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Revisiting the 23 February 1892 Laguna Salada Earthquake by Susan E

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Revisiting the 23 February 1892 Laguna Salada Earthquake by Susan E View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Caltech Authors - Main Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1571–1578, August 2004 Revisiting the 23 February 1892 Laguna Salada Earthquake by Susan E. Hough and Austin Elliot Abstract According to some compilations, the Laguna Salada, Baja California, earthquake of 23 February 1892 ranks among the largest earthquakes in California and Baja California in historic times. Although surface rupture was not documented at the time of the earthquake, recent geologic investigations have identified and mapped a rupture on the Laguna Salada fault that can be associated with high prob- ability with the 1892 event (Mueller and Rockwell, 1995). The only intensity-based magnitude estimate for the earthquake, M 7.8, was made by Strand (1980) based on an interpretation of macroseismic effects and a comparison of isoseismal areas with those from instrumentally recorded earthquakes. In this study we reinterpret original accounts of the Laguna Salada earthquake. We assign modified Mercalli intensity (MMI) values in keeping with current practice, focusing on objective descriptions of damage rather than subjective human response and not assigning MMI values to effects that are now known to be poor indicators of shaking level, such as liquefaction and rockfalls. The reinterpreted isoseismal contours and the estimated magnitude are both significantly smaller than those obtained earlier. Using the method of Bakun and Wentworth (1997) we obtain a magnitude estimate of M 7.2 and an optimal epicenter less than 15 km from the center of the mapped Laguna Salada rupture. The isoseismal contours are elongated toward the northwest, which is qualitatively con- sistent with a directivity effect, assuming that the fault ruptured from southeast to northwest. We suggest that the elongation may also thus reflect wave propagation effects, with more efficient propagation of crustal surface (Lg) waves in the direction of the overall regional tectonic fabric. Introduction Southern California and northern Baja California were The Mw estimate of 7.1 was thus considered a lower bound. relatively sparsely populated in the 1890s. Nonetheless, the Based on scarp degradation relationships, they further esti- earthquake of 23 February 1892 was strongly felt and doc- mated that the earthquake occurred within the past 100–200 umented in many locations throughout southern California. yr. They associated the rupture with the known historic event While surface rupture had been mapped following an 1887 on 23 February 1892, which had been previously estimated earthquake in northern Sonora, Mexico (DuBois and Smith, to be located farther west (Toppozada et al., 1981). 1980), the remote epicentral region of the 1892 event was In an exhaustive archival search, Strand (1980) com- apparently never investigated at the time of the earthquake. piled nearly 100 original accounts of the earthquake, pri- Based on the distribution of shaking effects, Strand (1980) marily from newspaper accounts but also from other sources concluded that the earthquake most likely originated on the such as letters. Strand (1980) also compiled accounts from Laguna Salada fault, along which very recent scarps had one foreshock and several of the larger aftershocks. In this been documented (Barnard, 1968). Mueller and Rockwell study we focus on only the mainshock. In his study, Strand (1995) mapped a 22-km segment of Holocene oblique- (1980) interpreted these accounts to obtain modified Mer- dextral rupture along the Laguna Salada fault in northern calli intensity (MMI) values (Table 1). In his interpretations Baja California (Fig. 1); they also documented a small com- he followed the scheme of Brazee (1979), who compiled lists ponent of normal rupture on the nearby northeast-striking of detailed indicators for each MMI level. In his interpreta- Canon Rojo fault. Based on the observed rupture length and tion, Strand (1980) generally assigned MMI values based on scarps with 3–4 m of vertical displacement, Mueller and the indicator(s) that corresponded to the highest intensity Rockwell (1995) estimated a moment magnitude of 7.1. level. For example, an MMI value of VI was assigned to Los They noted that slip might have extended an additional 10 Angeles (34:03Њ N, 118:15Њ W) based on accounts that de- km farther south; the rupture might also have extended far- scribed only modest objective effects, such as the swaying ther north, into an area that is covered by young sand dunes. of chandeliers, but also alarm and nausea on the part of some 1571 1572 Short Notes Salton Sea 33˚ California Baja California Laguna Salada Cerra