View source for 1991 Sierra Madre earthquake

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{{Infobox earthquake
| title = 1991 Sierra Madre earthquake
| local-date = {{Start date text|28 June 1991}}
| local-time = 07:43:55
| timestamp = 1991-06-28 14:43:55
| image = 
| caption = 
| map2 = {{Location map+ | USA California
|places =
{{Location map~|California|lat=36.18|long=-115.14|label_size=100|label=Las Vegas|position=top|mark=Green pog.svg}}
{{Location map~|California|lat=35.37|long=-119.02|label_size=100|label=Bakersfield|position=top|mark=Green pog.svg}}
{{Location map~|California|lat=35.37|long=-117.49|label_size=100|label=Randsburg|mark=Green pog.svg}}
{{Location map~|California|lat=32.72|long=-117.16|label=San Diego|label_size=100|position=left|mark=Green pog.svg}}
{{Location map~|California|lat=33.82|long=-116.53|label=Palm Springs|label_size=100|position=right|mark=Green pog.svg}}
{{Location map~|California|lat=34.43|long=-119.72|label=Santa Barbara|label_size=100|position=left|mark=Green pog.svg}}
{{Location map~|California|lat=34.24|long=-118.00|mark=Bullseye1.png|marksize=40}}
| relief = yes
| width = 250
| float = right
| caption = }}
| isc-event = 330437
| anss-url = ci2021449
| magnitude = 5.6 {{M|w|link=y}}<ref name="ISC-GEM"/><ref name="PAGER-CAT"/>
| damage = $33.5–40 million<ref name="PAGER-CAT"/><ref name=Hauksson_p1058/>
| intensity = {{MMI|VII}}<ref name=Hauksson_p1058>{{harvnb|Hauksson|1994|p=1058}}</ref>
| PGA = 0.58 ''[[peak ground acceleration|g]]'' at [[Cogswell Dam]]<ref name=Huang/>
| landslide = Yes
| aftershocks = 
| depth = {{convert|10|km|abbr=on|order=flip}}<ref name="ISC-GEM"/>
| location = {{coord|34.22|N|118.07|W|display=inline,title}}<ref name="ISC-GEM"/>
| type = [[Thrust fault|Thrust]]
| countries affected = [[Greater Los Angeles Area]] <br> [[Southern California]] <br> United States
| casualties = 2 deaths<ref name="PAGER-CAT"/> <br> 100–107 injured<ref name="PAGER-CAT"/><ref name=lat_Jun29/>
}}

The '''1991 Sierra Madre earthquake''' occurred on June 28 at {{tooltip|07:43:55 local time|14:43:55 UTC}} with a [[moment magnitude scale|moment magnitude]] of 5.6 and a maximum [[Mercalli intensity scale|Mercalli intensity]] of VII (''Very strong''). The [[thrust earthquake]] resulted in two deaths, around 100 injuries, and damage estimated at $33.5–40 million. The event occurred beneath the [[San Gabriel Mountains]] on the Clamshell–Sawpit Fault, which is a part of the Sierra Madre–Cucamonga Fault System. Instruments captured the event at a number of strong motion stations in [[Southern California]].

==Preface==
{{see also|California earthquake forecast}}

Due to its history of generating great earthquakes ([[1857 Fort Tejon earthquake|1857]], [[1906 San Francisco earthquake|1906]]) the focal point of earthquake hazard assessment in California has been the strike-slip [[San Andreas Fault|San Andreas System]] of faults. However, a system of faults, including the Sierra Madre–Cucamonga Fault System that runs along the border of the [[San Gabriel Valley|San Gabriel]] and [[San Fernando Valley]]s, has also caused a series of moderate to large events in the Greater Los Angeles Area. The westernmost portion of this fault system ruptured in [[1971 San Fernando earthquake|1971]] and has been identified as one of six major fault systems that pose a threat to the highly populated region.<ref name=Dolan>{{cite journal|title=Prospects for Larger or More Frequent Earthquakes in the Los Angeles Metropolitan Region|first1=J. F.|last1=Dolan|first2=K.|last2=Sieh|authorlink2=Kerry Sieh|first3=T.|last3=Rockwell|first4=R. S.|last4=Yeats|first5=J.|last5=Shaw|first6=J.|last6=Suppe|first7=G. J.|last7=Huftile|first8=E. M.|last8=Gath|year=1995|journal=[[Science (journal)|Science]]|volume=267|number=5195|pages=199–205|doi=10.1126/science.267.5195.199|pmid=17791339|bibcode=1995Sci...267..199D|s2cid=20501804}}</ref>

