OSMS 94-25


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A magnitude 5.3 (ML BRK; moment magnitude 5.4 BRK) earthquake occurred on December 26, 1994 at 6:10 am (PST), approximately 20 km west (offshore) of Eureka. The preliminary hypocenter is located at 40.76 N, 124.37 W at a depth of approximately 20 km (BRK, USGS).

The recorded peak horizontal accelerations from this earthquake are relatively high (over 0.5 g maximum), but the duration is short (1-2 seconds) and the ground response records show a single pulse. In contrast to the horizontal, the vertical accelerations were relatively low at the near-in stations.

Table 1 summarized the strong-motion data at 9 CSMIP stations, ranging in epicentral distances from 19 to 48 km. The location of these stations and other CSMIP stations in the Eureka area are shown on the map. The largest horizontal acceleration was 0.56 g, recorded at a ground response station in Eureka about 20 km from the epicenter. The vertical acceleration at this station was only 0.06 g.

The records from two structures are included in this report. A 5-story building and a lightly-instrumented bridge. The 5-story building, a reinforced masonry residential building, was strongly shaken, with recorded horizontal accelerations of approximately 0.5 g at the first floor and 0.9 g at the roof level.

Processed acceleration, velocity and displacement and response spectra (preliminary) are included in this release for the abutment channels of a Caltrans bridge over the Samoa Channel in Humboldt Bay near Eureka.