Information contained in this report is preliminary,
and should not be cited for publication. The on-line version of the quarterly isabbreviated. The complete quarterly is available in paper version on request.
Seismograph network operation in Washington and Oregon is supported by the state of Washington and the following contracts: U.S. Geological Survey Joint Operating Agreement 01-HQ-AG-0011 and Contract 259116-A-B3 from the Pacific Northwest National Laboratory, operated by Battelle for the U.S. Dept. of Energy
There were 2,029 events digitally recorded and processed at the University of Washington between July 1 and September 30, 2001. Locations in Washington, Oregon, or southernmost British Columbia were determined for 800 of these events; 665 were classified as earthquakes and 135 as known or suspected blasts. The remaining 1,229 processed events include teleseisms (192 events), regional events outside the PNSN (181), and unlocated events within the PNSN. Unlocated events within the PNSN include very small earthquakes and some known blasts. Frequent mining blasts occur near Centralia, Washington and we routinely locate some of them.
DATE-(UTC)-TIME LAT(N) LON(W) DEPTH MAG COMMENTS CIIM SHAKEMAP
yy/mm/dd hh:mm:ss deg. deg. km
01/07/01 05:44:12 47.67 117.41 0.0 2.8 0.4 km W of Spokane, WA
01/07/01 05:45:43 47.66 117.40 0.5 2.8 0.3 km SSE of Spokane, WA
01/07/01 06:07:13 47.67 117.40 0.0 2.3 0.5 km NW of Spokane, WA
01/07/02 17:48:28 47.67 117.42 0.5 0.6 1.5 km WNW of Spokane, WA
01/07/03 21:20:27 47.67 117.41 0.0 2.2 0.9 km NW of Spokane, WA
01/07/08 11:16:32 47.68 117.41 0.5 1.5 1.7 km NW of Spokane, WA
01/07/16 11:37:35 45.11 122.51 13.1 1.9 21.4 km SE of Canby, OR
01/07/22 15:13:52 47.08 122.68 52.4 4.3 16.3 km ENE of Olympia, WA x x
01/07/24 13:31:06 47.49 122.02 16.4 2.2 9.3 km N of Maple Valley, WA
01/07/29 06:26:53 47.74 117.46 0.6 2.1 8.9 km NNW of Spokane, WA x
01/07/29 06:37:58 47.72 117.45 6.2 1.3 7.5 km NNW of Spokane, WA
01/07/29 07:04:25 47.73 117.46 3.9 1.2 7.9 km NNW of Spokane, WA
01/07/30 20:35:09 47.73 117.46 0.6 1.8 8.2 km NNW of Spokane, WA
01/07/31 01:38:11 47.73 117.45 0.4 3.2 8.0 km NNW of Spokane, WA x
01/07/31 05:07:32 47.73 117.44 0.6 2.2 7.4 km NNW of Spokane, WA
01/07/31 05:24:33 47.74 117.45 0.5 1.5 9.2 km NNW of Spokane, WA
01/07/31 06:48:11 47.71 117.47 0.5 1.8 7.3 km NW of Spokane, WA
01/07/31 08:51:55 47.72 117.45 2.1 1.6 7.4 km NNW of Spokane, WA
01/07/31 16:27:43 47.72 117.45 0.6 1.8 7.4 km NNW of Spokane, WA
01/08/01 14:29:48 47.71 117.44 0.6 2.2 5.9 km NNW of Spokane, WA
01/08/09 13:31:24 47.73 117.46 0.5 1.5 8.2 km NW of Spokane, WA
01/08/19 06:17:32 48.25 121.61 1.7 3.0 1.0 km WSW of Darrington, WA
01/08/25 17:52:34 48.23 121.60 2.7 2.1 2.1 km S of Darrington, WA
01/08/30 03:47:31 48.23 121.62 4.8 2.7 2.9 km SW of Darrington, WA
01/09/28 18:34:53 47.68 117.38 1.8 2.8 1.8 km NE of Spokane, WA x
01/09/28 18:37:53 47.66 117.37 0.3 1.9 2.3 km ESE of Spokane, WA
01/09/28 18:38:37 47.67 117.40 0.6 2.6 0.7 km NNW of Spokane, WA
01/09/28 18:41:40 47.67 117.39 0.0 1.6 0.6 km NE of Spokane, WA
The first swarm, 5 km SSE of Mt. Hood, occurred mainly on August 20, when 9 events (none larger than magnitude 1.3, and mostly at depths <5 km) were located. The second swarm was more vigorous, longer lasting, and slightly deeper (most events 7-9 km deep). It was located in a different area 8 km SSW of Mt Hood, and began on August 21 with very small (M<1) events. Its most intense activity was Sept. 11-16, and included 5 events magnitude 2.0 or larger and 15 events magnitude 1.0 or larger. The largest event was magnitude 2.9 on Sept. 14.
