HR: 0800h
AN: S41A-0922    [Abstracts]
TI: Maximum Magnitude in Relation to Mapped Fault Length and Fault Rupture
AU: * Black, N
EM: nblack@ess.ucla.edu
AF: UCLA, 595 Charles Young Dr East, Los Angeles, CA 90095
AU: Jackson, D
EM: djackson@ucla.edu
AF: UCLA, 595 Charles Young Dr East, Los Angeles, CA 90095
AU: Rockwell, T
EM: trockwell@geology.sdsu.edu
AF: SDSU, 5500 Campanile Dr, San Diego, CA 92182
AB: Earthquake hazard zones are highlighted using known fault locations and an estimate of the fault's maximum magnitude earthquake. Magnitude limits are commonly determined from fault geometry, which is dependent on fault length. Over the past 30 years it has become apparent that fault length is often poorly constrained and that a single event can rupture across several individual fault segments. In this study fault geometries are analyzed before and after several moderate to large magnitude earthquakes to determine how well fault length can accurately assess seismic hazard. Estimates of future earthquake magnitudes are often inferred from prior determinations of fault length, but use magnitude regressions based on rupture length. However, rupture length is not always limited to the previously estimated fault length or contained on a single fault. Therefore, the maximum magnitude for a fault may be underestimated, unless the geometry and segmentation of faulting is completely understood. This study examines whether rupture/fault length can be used to accurately predict the maximum magnitude for a given fault. We examine earthquakes greater than 6.0 that occurred after 1970 in Southern California. Geologic maps, fault evaluation reports, and aerial photos that existed prior to these earthquakes are used to obtain the pre-earthquake fault lengths. Pre-earthquake fault lengths are compared with rupture lengths to determine: 1) if fault lengths are the same before and after the ruptures and 2) to constrain the geology and geometry of ruptures that propagated beyond the originally recognized endpoints of a mapped fault. The ruptures examined in this study typically follow one of the following models. The ruptures are either: 1) contained within the dimensions of the original fault trace, 2) break through one or both end points of the originally mapped fault trace, or 3) break through multiple faults, connecting segments into one large fault line. No rupture simply broke a fault end to end, without stepping over onto another fault. In nearly all of these earthquakes, ruptures propagated beyond the mapped endpoints of the faults and/or connected multiple fault segments into one large rupture. In many California earthquakes, the rupture propagated beyond the originally mapped fault traces. Examples include: the Hector Mine, Landers, and Elmore Ranch earthquakes. In all of these cases, and others (Knuepfer, 1988), faults are not mapped into Quaternary sediments, and therefore could be buried under alluvium. Furthermore, it is observed that multiple fault segments can rupture in a single event. These situations imply that fault lengths used in Southern California hazard estimates may be underestimated, because hazard analysis does not take into account fault segmentation or fault traces that die out in Quaternary alluvium. By underestimating fault lengths, the maximum magnitudes will also be underestimated. In order to access a faults maximum magnitude, the geometry must be accurately defined. To define this geometry, guidelines must be established that account for fault step-overs and buried fault segments. Adding these guidelines will provide a more accurate assessment of the maximum fault length, and a better understanding of the earthquake hazard. In the future we will use geophysical data in combination with structurally geology and geomorphology to constrain the fault geometry. We also plan to expand the data set to include regions outside of Southern California. Better constraints on fault geometry, and additional earthquake analysis, will lead to a more complete understanding of how fault length and fault rupture are related.
DE: 7223 Seismic hazard assessment and prediction
DE: 8015 Local crustal structure
DE: 7205 Continental crust (1242)
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting