Bilek – Mexico Project

Source Parameters for Mexican Earthquakes


The transition between the portion of the subduction megathrust that produces large/great earthquakes and the area of non-volcanic tremor (NVT) and slow slip events (SSEs) is a critical zone, as it provides an opportunity for understanding what limits typical seismogenic behavior and what allows recently discovered slower forms of fault slip.  Studying this transition can be difficult however, given typical trench locations and slab dip angles that place the seismogenic zone offshore, away from high quality land-based observations. Along the Oaxacan segment of the Middle America subduction zone, an existing seismic and geodetic network provides observations directly above this target zone, recording many aspects of the broad slip spectrum in a focused geographic area.  Importantly, on March 20, 2012 the network caught the first magnitude > 7 megathrust earthquake in a region where the patterns of NVT and SSEs are known a priori.  The resulting dataset represents a unique opportunity to examine the spatial, temporal, and spectral relationships between large earthquakes, small seismicity, slow slip and tremor.

This project will use this dataset to quantify earthquake occurrence and source parameters along 5 transects along the Oaxacan margin, using the results to test the following hypotheses:

There is a sharp spatial transition in seismicity rate and source properties along the megathrust as events approach the downdip seismicity limit due to increased fluid pressures linked to the generation of episodic tremor and slip.

– Slow slip events produce stress loading in adjacent regions that lead to increased seismicity rates and changes in source characteristics of small earthquakes.

– Large earthquakes are associated with temporal changes in earthquake occurrence and source characteristics in adjacent regions due to changes in stress leading up to and following the event.

Several seismological techniques will be employed: 1) construction of a uniform event catalog since recording began in 2006, 2) focal mechanism determination for the complete event catalog, 3) rupture duration and stress drop calculations for earthquakes using direct phase waveform modeling and coda-wave based spectral ratio techniques, and 4) investigation of seismicity rates over small fault patches by using a waveform template matching algorithm to determine the time history of repeating events.  The resulting catalogs of earthquake locations, occurrence rates, and source parameters will be used to critically evaluate the hypotheses above, which will ultimately advance our understanding of what controls seismogenesis.


 We will be using data collected on a temporary network of stations (grey triangles on map below) and analyzing earthquakes (subset in white open circles) that occur near the area of tremor (green diamonds), slow slip events (pink outlines) and other large earthquakes (rupture areas defined by black or blue outlines).




Susan Bilek, New Mexico Tech,, 575-835-6510

Michael Brudzinski,, 513-529-9758


This study is supported by NSF-EAR (EAR-1246903 to SLB, EAR-1246944 to MB). Graduate students have also been supported through NMT. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation or the Universities.