these earthquake scales mean.

Richter Scale

Consider, once again, the campfire. This temperature is measurable and absolute. When an earthquake occurs, the Richter scale measures the magnitude of the earthquake at its epicenter. The Richter scale was developed in 1935 as a way to quantify the strength of earthquakes. It is a logarithmic scale based on the amplitude of the waves recorded by seismographs. A logarithmic scale means a magnitude increase of 1 relates to an energy increase by a factor of 10. An earthquake measuring a 4.0 on the Richter scale is 10 times as strong as a 3.0!

Seismograph at Weston Observatory at Boston College, Weston, Massachusetts

Earthquake seismograph at Weston Observatory at Boston College, Weston, Massachusetts.

 

Modified Mercali Intensity Scale

Now, you know the closer to the campfire you sit, the hotter the flames feel on your skin. This generally holds true with earthquakes as well. Typically, the nearer the epicenter the stronger the ground shaking you would feel; however, there are other factors that affect the intensity of the earthquake you feel at your location. The type of earthquake, bedrock the shockwaves traveled through, and amplitude of the shockwaves from the earthquake are a few of these factors. The intensity you feel is measured on a scale called the Modified Mercali Intensity Scale (MMI). The MMI scale ranges from “Not Felt” and “Weak Shaking” up to “Violent” and “Extreme” with well-built structures suffering damage.

USGS map and intensity scale for 1971 San Fernando Earthquake (Magnitude - red-circled, epicenter - star, intensity - table)

USGS earthquake map and intensity scale for 1971 San Fernando Earthquake (Magnitude – red-circled, epicenter – star, Modified Mercali Intensity scale – table)

Other Scales Around the World

While the Richter scale is widely known and the MMI scale is used in the United States, there are other magnitude and intensity scales in use around the world. The Japanese Meteorological Agency uses a separate calculation for shallow earthquakes (depth < 60km) which has been shown to be reasonable when the magnitude is 4.5-7.5; however, this magnitude measurement has historically underestimated larger magnitude tremors. Additionally, Japan and Taiwan use the Shindo intensity scale which has significant correlation to the MMI scale. During the middle to late 20th century, the USSR, East Germany, and Czecholsovakia established and utilized the Medvedev-Sponheuer-Karnik scale (MSK) to evaluate shaking and effects from earthquakes. This scale was built upon in the 1990s by the European Seismological Commission as they shifted to implement the European Macroseismic Scale for European countries. The MSK scale continues to be employed in Russia, India, Israel, and the Commonwealth of Independent States.

You can read more about some of these other scales here:

JMA Shindo intensity scale: https://www.jma.go.jp/jma/en/Activities/inttable.html

MSK Scale: https://www.gktoday.in/gk/various-earthquake-scales/

 

Sources:

https://earthquake.usgs.gov/learn/topics/mercalli.php

https://www.japan-talk.com/jt/new/why-japan-doesnt-use-magnitude-for-earthquakes

Fault Connection Reveals Risk to Bay Area

Newly-discovered Fault Connection

It has been 27 years since the 6.9-magnitude Loma Prieta earthquake on the San Andreas fault rattled the San Francisco Bay Area, killing 63, injuring over 3,700, and famously interrupting that year’s World Series.  Four scientists with the United States Geological Survey have recently discovered a connection between two fault lines under the Bay that were previously believed to be unlinked, revealing a fault connection they say could lead to an even larger future earthquake.

Underwater surveys conducted in shallow portions of the northern San Francisco Bay discovered a section of the Hayward Fault that connects to the western segment of the Rodgers Creek Fault.  The Hayward Fault extends for 62 miles from San Jose to San Pablo Bay, passing directly under the densely-populated urban areas of Berkeley and Oakland.  The Rodgers Creek fracture runs north from the bay, 56 miles through the heart of wine country.

The worry is that the 188-mile connection between the two faults will make the effects of a rupture along either fault more intense and impact substantially more people.


7 million Could Be Drastically Affected

Explained in detail in a recent journal article, the study is the first evidence that the two major faults are linked. The fault connection discovery was published in the October 19th edition of the journal Science Advances. The USGS team led by Janet Watt stated that the next major earthquake to the strike the Bay Area will likely come from the (now-connected) Hayward and neighboring Rodgers Creek faults. The scientists used integrated geophysical interpretation and kinematic modeling to show that the Hayward and Rodgers Creek faults are directly connected at the surface (in San Pablo Bay), and the coinciding geometric relationship has significant implications for earthquake dynamics and seismic hazard.

They argue that the discovered link enables a simultaneous rupture along their combined 188 miles, potentially producing a quake as large as 7.4 in magnitude–five times stronger than the Loma Prieta event. According to their findings, the worst case scenario event would cause extensive damage and loss of life with global economic impact. An estimated 7 million people could be drastically affected.

 

Maps showing the Hayward and Rodgers Creek fault connection

Maps showing the Hayward and Rodgers Creek fault connection


Sources:

http://advances.sciencemag.org/content/2/10/e1601441

http://www.theverge.com/2016/10/19/13335816/earthquake-faults-san-francisco-oakland-bay-area-san-andreas-hayward

https://en.wikipedia.org/wiki/1989_Loma_Prieta_earthquake