Category Archives: Seismicity

Calaveras fault mystery!

As science is defined by philosphers as the procces of discovering patterns, however, sometimes those patterns break down revealing new exception! In fact, with time those exceptions form a new pattern.

In lithospheric dynamics, is well known that earthquakes occure in the brittle part of the crust, while creep occurs in ductile part of crust. That is not the case in Hollister in California where creep can be found on the surface!! However, geoscientists have different explinations for such phenomena. Check the following link which is a report  made by USGS summerized most of the geophysical findings in Calaveas fault creep in Hollister.

http://search.proquest.com.ezaccess.libraries.psu.edu/docview/1641132558?pq-origsite=summon

Seismicity of the 2014 Oso, WA Landslide

The banks of the Stillaguamish River near Oso, Washington have been known to landslide. In the last 50 years, 6 events were documented. On March 22nd, 2014 a catastrophic landslide occurred ~ 6.5 km from Oso causing 43 fatalities. The landslide traveled approximately 1.1 km covering a nearby highway, destroying several homes, and damming the Stillaguamish River. To investigate the slope failures a team of scientists analyzed short and long period signals from the landslide.

They found that the landslide was comprised of a series of multiple failures with two major collapses that occurred ~ 3min apart. The first event showed characteristic features of seismic signals generated by landslides, notably an emergent onset and lack of clear p and s waves. The second event was more impulsive with several discernible amplitude peaks.

Screen Shot 2015-05-06 at 1.45.00 PM

 

Shown above are the seismic signals from the 2 events filtered between 1-3 Hz and 3-10 Hz for (a&c) and (b&d), respectively. Figure from: Hilbert et al., 2014 (The entire report can also be found at that link.

The long period signals were used to invert for the forces acting at the source. Combined with remote sensing data they were able to estimate the volume of material displaced by the landslide. The first event displaced between 6.0e6 and 7.5e6 cubic meters of material. In total a volume of 7e6 and 10e6 cubic meters was mobilized during the landslide.

Coming soon, a seismometer on Mars

Insight, standing for Interior Exploration using Seismic Investigations, Geodesy and Heat Transport, is a NASA funded mission to place a single geophysical robotic lander on Mars. The lander is planned for launch on March 2016 and will be equiped with a seismometer and a heat flow probe. The main objective of this mission will be to study Mars’ deep interior and early geological evolution bringing a better understanding of the Solar System’s terrestrial planets and their evolutionary process. The seismometer will help determine whether there is any seismic activity in Mars as well as the size, thickness, density, velocity and overall structure of Mars’ crust, mantle and core. The seismometer is a broad-band instrument and is designed to detect sources such as quakes but also seismic ambient noise generated by atmospheric excitation and tidal forces from Mars’ satellite. However if any seismic activity is recorded, its source won’t be located because at least three seismometers are needed to locate the source of a quake. So besides the obvious answer of cost, I was wondering why they don’t plan on sending more seismometers. Would anyone have an idea ?

You can find more information on this topic on the NASA webpage dedicated to this mission : http://insight.jpl.nasa.gov/home.cfm

Seismic Cycle of the Himalayas

Given the recent damaging event in Nepal, I was interested in reading this paper on the seismic cycle of the Himalayas. Unfortunately, all but the abstract is in French. I was wondering if Maeva or any other French-speaking class member(s) would be willing to translate some of the key points of the article for the rest of us. It would be greatly appreciated!

Thanks.

Finite-Fault Model for the Apr 25, 2015 Mw 7.9 Nepal Earthquake

According the USGS web site, they have analyzed selected P, SH and long-period surface waves to construct a kinematic model of this large earthquake. The NEIC solution (Lon. = 84.7 deg.; Lat. = 28.2 deg., Dep. = 15.0 km), has a fixed depth at 15 km. The fault plane dips 10° and strikes  295° (just north of west). The maximum displacement is ~ 3m (Figure accessed Sunday 26 April, 2015).

http://earthquake.usgs.gov/earthquakes/eventpage/us20002926#scientific_finitefault

[From the USGS] Cross-section of slip distribution. The strike direction is indicated above each fault plane and the hypocenter location is denoted by a star. Slip amplitude is shown in color and the motion direction of the hanging wall relative to the footwall (rake angle) is indicated with arrows. Contours show the rupture initiation time in seconds.

[From the USGS] Cross-section of slip distribution. The strike direction is indicated above each fault plane and the hypocenter location is denoted by a star. Slip amplitude is shown in color and the motion direction of the hanging wall relative to the footwall (rake angle) is indicated with arrows. Contours show the rupture initiation time in seconds.

