Serving professionals in engineering, environmental, and groundwater geology
since 1957


Date: Tuesday, August 10, 2004
Location: Steven’s Steak House, 5332 Stevens Place, Commerce, California
Time: 6:00 p.m.-Social Hour; 7:00 p.m.-Dinner; 7:45 p.m.-Presentation

Cost: $25 per person with reservations, $30 at the door, $12 for students with a valid Student ID
Reservations: Call (949) 253-5924 ex 564, or email Brian Villalobos,
By noon, Monday, August 9, 2004

SPEAKER: Dr. Robert D. Jackson
TOPIC: “Maximum Magnitude for a Fault?”


Most fault-based forecasts assume the upper magnitude limit depends on fault length or area. For example, the source model in the CGS/USGS seismic hazard report for California assume the Wells and Coppersmith (1994; W&C) relation adjusted for uncertainty. However, the magnitudes estimated by applying W&C (or similar regression relationships) to mapped fault lengths are inconsistent with California earthquake data. First, several earthquakes violated the assumption that mapped fault dimension limits rupture size. Second, the magnitude distribution obtained using W&C for all of California is inconsistent with observations.

We examined previously mapped faults where California earthquakes might have occurred. None of the quakes on the W&C list ruptured a previously mapped fault or segment from end to end. Five events out of 16 ruptured past the ends and some others could not be associated with mapped faults. Whether unmapped faults or segments existed before the quakes is an open question, but mapped fault termini don’t stop earthquakes.

We constructed a statewide source model based on a new fault activity map. We jointed collinear faults with gaps under 20 km to make longer, single faults. Using observed slip rate and length, we estimated moment rate and magnitude distribution for each fault assuming a truncated Gutenberg-Richter magnitude distribution, W&S, and b=1. We then determined the a-value to match the tectonic moment rate for each fault, and summed all faults to get a theoretical statewide magnitude distribution.

This distribution forecasts a significantly higher rate than reported in the Toppozada catalogue in the magnitude range 6-7. Other models based on W&C share the same discrepancy.


Dr. Jackson is a professor at UCLA since 1981 and is currently the Chair of the Department of Earth and Space Sciences. He graduated from the Department of Physics at Cal Tech in 1965 and received his Ph.D. from the Department of Earth and Planetary Sciences at MIT in 1969.

Dr. Jackson has received numerous awards and honors and is a fellow of various professional societies. Dr. Jackson has served as an officer or on panels for the American Geophysical Union, National Academy of Sciences/Natural Research Council and Southern California Earthquake Center to mention a few. He has authored or co-authored over a dozen articles and papers on earthquakes including topics such as forecasting, probabilities, stress-strain, hazard source models and crustal deformation.