In the Klamath Falls area, 14 earthquakes occurred in the third quarter of 2001. Since 1994, most earthquakes in the Klamath Falls area have been considered aftershocks of a pair of damaging earthquakes in September of 1993. The 1993 earthquakes were followed by a vigorous aftershock sequence which decreased over time.
Elsewhere in Oregon, on Aug. 21 a M 1.9 earthquake at about 5 km depth was located near the Three Sisters volcanic area in Oregon. Earthquakes are uncommon in this area. Another interesting nearby event was a clear rockfall signal noted on seismograms HUO and TCO on Sept. 26. The rockfall was visually confirmed to be on South Sister. Bob Norris and Willie Scott of the USGS estimated the volume of the rockfall based on the signal (Norris) and visual observations (Scott) at between 10,000 and 50,000 cubic meters, large enough to leave a noticeable deposit on the Prouty Glacier. Mt. Rainier has rockfalls this size every few years or so, but they are rarer at South Sister, as it is a much smaller volcano and has only a few cliffs that are steep and tall enough to generate a rockfall of that size.
During the third quarter of 2001, 466 earthquakes were located between 45.5° and 49.5° north latitude and between 121° and 125.3° west longitude. Five earthquakes were felt this quarter in western Washington. Details are in Table 3A. The largest felt earthquake in western Washington was a magnitude 4.3 earthquake in the early morning hours of July 22. It was located at 52 km depth about 18 km NE of Olympia; close to the M 6.8 Nisqually earthquake on Feb. 28, 2001. This July 22 earthquake may be a late aftershock of the Nisqually earthquake. It was well recorded by strong motion instruments in the Puget Sound.
This quarter, on August 14 (PDT), a small volcanic lahar was recorded on three seismometers at Mt. Rainier as it flowed down Van Trump Creek and the Nisqually River. The event lasted for several hours. Seimograms and spectrograms are available on our web site at:
A total of 50 tectonic events (25 of these were smaller than magnitude 0.0, and thus are not shown in Fig. 4) were located within the region shown in Fig. 4. Of these, 33 were tectonic events located in the "Western Rainier Seismic Zone" (WRSZ), a north-south trend ing lineation of seismicity approximately 15 km west of the summit of Mt. Rainier (for counting purposes, t he western zone is defined as 46.6-47 degrees north latitude and 121.83-122 west longitude). The largest tectonic earthquake located near Mt. Rainier this quarter was the felt magnitude 1.8 earthquake on August 28, located about 27 km south-south-west of the summit at a depth of about 4 km.
This quarter, there were 10 (4 of them smaller than magnitude 0.0 and thus not shown in Fig. 4) higher-freq uency tectonic-style earthquakes within 5 km of the summit. The remaining events were scattered around the cone of Rainier as seen in Fig. 4.
This quarter, there were 36 (20 smaller than magnitude 0.0 and thus not shown in Fig. 4) higher-frequency tectonic- style earthquakes within 5 km of the summit. The remaining events were scattered around the cone of Rainier as seen in Fig. 4.
Two type "S" or "L" events were located at Mount St. Helens, and 356 "L" or "S" events too small to locate were recorded.
During the third quarter of 2001, 101 earthquakes were located in eastern Washington in the area between 45.5-49.5 degrees north latitude and 117-121 degrees west longitude.
The most unusual activity in eastern Washington this quarter was the continuation of a very unusual sequence of earthquakes in the Spokane urban area.
Because of the ongoing nature of the seismicity and its location in an urban area, on June 26 temporary station SPUD was installed at the Spokane County Public Works building (on W. Broadway between Monroe and Madison) to improve our ability to locate Spokane earthquakes. As the sequence continued, five additional stations were installed in Spokane between June 30 and July 2.
By the end of the third quarter, the PNSN had located a total of 61 events in the Spokane sequence. Another twenty-two events were recorded, but could not be located. Because a variety of seismograph configurations have operated, the accuracy of event locations has varied over the course of the sequence.
The Spokane sequence began with a M 2.0 foreshock on May 24, a month before the larger shocks. This quake was felt by residents, who also reported feeling other shaking around that time, likely due to earthquakes too small to be record by the PNSN. Reports of shaking in May and early June became known to us only after activity intensified in late June. On June 25 the two largest events in the sequence (M 3.9 and M 3.4 on 6/25) occurred, and twenty-three other events were located during the following week. Some of the locations on and after June 26 included readings from the quickly-installed temporary Spokane station SPUD, although SPUD had some telemetry problems and operated only intermittently during the sequence.
