Category Archives: Earth’s Interior

The 2013 Great, Deep Sea of Okhotsk Earthquake (Mw 8.3)

On May 24th, 2013 the largest known deep earthquake (~600 km) with Mw 8.3 occurred beneath the Okhotsk sea. Such an event may reshape our understanding of deep earthquakes.  I don’t know many details about the nature of deep earthquakes, I’m wondering if the mineralogical phase change associated with the 660-km discontinuity could contribute to these deep events? The more generally accepted explanation for deep earthquakes is nucleation by a phase transition within the subducted material, for example:

http://earthquake.usgs.gov/earthquakes/eqarchives/poster/2013/20130524.pdf

One interesting thing I found in the poster is that the shaking intensity distribution shows an interesting pattern. Can we use it to learn about the earth structure above the source?

Global Earthquake Depth Distributions

I was making some illustrations for class notes on the distribution of earthquakes with depth and decided to post some plots here with a few general comments. I used the GCMT Catalog as the source, so these are events with seismic moments greater than about \(10^{22}\) dyne-cm and the catalog is complete for events larger than about \(10^{24}\) dyne-cm.

First, lets look at numbers of events. The number of shallow earthquakes is much larger than those at depth, but the pattern with depth shows a small increase for depths between 500 and 650 km.

GCMT Depth Distribution

Distribution of the number of earthquakes as a function of depth (using 50-km wide bins and the Global CMT Catalog) and a linear (left) and logarithmic (right) scales. Numbers to the right of the bars indicate the event count for each bin. Figure by C. J. Ammon, Penn State.

The number of events is informative, but not directly reflective of the energy release in earthquakes. For a better approximation to the energy release, here’s the cumulative seismic moment in 50-km bins.

GCMT Moment vs Depth

Distribution of seismic moment in the GCMT catalog as a function of depth using 50-km wide bins. The number to the right of each bar represent the cumulative moment in the bar with units of 10^20 N-m. Figure by C. J. Ammon, Penn State.

Depending on how you look at it, the minimum between 200 and 550 km, or the maximum from 550 to 650 km is shifted down by one bin and more pronounced than in simple event-number counts. But again, the shallow activity dominates.

We can be more creative and communicate the depth distribution on a planetary scale if we plot the earthquakes on a cross section of Earth. First, I’ll spread the events out using event longitude, which requires a little thought to see the geographic correlations. But the relatively sparse geographic distribution of deep earthquakes (in the deep slabs) is illuminated.

Earthquake distribution by longitude and radius.

GCMT catalog earthquake distribution shown on a cross section of the planet (surface, 660km discontinuity, and the outer and inner core boundaries are shown). The earthquakes are group by longitude, which starts and zero on the right (“east”) and then increases counterclockwise around the circle. Figure by C. J. Ammon, Penn State.

One more view, showing earthquake depth distribution as a function of time. Think of the cross-section as a clock with 1975 at the top and increasing to the end of 2015 in a clockwise direction. Note that the GCMT catalog does not include all the deep events for a year and that you can see a hint of improvement in catalog completeness with time (as more and more seismometers were deployed). In general, the pattern is relatively stationary. The large earthquakes show an interesting drought in large, deep events from 1977 to 1994, when deep events occurred beneath Fiji and Bolivia.

GCMT catalog earthquake distribution shown on a cross section of the planet (surface, 660km discontinuity, and the outer and inner core boundaries are shown). The earthquakes are group by time. The top of the clock is 1975, and time increases clockwise, as labeled).

GCMT catalog earthquake distribution shown on a cross section of the planet (surface, 660km discontinuity, and the outer and inner core boundaries are shown). The earthquakes are grouped by time. The top of the clock is 1975, and time increases clockwise, as labeled. Figure by C. J. Ammon, Penn State.

Seismic wave attenuation: geometrical spreading, anelasticity, multipathing and scattering

As we discussed in class, seismic waves can lose energy through reflection, geometrical spreading and intrinsic attenuation, also referred as anelasticity.

Geometrical spreading depends on the distance r the wave has propagated from the source. In a uniform material, seismic waves propagate away from their source as spherical wave front of increasing area. Because of the conservation of energy, the energy per unit area of wave front decreases as the distance from the source increases. For surface waves, in the case of a homogeneous flat earth, the energy per unit area of wave front decreases as 1/r and hence the amplitude, which is proportional to the root square of the energy, decreases as 1/√r. For body waves, the energy per unit area of wave front decreases as 1/r2 and hence the amplitude decreases as 1/r.

On the other hand, anelasticity reduces seismic wave amplitudes by converting part of their kinetic energy to frictional heat by permanent deformation of the medium. Anelasticity is characterized by the frequency-dependent quality factor Q, which is a measure of the the energy lost per oscillation of the seismic wave : Q = 2πE/∆E . So the smaller Q, the larger the energy loss.The loss of energy will lead to exponential decay of the seismic wave amplitude : A(t) = A0*e-πft/Q . The smaller Q and the larger the frequency (i.e. more oscillations per second), the larger the attenuation and the seismic wave amplitude decay.

