REVIEWS OF GEOPHYSICS, SUPPLEMENT, PAGES 721-733, APRIL 1991 U.S. NATIONAL REPORT TO INTERNATIONAL UNION OF GEODESY AND GEOPHYSICS 1987-1990 Seismotectonics EGilL HAUKSSON Seismological Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California INTRODUCTION CALIFORNIA'S TRANSFORM BouNDARY Seismotectonics is the synthesis of earthquake, San Andreas Fault System geophysical, geodetic and geological data to deduce the Studies of the San Andreas fault itself have included tectonic framework of a region [Scholz, 1990]. This analysis of individual earthquake sequences and synthesis of approach has been applied successfully to active tectonic data from significant stretches of the fault. The debate over regions such as plate boundaries, regions of intraplate the strength of the fault continues although recent analysis seismicity, and active volcanoes throughout the world. of borehole breakout data and earthquake focal mechanisms Because more than 85% of the seismic moment release suggest that the fault in central California is weak and dip­ takes place in subduction zones [Scholz, 1990], these have slip motion is partitioned onto adjacent subsidiary faults abundant large and great earthquakes and are by far the most [Mount and Suppe, 1987; Zoback eta/., 1987]. popular study regions. The second most popular study Improved dates of great earthquakes along the Mojave region is the state of California, because the transform segment of the San Andreas fault indicate that the past 10 boundary, the San Andreas fault, is on land and easily earthquakes occurred in four clusters, each with two or three accessed [Wallace, 1990]. Studies of earthquakes along the events [Sieh et al., 1989]. Trenching and measurements of San Andreas fault system include the 1987 (Ms=6.6) the deformation of a drainage canal along the southern San Superstition Hills and the 1989 (Ms=7.1) Lorna Prieta Andreas fault in Coachella Valley revealed evidence for earthquake. During the last four years seismotectonic both low creep rates of 2-4 mm/yr and large prehistoric studies have also lead the way in studying fold and thrust earthquakes [Sieh and Williams, 1990]. A re-evaluation of belts in California. Two significant concealed thrust historic aseismic, post seismic and seismic slip along the faulting earthquakes, the 1983 (Ms=6.7) Coalinga and the Parkfield segment of the San Andreas fault showed that a 1987 (ML=5.9) Whittier Narrows earthquake have provided slip deficit of 3±0.2 m exists south of Gold Hill and only ample data for these studies. The 1988 (Ms=6.8) 0.3±0.3 m deficit exists to the north of Parkfield [Segall Armenian earthquake is another example of a damaging and Harris, 1987; Lienkaemper and Prescott, 1989]. partially concealed event, occurring along the southern edge Further north of Parkfield along the creeping segment of of the Lesser Caucasus where the north-south convergence the fault Perkins et al. [1989] found a slip rate of 22±6 occurs at 30 mm/yr [Pacheco eta/., 1989]. Seismotectonic mm/yr. They attributed the northwestward decrease in studies of earthquakes in the central and eastern U.S. are of long-term slip rate to transfer of slip to the Paicines and significant interest to U.S. researchers because this region Calaveras faults. In northern California the evolution of has a probability of approximately two-thirds that of the San Andreas fault system could be controlled by California to have an earthquake with similar damage in the thermal-mechanical processes associated with the next 30 years [Nishenko and Bollinger, 1990]. Volcanic development and evolution of a narrow "slabless window" seismicity is also of interest because volcanic eruptions in moving along the western edge of North America [Furlong Hawaii and Alaska continue to be associated with et al., 1989]. significant earthquake sequences. Volcanic seismicity also Bilham and King [1989] explained the morphologies of provides an important means to mitigate volcanic hazards the Parkfield region and the Coachella segment with simple and insights into the physics of near-surface magmatic distributions of slip on mapped surface faults. Using high processes. quality borehole microearthquake data to image the In this report that covers the U.S. contributions to Parkfield asperity, Malin et a/. [1989] showed that the seismotectonics during 1987-1990 I have followed the asperity or the strong patch along the fault, is devoid in organizational approach of Hill [1987]. I discuss only microearthquakes and stress drop of earthquakes is higher some of the interesting advances, while many more studies immediately adjacent to this zone. are also documented by the papers listed in the Oppenheimer eta/. [1988] used 946 fault plane solutions bibliography. of