2011 IRIS Undergraduate Interns

This summer I will primarily be using Antelope to look for and locate earthquakes in Jalisco, Mexico. The Jalisco Block of Southwest Mexico is a relatively young subduction zone, subducting both the Cocos and Rivera plates under the North American plate. However, Jalisco does not appear to be a truly traditional subduction zone nor does it appear to exhibit similar seismic behavior down-dip of the seismogenic zone as in the neighboring Oaxaca zone. Hopefully having a better database of hypocenters from Jalisco will help contribute to a better understanding of what is occurring in that region. 

There has been an ongoing debate about the nature of heterogeneities in the lowermost mantle. Different P and S-wave tomography models can be compared to obtain an ad hoc picture of the nature of heterogeneities (e.g. thermal versus chemical), but because such models are normally derived from different datasets and using different assumptions, such a comparison is a nuisance. Here we use existing data from deep and large magnitude earthquakes to analyze and compare differential travel times of PcP-P and ScS-S waves. This type of analysis is the most direct probe of the nature of heterogeneities in the lowermost mantle because the PcP and ScS waves sample the core-mantle boundary (CMB) region in the same way. The predicted PcP-P and ScS-S travel time differential is subtracted from the observed travel time difference, and that residual can then be plotted onto a map to show where parts of the CMB produce slower or faster PcP or ScS. By then comparing the PcP-P and ScS-S travel time residuals as well as their lateral gradients, we can infer what may be the nature of heterogeneity in a given area of the lowermost mantle.

The goal is to expand the data set by Tkalcic and Romanowicz (2002) by using seismic stations and earthquakes in areas not initially used. Because of the ten-year difference between the initial data set, we are able to utilize the new data recorded at the newer seismic stations, in particular in Australia and Antarctica. In this study, over 480 measurements were made. Although more observational work still needs to be done in finding events that produce good PcP-P and ScS-S, we are beginning to fill in the “holes” of the existing spatial coverage. Events in locations such as the Mediterranean Sea add to the velocity model particularly in the northern Atlantic Ocean. In the new data set, the largest negative PcP-P residuals are located along the coast of Antarctica and between Alaska and Russia. These larger negative residuals indicate significantly faster paths through the lowermost mantle. When compared to some whole-mantle P-wave velocity models, the acquired data appears to coincide with the models in many areas. The surface projections of the bounce points in east Asia and North America show fast patches at the CMB which fit previous models. After analyzing PcP-P, we moved onto ScS-S to compare the former residuals with the latter to presume the nature of the heterogeneities. The measurements vary in comparison to those of PcP-P, as some are very close to their PcP-P counterparts, others are very different in magnitude, and even opposite in sign. Because of the locations of the chosen events, a high percentage of the data relates to the CMB beneath Asia and the Middle East.

Further analysis is necessary in order to fill in the bigger “holes” in the spatial coverage of the lowermost mantle. Large areas, mainly in the oceans, have yet to be examined. The difficulty in doing so comes from the lack of seismometer stations in these areas and the noisy data often retrieved from stations on islands. A possible solution to this problem would be the installation of borehole ocean bottom seismometers at key locations.

Locating offshore seismic events with precision is very difficult due to a lack of nearby seismic stations, limited azimuthal coverage, and uncertain velocity structures. Thirty-four ocean bottom seismometers (OBSs) were deployed in August of 2010 in order to attenuate these issues and gain a better understanding of offshore seismicity patterns. I am scheduled to help recover these OBSs in September so that the data it has been collecting for almost a year can be examined and analyzed.

In preparation for the research cruise, I will map data from the Southern California Seismic Network (SCSN) earthquake catalog and other scientific sources using the Generic Mapping Tools (GMT). I will also calculate polarity-based focal mechanisms from waveform data using a Fortran computer program. The results from these two analysis techniques will hopefully show a correlation between offshore seismicity and known/suspected offshore faults. They will also help us gain a better understanding of offshore seismicity patterns, fault structure, and rupture dynamics.  

