The 1993 Klamath Falls earthquakes took place in

Klamath Falls, Oregon Klamath Falls ( ) is a city in and the county seat of Klamath County, Oregon, United States. The city was originally called ''Linkville'' when George Nurse founded the town in 1867. It was named after the Link River, on whose falls the city was ...

, beginning on Monday, 20 September at 8:28 p.m. The

doublet earthquake __NOTOC__ In seismology, doublet earthquakes – and more generally, multiplet earthquakes – were originally identified as multiple earthquakes with nearly identical waveforms originating from the same location. They are now characterized as sing ...

registered respective magnitudes of 6.0 and 5.9 on the

moment magnitude scale The moment magnitude scale (MMS; denoted explicitly with or Mw, and generally implied with use of a single M for magnitude) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment. It was defined in a 1979 pape ...

. The earthquakes were located at a depth of and tremors continued to be felt more than three months after the initial shocks.

Tectonics and previous activity

The tectonic structure of south-central Oregon is riddled with fault lines. The West Klamath Lake fault zone is capable of earthquakes up to Richter magnitude 7.3. The magnitude 5.4 Coos Bay earthquake which preceded a later 2002 Klamath Falls earthquake occurred along the northern segment of the Blanco Fracture Zone, which is separate from the inland Klamath Basin and Range geological province. These fault networks have caused several previous earthquakes in the Klamath Falls area. Records from 1945 indicate approximately fifteen seismic events, prior to the 1993 events, circulating within a radius of Klamath Falls. However, of those observed during the preceding fifty years, only six were considered significant. Previously most tremors measured less than 4 M_w with the strongest being 4.3 M_w in 1948.

Earthquake

The September 1993 earthquakes began striking at approximately 8:16 p.m. local time. The first major quake registered 5.9 on the

with a foreshock registering 4.2 M_w about twelve minutes before. The second major shock was at 10:45 p.m. Over the next five months at least 300 more tremors were recorded. The strongest aftershock occurred on 4 December 1993, and measured 5.4M_w. The tremors during the September 1993 Klamath Falls earthquakes were some of the strongest felt in Oregon's history over the past 50 years. The first shock was only 4.2 M_w. The two major quakes, which measured 5.9 and 6.0, were, up to that point in Oregon's history, the strongest recorded earthquakes and resulted in the most damage. The epicenter of both earthquakes was located northwest of Klamath Falls in the Mountain Lakes Wilderness. The epicenter of the second major shock was slightly more to the north than the previous tremors. The major shocks were felt as far north as

Eugene, Oregon Eugene ( ) is a city in the U.S. state of Oregon. It is located at the southern end of the Willamette Valley, near the confluence of the McKenzie and Willamette rivers, about east of the Oregon Coast. As of the 2020 United States Census, Eu ...

, and as far south as

Redding, California Redding is the economic and cultural capital of the Shasta Cascade region of Northern California and the county seat of Shasta County. Redding lies along the Sacramento River, north of Sacramento, and south of California's northern border wi ...

. All of the shocks, from the first foreshock to the last aftershock some two months later, were clustered around latitude 42^o20'N and longitude 122^o05'W, west northwest of Klamath Falls.

Damage and casualties

The destructive force of the 1993 Klamath Falls earthquakes was measured at VII on the

Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the eff ...

. There were only two recorded casualties, one of which involved a motorist who was crushed in a

rock slide A rockslide is a type of landslide caused by rock failure in which part of the bedding plane of failure passes through compacted rock and material collapses ''en masse'' and not in individual blocks. Note that a rockslide is similar to an avalanc ...

U.S. Route 97 U.S. Route 97 (US 97) is a major north–south route of the United States Numbered Highway System in the Pacific Northwest region. It runs for approximately through the states of California, Oregon, and Washington, primarily serving in ...

. The second death was a result of a heart attack. The tremors precipitated landslides and also caused structural damage to buildings, including the county courthouse, in addition to private residences, businesses and historic buildings in Klamath Falls. Damage was estimated at approximately $10 million.

Aftermath

In the wake of the earthquakes, several damaged buildings in Klamath Falls and elsewhere in Klamath County had to be repaired. The city of Klamath Falls obtained a Community Development Block Grant which was used to partially fund upgrades to local public buildings, including the courthouse annex. Within three weeks after the first major shocks four seismographs were installed in the area by the U.S. Geological Survey. These were helpful in recording subsequent seismic activity.

References

External links