[[Geodesy|Geodetic]] research has shown that [[Thrust tectonics|crustal shortening]] is occurring in the region of the Transverse Ranges at a rate of {{convert|8.5|mm|sp=us|order=flip}} per year. Although a number of earthquakes have occurred in the Los Angeles area since 1971, the rate of moderate events since then may not be typical, and few large events have occurred since records have been kept. The rate of contraction and too few moderate events that have occurred indicated to researchers that a deficit of seismic energy release exists, and that larger, but less frequent events may also contribute to the release of strain accumulation.<ref name=Dolan/>

==Tectonic setting==
{{see also|Los Angeles Basin}}

To the west of the San Andreas Fault, the southern edge of the [[Transverse Ranges]] has been in a state of contraction during the [[Quaternary|Quaternary Period]], with active [[Fault (geology)#Dip-slip faults|reverse faulting]] that has uplifted San Gabriel Mountains. The west-[[Strike and dip|striking]] (and range-bounding) Sierra Madre–Cucamonga Faults, as well as the [[San Cayetano Fault]] and Santa Susana Faults all exhibit a component of left-lateral [[Fault (geology)#Strike-slip faults|strike-slip]] motion and share a similar trend.<ref name=Powell>{{cite book|title=San Andreas Fault System: Displacement, Palinspastic Reconstruction, and Geologic Evolution|chapter-url=https://books.google.com/books?id=VOjSpEDAmqoC&pg=PA65|chapter=Balanced Palinspastic Reconstruction of Pre-Late Cenozoic Peleogeology, Southern California: Geologic and Kinematic Constraints on Evolution of the San Andreas Fault System|first=R.E.|last=Powell|year=1993|publisher=[[Geological Society of America]]|isbn=978-0813711782|page=65}}</ref>

The Malibu Coast–Santa Monica Fault lies at the southern flank of the [[Santa Monica Mountains]] and the Raymond Fault runs to the south of the [[Verdugo Mountains]] and meets the San Gabriel Mountains at [[Monrovia, California|Monrovia]]. The Clamshell–Sawpit Fault lies beneath the San Gabriel Mountains inline with the Raymond Fault. The characteristics of the various faults are difficult to interpret, but they all [[Strike and dip|dip]] steeply to the north with several thousand feet of displacement.<ref name=Powell/>

==Earthquake==
[[File:USGS Shakemap - 1991 Sierra Madre earthquake.jpg|thumb|USGS ShakeMap for the event]]

The shock occurred on June 28, 1991, at 07:43:55 local time, and was the result of [[thrust fault]]ing on the Clamshell–Sawpit Fault at the southern boundary of the San Gabriel Mountains. At less than {{convert|1|cm|abbr=on|order=flip}}, the total amount of uplift was very minimal, considering the {{convert|2|m|abbr=on|order=flip}} of vertical displacement that was observed during the [[1971 San Fernando earthquake]]. The uplift initiated about two million years ago, and is a result of the convergence of the [[Pacific plate|Pacific]] and [[North American plate]]s near the restraining bend of the [[San Andreas Fault]].<ref name=Hauksson>{{harvnb|Hauksson|1994|pp=1071–1073}}</ref>