Around the beginning of July, when the 5-station temporary array was installed in Spokane, activity dropped off. Between July 2 and July 12 only 16 earthquakes (mostly tiny events with data only from the temporary five-station array) were located. The proximity and number of the stations made these earthquakes the most accurately located of the sequence. All of the earthquakes located with the 5-station array are north and east of the Spokane River, inside the river's bend. This is also near where the two largest shocks on June 25 were located using regional seismograph data, and in the area where residents report feeling continued shaking, between Riverfront Park and Corbin Park. Figure 7 (11K) shows the location of seismometer stations and earthquakes located near Spokane between January and September, 2001.
The July 2-12 earthquakes appear to be extremely shallow, and residents reported feeling many, but not all, of the recorded earthquakes. Loud explosion-like sounds were often reported along with the shaking. Reports of shaking were also received for times when no activity was recorded, probably due to a heightened sensitivity to typical background vibrations that occur in the urban environment. No earthquakes were recorded between July 13 and July 28.
The temporary 5-station array was removed on July 25, and activity picked up again almost immediately. Fourteen events were located between July 29 and August 1. The largest event in this time period was magnitude 3.2, and at least a dozen events were reported felt between July 29 and August 1, much disturbing residents.
Following August 1, seismicity quieted. SPUD continued to operate and an additional permanent station, SFER at Ferris High School, was installed on August 9. Just one event was located on August 9, but it occurred in the early morning hours before the SFER installation. No further events were located in August or during the first three weeks of September.
SPUD was removed on September 25. It had been borrowed from another project and had to be returned. Seismicity picked up just a few days later, when four events occurred within a ten-minute period on Sept. 28. The largest was a magnitude 2.8 that was noticed by many people in the downtown area, as was the magnitude 2.6 that followed about 4 minutes later.
There is no history of damaging earthquakes in the Spokane area, nor any comparable sequence of small events. While the current sequence continued through the 3rd quarter, it is notable that since June the maximum magnitude and number of earthquakes in each burst of activity has declined and the time lapses between bursts of activity have lengthened.
Spokane Geology and Earthquake History:
Geologists have long suspected that the course of the Spokane River was structurally controlled. It flows east to west toward Spokane, where it abrubtly changes to a northwest direction. Hangman Creek (also called Latah Creek) flows into the Spokane River near the bend along the same NW striking lineament. This lineament is clearly expressed in the topography, paricularly along Hangman Creek, which is quite straight compared to the complex dendritic pattern more commonly displayed by other drainage in this area (see digital elevation map). Bob Durkey of the Washington State Dept. of Natural Resources has mapped the Hangman Creek watershed, and named this structure the Latah Fault. An brief discussion on the possible relation of the Latah Fault to the Spokane earthquakes has been published in Washington Geology (Derkey, R.E., and M.H. Hamilton, 2001, Spokane Earthquake point to Latah Fault?, Washington Geology, V. 29, No 1/2, pg. 42).
However, direct evidence for faulting is skimpy: At Hangman Valley Golf course, flood deposits are uptilted to 35 degrees. Well data from either side of Hangman Creek shows elevation or thickness differences in the Grande Ronde Columbia River Basalt (CRB) flow. However, the flow was deposited onto a Miocene landscape and it is difficult to determine whether differences result from flows interacting with the ancient landscape or from faulting. Likewise exposures of basalt along the Spokane River near "Bowl and Pitcher" in Riverside State Park differ from what is seen at "Five Mile Prairie" across the river. Again it is difficult to pinpoint the cause of this.
Very little is known about the seismic hazard to Spokane, since there is no history of large damaging earthquakes in the area. It is unusual to have any earthquake activity at all in Spokane, and no sequence like this one has been noted in the past. Looking back at Spokane history, minor damage has been caused by events outside the immediate Spokane area (e.g. Hebgen Lake quake of 1959), and there is also a history of a few small quakes felt only locally in Spokane. Events felt only in and around Spokane occurred in 1915, 1920, 1922, 1941, 1942, 1948, 1952, 1961 and 1962. In some instances the shaking was accompanied by explosion-like noises, and in some cases several events close together in time were reported. No other extended sequence like the one in 2001 is known.
Although never previously noted near Spokane, earthquake swarm activity (defined as a cluster of small close-together earthquakes without a distinct, sizable mainshock) has previously been seen in eastern Washington. One notable swarm occurred near Othello and lasted nearly a year. It began in November of 1987 and included about 200 located earthquakes, about 20 of them larger than magnitude 2.0. The largest event in the Othello sequence was magnitude 3.3.
