2001 North Coast Seismicity

2001 continued the pattern of relative seismic quiescence on the North Coast, although activity was somewhat elevated over last year. Twenty earthquakes of magnitude 4 or larger were reported during the year, just under the long-term average of 22 per year, however none were strong enough to knock items off shelves or cause damage. Three earthquakes were in the magnitude 5 range, the largest a magnitude 5.6 on January 13 offshore of Eureka that was felt lightly from Garberville to Crescent City. Perhaps the most interesting sequence of earthquakes occurred in early April on the Gorda Ridge about 130 miles west of Crescent City. At least thirteen earthquake of magnitude 3.5 and larger and nearly 4000 smaller earthquakes were recorded during a twelve day period. The earthquakes were associated with an undersea eruption on the Jackson segment of the Gorda Ridge. Submarine eruptions on the Gorda and Juan de Fuca ridges are relatively common, occurring on average about every two years. More information on monitoring eruptive activity offshore of the Pacific Northwest is posted at:http://www.pmel.noaa.gov/vents/.

2001 North Coast California Earthquakes of magnitude 4 or larger

(Northern Mendocino, Humboldt, Del Norte Counties)

DATE

TIME

LAT

LONG

ML

MW

LOCATION, COMMENTS

(Local)

°N

°W

11-Jan

10:10 AM

40.63

124.2

4

4.2

Gorda plate, subducted slab, 3 miles NNW Fortuna, felt Redcrest to Trinidad.

13-Jan

5:08 AM

40.74

125.3

5.4

5.6

Gorda plate, 62 miles W Eureka. Felt from Garberville to Crescent City.

1-Feb

12:01 PM

40.42

127.2

4.6

Mendocino fault, 153 miles W Petrolia. Not felt.

2-Feb

3:03 PM

39.73

122.8

4.2

4.2

Lake Mountain fault system, 7 miles NW of Alder Springs. Felt in Laytonville.

19-Feb

6:47 AM

40.38

127.2

4.1

Mendocino fault, 154 miles W Petrolia. Not felt.

22-Feb

12:45 AM

40.35

125.5

4.1

Mendocino fault, 63 miles W Petrolia. Not reported felt.

22-Mar

2:22 PM

40.35

126.4

4.6

Mendocino fault, 112 miles W Petrolia. Not reported felt

4-Apr

4:10 PM

42.06

126.9

4.5

Jackson segment of the Gorda ridge, 130 miles W Crescent City. This was the largest of at least 13 earthquakes of magnitude 3.5 and larger centered on the Jackson segment of the northern Gorda ridge. Over 3900 smaller earthquakes detected between Apr. 4

19-Apr

10:19 PM

40.62

125.4

4.3

4.4

Gorda plate, 55 miles W Ferndale. Felt by a few in Arcata.

23-Apr

11:05 AM

40.36

127.2

4

Mendocino fault, 155 miles W Petrolia. Not felt.

1-May

9:55 PM

40.33

124.7

4.1

Mendocino fault, 22 miles W Petrolia. Felt Whitethorn to Arcata.

21-May

5:40 PM

40.46

124.9

4.1

3.9

Southern Gorda plate, 32 miles WNW Petrolia. Felt lightly Ferndale to McKinleyville.

12-Jul

12:18 AM

42.09

126.1

4.2

Gorda plate, 100 miles WNW Crescent City. Not felt.

20-Sep

1:02 AM

40.44

125.3

4.5

5.1

Mendocino fault, 52 miles W Petrolia. Felt lightly Arcata to Petrolia.

10-Nov

10:55 AM

41.24

126.4

4

Gorda plate, 120 miles W Trinidad. Not felt.

20-Nov

8:58 AM

41.75

125.9

3.8

4.1

Gorda plate, 90 miles W Crescent City.

25-Nov

9:27 AM

42.06

126.8

4.4

Gorda ridge, 138 miles WNW Crescent City.

29-Nov

11:46 PM

40.49

126.9

4.7

5.1

Mendocino fault, 136 miles W Petrolia. Not felt.

7-Dec

6:29 AM

39.04

123.1

4.1

4.3

Maacama fault, 9 miles SSE Ukiah. Felt Healdsburg to Potter Valley.

14-Dec

1:41 AM

39.05

123.1

4

4.4

Maacama fault, 9 miles SSE Ukiah. Felt Willits, Ukiah, Fort Bragg, Hopland.

 

Other widely felt earthquakes:

22-Oct. 1:23 AM Mag. 3.5 7 miles SW of Trinidad.

Note: All times/dates are local PST or PDT. ML is the Local or Richter magnitude as determined by the U.S. Geological Survey; MW is the moment magnitude as determined by the Berkeley Seismological Laboratory. The Moment magnitude is calibrated to agree on average with the Richter magnitude. For large earthquakes, the moment magnitude is considered a better measure of energy released. The felt areas of the more widely felt earthquakes are posted on the web at: http://pasadena.wr.usgs.gov/shake/

Compiled by Lori Dengler, Humboldt Earthquake Education Center, Department of Geology, Humboldt State University from data supplied by the U.S. Geological Survey and the Berkeley Seismological Laboratory. January 12, 2002.