aftershocks of the 1984 Morgan Hill earthquake to infer a high-angle uniform stress field along the Calaveras fault. They combined this stress field with changes in the static stress field caused by the mainshock and were able to Copyright 1991 by the American Geophysical Union. explain the focal mechanisms and spatial distribution of aftershocks. Paper number 91RG00734. Jones [1988] used 138 focal mechanisms of small 8755-1209/91/91RG00734 $15.00 earthquakes to invert for the state of stress along the San 721 722 HAUKSSON: SEISMOTECTONICS Andreas fault from Parkfield to the Salton Sea. She found applied the technique of balanced cross section to explain remarkable agreement between the state of stress derived how the 1983 Coalinga earthquake contributed to the from the focal mechanisms and the late Quaternary growth of the Coalinga anticline. Eberhart-Phillips [1990 geologic deformation along the fault. In a more regional and 1989] determined the three-dimensional velocity study Sanders [1990] interpreted the depth distribution of structure of the Coalinga region and used it to constrain the small earthquakes in southern California and showed that pattern of active faulting and deformation in the Coalinga the depth of faulting is greater within major fault zones area. She demonstrated that the faulting consisted of than within the adjacent, more stable crustal blocks. southwest dipping thrusts uplifting blocks of higher­ The 1986 North Palm Springs earthquake in southern velocity material. The Coalinga mainshock caused the California occurred on the San Andreas fault and both maximum principal stress to rotate 15° to the north aftershock locations as well as local and teleseismic focal [Michael, 1987]. mechanisms showed that the San Andreas dips 50° to the The Eaton and Rymer [1990] model for thrust faulting northeast in Banning Pass [Jones eta/., 1986; Pacheco and earthquakes in the southern Coastal Ranges attributes the Nabelek, 19 8 8]. A finite fault inversion for slip thrust events to a component of apparent convergent distribution along the fault plane of the 1986 North Palm displacement across the San Andreas fault. The model also Springs earthquake revealed a 6x4 km2 patch of large suggests that both possible ramping up of detachment magnitude oblique reverse slip and a 3x5 km2 patch of zones into the brittle crust and presence of strong crustal large magnitude right-lateral strike-slip [Mendoza and blocks will contribute to occurrence of great earthquakes in Hartzell, 1988]. Aftershocks with high stress drops were this region. observed outside or near the edges of these patches of large In a more regional study Wong eta/. [1988] argued that magnitude slip [Mori and Frankel, 1990]. Following the the whole length of the northern and central Coast Ranges­ mainshock, Williams et a/. [1988] reported aseismic Sierran block boundary zone was seismically active. They triggered slip 44 to 86 km to the southeast along the San provided three possible explanations for the compression Andreas fault. within this zone: 1) the westward motion of the Sierran A moderate-sized thrust faulting earthquake (ML=5.3) block against the Coast Ranges; 2) slightly convergent occurred offshore in July 1986, near Oceanside in the motion along the plate boundary; 3) fault-normal Continental Borderland [Hauksson and Jones, 1988; compression across a weak San Andreas fault. Thrust Pacheco and Nabelek, 1988]. Either the thrust faulting faulting earthquakes also occur west of the San Andreas may be caused by a left step in the San Diego trough fault fault in the Coast Ranges, central California [Dehlinger cnJ or thrust faulting is more common within the Continental Bolt, 1987]. Borderland than previously thought [Legg eta/., 1989]. The 1987 (Ms=6.6) Superstition Hills earthquake Transverse Ranges sequence consisted of a foreshock sequence (Ms=6.2) on the Studies of the seismotectonics of the Transverse Ranges northeast striking Elmore Ranch fault and the mainshock­ are mostly based on analysis of seismicity, GPS (Global aftershock sequence on the northwest striking Superstition Positioning System) and other geodetic data, heat flow Hills fault [Magistrale et a/., 1989; Hanks and Allen, data, and geological studies of individual fault segments. 1989]. Modeling of both teleseismic and strong motion Using a combination of data from local earthquakes, data shows that the mainshock rupture was complex and teleseisms, and seismic refraction surveys Keller and consisted of at least two or three subevents [Hwang et a/., Prothero [1987] showed that crustal thickness increases 1990; Bent eta/., 1989; Frankel and Wennerberg, 1989; from 23 km thick in the Borderland to 31 km thickness Sipkin, 1989; Wald eta/., 1990].