Earthquake locations in the current sequence appear spread out along the trend of the Hangman Creek structure, but this is probably misleading as many of the far-flung locations are not well constrained. The best located earthquakes (using data from the 5-station temporary array) were immediately inside the bend in the river, approximately near the Corbin Park neighborhood.
A close examination of events that appear to be to the northwest indicates that those events either lacked a close-by station or, if the data included station SPUD, occurred after July 28. The internal clock of station SPUD may have drifted, and the drift would increase with time. SPUD waveforms from events apparently to the northwest appear no different than waveforms from events located closer to SPUD. It seems fairly likely that all events in the swarm have been located in a single cluster close to the immediate downtown Spokane area.
Eastern Washington Geology and Earthquake History
Placing Spokane in a wider context; it is important to realize that, although less active than western Washington, eastern Washington does have faults capable of producing damaging earthquakes. Historic damaging earthquakes in eastern Washington include the 1872 earthquake (M 6.8-7.3), near southern Lake Chelan. This earthquake was one of the largest earthquakes known in Washington, and it was widely felt in Washington and British Columbia and followed by many aftershocks. The same area, near Entiat, experienced a M 5.0 earthquake in 1959, and there is a persistent cluster of tiny earthquakes in the area.
An earthquake near Walla Walla in 1936 (M 6.4) caused extensive damage to chimneys and walls, ground cracking, and water level and well flow changes, with numerous aftershocks reported. Since 1936, several earthquakes around magnitude 4.0 have been located in the same area.
Large earthquakes at some distance from eastern Washington are sometimes felt and can even be damaging. For example, the 1959 M 7.5 Hebgen Lake earthquake in Montana was felt as far as Seattle, and caused minor damage in Spokane, as did the 1983 M 7.2 Borah Peak, Idaho earthquake.
Overall, the rate of seismicity in eastern Washington is low, although the seismology lab at the University of Washington locates 200-300 earthquakes per year in eastern Washington and northeastern Oregon. Only a few earthquakes each year are usually large enough to be felt. Earthquakes in eastern Washington are shallow, within the upper 10 miles or so of the Earth's crust. Although we know that damaging earthquakes can occur in eastern Washington, the low rate of seismicity makes evaluating earthquake hazards and identifying dangerous faults difficult. Events like the 1872 and 1936 earthquakes are likely to recur, and other faults may also break. We look for clues in the geology, but the processes that produce faults and deform, fold, and uplift rocks are slow and the traces they leave in the landscape can be difficult to interpret. Active faults may never break the surface, faults exposed at the surface may be inactive, and evidence of faulting can be eroded away or obscured by new deposits.
In eastern Washington the landscape includes older rocks in the Cascade highlands and geologically young (15 million years old) flood basalts covering the Columbia Plateau. The older (100 million years) Mesozoic crystalline rocks of the Cascade highlands contain faults, but the absence of young rocks makes it difficult to determine whether those faults are still active. Earthquakes of magnitude 4.5 - 5.0 have occurred in the Cascade highlands in recent years, and larger earthquakes cannot be ruled out. Complex geologic structures within the older rocks extend beneath the layered flood basalts. Large faults in the eastern Columbia Plateau, near the Spokane area and near Clarkston/Lewiston were active while the basalt flows were occurring, but show little evidence of significant recent activity. In the western part of the Columbia Plateau, numerous faults showing very recent activity distort the flood basalts. The Yakima Fold Belt is a series of east-west trending folded and faulted ridges in the basalts. The most prominent ridges are in the southwestern plateau, and include the Saddle Mountains, Rattlesnake Mountain, Frenchman Hills, and Horse Heaven Hills. Swarms of small earthquakes are common and there are large geologically recent thrust faults that may be capable of producing earthquakes larger than magnitude 7. Yakima Fold Belt structures also extend into the northwestern plateau, and are a feasible source for the 1872 earthquake.
Earthquake Mitigation for low-seismicity areas
Damaging earthquakes sometimes occur even in areas of low seismicity When there is not enough information to estimate the frequency or maximum magnitude of damaging earthquakes, a conservative approach is to plan for the possibility of a damaging earthquake of moderate size (magnitude 5.0). There are many low-cost mitigation measures such as family emergency plans, restraining gas water heaters and tall bookcases, etc. Some mitigation activities, such as bracing or removal of masonry parapets and anchoring of masonry walls can be implemented during other refurbishing projects. The 2001 sequence has demonstrated that there is an active seismogenic source in the Spokane area. Damaging earthquakes near Wenatchee (1872) and Walla Walla (1936), the presence of large geologically recent faults in the Tri-cities area, and the generally young geology of Washington suggest that basic mitigation acitivites should be considered by communities in eastern Washington.
Times, locations, and depths of felt earthquakes in the PNSN region are given in Table 3A.