Geoscience student Ellen Lamont (’11) will present her research findings on earthquakes in central Taiwan at the meeting of the American Geophysical Union in San Francisco in December, 2010. In recent years, the meeting has been attended by more than 15,000 geoscientists from around the world, including many international scientists. We anticipate that her findings will garner much interest.
Ellen is working with Dr. Jon Lewis on an NSF-funded project that aims to understand active crustal strain in the central Taiwan mountain belt. This is an area where the Luzon volcanic arc is colliding with what used to be the continental shelf of China. Ellen’s work sheds light on how the youngest part of the westward propagating mountain belt mechanically links to the older, more mature part of the collision.
Transient Upper Crustal Tear Illuminated by the Chi Chi Earthquake: Results from Strain Inversions in the Luliao Region, Taiwan E. A. Lamont1; J. Lewis1; T. B. Byrne2; J. M. Crespi2; R. Rau3
1. Geoscience, Indiana University of Pennsylvania, Indiana, PA, United States.
2. Geoscience, University of Connecticut, Storrs, CT, United States.
3. Earth Science, National Cheng Kung University, Tainan, Taiwan.
Modeling of earthquake focal mechanisms and coseismic GPS data from an area at the southern tip of the 1999 Chi Chi rupture suggests the existence of an evolving upper plate tear. The earthquakes occur in what we refer to as the Luliao seismic zone and define a steeply northeast-dipping tabular volume that extends from the surface to approximately 11 km. We find that the focal mechanisms from the six-month period following the 1999 Chi-Chi Earthquake yield best-fitting strain tensors that suggest the dominance of strike-slip faulting. Our strain inversions, using a micropolar continuum model, reveal orogen-perpendicular (NW-SE) minimum stretching (i.e., shortening) and orogen-parallel (NE-SW) maximum stretching. Additionally, our inversions indicate plane strain with positive, non-zero relative vorticity values, suggestive of counter-clockwise (map view) block rotations. Published coseismic GPS data provide additional evidence that this tabular volume of crust is the locus of strike-slip faulting accompanied by block rotation. Preliminary 2D strain inversions for GPS stations that span the inverted focal mechanisms reveal negative (counterclockwise) rotation values and principal strain axes that are generally consistent with our focal mechanism inversions. We interpret our findings to reflect an accommodation zone that is activated by differential westward expansion of the foreland fold and thrust belt. In particular, this zone separates an area of greater westward propagation near Taichung from an area of lesser propagation to the south near Chiayi. Differential expansion of the orogen appears to be influenced by an eastward pointing, lower-plate promontory south of the Sanyi-Puli seismic zone. Unlike the Luliao events, the Sanyi-Puli seismic zone extends from the near surface to approximately 30 km and we have interpreted it as a reactivated continental margin fracture zone inherited from South China Sea rifting. The lower-plate promontory is coincident with the widest and most distal portion of the Taiwan foreland fold and thrust belt as defined by the westward curving traces of the Changhwa and Chelungpu faults. The Luliao seismic zone lies to the south of the promontory and links the southern tip of the Chi Chi rupture with the area where the Hsuehshan slate belt pinches out southward against the Backbone Range. These relations coupled with our inverse models suggest that lower plate architecture plays a role in exhumation patterns and the evolution of this upper plate accommodation zone.