Important Terms & Concepts

Safety & Community Services Element

This element uses the following technical terms to discuss earthquakes and noise:

Earthquake Magnitude

Monterey Park lies within a region where earthquakes are not an uncommon occurrence. Earthquakes result from a shift or movement along weak points or contacts of geologic formations or structures. Scientists use the term magnitude to describe the relative energy release by such movement. An earthquake's magnitude is based on the size of the earthquake's seismic waves, which are recorded on a seismograph. Magnitude generally is rated and expressed using a logarithmic scale.

The amount of energy released, for example, from a 6.0 earthquake is ten times greater than that associated with a 5.0 event. Scientists consider a large earthquake as one having a magnitude of 7.0 or greater. For purposes of comparison, the 1987 Whittier earthquake registered a 5.9 magnitude, while the 1994 Northridge temblor measured 6.7 magnitude.

Magnitude vs. Intensity

Magnitude differs from earthquake intensity, which is the physical, observable effects an earthquake has on structures and people. News media generally do not report earthquake intensity according to scales or references; instead, the media rely upon pictures and comparisons to past events to show how an earthquake affects people and property. However, the Modified Mercalli scale has been developed to describe an earthquake's intensity relative to its magnitude. Table SCS-1 (PDF) presents the Modified Mercalli scale.

Mercalli Scale

The Modified Mercalli Scale represents a subjective measurement or description of ground shaking associated with a seismic event. The peak (maximum) horizontal ground acceleration, or PGHA, is used by seismologists to quantitatively measure ground shaking at particular locations. These values, expressed in units of g, which is a fraction or percentage of gravitational acceleration, provide useful information for determining how buildings must be constructed to withstand collapse or other damage in the event of an earthquake.

Noise Metrics

Noise generally is defined as unwanted or intrusive sound. Because noise consists of pitch, loudness, and duration, describing noise with a single unit of measure presents a challenge. The A-weighted decibel scale (dBA) has been developed to describe the loudness of a sound or sound environment based on the sensitivity of the human ear.

Figure SCS-1 (PDF) identifies typical noise levels associated with activities that occur in Monterey Park. Table SCS-2 (PDF) indicates criteria the State has established to reduce adverse noise effects on human health.

Noise Measurement Guidelines

The dBA descriptor only reports noise from a single source or combination of sources at a point in time. To allow a more comprehensive description of a noise environment, federal and state agencies have established noise and land use compatibility guidelines that use averaging approaches to noise measurement. Two measurement scales commonly used in California are the Community Noise Equivalent Level (CNEL) and the day-night level (Ldn).

In order to account for increased human sensitivity at night, the CNEL level includes a 5 decibel penalty on noise during the 7 p.m. to 10 p.m. time period and a 10-decibel penalty on noise during the 10 p.m. to 7 a.m. time period. The Ldn level includes only the ten decibel weighting for late-night noise. These values are nearly identical for all but unusual noise sources.