KCC Event Brief: 2014 La Habra Earthquake

Transcription

1 KAREN CLARK & COMPANY KCC Event Brief: 2014 La Habra Earthquake June COPLEY PLACE BOSTON, MA T: F:

2 Overview On Friday, March 28, 2014 at 9:09pm, a magnitude 5.1 earthquake occurred near the city of La Habra, California. According to the USGS the earthquake epicenter was one km east of La Habra, five km north of Fullerton and 33 km east of Los Angeles. Light to moderate ground shaking was felt in Orange, Los Angeles, Ventura, San Bernardino and Riverside counties. The event was preceded by two foreshocks, a M3.6 at 8:03pm and a M2.1 at 8:16 pm, and there have been more than 200 aftershocks to date, with magnitudes as great as 4.1. The earthquake was estimated to have a shallow hypocentral depth of 7.5 km on an oblique north dipping fault plane. The La Habra earthquake occurred on a blind thrust fault associated with the Puente Hills fault system. Earthquake ground motions approached 0.3g and a maximum intensity of VII was recorded in the city of Brea. Intensities of up to VI were reported in Fullerton, La Habra, La Mirada and Buena Park. Damage from the earthquake occurred in isolated pockets and included broken chimneys and building cracks in single family homes, foundation cracks in apartment buildings and broken water lines which displaced residents. Impacts By California standards the La Habra earthquake was small, and damage was limited to unreinforced masonry construction, non-structural building elements, and building contents. While this was a small event, it does provide an opportunity to observe damage from small magnitude earthquakes and to test modeled ground motions. For all significant earthquakes, the USGS creates an intensity map using data from their webbased Did You Feel It (DYFI) Program ( DYFI gathers information about shaking intensities from what people experienced, such as their perceived level of shaking and extent of damage. Users submit an online questionnaire which is then summarized and quantified into an intensity value for each affected zip code. The map below and on the right shows shaking intensities as reported by the USGS for the La Habra event. Based on the event parameters, KCC generated ground motions for the event using the new earthquake module in RiskInsight. The estimated ground motions are shown in the map below on the left the estimated total losses are less than $500 million with minimal insured losses. Copyright 2014 Karen Clark & Company 2014 La Habra Earthquake Page 2

3 La Habra Modeled Peak Ground Acceleration La Habra Reported Modified Mercalli Intensities from the USGS One of the most significant lessons learned from past California earthquakes is that unreinforced masonry (URM) construction is highly vulnerable to earthquakes. Masonry materials such as bricks and concrete blocks have historically been used in construction because they are easy to assemble and relatively strong under normal gravity loads. In URM construction the mortar is the glue used to hold the masonry together, and the strength of the mortar deteriorates over time. When an unreinforced masonry wall is exposed to an earthquake, the mortar can easily crack and fail if the ground motion results in shaking either perpendicular or parallel to the wall. While most of the URM buildings in California have been retrofitted, URM is still used in construction of fences and chimneys for both residential and commercial properties, and as expected this vulnerable material experienced damage during the La Habra earthquake. Chimneys are particularly vulnerable in earthquakes because in addition to being constructed of URM, they are relatively denser and taller than the buildings they are attached to and during the earthquake shaking will tend to sway out of synch with the main structure. If the chimneys are not properly connected this difference in motion with the main building can result in considerable damage to the chimney and main structure due to the falling masonry. URM fences can also collapse during earthquakes. Fences are prone to tipping over with ground motion perpendicular to the wall and cracking and crumbling due to ground motion parallel to the wall. Isolated instances of URM failures were identified in a number of areas in close proximity to La Habra. Copyright 2014 Karen Clark & Company 2014 La Habra Earthquake Page 3

4 Masonry wall damage Chimney damage Dave Mecham/KTLA AP Photo/The Orange County Register, Ken Steinhardt Non-structural elements of buildings are also particularly vulnerable in earthquakes. Well before the ground motion reaches levels that may result in structural damage, brittle non-structural materials such as plate glass windows and stucco or gypsum wall materials can crack or shatter when exposed to lateral movement during ground shaking. Reports of light damage to residential and commercial building windows were observed with 20 to 30 businesses suffering broken plate-glass windows along Whittier Boulevard in La Habra. Light damage to exterior stucco walls was observed in single family homes and apartment buildings. Residential plate glass damage in Buena Park Vanessa Limongelli /abc7 Commercial plate glass damage at Goodwill, La Habra Miguel Galindo/abc7 Copyright 2014 Karen Clark & Company 2014 La Habra Earthquake Page 4

