On August 9, the PNSN installed a permanent 4 component station (a short-period vertical seismometer and a 3-component strong-motion sensor) at Ferris High School in Spokane. Real-time data can be found on our main webicorder page.
On July 30, at 6:38PM PDT a magnitude 3.2 earthquake occurred near Spokane. This had been the largest event since June 25 2001 until November 11 when a magnitude 4.0 earthquake occurred at 8:00AM PST. On November 11, the magnitude 4.0 earthquake was followed by a magnitude 3.1 earthquake at 9:21AM PST and a magnitude 3.3 earthquake at 7:03PM PST.
Overview of the 2001 Spokane Sequence
In June, 2001, we became aware of an unusual sequence of earthquakes in the Spokane urban area. It is a bit difficult to pinpoint exactly when the sequence commenced, as Spokane is at the edge of the PNSN, and the nearest stations, NEW and DPW are more than 60 km away from the activity. The sequence included a possible foreshock, magnitude 2.0, on May 24, 2001. It is not clear whether the May 24 event was felt, but numerous people have informed us that shocks were felt on June 24, and possibly earlier. On June 25 2001, the largest earthquake in the early part of the sequence occurred. It was felt by hundreds of people throughout Spokane, and located beneath downtown Spokane near Gonzaga University. Magnitude estimates were 3.9 (Mc) and 3.7 (Ml using localmag). The June 25 2001 mainshock caused some minor damage as bricks tumbled from a few chimneys, and a few dishes fell from shelves, etc. By the end of June, The PNSN had located 21 earthquakes (11 reported felt) within about 11 km of Gonzaga University.
Because of the poor station coverage in the area, on June 25 2001 the PNSN deployed equipment already on hand (for a CREST station) and installed a 6-component broad-band and strong-motion station at the Spokane Public Utilities Dept (the station was named SPUD) with continuous internet telemetry to the UW.
Reports of unrecorded felt earthquakes continued despite the new station, and the PNSN continued to record aftershocks of magnitude 2.0 and larger through June. In addition to the recorded earthquakes, residents of certain neighborhoods in Spokane continued to report feeling shaking at times when our instruments recorded nothing. Most of these shaking reports were confined to a limited geographic area in the part of the Spokane immediately north of downtown. Reports of explosion-like sounds accompanying the shocks are common.
These features of the Spokane quakes (noise, not being recorded by PNSN instruments, and being felt over tiny areas) mean that they are probably extremely shallow and very close to the areas where they are being felt and heard.
Because of this continuing activity, 5 additional temporary stations were requested from the Program for the Array Seismic Studies of the Continental Lithosphere (PASSCAL) project, and were installed between June 30 and July 2. These stations recorded data on-site only (no live telemetry to the UW). Data from these stations has been downloaded from the instruments, and has been merged with data from the PNSN permanent stations. If even one earthquake was well-recorded by all the stations, it can be used to improve the locations of all quakes in the area.
Felt earthquakes continued into early July, then tapered off. The PASSCAL stations had been committed for another project, and they were removed from Spokane on July 23 and 24th 2001. On July 28 2001 at 11:26 PDT, another felt earthquake, magnitude 2.1 was recorded, and Spokane inhabitants again reported feeling additional shaking at other times when no earthquake was recorded. Fourteen earthquakes were located between July 29 and August 1 2001, the largest being a magnitude 3.2.
Our permanent Spokane station, SFER (Ferris High School), was installed on August 9. No earthquakes were located the rest of August or the first three weeks of September. SPUD was removed on September 25 2001; it had been borrowed from another project and had to be returned. Four earthquakes occurred on September 28, ranging in magnitude from 1.6 to 2.8. Between November 11th and April 23, 2001 time, the PNSN has recorded and located 42 earthquakes in Spokane. The largest of these was a magnitude 4.0, which occurred on November 11th at 8:00am PST. Since November the earthquakes have decreased in magnitude. Despite their very small magnitudes, many of these earthquakes been reported felt by residents of Spokane.
On November 15th, 2001 a new 3-component strong motion seismograph,
SGAR (Garfield Elementary), was installed. Another 3-component strong motion
seismograph was installed at Westview Elementary (SWES) on November 16th.
Real-time data is
being telemetered to the UW and displayed on our webicorders along with our
permanent Spokane station, SFER (Ferris High School). Currently SFER can
be viewed on our main webicorder page, whereas SGAR and SWES can be found
under the "School Network" on our main webicorder page.
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. The earthquake hazard to Spokane is very poorly understood.
The current activity has made it very clear that evaluation of possible seismogenic geologic structures in the Spokane area is necessary.
Although never previously noted near Spokane, earthquake swarm activity; defined as a cluster of small close-together earthquakes without a very 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.
None of the past damaging earthquakes in Washington or Oregon was preceded by any noticable foreshock activity.
Spokane Earthquake History and Geology:
Mike Zientek of the USGS Spokane Office has informed us that 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 Derkey of the Washington State Dept. of Natural Resources has mapped the Hangman Creek watershed, and named this structure the Latah Creek Fault.
However, direct evidence for faulting is skimpy:
Looking back at the history of EQs in the area, 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 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 extended sequence like the current activity is known.
The Spokesman Review (copies of articles kindly provided by Mike Prager of the Spokesman Review) published stories about the 1915, 1922, 1948, and 1952 events that provide summaries of what neighborhoods the events were felt in. Each earthquake was felt most strongly at one point or another along the same NW striking lineation that appears to control the paths of Hangman Creek and the Spokane River.
Locations of earthquakes in the current sequence appear to suggest that some of the the aftershocks may have been northwest of the mainshock and along the trend of the Hangman Creek structure. This may be misleading, as the locations are not well constrained. Data from the temporary stations as well as our newly installed strong motion stations have already improved the event locations and show that all of the events have been occurring in more of a cluster just north of the river, rather than spread out in the northwest trend of the Hangman Creek structure.
Other types of geologic studies such as aeromagnetic and gravity studies, and drilling or trenching across the suspected fault, may reveal the dimensions of the fault (this is used to estimate maximum likely magnitude) and the long-term displacement rate across the structure (providing a crude estimate of likely frequency).