Preliminary Report on the 29 July 2008 Mw 5.4 Chino Hills, Eastern Los Angeles Basin, California, Earthquake Sequence
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Preliminary Report on the 29 July 2008 Mw 5.4 Chino Hills, Eastern Los Angeles Basin, California, Earthquake Sequence Egill Hauksson, Karen Felzer, Doug Given, Michal Giveon, Susan Hough, Kate Hutton, Hiroo Kanamori, Volkan Sevilgen, Shengji Wei, and Alan Yong Egill Hauksson,1 Karen Felzer,2 Doug Given,2 Michal Giveon,1 Susan Hough,2 Kate Hutton,1 Hiroo Kanamori,1 Volkan Sevilgen,2 Shengji Wei,1 and Alan Yong2 INTRODUCTION The Chino Hills sequence was widely felt across southern California, although damage was minimal. Relatively strong The 29 July 2008 Mw 5.4 Chino Hills earthquake was the shaking was recorded to the north in the Diamond Bar area and largest event to occur within the greater Los Angeles metro- to the northwest in the eastern Los Angeles basin, as demon- politan region since the Mw 6.7 1994 Northridge earthquake. strated in the ShakeMap (Wald, Quitoriano, Dengler et al. 1999) The earthquake was widely felt in a metropolitan region with a and actual strong-motion records, which were made available via population of more than 10 million people and was recorded http://www.strongmotioncenter.org. The initial ShakeMap was by hundreds of broadband and strong-motion instruments. In available 12 minutes after the origin time, in part due to a com- this report we present preliminary analysis of the event and dis- puter malfunction, but normal production time is about five min- cuss its significance within the seismotectonic framework of the utes; six updates of the ShakeMap followed during the next hour northern Los Angeles basin as revealed by previous moderate as more data arrived from near real-time stations (Figure 2). The earthquakes. final map included amplitudes from 526 California Integrated The Chino Hills mainshock-aftershock sequence began at Seismic Network (CISN) stations. Over 40,000 people filled out a depth of about 15 km in the east Los Angeles area at 11:42 the Internet “Did You Feel It?” form to describe the effects of the am (PST) (Figure 1). The epicenter is between two mapped earthquake at locations throughout southern California. faults: the Whittier fault to the west and the Chino Hills fault Five other moderate-sized mainshock-aftershock sequences to the east. The focal mechanism indicates a mixture of strike- have occurred in the general vicinity of the Chino Hills earth- slip and thrust faulting on a west-southwest or a west-northwest quake since 1987. The largest event was the Whittier Narrows striking nodal plane. The mainshock was followed by only two earthquake of 1 October 1987, which was located about 30 km aftershocks with M >3; M 3.8 at 11:52 am (PST) and M 3.6 west-northwest and had a magnitude of 5.9. It caused three at 13:40 (PST). In the first two hours, 37 smaller aftershocks direct fatalities and over $358 million in damage. The Whittier were also recorded in the magnitude range of 1.3 to 2.8. By 14 Narrows earthquake resulted from thrust faulting on the Puente August the Southern California Seismic Network (SCSN), a Hills thrust (Shaw and Shearer 1999). The other four earth- joint project of the California Institute of Technology (Caltech) quakes were the M 4.6 1989 Montebello, M 5.0 1988 Pasadena, and the U.S. Geological Survey (USGS) had recorded ~ 150 M 5.2 1990 Upland, and M 5.8 1991 Sierra Madre. The 1989 aftershocks of M ≥ 1.0. The mainshock was not preceded by Montebello earthquake was caused by thrust faulting, similar to foreshock activity. the Whittier Narrows mainshock (Hauksson 1990). Both the During the 2008 Chino Hills sequence, the SCSN auto- Pasadena and the Upland earthquakes exhibited west-southwest matically processed real-time waveform data from 370 stations left lateral strike-slip faulting while the Sierra Madre earthquake across southern California. The first location and ML magni- exhibited thrust faulting (Jones et al. 1990; Hauksson and tude estimate of 5.6 were released ~ 80 s after the origin time. Jones 1991; Hauksson 1994; Shearer 1997; Astiz et al. 2000). An updated location and finalML 5.8 were released after ~ 140 Thus the crustal deformation associated with the 2008 Chino s. The automatic moment tensor and the Mw estimate of 5.4 Hills earthquake is similar to deformation associated with the were available ~ 10 minutes following the origin time. These previous events. SCSN rapid notifications were posted on the Web, and data were made available via http://www.data.scec.org and http:// MAINSHOCK MOMENT TENSOR earthquake.usgs.gov. A real-time SCSN moment tensor solution (Clinton et al. 2006), which was derived using an automated analysis of wave- 1. California Institute of Technology forms from six stations and the inversion method of Dreger and 2. U.S. Geological Survey Helmberger (1993), revealed a strike of 291°, a dip of 59°, and doi: 10.1785/gssrl.79.6.855 Seismological Research Letters Volume 79, Number 6 November/December 2008 855 2008 Chino Hills Mw5.4 Earthquake Sequence San Andreas Flt 34°20′ 06/28/1991 M5.8 San Gabriel Mtns Northridge 02/28/1990 12/03/1988 M5.2 M4.9 Sierra Madre Fault Zone 34°10′ PA UP 10/01/1987 Santa Monica Mtns 06/12/1989 M5.9 Los M4.4 Angeles San Jose Fault ′ Newport-Inglewood Fault M5.4 34°00 MB 07/29/2008 Chino Fault LAX Whittier Fault Orange 33°50′ County Yorba Linda trend Long km Beach Palos 0 5 10 Verdes −118°30′ −118°20′ −118°10′ −118°00′ −117°50′ −117°40′ −117°30′ ▲ Figure 1. Map of (1981 to 2005) seismicity recorded by the SCSN and some recent sequences in the Los Angeles basin, including lower hemisphere focal mechanisms of the moderate-sized mainshocks. The 2008 Chino Hills mainshock is shown as a red star and the after- shocks as red circles. LAX—Los Angeles Airport, MB—Montebello; PA—Pasadena; UP—Upland; WN—Whittier Narrows. a rake of 142°. The double-couple solution was also determined phase inversion (Kanamori and Rivera, 2008), using 18 broad- using the “cut-and-paste” technique (Zhao and Helmberger band SCSN stations with a pass-band of 50 to 150 s, yields a 1994; Zhu and Helmberger 1996). In the “cut-and-paste,” we similar mechanism and Mw ((strike/dip/rake) 51°/38°/32°, analyzed the broadband waveforms of regional seismograms 294°/71°/123°, Mw = 5.4). The main difference between the from 22 stations with epicentral distances between 100 and 200 global CMT solution and the WP inversion is in the dip angle km and maximum azimuth gap of 32° (Figure 3A). The errors in of the southeast-dipping plane. These solutions represent the the strike, dip, and rake of the focal mechanism are about ±3°, long-period characteristics of the source, and the good agree- ±6°, and ±15°, respectively. ment between the regional, global, and long-period solutions The Green’s functions were generated from the FK tech- suggests that the source was associated with no significant nique (Zhu and Rivera 2002) and were based on a 1-D south- anomalous long-period (up to 100 s) deformation. ern California velocity model (Zhao and Helmberger 1994). A source depth of 15 km and Mw 5.4 correspond to minimum RELOCATIONS AND FOCAL MECHANISMS error between the real and synthetic waveforms (Figure 3B). The high cross-correlation coefficients (Figure 3C) confirm We relocated the mainshock and aftershocks, using a 3-D veloc- the similarity of the synthetics with the data, both for the Pnl ity model and HypoDD (Waldhauser and Ellsworth 2000). The waves and the surface waves. Because the “cut and paste” allows relocations reveal minimal clustering around either mainshock time shifts between portions of seismograms and synthetics, the nodal plane (Figure 4). In addition, the aftershocks occurred in a results of this method are relatively insensitive to minor changes limited depth range from 13 to 16 km and form a tight subhori- in the velocity model and to possible lateral crustal variations zontal distribution. It is thus not easy to determine from the spa- (Zhu and Helmberger 1996). tial aftershock patterns which of the two steeply dipping nodal The global centroid moment tensor (CMT) solution planes ruptured in the mainshock. Further, the mainshock is (http://www.globalcmt.org/CMTsearch.html) for this event located at a depth of 15 km, midway between the Whittier and yields a best double-couple with the nodal planes (strike/dip/ Chino faults, and because of its large focal depth, projecting the rake) 43°/62°/31°, 297°/63°/149° with Mw = 5.4. The W nodal planes up to dipping surface faults is difficult at best. 856 Seismological Research Letters Volume 79, Number 6 November/December 2008 CISN ShakeMap : 4.0 mi SE of Diamond Bar, CA Tue Jul 29, 2008 11:42:15 AM PDT M 5.4 N33.96 W117.76 Depth: 13.7km ID:14383980 34.5° Victorville Santa Clarita Northridge Big Bear Los Angeles 34° Riverside Long Beach Hemet Mission Viejo 33.5° Temecula km Avalon 0 50 –119° –118° –117° Map Version 7 Processed Wed Jul 30, 2008 12:08:07 PM PDT, PERCEIVED SHAKING Not felt Weak Light Moderate Strong Very strong Severe Violent Extreme POTENTIAL DAMAGE none none none Very light Light Moderate Moderate/Heavy Heavy Very Heavy PEAK ACC.(%g) <.17 .17–1.4 1.4–3.9 3.9–9.2 9.2–18 18–34 34–65 65–124 >124 PEAK VEL.(cm/s) <0.1 0.1–1.1 1.1–3.4 3.4–8.1 8.1–16 16–31 31–60 60–116 >116 INSTRUMENTAL INTENSITY I II–III IV V VI VII VIII IX X+ ▲ Figure 2.