Seismo-Watch

Special Earthquake Report No: 00-008

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NAPA-SONOMA, NORTHERN CALIFORNIA

ML 5.2 (NCSN)

Mw 5.0 (UC Berkeley)

00/09/03
08:36:30 (1:36 a.m. local time)
38.377N
122.414W
9.4 km (5.8 miles)
A

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(Napa M5.2)

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Napa-Sonoma, California, September 03, 2000 (Seismo-Watch) -- Preliminary data indicates that a moderate earthquake registering M5.2 shook the North San Francisco Bay Area early this morning, jarring hundreds of thousands of people awake, injuring at least 25 people - three seriously, and causing widespread minor damage totaling between $5-15 million.
 
The earthquake occurred at 1:36 a.m. about 6 miles northeast of Sonoma and about 6 miles northwest of Napa along the eastern slope of Mt. Veeder, in upper Pickle Canyon, near Lokoyo (See Map). The focal point was placed at 9.4 km beneath the surface and the preliminary fault plane solution from the U.C. Berkeley suggested pure strike-slip motion along either a northwest or northeast trending plane. There have been no significant aftershocks at this time, just a few small M1's and a M2's.
 
The main jolt was strongly felt in the surrounding region, and as far away as the northern San Francisco Peninsula, Sacramento and Livermore. Damage reports indicate Napa and Yountville were the hardest hit where windows shattered, plaster walls cracked and items were thrown from tables and shelves, littering isles in commercial stores and floors of thousands of residents. Most of the damage was nonstructural and there were no reports of collapsed buildings
 
The Queen of the Valley Hospital in Napa reported at least 25 people were treated at the Emergency Room, including a two people in serious condition and a 5-year-old boy who is in critical condition, both with injuries from falling objects. Other injuries were mostly of lacerations due to shattered windows. Medical technicians also responded to several people complaining of heart conditions.
 
The young boy, Nathan Schank, was having a camp out in the living room with his brothers when the quake struck. The brick facade on the fireplace collapsed on top of him, fracturing his hip and an arm and causing intense internal bleeding. He is at Children Hospital in Oakland now where his condition is improving.
 
People described the quake as two swift jolts followed by a long rolling motion lasting from a just few seconds to as long as 10-15 seconds. Many people were frightened and some were terrified. One man reported his wife and kids were screaming while the shaking occurred. Others opened any eye, looked for damage and upon not seeing any, went back to sleep. Many told about the loud noise associated with the earthquake, like a freight train running through their home.
 
PG&E reported about 6,000 customers were without power for about three hours in Napa County. There have been some reports of gas leaks and there have been no fires. PG&E said they will be out checking the region during the next few days. There have been some sketchy reports of severed water lines and there has been no news regrading the natural springs of the area. (Please contact Seismo-Watch if you have any news)
 
It is not clear at this time which fault produced the earthquake as it occurred off the major faults of the region. Preliminary analysis suggests the quake occurred in the hills between the Carneros and the West Napa fault systems where no active faults have been geologically mapped. See below for more on the probably fault responsible for the earthquake.
 
Although the Napa-Sonoma M5.2 earthquake has not produced any significant aftershocks yet, the USGS and UC Berkeley have issued an automatic aftershock warning for earthquakes in the area. Based upon the seismological characteristics of the region, earthquakes at this magnitude have a 20% or 1 in five chance of producing quakes of equal magnitude or stronger within the next 7 days. There is also a statistical chance (5% or one in 20) that a temblor of larger magnitude will occur. The statistics also anticipate around 3-20 M3's could occur from this event, yet this temblor does not seem to be in the mood for any aftershocks at this time.
 
This morning's temblor was the largest quake in the San Francisco Bay Area since a the Bolinas (Marin County) M5.0 earthquake on August 17 of last year. Aside for the four events in The Geysers geothermal area, there have been eight quakes registering M4.0 or stronger north of Hayward and south of Willits since 1990 (See list). There have been eight M5's recorded in this area in the last 50 years, but only three of those triggered north of the San Francisco Bay, the Willits M5.2 earthquake in 1962 and the Santa Rosa M5.6 and M5.7 earthquakes in 1969. These latter temblors caused significant damage at Santa Rosa, essentially forcing the rebuilding of the downtown area. (See this link)
 
In the last 200 years, there have been 28 earthquakes measuring M5.0 or better, including six M6's and one in the M7 range - the powerful San Francisco M7.8 earthquake in 1906. There has not been a M6 jolt in this area since the San Francisco earthquake.
 
