Bilek – Research

My main research interests focus on earthquake processes. Current study areas include one close to home in the New Mexico area, and more far-flung areas in the shallow subduction zones around the world. One common thread to most of my work is to examine important factors influencing the nature of seismicity.

Shallow Subduction Zone Earthquakes and Slow Ruptures
Much of my research deals with shallow seismicity in global subduction zones, with a particular focus on areas that have produced slow ruptures. A recent project investigates this issue along the Nicaragua/Costa Rica margin, in the area of the 1992 tsunami earthquake and recently observed slow slip events (more information here). One recently completed NSF-funded project (with Heather DeShon at U. Memphis (CERI) and Bob Engdahl (U. Colorado) involved relocating earthquakes in the regions of past tsunami or slow ruptures and determining source characteristics of these relocated events to assess how common slow/long duration earthquakes are in these areas.

 

Cascadia:  we are using a combined on-shore/off-shore seismic array to search for small earthquakes on the recently quiet megathrust fault. The Cascadia subduction zone has produced great earthquakes and tsunami in the past, last in 1700, but has been seismically quiet since then.  Based on GPS data though, we know that the fault does move in slow slip events.  Using the Cascadia Initiative dataset and subspace scanning tools (in conjunction with Charlotte Rowe, LANL), we are searching for small earthquakes that may have been missed by more traditional land-based seismic networks.  Initial results suggest specific regions, namely in central Oregon, do produce small magnitude earthquakes not previously detected that may be related to seamount-related deformation.  More information about this project can be found here

 

Description of the Cascadia subduction zone and geometry for the dataset that we are using to search for small magnitude earthquakes.

 

Costa Rica/Nicaragua: This subduction zone produces a range of slip processes, from large earthquakes to slow slip and tremor.  In addition, there is significant diversity in the geology of the subduction zone region, with variable origin in the incoming Cocos plate as well as variable roughness on the incoming plate.  In collaboration with Susan Schwartz (UCSC), Andrew Newman and Zhigang Peng (GA Tech), and Scott Phillips (LANL), we use seismic data from a range of local and regional networks to explore how earthquake source parameters vary depending on incoming plate conditions and past slip.

 

Costa Rica earthquakes, defined into specific clusters, we use for source parameter analysis.  The star indicates location of the M 7.6 earthquake that occurred in 2012; the clusters we use here are aftershocks of that event.

Earthquake stress drop along northern Costa Rica and southern Nicaragua, spanning the southern portion of the 1992 tsunami earthquake rupture zone.  Earthquakes outside of the rupture zone (dashed box) have higher Δσ than those inside the rupture zone.  The area of c1 and c2 is within a low rupture velocity, high moment release patch of Ihmlé [1996]; this is also the area of imaged subducted seamount [McIntosh et al., 2007]. Citation: Bilek, S.L. Rotman, H.M., and Phillips, W.S., Low Stress Drop Earthquakes in the Rupture Zone of the 1992 Nicaragua Tsunami Earthquake, Geophys. Res. Lett., 43, doi:10.1002/2016GL070409, 2016.

 

Mexico: Similar to Costa Rica, the Mexico subduction zone also exhibits a range of slow earthquakes, tremor, and large earthquakes.  In collaboration with Mike Brudzinski (Miami University), we are looking at temporal changes in earthquake source properties around the time of a large megathrust earthquake in the Oaxaca portion of the margin.

 

Oaxaca segment of the Mexican subduction, showing the diversity of slip, including small seismogenic earthquakes (black dots), large megathrust events (black outlines), slow slip events (SSE, pink outlines), and tremor (green).  We use recordings from small earthquakes, recorded by the local seismic network (OXNET, triangles), to explore how source parameters vary spatially relative to the other slip processes.

Socorro Magma Body Studies

The Socorro Magma Body, a mid-crustal magma sill within central New Mexico, produces slow uplift and regional seismicity.  Several NMT faculty and students study various aspects of the magma body; Bilek’s research group focuses on characterizing the seismicity and structure of the SMB.  Long-term seismic networks have captured decades of seismicity, and our group is working on improving seismic velocity models and relocating the associated seismicity.  We recently deployed a short term, large N (over 800 stations) seismic network (in collaboration with Brandon Schmandt and Lindsay Worthington (UNM) and Rick Aster (CSU)) in the northern portion of the SMB to explore seismicity and structure in the area of maximum uplift.

 

Cartoon of the Socorro Magma Body in central New Mexico with the magma sill sitting at ~19 km depth beneath Socorro, and seismicity (yellow dots) concentrated above the SMB.

 

In Feb 2015, we deployed a mixed array of both broadband sensors and 800 single channel node seismometers above the Socorro Magma Body to study the earthquakes and structure associated with the magma body deformation.  This project, in conjunction with UNM, Colorado State and IRIS PASSCAL, highlights the use of new instrument technology and deployment strategies.  Learn more about this project here and the deployment here. 

Map of NMT seismic network (red stars) and 2015 deployment of 800+ short period geophones (nodes, red triangles) placed within the northern section of the SMB (outlined by dashed line).

Induced Seismicity and Other New Mexico Earthquakes
Earthquakes linked to hydrofracting or wastewater injection is an area of high interest nationally, and my students and I are currently working on better characterization of possible induced seismicity in several locations within the state of New Mexico and elsewhere.  During 2016, I participated in a community-led deployment of seismometers in central OK to explore induced seismicity in that state.  You can find out more about that experiment at  http://www.iris.edu/hq/site/PAGE_static/4261
Using Seismology to Explore Hydrogeologic Processes
Karst aquifers are important water supplies for much of the globe.  We use non-invasive geophysical techniques to monitor water flow in karst aquifers to provide information on important recharge events.  We (with Andrew Luhmann and Ronni Grapenthin) conducted a pilot experiment to record seismic signals associated with both artificial and natural recharge events into a karst system in MN; we are currently relating these signals to changes in hydrologic data such as discharge.  Our upcoming project (see https://goo.gl/afvHAR ) is a multi-year project to monitor the Santa Fe Rise system in central FL with a variety of geophysical and hydrologic data collection to better understand the recharge processes.
Photos below are from the pilot experiment in MN, where we deployed 12 seismometers to record artificial recharge events from a large inflatable pool as well as a well-timed large rainstorm.