GEOLOGIA

On May 3, 1887 Arizona and the Southwest experienced a major

earthquake that had an estimated magnitude of 72 on the Richter scale

(DuBois and Smith, 1980). The epicenter was in Sonora, Mexico

approximately 40 miles south of Douglas, Arizona. The earthquake

caused several dozen deaths, damaged buildings as far away as Phoenix,

generated rockfalls and fires triggered by rockfalls in the mountains, and

caused panic among the population. This year is the 100th anniversary

of the only earthquake that caused considerable damage in Arizona in

historic times.

Although earth scientists know much more now regarding the

mechanisms of earthquakes than they did 100 years ago, reliable

earthquake prediction is still in its infancy. It is known that the crust and

uppermost mantle of the earth is divided into approximately a dozen

major sections or "plates" that are slowly moving. Rates of relative

movement range up to several inches per year. It is along the plate

boundaries that the most earthquakes occur. The San Andreas fault of

California is a plate boundary along which the Pacific plate is moving

northwestward with respect to the adjacent North American plate.

Because of friction along plate boundaries, plates do not smoothly slip

past each other. As a consequence, resistance to movement allows stress

to accumulate. When stress builds to the point at which it overcomes the

resisting forces, energy is released causing ground motion, or an

earthquake.

Although southeastern Arizona is several hundred miles from the

San Andreas fault system, it is not immune to earthquakes. No region can

be considered completely earthquake free; in fact, worldwide there are

approximately 1 million detectable earthquakes annually (Gilluly and

others, 1968). The majority of these are small shocks that cause no

damage. The large, dangerous earthquakes occur less frequently, on the

average of only several per year, and are usually concentrated along plate

boundaries. By the time the surface waves of these large events reach

southeastern Arizona, the energy has dissipated so that little or no motion

is felt except by sensitive recording devices.

The 1887 event was, however, close enough and strong enough to

cause major damage and loss of life in the southem portion of the State.

The earthquake occurred along a south-trending fault approximately 30

miles in length located south of Douglas, Arizona (Figure 1). This surface

rupture, named the Pitaycachi (pronounced ti ka che) fault, is one of

several surface faults in the region that are thought to have been active

during the last 100,000 years (Pearthree, 1986). These faults are located

along the margins of south-trending ranges in the southeastern Arizona -

southwestern New Mexico border region and extend into Sonora, Mexico.

It is estimated that the 1887 Sonoran earthquake released twice as

much energy as any of the other earthquakes recognized in this region

Figure 1. Aerial view, looking northward, of 1887 scarp along Pitaycachi fault, Sonora,

Merico. The fault extends from about 8 kilometers south of the Arizona border for 50

kilometers to and beyond Colonia Morales in the San Bemardino Valley. Photo by Peter

Kresan.

(Pearthree, 1986). Firsthand accounts reported that two violent shocks

were preceded by low rumbling noises. This rumbling sound was reported

in Tucson and as far awavas Phoenix (~Fi"au re 2,) .E stimates of the duration

of ground motion vary f;om a few seconds to approximately 10 minutes,

with 1 to 3 minutes being the time most frequently reported. People

throughout the region ran into the streets, some fainted, and others were

thrown to the ground (DuBois and Smith, 1980). Numerous rockfalls

were reported in the mountain ranges of southeastern Arizona and

northern Sonora. Sparks from the crashing boulders ignited dry brush

and grass, and fires quickly spread to the forests. Nearly all the valleys

experienced changes in water conditions. Wells that had been excellent

UNI TED STA TES

' UTAH 1 J..- / ARIZONA I

Figure 2. lsoseismal map of the 1887 earthquake (from DuBois and Smith, 1980).

lsoseismal lines connect points on the Earth's suface at which earthquake intensity is the

same; they are usually closed curves around the epicenter (the black ova1;shaped area

shown in the map). The severity of an earthquake can be expressed in tux, very different

ways: by magnitude and by intensity. Magnitude measures the amount ofseismic energy

released at the focus of an earthquake It is determined from the logarithm ofthe amplitude

of earthquake waves recorded by seismographs. Magnitude is expressed on the Richter

scale in whole numbers and decimal fractions (e.9.. 7.2, the magnitude of the 1887

earthquake). Theoretically this scale has no upper limit howewr, the largest earthquake

ever recorded, in Chile in 1960, had a magnitude of 9.5 (DuBois, 1979).

Intensity is an arbitrq measure of the observable effects of an earthquake on

humans and structures at a specific site. It van'es from place to place depending on the

strength of the earthquake (magnitude), the distance from the epicenter, and the local

geology. The intensity scale currently used in the United SWes is the Modified Mercalli

(MM) Intensity Scale. This scale composed of 12 levels of intensity that range from

impe~eptible shaking (I) to catastrophic destruction (MI), is designated by Roman

numerals, as shown in the map above. The lower numbers of the MM intensity scale

generally deal with the manner in which the earthquake is felt by persons; the higher

numbers are based on obsemed structural damage For instance the MM rating of 111,

recorded in Yuma during the 1887 earthquake, is based on the following MM

characteristic: "Felt noticeably indoors, but not always recognized as earthquake." The

rating of VI, recorded in Phoeniu, is based on these observations: 'Felt by all, many

frightened and run outdoors;fallingplaster, moving furniture; damageslight" Tucson was

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