GROUND RUPTURE, COALlNGA EARTHQUAKE OF 10 JUNE 1983

from California Geology, August 1983, Vol. 36, No. 8.

GROUND RUPTURE, COALlNGA EARTHQUAKE OF 10 JUNE 1983

FRESNO COUNTY, CALIFORNIA

RICHARD D. MCJUNKlN

and

EARL W. HART

Geologists

California Division of Mines and Geology

OVERVIEW

A moderate earthquake occurred on 10 June 1983 at 20:10 (PDT) approximately 15 km northwest of Coalinga, California (Figure 1). Preliminary data indicate that the earthquake was a ML= 5.0 event (located near Longitude 36.26° N - Latitude 120.46°W) at a depth of approximately 2 km, according to the UC Berkeley Seismographic Station. Fault rupture associated with this event was located on Il 11 June by E.J. Fowkes, Westhills Community College (personal communication, 13 June 1983). The rupture was subsequently mapped by the authors between June 13 and 15 (Photos 1l and 2). This is the first located fault rupture from three ML 5.0 earthquakes that have occurred in this region beginning with a ML 5 event northwest of Coalinga on 25 October 1982 (Figure 1).The largest of these earthquakes (ML=6.5) was on 2 May 1983 and it destroyed or damaged many of the buildings in Coalinga and throughout the surrounding region. In addition, ground failure cracks and landslides were common within 30 km of the epicenter.

Figure 1. Regional location map of Coalinga earthquakes.

Photo 1. View looking north: steeply-east dipping fault rupture crossing farm road. The scarp height, measured bottom to top of scarp, is about 18 cm.

Fault rupture from the 10 June earthquake preduced a 15 cm-high west-facing scarp in Los Gatos Road (E.J. Fowkes, Westhills Community College, personal communication, 13 June 1983). A conglomerate bed is truncated in a roadcut along the projection of this fault (Photo 3) suggesting that the fault pre-existed the 10 June earthquake. During subsequent field mapping, a fresh zone of discontinuous northerly-trending fault rupture that was traceable for approximately 2.1 km was located from Los Gatos Road to the north. The same zone may also extend 0.6 km southward toward Los Gatos Creek (personal communications from E.J. Fowkes, Westhills Community College, 13 June 1983; and R.C. Erickson, Chevron USA, 20 June 1983).

The 10 June earthquake generated two segments of northerly-trending fault rupture (Figure 2). Topographic expression, west-facing scarplets in soil, linear to sinuous mounds of soil pushed up by the fault, and a well-defined mole track indicate that faulting resulted from reverse motion on a steeply east-dipping fault. A scarp height of 48 cm was measured at two locations along the northern fault segment; most areas where the fault trace was well-defined had 10 to 20 cm of east-side-up offset.

Evidence of right lateral slip associated with the 10 June earthquake was noted at a fence which crossed left-stepping cracks along the southern boundary of section 33, T19S, R14E (Figure 2). At this location the fence Was offset about 8 to 1O cm right laterally (Photo 4).

Groound cracking and landsliding from the 10 June 1983 earthquake shaking were common and pervasive in sections 4 and 33 (Figure 2). Abundant extensional cracks were present in soil on five- to ten-degree slopes; steeper slopes were extensively fissured. In addition, linear cracks and scarps, locally open or as high as 15 cm, were present on most ridges. Soil slumps, rock falls, and translational land-slides generated by the 10 June 1983 earthquake were numerous in the region of fault rupture. The abundance of shaking failures locally observed is consistent With the reported location and shallow focus of this earthquake. Similar land-slides and soil failures were also triggered in this area by the larger 2 May 1983 earthquake.

Figure 2. Local map of ground rupture from 10 June 1983 earthquake

Photo 2. View looking west; fault rupture defined by disturbed line of high grass and weeds. Fault rupture, well-defined in this area, dips steeply east to form a side hill bench.

INTERPRETATIONS

Ground rupture from the 10 June earthquake appears to be spatially associated with pre-existing north- to northwest-trending faults mapped in the area (Dibblee, 1971; Jennings, 1977; R.C. Erickson, Chevron USA, unpublished mapping). Geologic investigations by the California Division of Mines and Geology and other agencies and organizations may reveal additional fault rupture from this earthquake.

Numerous earthquakes, including the three ML> 5 events (25 October 1982, 2 May 1983, and 10 June 1983) that have recently occurred in the Coalinga area, indicate that the region is being subjected to compression manifested by seismic activity and revesse faulting. Faulting from the 10 June event is structurally within the upper plate of the thrust fault that may have generated the 2 May Coalinga event. The 2 May event is interpret(tr1 to have originated on a shallow-angle southwest dipping fault, representing one of two focal plane analyses for this event (Murtha and O'Connell, in preparation). Photo 3. View lookjng north-northeast: a fault exposed by a roadcut where Los Gatos Road was displaced by fault rupture from the 10 June 1983 earthquake. Note that the road surface has been smoothed over and resurfaced with gravel. Fault rupture occurring in the road at this location trends into the roadcut-exposed fault.

The second focal plane analysis places the fault as having a northwest strike and dipping steeply to the east. Fault rupture associated with the 10 June earthquake is not concordant with a low angle south-west dipping fault interpreted from first motion data.

The geometry and mechanisms of seismic events occurring in this area during the past few months need to be further evaluated for refined interpretations as to the origin of compression and release of regional strain. These data will increase the understanding of the regional tectonics and mechanisms of fault rupture.

Photo 3. View looking north-northeast; a fault exposed by a roadcut where Los Gatos Road was displaced by fault rupture from the 10 June 1983 earthquake. Note that the road surface has been smoothed over and resurfaced with gravel. Fault rupture occuring in the road at this location trends into the roadcut-exposed fault.

Photo 4. View lookjng west; fence along southern boundary of section 33,T19S,R14E, is offset approxjmately 9 cm in a right lateral sense by left-stepping fault rupture trending beneath the fence.

REFERENCES

Dibblee. T.W.. Jr.. 1971. Geologic maps of the Coalinga and Joaquin Rocks l5 minute quadrangles: U.S. Geological Survey Open File Report 71-87. scale 1:62.500.

Jennings. C.W., 1977. Geologic map of Californta: Californta Division of Mines and Geology. Geologic Data Map No. 2. scale 1:750.000.

Murtha. P.E. and O°Connell. D.R.. 19t3. Coalinga aftershock parameters from UCB strong-motion instruments: Earthquake Engineering Research Institute. Reconnaissance Report. in preparation.