iPlex Lunch - spring-2012

April 11, 2012
1 p.m. - 2 p.m.
1707 Geology

Presented By:

• Zacharie Duputel - Caltech

Seminar Description coming soon.

April 18, 2012
noon - 1 p.m.
1707 Geology

Presented By:

• Eldon Gath - Earth Consultants International

Five years ago, Central Panama was considered relatively aseismic compared to the other Central American countries. Our recent tectonic geomorphic mapping and paleoseismic trenching studies in central Panama have led to a paradigm shift in the neotectonic deformation and seismic hazard of this Caribbean-bounding country. To date, we have conducted strip mapping and 2-D and 3-D trenching studies of several crustal faults, including the Gatún, Limón, and Pedro Miguel faults, and have demonstrated that each fault has had multiple Holocene surface ruptures. The Gatún fault is a left-lateral, E-W-trending strike-slip fault that forms the southern margin of the Sierra Maestra. The fault is at least 40-50 km long from the Gatún Lake eastward to the San Blás Islands, and possibly another 50 km westward across Gatún Lake into the interior of Panama. Our trenching studies indicate a sinistral slip rate of 6.0±3.0 mm/yr, with three 0.7-1.0 m displacement events in the last ~500 years, and the most recent event (MRE) likely in 1849. The Limón fault is a N-S-trending, right-lateral strike slip fault, about 28 km long, terminating to the north at the Gatún fault, and possibly stepping onto the Pedro Miguel fault at its southern end. From our trenching studies we determined a dextral slip rate of 5.0±1.0 mm/yr, at least three surface ruptures in the past ~950 to 1500 years, average displacement per event of about 2 meters, and the MRE possibly in 1873. The Pedro Miguel fault may be the southern extension of the Limón fault, extending 48 km N-S from the south end of the Limón fault at the Chagres River to the Pacific Ocean, and likely another 12 km south to Taboga Island, where it may be responsible for the island’s uplift. Based on our trenching studies, the Pedro Miguel fault has a dextral slip rate of 5.0±2.0 mm/yr, producing three surface-rupturing events in the past ~1500 years totaling at least 8.1 meters displacement, with the last event, a 3.0±0.2 meter rupture, on May 2, 1621.

April 25, 2012
noon - 1 p.m.
1707 Geology

Presented By:

• Romain Jolivet - Caltech

The Haiyuan fault system is a major left-lateral fault system bounding the tibetan plateau to the north-east. Two M~8 earthquakes ruptured that fault system in the past hundred years: the 1920, Haiyuan earthquake and the 1927, Gulang earthquake. Here, we use Synthetic Aperture Radar interferometry to explore the spatial and temporal variations of the interseismic deformation across the Haiyuan fault, over a broad (150*150 km2) area covering the 1920 rupture zone and the millennial Tianzhu seismic gap. Using a small baseline approach, we process five SAR images time series acquired by the Envisat satellite along descending and ascending orbits, spanning the 2003-2009 period. The resulting mean Line-Of-Sight velocity maps are, in overall, consistent with left-lateral motion across the fault and reveal lateral variations of the velocity gradient in the near fault zone. We invert these mean LOS velocity maps for the short-term loading rate on the fault plane at depth and for the shallow slip distribution along the seismogenic part of the fault. The short-term loading rate is about 5 mm/yr. The shallow sections of the fault, that ruptured in 1920 and the most part of the Tianzhu seismic gap are currently locked. In between, a 35~km-long section, that experiences a strong micro-seismic activity, is creeping at a mean horizontal rate of almost 5 mm/yr. However, the shallow creep rate varies along the fault strike and locally reaches values higher than the deep loading rate. This suggests temporal fluctuations of the observed aseismic slip. The comparison of InSAR-derived averaged profiles of the fault parallel velocity, spanning the 1993-1998 (ERS data) and 2003-2009 periods, suggests an upward migration of the creep over the 20 years-long observation period. A time series analysis on the Envisat dataset using a temporal smoothing reveals a creep rate increase during the year 2007. This rate increase follows and may have been triggered by a Ml 4.7 earthquake that occurred on the creeping patch. We finally investigate the relationship between the spatio-temporal evolution of the surface creep and the roughness of the surface fault trace with a multiscale analysis. We show the control of the elastic properties of the brittle crust on the fault roughness, that in turn exerts a direct control on the surface aseismic slip distribution. The aseismic slip is made of locally interacting bursts that follow a power law, somehow similar to the Gutenberg-Richter law for earthquakes.

May 2, 2012
noon - 1 p.m.
1707 Geology

Presented By:

• Victor Tsai - Caltech

Seminar Description coming soon.

May 9, 2012
noon - 1 p.m.
1707 Geology

Presented By:

• Thomas Ader - Caltech

Seminar Description coming soon.

May 16, 2012
noon - 1 p.m.
1707 Geology

Presented By:

• Dimitri Zigone - ISTerre, Grenoble and USC

Seminar Description coming soon.

May 23, 2012
noon - 1 p.m.
1707 Geology

Presented By:

• Elizabeth Cochran - USGS

New developments in sensor technology and the ubiquity of networked computers provide an opportunity to record earthquakes at much denser scales. The Quake-Catcher Network (QCN) makes use of very low-cost micro-electro-mechanical systems (MEMS) accelerometers installed in homes, schools, and businesses to record earthquakes, with over 2000 participants worldwide. These sensors augment existing seismic networks for rapid earthquake detection, as well as for studies on seismic source- and site-related phenomena. Following the 3 September 2010 Mw7.1 Darfield earthquake, over 180 QCN stations were installed in a dense array to record the on-going aftershock sequence in and around the city of Christchurch. Using this network, we recorded hundreds of aftershocks from M2.6 – M6.3. We are using the data to automatically detect earthquakes and rapidly determine their location and magnitude. We also compare the records from the QCN sensors to nearby traditional network stations and find that the observed ground motions are similar, suggesting that these low-cost sensor provide reliable seismograms. We will be installing 6,000 sensors in the seismically active regions of the US, including Southern California, over the next two years to improve sensor coverage before the next moderate to large earthquake.

May 30, 2012
noon - 1 p.m.
1707 Geology

Presented By:

• Ed Nissen - Arizona State University

Seminar Description coming soon.

June 6, 2012
noon - 1 p.m.
1707 Geology

Presented By:

• Xyoli Perez-Campos - UNAM

Mexico is such an interesting country, tectonically speaking. This seismic tour will visit two regions: the gulf of California and central-southern Mexico. We will review the latest results in terms of seismotectonics and structure, based mainly in receiver functions, though other studies will be mentioned. Our first stop will be at the gulf of California, where Farallon subduction ceased 12 Ma and opening of the gulf started. The northern gulf is under diffuse continental deformation while the south is characterized by oceanic spreading. Results from receiver functions have shown the thinning of the crust and lithosphere, and a possible correlation between the surface tectonic provinces with the upper mantle structure. The second stop will be in central-southern Mexico, affected by the subduction of Rivera and Cocos plates beneath North American plate. Recent seismic experiments have mapped the geometry of the subducting slab. In particular, we will visit the upper mantle transition zone and its correlation with the subducted slab.