Probable Fault Discussion
 
Using the fault plane solution we know the September 3 Napa M5.2 earthquake occurred on either a northwest or northeast trending strike-slip fault located somewhere on the southwest flank of Veeder Mountain, near upper Pickle Canyon. But, there are no known active fault traces in the region to pin which one produced the earthquake (see map cut out).
 
The usual "smoking gun" for identifying the trend of the fault is the aftershock sequence as several tremors will nicely outline the rupture trace of the earthquake, showing both trend and length of rupture. Unfortunately, there have been few aftershocks recorded at this time for the main jolt, basically eliminating this approach. Perhaps if more aftershocks are recorded through the next week weeks this may become an option; so until then, geologists must use other lines of evidence to come up with some likely candidates.
 
Sometimes when faults rupture they produce more damage in one direction of the fault than another. This suggests a direction by which the earthquake energy was focused. This "directivity" is an indication of (1) the probable fault rupture orientation and (2) the direction the fault rupture process.
 
Since most of the residents of the area are concentrated in Napa, it is hard to assess the damage by the shear number of damage reports. Instead, geologist look for the intensity of damage, such as toppled chimneys, shattered windows, rock falls, etc., things that take more energy to topple or to be damaged.
 
While still early, the most intense damage reports have come in from Yountville and Napa, which are either northeast of southeast from the epicenter, weeding out a northwest or southwest directivity - but still not giving clear clues to the orientation of the fault. Being the closest to the epicenter, we would expect Yountville to have a high shaking intensity and Napa sits on loosely compacted alluvial sediments and old San Francisco Bay fill and we would also expect shaking to be more intense there as well. Further field mapping of damage intensity will needed in these areas.
 
Another clue to the orientation of the likely fault the ruptured can be found in the the geologic structure of the region (see map cut out). Most of the faults in the Northern California trend northwest-southeast, parallel to the San Andreas Fault system. The Carneros fault zone (centered just west of the epicenter ) trends about N40°W but the West Napa fault system (located to the east of the epicenter) trends more northerly, about N20°W, as it skirts around the hills on the west side of Napa Valley (thus the name of the fault zone). The fault plane solution indicated a N35°W or N55°E strike to the potential fault slip suggesting the Carneros fault system as the more likely candidate for the likely fault.
 
But not all the faults in Northern California trend in the northwest direction. Close examination of the Geologic Map of the Santa Rosa Quadrangle, 1:250,000, Circa 1982, (see map cut out) shows a small northeast trending unnamed fault in the hills southwest of Yountville. Its orientation is about N40°E and its southerly projection extends right to the M5.2 epicentral area. Interestingly, the fault seems to bend both Dry Creek and Pickle Creek Canyons in a westward or left-lateral direction (as one moves up canyon). The next canyon to the west is Redwood Creek canyon and it too has a conspicuous westward bend at the southwest projection of the unnamed fault, but it is more defuses - perhaps because this is where the unnamed fault would intersect with the Carneros fault system.
 
Unfortunately, no fault activity analysis has been performed for this unnamed fault beyond labeling it as less than Quarternary (<2,000,000) in age. Its orientation and geomorphology and the directivity damage evidence, however, is highly suggestive as it to be the likely candidate for the Napa M5.2 earthquake. Perhaps further geologic field work in the area will reveal more information about this unnamed fault.
 

Sources: CDMG, USGS, UCB, Santa Rosa Press Democrat, Associated Press, personal communications.

Update: 12:45 p.m., September 5, 2000.

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Location Quality: A (good), B (fair), C (poor), D (bad)
Magnitude: Ml (local or Richter magnitude), Lg (mblg), Md (duration), Mb (body wave), Ms (surface wave), Mw (moment)

Standard Sources Include:
U.S. Geological Survey, National Earthquake Information Center, Golden, CO (NEIC)
Harvard Geophysical Observatory, Harvard University, Cambridge, MA (HRV)
U.S. Geological Survey, Northern California Seismic Network (NCSN)
University of California, Berkeley, Seismological Laboratory (UCBSL)
Southern California Seismic Network (USGS & Caltech), Pasadena, California (SCSN)
California Institute of Technology, Pasadena, California (Caltech)
various wire reports and/or personal communications

All data are preliminary and subject to change.
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