Hello Chris,

Not sure what this means, but that area is very quiet for 4 mag and up. Pulled up the PDE back to 1973 and found this;

NEIC: Earthquake Search Results

U. S. G E O L O G I C A L S U R V E Y
E A R T H Q U A K E D A T A B A S E

FILE CREATED: Mon Sep 3 14:35:29 2007
Circle Search Earthquakes= 5
Circle Center Point Latitude: 33.733N Longitude: 117.476W
Radius: 30.000 km
Catalog Used: PDE
Magnitude Range: 4.0 - 10.0
Data Selection: Historical & Preliminary Data

CAT YEAR MO DA ORIG TIME LAT LONG DEP MAGNITUDE IEFM DTSVNWG DIST
NFPO km
TFS

PDE 1973 10 23 045757.10 33.82 -117.72 8 4.90 mb GS .F . ....... 24
PDE 1974 09 21 103742.70 33.77 -117.25 8 4.20 mb GS 5D . ....... 21
PDE 1975 01 03 055531.60 33.55 -117.65 5 4.30 mb GS 4F . ....... 25
PDE 1978 04 29 033211.80 33.83 -117.73 5 4.60 mb GS 4F . ....... 26
PDE 2000 03 07 002028.18 33.81 -117.71 11 4.00 MLPAS .F . ....... 23

Looks like a rare hit area. Not sure about that cluster of quakes in the 70's.
If I make the circular larger, then I run into Whittier Narrows, and the after shock sequence there. Full credit to USGS for the stats.

Glen