Geophysics

Faculty

Bilek – Subduction zone studies

– 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.

 

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.

 

Bilek – Socorro Magma Body

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.

 

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).

 

Bilek – Seismic monitoring of karst aquifers:

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.

Pilot experiment in MN – example of an artificial recharge event where we poured a large volume of water into the karst system.

 

Seismograms from the pilot experiment, recording one of the artificial recharge events.  Vertical line indicates start time of the pour.

 

Other Items:

 

IRIS-PASSCAL (https://www.passcal.nmt.edu )

NMT hosts the IRIS PASSCAL instrument center on campus. This facility serves as a center of excellence for portable seismology, supporting PI seismic experiments around the world. The group provides pre-experiment support, instrument use training, logistics support, data handling and data archiving assistance.  The facility employs a professional staff as well as both NMT undergraduate and graduate students.  For more information, contact Susan Bilek, PI of the PASSCAL facility.

 

New Mexico Seismic Network

The Geophysics group and the NM Bureau of Geology operate a seismic network within the state of NM.  There are two primary focus areas of the network: around the Socorro area of central New Mexico and around the Carlsbad area of southeastern New Mexico.  Earthquake information is routinely posted at http://www.ees.nmt.edu/outside/NMTSO/quakelist.html , and more information about the network is available here (http://www.ees.nmt.edu/outside/NMTSO/ ).