This proposal funds a study of historic records of earthquakes prior to 1928 in Washington and Oregon. The main objectives are to estimate magnitudes and locations for historic earthquakes, search for aftershock sequences, and make information on early historic earthquakes in Washington and Oregon accessible via the web.

An event's magnitude and location can be estimated if the strength of shaking is known at a number of different places. Large earthquakes are felt over wide areas and shaking generally falls off with distance from the earthquake. Shaking strength is described by the Modified Mercalli intensity (MMI) scale, which assigns values from 0 to 12 depending on severity of shaking. Older earthquakes that took place before seismometers were installed were often reported felt at many communities, and the geographic distribution of intensities estimates the distribution of ground acceleration.

To determine the location and magnitude of older earthquakes, we first use earthquakes with known locations and magnitudes to estimate how shaking weakens with distance. That "attenuation relation" is then used in a large number of trials with various location/magnitude combinations. Using a trial location and magnitude, each observed intensity is compared to an intensity calculated from the trial location/magnitude. By testing many possibilities, the location/magnitude combination that best fits the observed data is determined.

Other aspects of this study include conducting a systematic search for accounts of aftershocks for some of the larger earthquakes, and reformatting multiple catalogs and hundreds of newspaper articles (collected under previous funding and augmented under this study) for convenient viewing via internet.

Results

Using a previously complied database of newspaper clippings and records of weather observers, we have assigned intensities and matched the intensity locales to geographic coordinates. We are currently selecting events with enough intensity values for location/magnitude estimation, and searching for additional newspaper accounts to improve the estimations and expand the number of events for which estimates can be made.

Due to staffing issues, we are slightly behind schedule, and have requested a six-month no-cost extension to continue refining the intensity database, complete the search for aftershocks of larger events, and export information from the data-base to web-compatible format.

An additional task, developing outreach materials based on Native American earthquake and tsunami stories, was proposed as part of this study in cooperation with emergency managers in Washington and Oregon but not funded. A small part of this work has gone forward under other support. Several trips were made to the Washington Coast to assist in developing sources and material for a K-12 video using Native stories to teach about the tsunami hazard on the Washington coast. Landslide-related Native stories from Puget Sound came to our attention, and an abstract and poster were presented at the 2003 Geological Society of America meeting in Seattle.