This summer I will be researching with Anne Trehu at Oregon State University. On August 28-30 an earthquake swarm occured off the northern Juan de Fuca ridge. There were at least 75  earthquakes recorded onshore with the largest being a magnitude 5.9. The earthquake swarm was also recorded by an array of 16 ocean bottom seismometers called Central Oregeon Locked Zone Array (COLZA). Hundreds of T-phases were recorded on COLZA. We will locate the T-phase source using cross correlation techniques that are usually used for locating seismic tremor. The locations of the earthquakes can give a better understanding of the source of the swarm. The migration of seismic activity can be related to magmatic processes on the mid ocean ridge.

I will also be going on a research cruise in July to help deploy ocean bottom seismometers off the coast of Oregon and Washington.

As oceanic plates subducts down into the mantle at convergent plate boundaries, friction on the interface with the overriding plate causes a stick-slip behavior. The overriding plate is pulled down by the subducting plate in areas of strong coupling, accumulating strain on the megathrust fault until slip occurs and the overriding plate pops back up. This process generates major earthquakes such as the 2004 tsunami-generating Sumatra-Andaman event. Recent observations have also revealed slow slip episodes on the deeper plate interface with the motion indicating the release of accumulated strain, but these episodes typically last much longer than an earthquake. Corresponding seismic tremor composed of relatively small, monotonic signals also appear to occur near the deeper plate interface. Together the correlated strain and seismic observations characterize episodic tremor and slip (ETS) events, which recur with regular intervals that range from months to years. The processes that govern ETS and the potential relationships to major earthquakes remain unknown. Nevertheless, these events have been proposed to have significant impact on the likelihood of megathrust earthquakes since they appear to occur over adjacent depth ranges along the same plate interface. This project seeks to examine whether there are observable characteristics of the plate interface that could influence the frictional behavior and hence what type of seismic activity occurs. 

This summer I will be working with Dr. John Nabelek at Oregon State University. Dr. Nabelek and his colleagues deployed a densely spaced array of seismometers in the Central Nevada Seismic Belt (CNSB). The CNSB is characterized by extension and normal faulting. One such normal fault is the Pleasant Valley fault, which ruptured in 1915 causing a magnitude 7.6 earthquake. This array crosses the Pleasant Valley fault, and records waves generated by teleseismic earthquakes, as well as high frequency waves from nearby mine blasts. We conduct receiver functions on these high frequency signals to image the crust. We hope to gain a better understanding of the structure of the CNSB, and of normal fault geometry in general. In particular we aim to determine the dip of these faults, whether they are listric or planar, and whether they bottom out in the mid crust or whether they become ductile shear zones at depth and continue through the lower crust and possibly into the mantle.

My project is focused on many years of data collected in southeast Alaska as part of STEEP. I will be calculating teleseismic P-wave travel time delays using dbxcor for many different events on multiple seismic stations. This data processing will take up the first few weeks of my internship. Once all the data has been processed a 3D tomography model will be made of the area to get a better understanding of the plate tectonics in the area, which are quite complicated. There have been other studies that have collected data in the area including some active source studies. If there in enough time the hope is to integrate that data into the tomography model as well.

This summer I am creating receiver functions for seismic stations along the Ryukyu arc in southeastern Japan. After I have made the receiver functions I will be doing some forward modeling to try and determine whether any serpentinization has occured in the upper mantle of the subduction zone. I am working under the guidance of Dr. Maureen Long with the help of one of her grad students Erin Wirth.

I am working for Dr. Maria Beatrice Magnani at the Center for Earthquake Research and Information (CERI) in Memphis, Tennessee. During the internship I will be assisting Dr. Magnani in her research on the fault structure in the subsurface of the New Madrid Seismic Zone (NMSZ). The presence of magnitude 7.0, and greater, earthquakes within the NMSZ have led researchers to believe that there is a complex fault system in the near subsurface zone of the lithosphere and that this system is presently active. Considering the damage of the earthquakes that occurred in 1811-1812 in the region, researchers believe that it would be important to map the fault system to better understand the following: where exactly the fault zones are located and their relation to each other, what parts of the system are active, the average resonance period for earthquakes at the fault zones, and probable causes of the stress in the system.

