Glendale (PDF) - Hazard Mitigation Web Portal - State of California

Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 6 – EarthquakesDip-slip faults are slanted fractures where the blocks mostly shift vertically. If the earth above an inclined faultmoves down, the fault is called a normal fault, but when the rock above the fault moves up, the fault is called areverse fault. Thrust faults are reverse faults with a dip of 45 ° or less.Few faults are simple, planar breaks in the Earth. They more often consist of smaller strands, with asimilar orientation and sense of movement. Sometimes geologists group strands into segments,which are believed capable of rupturing together during a single earthquake. The more extensive thefault, the bigger the earthquake it can produce. Therefore, multi-strand fault ruptures producelarger earthquakes.The bigger and closer the earthquake, the greater the likelihood of damage. Thus fault dimensionsand proximity to urban centers are key parameters in any hazard assessment. In addition, it isimportant to know a fault’s style of movement (i.e. is it dip-slip or strike-slip), the age of its mostrecent activity, its total displacement, and its slip rate (all discussed below). These values indicatehow often a fault produces damaging earthquakes, and how big an earthquake should be expected thenext time the fault ruptures.Total displacement is the length, measured in kilometers (km), of the total movement that hasoccurred along the fault over as long a time as the geologic record reveals. It is usually estimated bymeasuring distances between geologic features that have been split apart and separated (offset) bythe cumulative movement of the fault over many earthquakes. Slip rate is a speed, expressed inmillimeters per year (mm/yr). Slip rate is estimated by measuring an amount of offset accruedduring a known amount of time, obtained by dating the ages of geologic features. Slip rate data alsoare used to estimate a fault’s earthquake recurrence interval. Sometimes referred to as “repeattime” or “return interval”, the recurrence interval represents the average amount of time that elapsesbetween major earthquakes on a fault. The most specific way to derive recurrence interval is toexcavate a trench across a fault to obtain paleoseismic evidence of earthquakes that have occurredduring prehistoric time.In southern California, ruptures along thrust faults have built the Transverse Ranges geologicprovince, a region with an east-west trend to its landforms and underlying geologic structures. Thisorientation is anomalous, virtually unique in the western United States, and a direct consequence ofthe plates colliding at the Big Bend. Many of southern California’s most recent damagingearthquakes have occurred on thrust faults that are uplifting the Transverse Ranges, including the1971 San Fernando, the 1987 Whittier Narrows, the 1991 Sierra Madre, and the 1994 Northridgeearthquakes. Thrust faults can be particularly hazardous because many are blind thrust faults, thatis, they do not extend to the surface of the Earth. These faults are extremely difficult to detectbefore they rupture. Some of the most recent earthquakes, like the 1987 Whittier Narrowsearthquake, and the 1994 Northridge earthquake, occurred on blind thrust faults.When comparing the sizes of earthquakes, the most meaningful feature is the amount of energyreleased. Thus scientists most often consider seismic moment, a measure of the energy releasedwhen a fault ruptures. We are more familiar, however, with scales of magnitude, which measureamplitude of ground motion. Magnitude scales are logarithmic. Each one-point increase inmagnitude represents a ten-fold increase in amplitude of the waves as measured at a specific location,2006 PAGE 6 - 5