Category Archives: Uncategorized

Papua New Guinea

In the past 7 days the area near Papua New Guinea has had 56 earthquakes larger than 4.3.  This included a magnitude 7.1 and a magnitude 7.5.  This area experiences large earthquakes quite frequently.  In the past 5 years there have been 40 events larger than magnitude 6 and 2651 events greater than magnitude 4.

l downloaded csv files from the USGS and made a couple histograms and a plot of time vs depth for the past 7 days. These plots can be seen here.

Deterministic vs. undeterministic earthquake magnitude based on it’s rupture!!

The dominant rupture process theory implies that we can’t determine the earthquake magnitude from it’s rupture initiation. However, the new findings with the following paper suggest that we could determine the earthquake magnitude by studying the frequency content for the first few seconds of the rupture.

Please visit the paper by the following link:

http://www.nature.com/nature/journal/v438/n7065/abs/nature04214.html

Historical Seismicity of Nepal!!

I found this article to be very interesting-

http://www.bbc.com/news/science-environment-32472310

In summary-

Larent Bollinger and his colleagues anticipated a major earthquake to happen in the exact location that last Saturday’s event occurred. Bollinger’s team dug trenches across the main earthquake fault where the fault met the surface and “used fragments of charcoal buried within the fault to carbon-date when the fault had last moved.” They discovered that this segment of the fault had not erupted for a very long time, 1344 to be exact. They also discovered that the 1344 event was preceded by large event on a neighboring fault in 1255. They theorized that the movement from 1255 event caused strain to be transferred westward along the fault, which was finally released in 1344 (89 years later).

In 1934, a large earthquake, taking over 17,000 lives, ruptured the segment of the fault where the 1255 took place. The Nepal event that just recently happened, some 81 years later, seems to follow this historic pattern.

Another article that goes more in depth with regards to the historical seismicity of Nepal.

http://www.lebret-irfed.org/spip.php?article787

Inner core structure based on seismic waves study

Our beliefs are changing through time, because some of them had been chosen based on our world view and what can fit with our previous background rather than what can represent nature itself. However, one of those beliefs is about earth’s inner core structure and iron crystals orenintation. Geoscientists used to represent the inner core as a cylindrical symmetry that aligned towards the poles (north-south). In fact, they had no evidence for that except that it fit their general model for the earth interior.

Recently, a group of scientists studied carefullay the seismic waves that penetrate the inner core and tried to discover the iron crystal orientation. The results were astonished when they found two orientations within the inner core. The inner part of the inner core shows iron crystal alinement toward the equator while the outer part of the inner core is almost perpendicular in which it shows north-south alinement. What can such finding tell us about Earth? In fact, that would improves our understanding about how our planet from in one hand, and give an important insight about inner core dynamics in the other hand.

Please visit the following link for more details:

http://sk8es4mc2l.search.serialssolutions.com/?ctx_ver=Z39.88-2004&ctx_enc=info%3Aofi%2Fenc%3AUTF-8&rfr_id=info:sid/summon.serialssolutions.com&rft_val_fmt=info:ofi/fmt:kev:mtx:journal&rft.genre=article&rft.atitle=Equatorial+anisotropy+in+the+inner+part+of+Earth%27s+inner+core+from+autocorrelation+of+earthquake+coda&rft.jtitle=NATURE+GEOSCIENCE&rft.au=Wang%2C+T&rft.au=Song%2C+XD&rft.au=Xia%2C+HH&rft.date=2015-03-01&rft.pub=NATURE+PUBLISHING+GROUP&rft.issn=1752-0894&rft.eissn=1752-0908&rft.volume=8&rft.issue=3&rft.spage=224&rft.epage=227&rft_id=info:doi/10.1038%2FNGEO2354&rft.externalDBID=n%2Fa&rft.externalDocID=000350770900022&paramdict=en-US

What’s all this talk about (interplate) Earthquakes?