To better understand this fault system, Dr. Magnani started a project in 2007 with the goal of running seismic surveys along the Mississippi, which runs directly through the NMSZ, to map out the fault system. Along with a lot of preparation, this project included three summers of cruises down the Mississippi, each one mapping about 300 kilometers of the river. This summer marks the last of the cruises and the conclusion of data collection.

So, for the first three weeks of the internship and I will be drifting backwards down the Mississippi collection seismic data from a hydrophone active source array (an airgun is used as the source) and a Manta Ray sized CHIRP sensor. The shore component will consist of primarily data analysis and interpretation (basic processing is performed on the boat during the day). I am so excited!

For more information, you can look at the Mississippi River Project website at: http://www.memphis.edu/riverproject/

Since 2007 seismological data collected from Costa Rica's Nicoya Peninsula has shown the existence of slow-slip events (SSE) and tremor. For a year and half, starting in 2000, data was collected on the peninsula and off-shore near the subduction zone of the Cocos plate and Caribbean Plate. This was before more modern techniques for tremor and SSE identification were developed.

I will assist in analyzing data from 2000-2001 to expand the known time-series of tremor and SSE events in Costa Rica's Nicoya Peninsula.

Slow-slip events differ from the news-headline earthquakes that people fear. SSE may release the same magnitude of energy as earthquakes, but they do so over a longer time period (several days, weeks, maybe longer) and they do so aseismically- they literally are slowly slipping along.

The relationship between SSEs and tremor is poorly understood. In this case, the term tremor does not refer to anything associated with volcanoes or underground aliens, but instead represents very small strange earthquakes (only noticeable through seismometers or other modern instruments). Unlike tremor, “normal” small earthquakes emit high frequencies, but for reasons not yet fully understood, tremor emits very low frequencies.

Expanding the scientific community's knowledge on SSE and tremor occurrence is vital for developing an understanding of the processes that control these events. A better understanding of these events may lead to a better understanding of earthquakes and other earth disasters that make the evening news.

I am working on the Source Physics Experiment at the Nevada Test Site.  The goal of SPE is to develop better ways of detecting small explosions so that we can reinforce the Comprehensive Nuclear Test Ban Treaty once it is adopted.  I am using geophone data from the last SPE shot  to better map the test area.  The part of the test site where we are testing is called Climax Stock and is made of Granitoid rock.  It has a perched water table and faults in it, which perhaps I will be able to see in the refraction data.  One thing that knowing the layout of the stock will help is trying to determine the cause of the S waves that have been found from previous explosions.

This summer I will be working with Dr. Alberto Venegas. We are attacking the problem of developing an computer program that detect focal mechanisms for local and regional earthquakes based on first motion phase descriptions in real time. The computation of focal mechanisms is a critical aspect of the daily operations at PRSN (Puerto Rican Seismic Network).

This summer I am working at Harvard University with Dr. Miaki Ishii on developing a continuous back projection of the Moro Gulf earthquake sequence from the summer of 2010. There were three large-magnitude(M7.3, M7.6, M7.4), deep (585-640km) earthquakes that ruptured in the same area within an hour and a half of each other. 'Triplet' earthquakes at this depth or magnitude have not been recorded, which makes this sequence especially unique.

One 
of
 my 
major 
roles
 is 
to
 acquire
 data 
and
 perform
 any 
preprocessing 
steps 
that 
are
 necessary 
for
 this
 type
 of 
analysis.
In 
addition,
I am responsible for 
running
 the 
back‐projection 
codes 
and 
plotting
 the 
results
 for
 interpretation
 by
 the 
group.
I will be putting in a lot of hours in the lab working mainly with UNIX, MATLAB, GMT, and some Fortran. Miaki also has a graduate student, Eric Kiser, who acts as my project leader and exposes me to all of the necessary information and resources to complete this analysis. I am extremely excited about all of the opportunities that this summer is going to bring.