Prieto Faul Laguna fault Salada 32˚ t miles 0 50 31˚ -118˚ -117˚ -116˚ -115˚ Figure 1. Map of northern Baja California and southern California. Location of Holocene surface rupture identified by Mueller and Rockwell (1995) is indicated by dashed box. Mapped faults, in this and other maps, are from Jennings (1994). observers. Strand (1980) also assigned a number of high for which MMI cannot have been above perhaps V (Musson, based on accounts that described rock- 1998). The distribution of rockfalls will moreover largely (םMMI values (VIII falls, liquefaction, changes in water level, and so on. Many correspond to the distribution of rocks, some of which will of these accounts were from remote desert regions in and fall in response to low levels of shaking (as, in fact, some around the Salton Trough, where no (or very few) buildings rockfalls occur in the absence of shaking). Unfortunately, existed at the time. For example, at Storm Canyon (32:55Њ because northern Baja California was so sparsely populated N, 116:26Њ W) an account described rockfalls and the drying in 1892, many of the relatively close-in accounts do not in- was assigned to this ac- clude any information about damage to structures. We were םup of a small spring; MMI VII count. thus able to assign intensity values for only 74 of the 98 Strand’s (1980) interpretations differ from the current locations at which intensities were assigned by Strand practice of intensity determinations in two critical respects. (1980); we also assign not-felt values for three locations at First, especially in modern analyses, such as those imple- which it was reported that the earthquake was not felt. These mented by the U.S. Geological Survey Community Internet 77 values provide reasonably good coverage of the intensity Intensity Mapping project (Wald et al., 1999), MMI values field throughout southern California (Fig. 2). Figure 2 also are assigned based on the overall effects at a location rather indicates locations at which landslides and rockfalls were than the most severe reported effects. Hough et al. (2000) observed; we will discuss these results in a later section. argued that this approach is especially important to avoid The isoseismal contours shown in Figure 2 were cal- overemphasis of subjective human response (people fright- culated by the gridding algorithm used in the surface utility ened, etc.) during large regional earthquakes, for which of the Generic Mapping Tools (Wessel and Smith, 1991). shaking can be dramatic even when damage is very low. This algorithm uses a tension factor, T, to control the degree can ,0 ס A second difference between our approach and that of of curvature. The minimum curvature solution, T will generate 1 ס Strand (1980) reflects the growing awareness that certain generate unrealistic oscillations, while T indicators do not reflect reliably the level of shaking at a site. a solution with no maxima or minima away from control which allows the algorithm to ,0.5 ס Such indicators include rockfalls, liquefaction, and water- points. Here we use T level changes in wells (e.g., Ambraseys and Bilham, 2003). find a maximum in the near-field region. As shown in Figure Formerly, for example, liquefaction was sufficient to assign 2, the global maximum is found to be at the southern end of MMI values of at least VIII, whereas recent studies have the Laguna Salada rupture as mapped by Mueller and Rock- documented liquefaction in earthquakes as small as M 3.5, well (1995). Short Notes 1573 Table 1 Accounts and Intensities of the Laguna Salada Earthquake Location Latitude (N) Longitude (W) Map Table Rev. MMI Most severe reported effects Many frightened, rumblings like carriage 5 7 7 116.767מ Alpine 32.833 Hanging objects swung, felt by almost all 4 6 6 117.917מ Anaheim 33.833 Not felt 1 119.004מ Bakersfield 35.357 Clocks stopped, duration estimated 4 6 7 117.233מ Ballast Point Light Station 33.683 Described as strong 4 5 4 116.867מ Banning 33.933 All frightened, loud sounds 5 7 7 116.700מ Barret Valley 32.617 Hanging pictures displaced, liquids sloshed 4 5 5 116.983מ Beaumont 33.933 Described as strong 5 6 7 117.050מ Bernardo 33.033 Small objects moved, animals frightened 5 5 5 118.200מ Boyle Heights 34.033 Rockfalls, difficult to stand 6 7 7 116.750מ Bratton Valley 32.683 Changes in wells/springs, loud sound 6 7 8 116.483מ Buckman Springs 32.767 All frightened ם7 8 116.467מ Cameron Corners 32.633 Some damage to poorly built masonry 7 8 8 116.483מ Campo 32.600 Damage to poorly built masonry 8 ם8 9 116.050מ Carrizo Station 32.883 Not felt 1 מ1 119.767מ Carson City, Nevada 39.167 Alarm/panic, groundwater ejected 7.5 ם9 9 115.267מ Cerro Prieto Mud Volcano Region, Baja 32.400 Felt by all, some furniture jostled 5 6 6 117.700מ Chino 34.017 Roaring sounds, rumblings like carriage 4 ם6 7 116.783מ Chocolate Canyon 32.867 Many
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