5 Stucco wall damage Single Family Home in Fullerton Stucco wall damage to Fullerton Apartment Complex Irfan Khan / Los Angeles Times Reuters/Gene Blevins Low levels of ground motion can also result in damage to improperly secured nonstructural elements such as hanging lights or hung ceiling panels, as well building contents including shelving, stored materials, and electronics. Reports of contents damage were widespread in the La Habra earthquake with store merchandise, restaurant plateware and glassware, office materials, and home electronics damaged across the impacted region. Light damage to non-structural ceiling components was also observed at several commercial operations. Suspended ceiling damage at Ross Store in La Habra Brandy Stowell/ ABC7 Suspended ceiling and lighting damage at motorcycle dealer in La Habra Allen J. Schaben / Los Angeles Times Copyright 2014 Karen Clark & Company 2014 La Habra Earthquake Page 5

6 Merchandise damage at Albertsons grocery store in Miranda Contents damage at restaurant near La Habra Damage to office contents in La Habra Maria Kimura/abc7 Entertainment center damaged in residential home in Fullerton Miguel Galindo /abc7 Natasha Hazou /abc7 As the figure below indicates, damage was observed at isolated locations primarily within Orange County where ground shaking was light to moderate. Locations of Observed Damages Copyright 2014 Karen Clark & Company 2014 La Habra Earthquake Page 6

7 Earthquake Hazard in Los Angeles The Greater Los Angeles area sits above a complex network of faults, some of which are capable of producing significant events. All fault types are found within this region, primarily strike-slip and blind thrust faults. The region is bordered to the north by the Southern San Andreas Fault and includes major faults such as the Newport-Inglewood, Elsinore, and San Gabriel with fault lengths of more than 60 km. The La Habra earthquake ruptured a small portion of a fault associated with the Puente Hills fault system. Despite the location of the hypocenter, it did not affect other faults within the Puente Hills fault system where larger earthquakes are likely to occur. The Puente Hills fault is a 40 km blind thrust fault that runs from the suburbs of northern Orange County through the San Gabriel Valley under downtown Los Angeles and terminates in Hollywood. The Puente Hills fault is one of the most hazardous in the Los Angeles area because of its orientation and shape and its proximity to highly populated areas. The fault surface is relatively horizontal and slopes underneath a large region of the Los Angeles basin. Other faults in the area, such as the Newport-Inglewood fault, are almost vertical in orientation, and therefore will likely affect a smaller surface area. A large earthquake on the Puente Hills fault could cause intense shaking felt over a much larger area resulting in an even more devastating earthquake than a rupture on the San Andreas Fault. Faulting within the Los Angeles Basin Since 1900, the Los Angeles basin has been affected by more than 30 earthquakes with a magnitude of 5 or greater. The earthquakes with magnitudes greater than 5 are shown below. Prior to La Habra, the 1994 Northridge earthquake was the last significant event to occur in this region. Copyright 2014 Karen Clark & Company 2014 La Habra Earthquake Page 7

8 Significant Los Angeles Earthquakes: Present 1925 Santa Barbara 1971 San Fernando 1994 Northridge 1923 N. San Jacinto 2014 La Habra 1933 Long Beach The 1994 Northridge earthquake was a 6.7 magnitude event that occurred on a blind thrust fault, 30 km northwest of Los Angeles. Maximum intensities of IX were observed and peak ground acceleration of up to 0.7g recorded with ground shaking lasting close to 20 seconds. Because the greater Los Angeles area is underlain by a deep coastal sedimentary basin, ground motions from the earthquake were significantly amplified. The amount of amplification at any given site within the basin varied resulting in a wide range of ground motion intensities. At least two magnitude 6 or above aftershocks followed the event. Damages were observed in Los Angeles, Ventura, Orange and San Bernardino Counties and resulted in economic losses exceeding $50 billion at the time Northridge Earthquake Ground Motion Intensities from USGS Previous earthquakes of magnitude greater than 6 include the 1925 magnitude 6.8 Santa Barbara Earthquake and the magnitude Long Beach Earthquake. Both of the events Copyright 2014 Karen Clark & Company 2014 La Habra Earthquake Page 8

