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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.

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

the equivalent relationship between slip and length is

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

What’s all this talk about earthquakes? Part II

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Moonquakes – a review of lunar seismicity

Beginning in 1969, a mere 66 years after the first ever engine-powered flight, the crew of the Apollo 12 mission deployed a set of seismic instruments on the surface of the Moon. Data was collected and radioed to Earth for 8 years until the machines were switched off in 1977.

Buzz Aldrin deploys a seismometer in the Sea of Tranquillity. Image taken from science.nasa.gov

The Moon is as close of a partner to Earth as it is a satellite, but there is still one large difference between the two bodies, active tectonics. The moon is considered to be tectonically dead; no new crust has been made since mare basalt formations roughly 3 billion years ago. But that doesn’t mean that we didn’t see seismicity!

Seismic instruments on the Moon recorded four fundamental sources of seismic energy.

deep moonquakes ~700km below the surface
impact vibrations
thermal quakes caused by the expansion of the crust due to exposure to light after a two weeklong, deep freeze night
shallow 20-40km depth seismicity

Seismic energy release on the moon is on the order of a billion times less than on Earth and is almost all a result of tidal forces from the earth.

Moon cartoon showing depth distribution of moonquakes. Image taken from the-moon.wikispaces.com

All of this activity helped lunar seismologists to explore the interior of the planet. They found a solid inner core, fluid outer core, a layer of partial melt atop the fluid outer core and of course a large mantle. They also found that the moon as a whole is much more rigid than Earth. The largest of the 28 shallow moonquakes recorded between 1972 and 1977 had a magnitude of 5.5 and energy was recorded for over 10 minutes (the Moon exhibits very little attenuation).

First person to guess why the moon is so much more rigid than the earth gets a high five!

What’s all this talk about earthquakes? Part I

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The detection and source characteristic of glacial earthquakes or “icequakes”.

Cryoseismic events are distinct from the earthquakes we have been characterizing and modeling so far in class. This wasn’t recognized until the latter part of the last century when a number of earthquakes were giving unusual results due to their atypical amplitude spectra.

What was obvious in these events, however, was their lack on high-frequency energy. As earthquakes scale in magnitude, typically the larger events lack in high-frequency signal; these smaller events, though, were breaking the rules. The answer to the puzzle was actually quite simple. The reason larger events lack the high-frequency energy is due to their long duration. Small events with short durations are very efficient at releasing short-period energy. Think about the Fourier Transform of an impulse function as an extreme. While these events (4.6 < M < 5.0) weren’t scaling up in magnitude, they were in duration.

As it turns out, a typical duration of an icequake is between 30 and 60 seconds – much longer than even many large earthquakes, which is great at quelling the high-frequency signal and the reason these events weren’t caught on traditional monitoring equipment using previous methods.

Later, attempts were made by Ekström et al. to invert the seismograms using the global-moment-tensor method to characterize the slip event. As expected, they ran into hurdles and the inversions were unstable. Their solution was to parameterize their inversion in terms of a centroid single force (CSF). A centroid single force model is a distribution of single forces equal but opposite of the slip direction. This is related to the event energy source being gravitational potential energy as opposed to elastic strain energy. Centroid single forces are also used in the characterization of landslides. CSF analysis can provide the product of mass and sliding distance, but neither independently.

Can anyone give a better description of a CSF?

Reference: Ekström, Göran, Meredith Nettles, and Geoffrey A. Abers. “Glacial earthquakes.” Science 302.5645 (2003): 622-624.

Earthquakes – In the News

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Roughly 11:45 pm local time on March 29, 2015 a M7.5 event occurred in the New Britain region of Papua New Guinea at a depth of roughly 40 km (quite close to the systems minimum depth). The thrust type event occurred along the New Britain trench, part of the Australia-Pacific Boundary with a strike of 259 degrees dipping at 33 degrees. Half duration of the event was reported to be 12.6 seconds and shaking was reported to have occurred for a duration of 5 minutes in the city of Rabual roughly 40 miles away. The Pacific Tsunami Warning Centre issued a tsunami warning for the region, but very little tsunami energy was recorded.

Along this boundary, the northward subduction of Australia occurs at a pace of roughly 95 mm/yr and the area is well known for M7+ thrust, in fact, since 1990 this segment of the “Ring of Fire” has experienced 13 M7.5+ slip events (36 M7+ events within 250 miles of the epicenter since 1900).

Numerous M4 and M5 events occurred in the hours following the main shock. These aftershock motions could either have been accommodated by the main fault or along faults within the altered strained volume surround the main fault. There are a few different laws that describe the occurrence of aftershocks following a large magnitude event. The modified Omori’s law states that that the rate of aftershocks is proportional to the inverse of the time since the main shock, so we should expect to see fewer and fewer events in this locality over the coming days.

Questions for the class:

If shaking was felt for 5 minutes 40 miles away, what part of the wavetrain were they feeling?

Can anyone come up with an estimate of D, or average slip distance, for this event?

Helpful links:

USGS: http://earthquake.usgs.gov/earthquakes/eventpage/us10001rvu#general_summary

GCMT: http://www.globalcmt.org/cgi-bin/globalcmt-cgi-bin/CMT4/form?itype=ymd&yr=2015&mo=3&day=1&otype=ymd&oyr=2015&omo=3&oday=30&jyr=2005&jday=1&ojyr=1976&ojday=1&nday=1&lmw=7.4&umw=10&lms=0&ums=10&lmb=0&umb=10&llat=-90&ulat=90&llon=-180&ulon=180&lhd=0&uhd=1000&lts=-9999&uts=9999&lpe1=0&upe1=90&lpe2=0&upe2=90&list=0

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