T41B-01
Using a microfossil-based approach to constrain megathrust-induced coseismic land displacement in coastal Oregon, USA
Paleoseismologists infer the amount of coseismic subsidence during plate-boundary earthquakes from stratigraphic changes in microfossils across sharp peat-mud and peat-sand contacts. However, the use of lithostratigraphic-based reconstructions is associated with a number of limitations, and these become particularly significant when examining low amplitude, short period variations that occur during a plate-boundary earthquake. To address this, paleoecologists working in the coastal zone have recently adopted a transfer- function approach to environmental reconstruction. Continuing subduction of the Juan de Fuca plate beneath the North America plate constitutes a major seismic hazard in the Pacific Northwest. The subduction zone interface presently lacks seismicity. The timing of the last great earthquake along the Cascadia subduction zone (1700AD) is now well refined by Japanese records of an orphan tsunami (no causal earthquake was felt in Japan) that was generated from an earthquake off the Pacific Northwest on the evening of January 26th 1700AD. I will apply the transfer function to modern foraminiferal datasets along coastal Oregon to analyze the fossil record and quantitatively determine the amount of vertical land movement associated with the 1700AD earthquake event. To date, we have collected 7 modern transects totaling 132 samples from the intertidal zone to the upland. We have also collected 9 cores recording the 1700AD earthquake. Furthermore, a 4m vibracore was collected and contains between 3 and 5 potential earthquake horizons. The 1700AD earthquake in the vibracore shows a distinct litho- and biostratigraphical change representing an instantaneous episode of subsidence of approximately 1m. However, development and application of the transfer function to such events will provide quantitative constrained estimates of coseismic land movement. Measurements that are more accurate are necessary to help modelers develop simulations that are more realistic in order to better assess earthquake and tsunami hazards. This will enable efficient and effective mitigation planning and preparation to minimize the personal and economic costs associated with such hazards.
T41B-02
Macroseismological And Paleoseismological Studies In The Active Segments Of The Morelia Acambay Fault System, Mexico
Paleoseismological studies along several faults of the Morelia-Acambay system show clear evidence of its seismicity. In the Acambay region, four seismic events were identified during the Pliocene Pleistocene; two linked to large sub aquatic landslides, and two to liquefaction processes. The two seismic events associated to slumps were also recognized in the Ixtlahuaca, Mexico, region; meaning these were relevant seismic events at a regional level. Their magnitudes were above 5 degrees. Because these latter events are located in the same column, they could be of aid in knowing recurrence periods. In the southern portion of the Lake of Patzcuaro region a collapse was identified generating a rock avalanche nearly 29,000 years ago. This collapse is associated to an earthquake of magnitude above 7 degrees, which decreased the lake's extension and resulted in morphology of small hummocks. The Purhepecha term Jaracuaro means "place that emerges" and is the name of a former island where Pre Hispanic settlements occurred. The island is exclusively made up of lacustrine sequences that rose above 50m in height, georadar and vertical electrical signal (VES) studies do not reveal intrusive bodies associated to this deformation. Isosists of the 1845 and 1858 seismic events were reconstructed in the modified Mercalli scale through several historical studies in a number of municipalities in the State of Michoacan. The results indicate isosistes of IX degrees in Patzcuaro, where the earthquake caused the collapse of the basilica. During the 1858 earthquake the water level in the southern portion of the Lake of Patzcuaro raised over 2m and the destruction of 120 adobe houses is related to the generation of a tsunami. This seismic event is being characterized in wells and ditches dugs around Patzcuaro Lake. The region of Patzcuaro has experienced a number of magmatic and tectonic events that undoubtedly modified the conditions of the lake regarding its sedimentology and anthropogenic development.
T41B-03
Can the seismic slip direction be retrieved from pseudotachylyte veins?
Pseudotachylytes can form by frictional melting as a result of seismic slip along a fault plane. During a brief seismic event, the fault plane material initially deforms by cataclastic flow until melting occurs and stress drops. The partially molten material then flows as a solid-melt suspension. Immediately after the seismic event, the pseudotachylyte vein consists of a mix of irregularly shaped, sub-equant, randomly oriented clasts, glass and a few newly formed microlites. This study aims to investigates the origin of the anisotropy of magnetic susceptibility (AMS) in pseudotachylyte veins and to assess whether the AMS records syn-seismic or post-seismic flow. The pseudotachylytes of the Val Gilba (Dora Maira Massif, Western Alps) are chosen as an example of syn- exhumation veins. They formed at about 20 Ma during the unroofing of mylonitic gneisses that had recorded a pressure peak of metamorphism at ca. 35 Ma. These veins, up to 25 mm in width, are occur parallel to the mylonitic foliation and are continuous over several tens of meters. The magnetic fabric of both host and vein are very consistent, yet distinct. The AMS of the host gneiss is controlled by magnetite and phyllosilicates (P = 1.32) and lies parallel to the pervasive stretching lineation. In contrast, the AMS of the pseudotachylyte, controlled almost entirely by elongated magnetite grains (P = 1.08), is oblique (30°) to the stretching lineation. The AMS fabric in the gneiss is strongly planar (T = 0.886), whereas in the pseudotachylite, the fabric is more plano-linear (T = 0.373). The pseudotachylite oblique fabric might be interpreted as a result of imbrication. Microstructures of the pseudotachylyte indicate that deformation was plastic and occurred at high temperature. The AMS in the pseudotachylite is interpreted to result from syn-seismic melt flow, as suggested by the laminated internal structure of the veins and by small drag folds affecting the laminae. These new results open unprecedented possibilities for paleoseismic studies regarding earthquake focal mechanism of prehistoric events.
T41B-04
Further Tests of the Seismo-Lineament Method for Recognizing Seismogenic Faults at the Ground Surface
The importance of locating the surface trace of faults that can produce earthquakes is self-evident, particularly in California where avoidance of ground-rupture hazards is a legal requirement. We have developed a method that utilizes earthquake focal mechanism solutions coupled with field reconnaissance to locate the surface trace of probable seismogenic faults. We project a fault-plane solution from the boundaries of the uncertainty region around the earthquake focus to the surface of a DEM to define a seismo-lineament -- a zone within which the surface trace of the fault associated with the earthquake is likely to be located. Field work is then undertaken to evaluate the hypothesis that a seismogenic fault exists within the seismo-lineament. If a fault is found within the seismo-lineament, the fault’s orientation and direction of slip are statistically compared with the orientation and slip data from the fault-plane solution to complete the spatial correlation of the fault with the earthquake. To evaluate the effectiveness of this procedure, we selected 6 historic earthquakes that caused fault displacement of the ground surface and used the seismo-lineament method to indicate the probable location of the surface trace of the fault. Earthquakes analyzed in this study include the Parkfield (2004, M6), Denali (2002, M7.9), Hector Mine (1999, M7.1), Superstition Hills (1987, M6.2 and M6.6), and Borah Peak (1983, M7.3) earthquakes. In all 6 test cases, the actual ground-rupture zone associated with the main shock was located within the seismo-lineament. In addition to using focal-mechanism solutions associated with the main shocks to define seismo-lineaments, we have used data from several major aftershocks associated with these events. Seismo-lineaments defined by aftershocks also coincided with the surface trace of the seismogenic fault. Based on results from this study, the seismo-lineament method is likely to be useful in identifying probable seismogenic faults in areas where high-quality focal-mechanism solutions are available for small earthquakes that did not cause ground rupture. This method will be particularly useful in locating probable seismogenic faults that reach the ground surface in highland areas where there is no in-situ Holocene material available to be analyzed through trench studies.