9 caused widespread damage particularly to URM buildings many of which suffered complete collapse. Perhaps of greatest concern was that many of the URM structures impacted by the 1933 earthquake were schools 70 were destroyed and over 120 suffered major damage. Fortunately the schools were not in session when the earthquake hit and the event motivated public officials in California to adopt more stringent construction standards including the prohibition of new URM construction. In subsequent California earthquakes damage to URM buildings built prior to 1933 continued to be observed, and building code requirements expanded to require retrofitting of URM structures. A 2007 study suggests approximately 70% of the URM buildings in California have been retrofitted, with higher rates observed in Los Angeles and Orange counties. This evolution in codes has dramatically reduced the number of URM buildings in California and mitigated the potential losses from the La Habra event. Hotel Californian Damage, 1925 Santa Barbara Continental Banking Company, 1933 Long Beach NISEE, U.C. Historical Society of Long Beach Morrison Apartments, 1933 Long Beach Franklin Junior High School, 1933 Long Beach Historical Society of Long Beach Historical Society of Long Beach Copyright 2014 Karen Clark & Company 2014 La Habra Earthquake Page 9

10 Possible Future Earthquake Scenarios Despite the location of the La Habra earthquake, there were no reported seismic movements on the Puente Hills fault. The Puente Hills fault has a characteristic magnitude of 7.0 and if the La Habra earthquake had initiated activity on the Puente Hills fault, an earthquake of this magnitude or greater could have occurred. To evaluate impacts of potential events in this region, KCC scientists and engineers conducted analyses on the impacts of magnitude 6.1 and 7.1 earthquakes on the Puente Hills fault. A 6.1 magnitude event could be associated with a 10km rupture on the fault, causing shaking intensities approaching VIII and peak ground motions of 0.45g. This earthquake could impact a region over 750 square km in area from Los Angeles to Laguna Beach. The simulated ground motions for this scenario, indicated below, show peak ground motion acceleration of over 0.4g in the cities of La Habra, La Mirada and Brea and up to 0.1g in Los Angeles. This scenario would result in estimated total losses of more than $20 billion. If a magnitude 7.1 earthquake occurred on the fault, shaking intensities of IX and peak ground acceleration exceeding 0.7g would likely be experienced. An earthquake of this magnitude would rupture the entire length of the 40 km fault, impacting an area of over 1,000 square kilometers, extending from Ventura through downtown Los Angeles to Riverside. In the ground motions simulated for this scenario, as indicated below, peak ground accelerations greater than 0.5g could occur in the downtown Los Angeles area with ground motions of 0.4g occurring from downtown Hollywood to as far south as Fullerton. This 7.1 magnitude scenario would cause over $300 billion in total losses. Simulated Ground Motions for 6.1 Earthquake Simulated Ground Motions for 7.1 Earthquake M6.1 Scenario M7.1 Scenario Copyright 2014 Karen Clark & Company 2014 La Habra Earthquake Page 10

11 Summary The magnitude 5.1 earthquake that struck just east of La Habra, California resulted in maximum observed intensities of up to VII on the MMI scale. The event had a shallow depth of 7.5 km and occurred on blind thrust fault associated with the Puente Hills fault system. The event caused light to moderate damage in isolated locations in Orange County. Although the event was relatively small by California standards, it serves as a reminder that it is only a matter of time before a major earthquake impacts the Los Angeles region. Copyright 2014 Karen Clark & Company 2014 La Habra Earthquake Page 11

12 Appendix Below is a sample of the USGS Did You Feel It online questionnaire where users can report their experience during an earthquake. Copyright 2014 Karen Clark & Company 2014 La Habra Earthquake Page 12

13 Copyright 2014 Karen Clark & Company 2014 La Habra Earthquake Page 13