The 1990 Upland earthquake occurred at on February 28 with a

moment magnitude 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 ...

of 5.7 and a maximum

Mercalli Intensity 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 ...

of VII (''Very strong''). This left-lateral strike-slip earthquake occurred west of the San Andreas Fault System and injured thirty people, with total losses of $12.7 million. Many strong motion instruments captured the event, with an unexpectedly high value seen on water tank near the epicentral area.

Tectonic setting

In the

Transverse Ranges The Transverse Ranges are a group of mountain ranges of southern California, in the Pacific Coast Ranges physiographic region in North America. The Transverse Ranges begin at the southern end of the California Coast Ranges and lie within Santa B ...

west of the right-lateral San Andreas Fault System, some faults exhibit left-lateral slip, including some at the boundary of the

Los Angeles Basin The Los Angeles Basin is a sedimentary basin located in Southern California, in a region known as the Peninsular Ranges. The basin is also connected to an anomalous group of east-west trending chains of mountains collectively known as the Tr ...

and the Transverse Ranges.

Earthquake

While several examinations of earthquake focal mechanisms in the Upland area showed both thrust and strike-slip faulting on opposite ends of the San Jose Fault, the mechanism of the 1990 event showed pure left-lateral motion on a steeply-dipping (70°) and northeast-striking fault. Field investigations revealed that the bilateral rupture did not reach the surface.

Strong motion

The shock was felt from Santa Barbara in the west and to

Las Vegas Las Vegas (; Spanish for "The Meadows"), often known simply as Vegas, is the 25th-most populous city in the United States, the most populous city in the state of Nevada, and the county seat of Clark County. The city anchors the Las Vegas ...

in the east. It was also felt in

Ensenada, Baja California Ensenada is a city in Ensenada Municipality, Baja California, situated on the Pacific Coast of Mexico. Located on the Bahía de Todos Santos, the city had a population of 279,765 in 2018, making it the third-largest city in Baja California. The ...

and it triggered 82 strong motion instruments at distances of . Typical ground accelerations were around 0.1 ''g'' at distances of up to , but some closer stations had significantly higher responses.

Damage

Rockslides left some roads closed in the

San Gabriel Mountains The San Gabriel Mountains ( es, Sierra de San Gabriel) are a mountain range located in northern Los Angeles County, California, Los Angeles County and western San Bernardino County, California, United States. The mountain range is part of the Tr ...

and Pomona city hall was heavily damaged. In Claremont some buildings were damaged, but only one building in Upland's historic downtown area was tagged for non-occupation. Those that were injured suffered cuts and bruises (mainly from falling objects) but two men were injured as a result of a landslide.

References

External links

M5.5 – 6km NNE of Claremont, CA
–

United States Geological Survey The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...