===Damage===

The earthquake was felt from [[Santa Barbara, California|Santa Barbara]] in the west, to [[Palm Springs, California|Palm Springs]] in the east, and south to the [[Mexico–United States border]]. It was centered about {{convert|7.5|mi}} northeast of [[Sierra Madre, California|Sierra Madre]]. Hundreds of homes were damaged and at least three roads leading up to [[Mount Wilson (California)|Mount Wilson]] were blocked by [[landslide]]s. A woman was killed when a steel beam fell at the [[Santa Anita Park]] racetrack in [[Arcadia, California|Arcadia]] and another woman died as a result of a heart attack in [[Glendale, California|Glendale]].<ref name=lat_Jun29>{{cite news|title=6.0 Earthquake Rocks Southland; 1 Dead, 107 Hurt : Temblor: Epicenter is near Sierra Madre. Hundreds of homes are damaged, and historic buildings in Pasadena are cracked. Some mountain roads are closed|url=https://www.latimes.com/archives/la-xpm-1991-06-29-mn-1291-story.html|first1=Berkley|last1=Hudson|first2=Claire|last2=Spiegel|first3=Vicki|last3=Torres|date=June 29, 1991|newspaper=Los Angeles Times}}</ref>

Also in Arcadia, a [[Motel 6]] was badly damaged. Other structural damage resulted from the shock. For example, both [[Monrovia, California|Monrovia]] and [[Pasadena, California|Pasadena]] each had more than a dozen buildings that were uninhabitable, but for the most part damage was limited to cracked [[plaster]] and broken windows. The [[Pasadena City Hall]] had moderate damage consisting of a cracked wall, dislodged [[marble]] barriers in the restrooms, and an offset [[dome]]. In Sierra Madre, four apartment buildings and nine homes were also tagged as uninhabitable. Of the more than 100 injuries that were reported in Arcadia, Pasadena, Glendale, and Sierra Madre, most were cuts, bruises, and sprains, but some serious injuries also occurred.<ref name=lat_Jun29/>

===Intensity===
With an area of perceptibility of around 58,500&nbsp;km<sup>2</sup>, the shock was felt from [[Santa Barbara, California|Santa Barbara]] and [[Bakersfield, California|Bakersfield]] to [[San Diego]], [[Palm Springs, California|Palm Springs]], and [[Las Vegas]]. This is about half the felt area that was seen during the [[1987 Whittier Narrows earthquake]], which was felt over an area of 110,000&nbsp;km<sup>2</sup>. The maximum intensity observed in the urban areas that were affected was VII (''Very strong''), but the intensity in the epicentral area is unknown because of its remote location in the mountainous [[Angeles National Forest]], though rock slides and landslides occurred there as a result of strong shaking. Intensity VI (''Strong'') effects were observed over 1,100&nbsp;km<sup>2</sup>, from [[Altadena, California|Altadena]] and [[La Verne, California|La Verne]] along the base of the mountains, to East Los Angeles.<ref>{{citation|title=Preliminary isoseismal map for the Sierra Madre, California, earthquake of June 28, 1991 UTC|first1=C. W.|last1=Stover|first2=B. G.|last2=Reagor|year=1991|series=Open-File Report 91-388|publisher=[[United States Government Printing Office]]|pages=1–6|citeseerx=10.1.1.915.5871}}</ref>

===Strong motion===

The [[California Strong Motion Instrumentation Program]] (CSMIP) obtained 120 records from 48 ground response stations, 38 buildings, six [[dam]]s, an airport control tower, and a power station. The individual stations that reported were as close as {{convert|4|km|sp=us|order=flip}} ([[Cogswell Dam]]) to as far as {{convert|80|km|sp=us|order=flip}} ([[Neenach, California|Neenach]]), but most were within {{convert|40|km|abbr=on}}. In downtown Pasadena, accelerations of 0.20 ''g'' were measured at the bottom floor of three buildings, but the duration of strong motion was only about two seconds. At the [[Altadena, California|Altadena]] station ({{convert|13|km|abbr=on|order=flip}} from the epicenter) 0.46 ''g'' was recorded. The Cogswell Dam station recorded a peak horizontal acceleration of 0.58 ''g'', but no major damage occurred.<ref name=Huang>{{citation|title=CSMIP strong-motion records from the Sierra Madre, California earthquake of 28 June 1991|url=https://www.conservation.ca.gov/cgs/Pages/Program-SMIP/Reports/EQData/osms91-03.aspx|year=1992|first1=M.|last1=Huang|first2=A.|last2=Shakal|first3=R.|last3=Darragh|first4=C.|last4=Ventura|first5=T.|last5=Cao|first6=R.|last6=Sherburne|first7=P.|last7=Fung|first8=J.|last8=Wampole|first9=M.|last9=DeLisle|first10=C.|last10=Petersen|series=OSMS 91-03, California Strong Motion Instrumentation Program|publisher=[[California Geological Survey|California Division of Mines and Geology]]|pages=1–3}}</ref>{{clear}}