The Virginia Earthquake – An Intraplate Event

The Virginia Earthquake (M5.8) occurred at roughly 2 pm on August 23, 2011 in the Virginia Seismic Zone (central Virginia). In Mineralogy lab on the fifth floor at Virginia Tech, my microscope nearly toppled over. After the shaking ceased, students rushed down the stairs to check out the live stream from nearby seismographs. Needless to say, this earthquake struck close to home.Screen Shot 2015-05-04 at 8.46.36 PM

(USGS)

A surprise to many, the Virginia Earthquake was an unexpected, relatively large East Coast intraplate event. Unlike most of the events that occur in the North America (Californa, Alaska, etc.) this event happened far from any active tectonic boundary. Events like this remind us that the stresses and strains we use to describe such boundaries, are actually present all over.

Screen Shot 2015-05-04 at 8.44.05 PM

Figure: Seismic events (Mw > 4.0) since 1980 for eastern North America. Location of the Virginia Earthquake is shown in blue. (USGS)

Intraplate earthquakes are technically unrelated to plate boundaries and have an interval on the order of a thousand years or much, much more. Intraplate earthquakes have different source properties than interplate earthquakes. For example, while both types of events have a moment relationship to length of

Screen Shot 2015-05-04 at 9.45.25 PM

the equivalent relationship between slip and length is

Screen Shot 2015-05-04 at 9.45.59 PM

and includes different values for alpha. For interplate earthquakes, values of alpha are usually around 1×10-5 whereas for intraplate events values for alpha fall closer to 6×10-5. So while both sets of earthquakes “obey the same scaling laws, intraplate earthquakes have, on average, 6 times more slip on the fault.” This further indicates that stress drop is much larger for intraplate earthquakes.

These differences are likely a direct result of differences in frictional properties between the two types of earthquakes. Those occurring away from plate boundaries have more slip on shorter lengths with slower slip velocities.

Another interesting aspect of East Coast intraplate earthquakes is the distance from which they are able to be felt. For similar magnitude events in California, shaking is usually felt within the region, like most middle magnitude earthquakes in the East Coast, the Virginia Earthquakes was felt at much larger distances. This is a result of crustal difference between the eastern United States and the West Coast. The older, denser crust in the East allows seismic energy to propagate much further.felt-comparisons

For more information, see Scholz et al. JGR 1986

 

Summary: Mw=7.8 Earthquake Central Nepal (25 April 2015)

This summary, from http://cires1.colorado.edu, describes the previous seismicity an deformation before the Nepal earthquake, for instance some seismicity close the rupture area occurred ~ 24 before the main shock. Thus, this article shows the historical big ones as well as describe zones for future potential big earthquakes along the Himalayas.

http://cires1.colorado.edu/~bilham/2015%20Nepal/Nepal_2015_earthquake.html

Seismological Grand Challenges

As we finishing the semester, I thought about having a list about the most difficult challenges in seismology based on what we have learned so far. The following pdf file represents a national report that combined a couple of question that seismologists could not answer yet. In my opinion this is good way to think about what is unknown in seismology and earth dynamics as well.

http://www.iris.edu/hq/lrsps/seis_plan_final.pdf

Coseismic interferogram -Nepal earthquake -InSAR and GPS observations.

Here I posted the InSAR observations from ESA -European Space Agency where they have posted several coseismic interferograms to track the ground deformation on Nepal area.

They compute several interferograms that are available to download. The best interferometric solution, combination of, 17.Apr – 29.Apr (descending), showed around 34 fringes or 1 meter line-of-sight displacement, due the 7.8 earthquake.

http://insarap.org

http://www.bbc.com/news/science-environment-32515059

On the same way, the continuous GPS sites close to the Nepal Earthquake recorded the ground motion.  Then, according UNAVCO report, they have compute some seismogeodetic seismograms using GPS data at 1 Hz and and 30 s. The GPS sites are located 200, 400 and ~ 60 km from the earthquake.

http://www.unavco.org/voce/viewtopic.php?f=59&t=1390&sid=e13c8d62c6054d63cfe308aaf5a7c78c#p2507