Aftershock triggering model using revised rate and state friction law

The rate- and state- dependent friction laws (RSF) are empirical relations based on laboratory experiments that have been used to model a variety of earthquake behaviors, including the mechanics of a seismic cycle, episodic aseismic slip, and triggered seismicity (Kame et. al, 2013). These laws describe variations in friction based on the loading rate and state of the sheared zone. There are several forms of the RSF laws. The paper summarized below is based on the RSF law proposed by Dieterich (1979) and a more recently revised version proposed by Nagata et al. (2012).

In 1994, Dieterich modeled aftershock seismicity after an imposed stress step using his RSF model. His model can predict the observed 1/t decay of aftershock rate but there are two major observational gaps: (1) The model under predicts the amount of aftershock productivity and (2) The model predicts too long a delay time before the onset of decay. In a recent paper, Kame et al. (2013) hoped to address these gaps by running similar models using the Nagata RSF law.

Dieterich’s model considered a fault of fixed size embedded in an elastic medium. He was able to solve for the aftershock rate analytically. Kame et al. (2013) applied the Nagata law, which contains a stress weakening effect, to a similar model but found that the problem required a numerical solution.

Main observations from Kame et al. (2013) study:

1) Although the revised model produced greater seismicity and shortened delayed times, these improvements were only by a small factor compared to the disparities with natural observations that span several orders of magnitude.

2) Unlike the Dieterich model , in which a stress step always advances the timing of an earthquake, the revised model showed two different types of behavior. In most cases, the timing of the earthquake was advanced. However, if the stress step occurred at a specific time in the loading history of the fault , oscillatory slow slip cycles began, effectively delaying  the onset of the earthquake.


For more details on this study see:

Kame, Nobuki, et al. “Effects of a revised rate-and state-dependent friction law on aftershock triggering model.” Tectonophysics 600 (2013): 187-195.
http://www.sciencedirect.com/science/article/pii/S004019511200755X

Other sources:

K. Nagata, M. Nakatani, and S. Yoshida. A revised rate- and state-dependent friction law obtained by constraining constitutive and evolution laws separately with laboratory data, 2012.
http://onlinelibrary.wiley.com/doi/10.1029/2011JB008818/abstract

J.H. Dieterich. A constitutive law for rate of earthquake production and its application to earthquake clustering, 1994. http://onlinelibrary.wiley.com/doi/10.1029/93JB02581/abstract

J.H. Dieterich. Modeling of rock friction 1. Experimental results and constitutive equations, 1979.
http://onlinelibrary.wiley.com/doi/10.1029/JB084iB05p02161/abstract

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?

What’s all this talk about earthquakes? Part II

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.

A11setup5-2

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.

quakes

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!

Origin of the Earth’s hum

Earthquakes can cause the Earth to vibrate over period of days to months. However even in the absence of earthquakes, the Earth keeps vibrating at very low frequencies. This continuous vibration generated by very slow seismic waves with periods greater than 50 s was first discovered in the late 1990s and since then several explanations involving ocean wave propagation have been proposed to explain the origin of this phenomenon. One of the theories suggests that this continuous hum is generated by the constructive interference of microseismic waves that are created during the collision of ocean waves moving in opposite directions. However the period of these microseismic waves is lower than 13 s. So the origin of this Earth’s hum remained unexplained until recently. In February 2015, a group of french researchers proposed the interaction of long ocean waves with the seafloor as a possible explanation and by integrating this new theory into their models, were able to generate microseismic signals with periods ranging from 13 to 300 s. They concluded that both the collision of opposing ocean waves and in a larger extent the movement of long ocean waves over the ocean bottom are responsible for the Earth’s hum.

To learn more about this study, please see the following paper: Ardhuin, F., Gualtieri, L., & Stutzmann, E. (2015). How ocean waves rock the Earth: two mechanisms explain microseisms with periods 3 to 300 s. Geophysical Research Letters.

Reference : http://blogs.agu.org/geospace/2015/04/07/new-study-explains-source-of-earths-mysterious-ringing/

Counteracting the effects of an earthquake with a seismic metamaterial

In 2013, a team of scientists at a french construction firm published a paper describing a new way of counteracting the destructive effects of an earthquake using a seismic metamaterial. In this paper, they explain how they tried to attenuate the amplitude of seismic waves at the free surface by modifying the energy distribution. Their experiment consisted in simulating an earthquake and using a metamaterial made of a grid of vertical and empty cylindrical columns bored into soil near the earthquake source to attenuate the energy released by the surface waves. They were able to reflect the energy of the incoming surface waves and hence significantly dampen their energy.

To learn more about the details and the results of this experiment, please see their paper:http://arxiv.org/pdf/1301.7642v1.pdf