Title

The 1990 Nooksack Forks, Washington, Earthquake Sequence: Sequence Geometry and Temporal Characteristics

Publication Date

5-1992

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geophysics

Department

Geosciences

Major Advisor

James E. Zollweg

Abstract

The April 14, 1990 Nooksack Forks earthquake occurred in the Deming area, Washington state. Strong foreshock and aftershock activity was associated with the earthquake. The main shock of the sequence, with magnitude 5.2, was the largest thrust earthquake in the Pacific Northwest in the last 30 years. The sequence has been interpreted as occurring on shallow (less than 4 km depth) conjugate thrust faults.

Deming is located near the confluence of the three principal forks of the Nooksack River. It is approximately 20 km northeast of Bellingham and 30 km west of Mount Baker, a major active stratovolcano of the northwest Cascades. Little or no seismic activity had been recorded in the immediate epicentral region since the first seismic telemetry network in Washington was installed in 1969. However, prior to 1990, some sporadic minor earthquakes had occurred about 7 km to the east.

The data analyzed in this study of the Nooksack Forks earthquakes were recorded on a temporary seismic network installed by J. E. Zollweg. A total of 115 aftershocks were selected for location and 53 extremely well constrained locations were used to examine the sequence geometry. Events were chosen based on size, clarity and the number of recording stations. All events were read by the author to minimize reader's bias.

Hypocentral depth error estimates were determined from the shape of the residual error versus depth curve calculated at a finite number of points. Epicentral errors were estimated from 95% confidence intervals calculated previously by J. E. Zollweg, and from a general observation that epicentral errors are usually about half the depth error. An accuracy of better than 100 meters in epicenter and 200 meters in focal depth is estimated for well-constrained events. The estimated errors indicate that structural features smaller than 200 meters could not be resolved.

The hypocentral depths of the Nooksack Forks earthquakes lie within 3.2 km of the surface. Most of the aftershocks have depths less than 2.5 km. The pattern of hypocentral distribution suggests that events are occurring on two conjugate faults in the lower Chuckanut Formation and in the underlying Shuksan metamorphic assemblage. Conjugate fault sequences are believed to be uncommon. To the author's knowledge, the Nooksack Forks sequence is one of only five sequences involving conjugate faulting to have been reported in North America.

Coda length magnitudes ML(coda) were estimated for 1780 foreshocks and aftershocks (in the magnitude range -1.1 through 3.9) measured from a single station. Of these, the 333 largest events were used to study the behavior of b-values in different parts of the sequence. The cumulative b-value for the entire sequence is about 0.98, which is within the normal range (0.8 - 1.2) for aftershock sequences. No significant changes in the cumulative b-values were detected between the foreshocks and immediate aftershocks. Therefore, the foreshock b-value could not have been used to predict this earthquake.

The temporal behavior of the b-value was investigated using windows of 100 events sliding by 20 events. Abrupt increases in temporal b-values started 3 days after the main shock. The increase in b-value with time could be related to changes to lower stress levels in the aftershock zone, or to differences in stress levels between the two conjugate planes. To the author's knowledge, this is the first time the temporal b-value behavior of a conjugate fault sequence has been examined.

Files over 30MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS