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MW 4.4 Encino Earthquake - 3/17/14

A MW4.4 Earthquake Occurred at 6:25AM in Encino on March 17th, 2014

CISN/SCSC Executive Summary (A. Guarino, E. Hauksson, N. Scheckel - Caltech Seismological Laboratory; V. Thomas, L. Jones, R. Graves, R. Dollar - USGS)

Version 1 - 9:58AM 3/17/2014

ShakeMap for the M4.4 Encino Oaks Earthquake

The maximum observed instrumental intensity was V, recorded in Santa Monica, although the ShakeMap shows interpolation of maximum intensity of VI.

A M4.4 earthquake occurred at 6:25AM on March 17, 2014, located 2 miles SSE of Encino, CA, in the city of Sherman Oaks. The event was felt widely throughout Ventura, Los Angeles, Orange, Riverside, and San Bernardino counties. There have been seven aftershocks as of 10:13AM, the largest of which was a M2.7 at 7:23AM, and was felt locally near the epicenter. The aftershock sequence may continue for several days to weeks, but will likely decay in frequency and magnitude as time goes by.

The maximum observed instrumental intensity was V, recorded in Santa Monica, although the ShakeMap shows interpolation of maximum intensity of VI. The maximum reported intensity for the Community Internet Intensity Map (Did You Feel It?) was repoted at VI in the epicentral area. The CIIM map is shown below.

CIIM

The moment tensor shows oblique strike-slip faulting, and the aftershocks create a linear feature that is on the nodal plane striking nearly N-S. The causative fault is difficult to identify, due to the lack of surface rupture and size of the event. The event has a hypocentral depth of 8km.

MT

 

HaukssonMap

Map showing 1994 Northridge earthquake and aftershocks (larger aftershocks are red stars), with the M4.4 Encino earthquake and aftershocks (beach ball shows moment tensor, star shows epicenter of mainshock, circles are aftershocks)

Approximately 5% of all earthquakes in Southern California are foreshocks for larger events, although this probability decays rapidly with time. Sometimes larger earthquakes may function as triggers for more distant earthquakes, but generally those events are very large, on the order of several magnitudes larger than this event.

Historically, there have been events of this magnitude in the Los Angeles basin. The most recent was a MW4.7 in Inglewood on 5/17/09. The last event >M5 was the July 29, 2008 MW5.4 Chino Hills Earthquake. The list below shows all events within the epicentral area, to 20 km since the 1994 Northridge earthquake:

#EVID     MAG  YYYY/MM/DD HH:mm:SS LAT       LON        DEP     LOCATION
3159411  5.2  1994/03/20 13:20:12 34 13.9 N 118 28.5 W 13.1    3 km ( 2 mi)  WNW  of Panorama City, CA    
3195727  4.5  1994/12/05 19:48:34 34 17.6 N 118 23.3 W  9.0    3 km ( 2 mi)  NW of Lake View Terrace, CA    
3247748  4.2  1995/12/27 01:42:58 34 14.7 N 118 35.9 W  6.0    1 km ( 1 mi)  SSE  of Chatsworth, CA    
9173365  4.3  2001/01/13 18:26:14 34 17.0 N 118 24.2 W  8.8    3 km ( 2 mi)  E of San Fernando, CA    
9173374  4.1  2001/01/13 18:50:53 34 17.3 N 118 24.2 W  8.4    3 km ( 2 mi)  E of San Fernando, CA    
9703873  4.2  2001/09/09 16:59:18 34  3.5 N 118 23.3 W  7.9    2 km ( 1 mi)  SE of Beverly Hills, CA   
15476961 4.4  2014/03/17 06:25:36 34  8.1 N 118 29.1 W  9.9    3 km ( 2 mi)  SSE  of Encino, CA   

UPDATE - 12:08PM, March 18, 2014

A causative fault is not immediately obvious for the M4.4 earthquake. As there was no surface rupture, research would likely be inconclusive. The two nodal planes of the mostly strike-slip focal mechanism strike west-northwest and north. The west-northwest striking plane for this event was dipping to the south, which means it is not associated with known north-dipping faults in the area (e.g. the Santa Monica fault). There are no mapped north-trending faults in the area, and that is the orientation of the second nodal plane. Additionally there is no evidence that the north-trending Newport-Inglewood fault extends beyond the Santa Monica fault to the north. Small quakes can happen anywhere in southern California, and this quake was simply a reflection of the regional buildup of stress associated with the Pacific and north America plate motion.

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