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- BURNEY, CALIFORNIA, December 23, 2000 (Seismo-Watch) -- Two moderate earthquakes registering M4.6 and M4.3 occurred in the late afternoon and evening hours of December 20, 2000 in a remote region of northeastern California near Burney. They were the largest events of a brief, but vigorous earthquake swarm which began earlier in the week.
- The activity was centered about 46 mi. north-northeast of Redding, about 8 mi. north of Burney and about 2 miles west of Four Corners in upper Long Valley (See Maps).
- It essentially began with a M3.2 event on December 15, but it broke into a rapid clip the afternoon of December 20 See a list of events. At 2:56 p.m. and 2:59 p.m., two jolts measuring M3.8 which were locally felt jump started the sequence. They were followed by a few M1's and M2's, and at 3:23 p.m., a M4.3 earthquake shook the region. It was followed by a few more tremors, one of which registered M3.1, but at 3:39 p.m., the largest quake in the sequence occurred, a M4.6 (Mw 4.4) temblor.
- The sequence raced on at a clip of 20 eq/hr for that first hour, then declined to about 2-5 eq/hr up till about 10:30 p.m. that night when it suddenly stopped. A few quakes have been recorded in the subsequent days, all in the mid M2 range, but no more swarm activity has been recorded. In all, at least 39 events have been recorded in the swarm thus far (to December 23), with a total of eight registering M3.0 or stronger, including the pair of M4's. See a list of events.
- Focal depths for the action range from 5-19 km deep and are not well constrained because of the poor network resolution in the area (i.e.; don't put a lot of credit in the depths at this early stage of data processing). The type of fault motion was predominantly normal-slip (down and away) with a minor oblique component. See fault plane solutions by UCBSL. This agrees well with the general seismotectonic conditions of the region.
- Earthquake Effects.
- Shaking from the larger events was felt over a wide region of Northeastern California, from Redding and Weaverville to Susanville and Chico. One report from Eureka is skeptical.
- Local residents reported items were tossed from tables and shelves, some hair-line cracks occurred in walls, and cabinet doors were shaken opened, spilling a few things. Some pictures fell from walls, hanging plants swung, house pets ranch animals were spooked, and some people were frightened. A few reports indicated a roar before the shaking, which caught their attention.
- There have been no reports of well water changes at this time.
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- Background
- The activity was centered along the northern trace of the Hat Creek Fault Zone, a broad set of normal-slip faults which form the prominent Hat Creek Graben. A graben is a structure caused by tension or a pull-apart action whereby a center block or set of interior blocks drop down forming a basin or a valley. The Hat Creek Fault Zone is the southwestern portion of the province-bounding fault zone which separates the Cascade and Klamath geologic provinces from the Modoc Plateau, a subprovince of the Basin and Range province characterized by numerous long, linear grabens similar to the Hat Creek Graben.
- The Hat Creek Graben roughly extends from about Lassen Peak, a Cascade-type volcano, north-northwestward to just north of the Pitt River. Highway 89 north of the Highway 89/44 junction travels along the western margin of the graben and the prominent Hat Creek Rim forms the eastern margin. The steep ascent eastward from the Highway 89/44 junction climbs the escarpment created by the fault zone on the eastern side of the graben.
- Recent studies by PG&E, the US Geological Survey and others, have revealed that the numerous fault traces in the Hat Creek Graben show relatively young (Holocene <10,000 years?) displacements, indicating that large (M6.5+) earthquakes have occurred here in the not so distant past. This suggests a much larger seismic hazard than previously recognized.
- Historical seismicity in this area is poorly understood because of its remote location. The detail of small tremor action is not well refined below about M2.0 level. The Northern California Seismic Network catalog shows the Hat Creek Graben has experienced about two dozen events registering M3.0 or stronger since 1977, including a M4.4 on June 14, 1991. Most of the M3+ action occurred in bursts and swarms similar to the December 2000 sequence. The UC Berkeley Earthquake Catalog which extends back to nearly the turn of the 1900 century shows several swarms back to about the 1940's, some of which included M4's, however, this week's M4.6 event was the largest on record.
- Volcano Country
- This area is volcano country and these magmatic process are due to two complex processes, 1.) the Cascade volcanoes, and 2.) the Basin and Range volcanoes. West of Eureka, the Gorda tectonic plate is being overrun by the much larger North American plate. It is being forced under the continent and into the mantle at a shallow angle and when sufficiently hot enough, begins to melt. The melted rock is then allowed to come to the surface through a series of faults and fractures within the North American interior which have been facilitated by this tectonic collision. Lassen Peak and Mount Shasta are to of the southern volcanoes produced by this process. They are referred to as Cascade Volcanoes and are similar to Mt. Rainier and Mt. St. Helens.
- Beneath Redding, the Gorda plate is at a depth of 60 km and under Lassen Peak it is about 90 km deep. Here temperatures and pressures are high enough to cause the rocks to melt and it is the faults and fractures which allow the magmatic fluids to come to the surface.
- The Basin and Range has been created by a massive torque placed upon the western edge of the North American plate because of the relative plate motions with it and the Pacific plate. This torque has pulled the North American plate in a northwest direction when its principal direction of motion is westward. The tension has created numerous graben-like geologic features, hence the name Basin and Range.
- Sometimes prominent fractures of the graben extend deep enough where they can tape the magma of the mantle. The fractures act like conduits and allow the magma to rise and sometimes come to the surface. The Basin and Range has several volcanic areas, including the Coso Volcanic Field along the Southern Sierra and the Modoc Plateau in Northeastern California, which has hundreds of such volcanoes and is being recognized as a subprovince of the much larger Basin and Range province.
- Inside the Hat Creek Graben are a row of small volcanoes that extend from near the Highway 395/44 northward to about Cassel, a small town east of Burney. These volcanoes are Basin and Range volcanoes and the faults and fractures of the Hat Creek Graben, and the earthquakes which created them, have played a key role in allowing the magma to come to the surface in this area.
- While this is a remote region with a relative low population, it is a pivotal location of active tectonic processes, both seismically and volcanically. The December 20, 2000 Burney earthquake swarm serves as wake-up call, bringing attention away for the plate boundaries to other areas of geologic importance, critical in understanding the larger scheme of the North American tectonic picture.
Sources: USGS, UCBSL, personal communications.
Update: 10:00 a.m., December 24, 2000.
Information contained within this report may not be disseminated without prior written consent from Advance Geologic Exploration, Inc. and Seismo-Watch.
Copyright (c) Advanced Geologic Exploration 2000
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