==Recurrence interval==
{{further|Return period}}

Although the long term slip rate of the constituent faults that make up the [[Sierra Madre Fault Zone]] are not precisely known, several studies have put forth scenarios for possible future activity. Two independent reports suggested that if the complete ({{convert|100|km|abbr=on|order=flip}}) fault system were to rupture, it could result in a {{M|s|link=y}} 7.7 shock similar to the [[1952 Kern County earthquake]]. One researcher stated that the recurrence interval for such an event would be greater than 5,000 years. Another proposal was for one of the seven individual segments to rupture in a M6.4–6.6 event, comparable with the 1971 San Fernando earthquake. The recurrence interval for this type of event was placed at 200 years.<ref name=Hauksson/>

==See also==
{{Portal|California}}
*[[List of earthquakes in 1991]]
*[[List of earthquakes in the United States]]
*[[List of earthquakes in California]]

==References==
{{reflist|refs=
<ref name="ISC-GEM">{{citation|title=ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009)|url=http://www.isc.ac.uk/iscgem/index.php|author=ISC|date=19 January 2015|publisher=[[International Seismological Centre]]|series=Version 2.0}}</ref>
<ref name="PAGER-CAT">{{citation|title=PAGER-CAT Earthquake Catalog|url=https://earthquake.usgs.gov/static/lfs/data/pager/catalogs/|archive-url=https://web.archive.org/web/20200313112456/ftp://hazards.cr.usgs.gov/web/data/pager/catalogs/|url-status=dead|archive-date=2020-03-13|date=September 4, 2009|publisher=United States Geological Survey|series=Version 2008_06.1}}</ref> 
}}

'''Sources'''
{{refbegin}}
* {{citation|title=The 1991 Sierra Madre Earthquake Sequence in Southern California: Seismological and Tectonic Analysis|url=http://www.bssaonline.org/content/84/4/1058.abstract|first=E.|last=Hauksson|year=1994|journal=Bulletin of the Seismological Society of America|publisher=[[Seismological Society of America]]|volume=84|number=4|pages=1058–1074|doi=10.1785/BSSA0840041058|bibcode=1994BuSSA..84.1058H |s2cid=131329568|access-date=2015-02-19|archive-date=2015-09-23|archive-url=https://web.archive.org/web/20150923194948/http://www.bssaonline.org/content/84/4/1058.abstract|url-status=dead}}
{{refend}}

==External links==
* [https://earthquake.usgs.gov/earthquakes/eventpage/ci2021449#executive M 5.8 – 13km NNE of Sierra Madre, CA] – United States Geological Survey
* [https://web.archive.org/web/20140322134634/http://www.data.scec.org/significant/sierramadre1991.html Sierra Madre earthquake] – Southern California Earthquake Data Center
* [https://web.archive.org/web/20031211072749/http://news.nationalgeographic.com/news/2003/12/1208_031208_earthquakes.html California Quake Zones May Be More Lethal Than We Thought, Studies Say] – ''[[National Geographic]]''
* [https://www.sciencedaily.com/releases/2005/05/050519141339.htm Los Angeles 'Big Squeeze' Continues, Straining Earthquake Faults] – ''[[ScienceDaily]]''
* {{EQ-isc-link|330437}}

{{Earthquakes in 1991}}
{{Earthquakes in California}}
{{Earthquakes in the United States}}

{{DEFAULTSORT:Sierra Madre Earthquake, 1991}}
[[Category:1991 earthquakes]]
[[Category:Earthquakes in California|1991]]
[[Category:San Gabriel Mountains]]
[[Category:Sierra Madre, California]]
[[Category:1991 in California]]
[[Category:1991 natural disasters in the United States]]
[[Category:Disasters in Los Angeles]]<!--LA County-->
[[Category:Geology of Los Angeles County, California]]
[[Category:June 1991 events in the United States]]