Two other processes can also reduce seismic wave amplitudes: multipathing and scattering. Multipathing and scattering can be thought of as elastic processes. They conserve energy and decrease or increase the amplitude of an incoming wave by shifting its energy to an earlier or later arrival.

Seismic wave multipathing is caused by velocity variations within the medium of propagation. According to Fermat’s principle, seismic waves follow the least-time path of propagation between two points in a medium. Lateral velocity variations in the medium will then cause seismic waves to focus in high velocity regions and defocus in low velocity regions.The spacing between seismic rays in a region represents the energy density in this region. The further apart the rays are, the lower the amplitudes of the recorded wave. By contrast, the closer the rays are, the larger the wave amplitudes. So the seismic waves arriving at a station have usually followed different ray paths in addition to the ideal, direct path and the region of the earth they sampled forms a volume called Fresnel zone. Multipathing can be a significant attenuation effect because most seismic activity occur at plate boundaries and velocity heterogeneities are important in these regions.

Likewise, heterogeneities within the propagation medium cause a propagating wave field to be scattered. These heterogeneities can be velocity anomalies but also material heterogeneities such as mineral boundaries, pore edges, cracks… Scattering will cause part of the energy released by an earthquake to arrive later at a receiver (i.e. after the initial pulse) as a coda (i.e. tail of incoherent energy that decays over a few seconds to a few minutes). Whether a seismic wave will be scattered or not when encountering a heterogeneity depends on the ratio of the heterogeneity size to the wavelength and the propagation distance in the heterogeneous medium. If the heterogeneity is large compared to the wavelength, the seismic energy will follow a different ray path (i.e. multipathing effect). However if the heterogeneity and the wavelength have the same order of magnitude, the seismic energy will be scattered. Heterogeneities much smaller than the wavelength will just change the medium’s “bulk” properties. Scattering can be significant in the continental crust because of the presence of many small-scale geologic structures that can significantly affect short wavelength waves (i.e. tens of kilometers or smaller).

Reference : Stein, S., & Wysession, M. (2009). An introduction to seismology, earthquakes, and earth structure. John Wiley & Sons.

Frontier beneath our feet: Seismic study aims to map Earth’s interior in 3-D

Frontier beneath our feet: Seismic study aims to map Earth’s interior in 3-D

Seismic waves carry information about the Earth’s structure. Thus seismologists combine seismology and computer science to map the Earth’s interior. Here, the Princeton University attempts to map the deep structures on 3D-map. The project will use M>5 worldwide earthquakes recorded on thousands of seismic stations through NSF and research institutions for seismology.

Scientist from Princeton University are interested on map the mantle up welling and plumes, so it will be great make some cross correlations with the Africa rift system currently studied by Andy Nyblade’s group.

http://www.princeton.edu/main/news/archive/S42/59/33Q27/index.xml?section

Pennsylvania P-Wave Travel Times

For this blog post I figured I would use some of my prior work and construct a P wave travel time curve.  These travel times are from small seismic events recorded across Pennsylvania. The travel time versus distance observations are shown below.

Observed P-wave travel times for small earthquakes across Pennsylvania (Kyle Homann, 2015).

Observed P-wave travel times for small earthquakes across Pennsylvania (Kyle Homann, 2015).

This curve is about what I expected, with some picks that could be questionable, but the majority are consistent.  I think that around 150 km distance, some S wave picks managed to sneak into data.  The slope is slightly less at greater distances along with a “bulge” around 200 km.  This may be a result of Pn overtaking Pg.  If you recall, Pg is the P wave that travels within the crust and Pn is the refracted wave from the crust-mantle boundary. As expected, the Pg wave speed indicated by the slope of the graph is ~6 km/s.

Books & Videos – Tsunami, Observational Seismology, & Programming

Penn State has good libraries and learning resources that you should take advantage of and immerse yourself in your studies. Here are links to two books available from PSU that you may find interesting (or that you should, since you are in this class…). Browse them and read what you can.

Tsunami, The Underrated Hazard (Second Edition) by Edward Bryant

Routine Data Processing in Earthquake Seismology With Sample Data, Exercises and Software by Jens Havskov and Lars Ottemoller

And if you are new to programming, but interested enough to invest some time to learn, try this video from http://psu.lynda.com, which requires about 5 hrs of investment, but my help clarify some of the fundamentals for you. I don’t think you need to learn to program in javascript, but the ideas of loops, conditionals, etc are common to all languages.

Potential evidence for a two-layered inner core

A research group at the University of Illinois and collaborators at Nanjing University in China suggest the Earth’s inner core consists of an inner and an outer layer. Using autocorrelation of earthquake coda measured by global broadband seismic arrays between 1992 and 2012, the team finds evidence of seismic anisotropy in the inner inner core and the outer inner core.

The article, Equatorial anisotropy in the inner part of Earth’s inner core from autocorrelation of earthquake coda, was published online in nature geoscience on February 9, 2015, but I have not been able to access more than a preview. If anyone can find and share the full paper, I would appreciate it.