Special Report: The Hector Mine Earthquake, 10/16/1999
(Updated 10/21/99)
A M7.1 earthquake occurred at 2:46 a.m. local time on
10/16/1999. The event was located in a remote,
sparsely-populated part of the Mojave desert, approximately 47
miles east-southeast of Barstow and 32 miles north of Joshua
Tree. The initial magnitude estimate of 7.0 was upgraded to 7.1
on October 18, 1999, based on in-depth analysis of "teleseismic"
data recorded worldwide. Like music, earthquake waves include
both "high tones" and "low tones"--the
latter, which are recorded by sensitive seismic instruments
around the globe--are crucial in constraining the magnitude of
large events, but are not analyzed quite as quickly as data from
the immediate southern California region.
The Hector Mine earthquake is not
considered an aftershock of the M7.3 Landers earthquake of 1992,
although subsequent analysis will explore the relationship
between these two events.
On average, an earthquake of M7.0
is expected to generate approximately seven aftershocks of M5 or
larger within the first week. Three M>5 events occurred on
the first day of the sequence: a M5.3 event at 2:49 local time,
a M5.8 event at 5:57 a.m., and a M5.0 at 10:38 a.m. 25
aftershocks of M4 or larger occurred within the first 36 hours
of the sequence. The rate of aftershocks is very close to the
average expected for a mainshock of this magnitude.
The earthquake occurred on the
Lavic Lake fault, one of a series of north-northwest trending
faults through the eastern Mojave shear zone. Geologists from
the U.S. Geological Survey and Southern California Earthquake
Center were able to fly over the rupture (which is within the
Twenty Nine Palms Marine Base) on the afternoon of October 16
and documented a 40-km long surface rupture. The aerial
photos show an apparent maximum offset of 3.8 - 4.7 meters.
These preliminary estimates for both fault length and slip are
consistent with expectations for a M7.0 earthquake, although the
slip is perhaps somewhat larger than average.
The Lavic Lake fault had been previously mapped in the Bullion
Mountains by Thomas
Dibblee, Jr., of the USGS, but the fault was not named. It
is one of a series of closely spaced, northwest-trending,
right-lateral strike slip faults that traverse this portion of
the Mojave Desert. Together, these faults within what is termed
the Mojave Shear Zone serve to relieve a small portion of the
stresses that build up along the boundary between the Pacific
and North American tectonic plates.
Previous evaluations of the Lavic
Lake fault by the California Division of Mines and Geology have
shown that the fault had not produced a large earthquake within
the last 10,000 years. As a result, the fault was not considered
"active" according to the regulatory criterion set
forth in the Alquist-Priolo Earthquake Fault Zoning Act of 1972.
This statute requires extensive follow-on fault studies and
special building setbacks before development can proceed along
"active" faults. However, movement along faults within
the Mojave Shear Zone is typically quite slow, and the
individual faults produce earthquakes only infrequently. In this
type of setting, then, the 10,000 year criterion for fault
activity, established nearly 30 years ago, may not be
particularly appropriate.
An interesting aspect of the
Hector Mine earthquake is that it occurred only 7 years after
the 1992 Landers and Joshua Tree earthquakes, which occurred on
similar faults within the Mojave Shear Zone. This apparent
clustering of earthquakes may be purely coincidental, but
scientists are conducting research to see if this kind of
earthquake behavior is typical of this region. That is, perhaps
many of the Mojave Shear Zone faults produce earthquakes within
a short time interval (perhaps spanning several hundred or more
years), followed by several thousands of years of quiescence.
The Hector Mine earthquake will provide important data that will
enable scientists to answer this and other questions about
faulting and earthquake behavior.
Geologists are working to map the
surface rupture in detail. Scientists with Caltech and the US
Geological Survey are continuing to analyze the abundant seismic
data recorded from the Hector Mine Earthquake and its
aftershocks. In addition to the classic aftershock sequence, the
earthquake has apparently triggered
small earthquakes as far south as the California-Mexico
border. Scientists are monitoring these events carefully, as
some of them have occurred close to the southern end of the San
Andreas fault.
The Southern California Seismic
Network is in the process of a major upgrade involving
installation of state-of-the-art instrumentation to provide far
more detailed information about shaking than has been available
in the past. Real-time results from the new Trinet
network were able to produce ShakeMaps
showing the distribution of shaking over Southern California. In
addition, over 23,000 people have electronically filled out a
questionnaire for the Did
You Feel It? Community Internet Intensity Map (CIIM).
The SCIGN Project is
also in the process of installing a state-of-the-art integrated
GPS monitoring system in southern California. The SCIGN
results for the Hector Mine earthquake show the co-seismic
displacement associated with the earthquake.
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California Earthquake History 1769-Present
Includes California, Baja California, and Nevada events
The magnitude listed here
is the "summary magnitude". For most events prior to
1898 this is the adjusted intensity magnitude, and for events
after 1898 it is the surface wave magnitude. The list includes
known earthquakes with a magnitude of at least 6 and selected
smaller events. The smaller events since 1898 all have at least
one reported magnitude of at least 5.8, even if the summary
magnitude is smaller. Some of these magnitudes may be different
than what is reported in the Southern California or Northern
California Earthquake Catalog. In the future, we will adjust
some of these magnitudes to reflect the best measure of the
earthquake and label the type of magnitude being reported.
The date and time for each
earthquake are given as "24 hour" time referenced to
Greenwich Mean Time (now UTC). To convert a time to Pacific
Standard Time (PST), subtract 8 hours. To convert a time to
Pacific Daylight Time (PDT), where appropriate, subtract 7
hours. Example: The 1989 Loma Prieta earthquake occurred
at 00:04 UTC on October 18, 1989 or 5:04pm PDT on October 17,
1989.
DATE TIME(GMT) LATITUDE LONGITUDE MAG LOCATION
year mth dy hr min (N) (W)
============================================================
1769 7 28 0 0 34 0.00 118 0.00 6.0 Los Angeles Basin
1800 11 22 2130 33 0. 117 18.00 6.5 San Diego region
1808 6 24 0 0 37 48.00 122 30.00 6.0 San Francisco region
1812 12 8 15 0 34 22.00 117 39.00 7.0 Wrightwood
1812 12 21 19 0 34 12.00 119 54.00 7.0 Santa Barbara Channel
1827 9 24 4 0 34 0. 119 0. 5.5 Los Angeles region
1836 6 10 1530 37 48.00 122 12.00 6.75 Hayward Valley
1838 6 0 0 0 37 36.00 122 24.00 7.0 San Francisco Peninsula
1852 11 29 20 0 32 30.00 115 0. 6.5 Volcano Lake, B.C.
1855 7 11 415 34 6.00 118 6.00 6.0 Los Angeles region
1856 2 15 1325 37 30.00 122 18.00 5.5 San Francisco Peninsula
1857 1 9 16 0 35 42.00 120 18.00 8.25 Great Fort Tejon earthquake
1857 9 3 3 5 39 18.00 120 0. 6.25 W. Nevada or E. Sierra Nevada
1858 11 26 835 37 30.00 121 54.00 6.25 San Jose region
1858 12 16 10 0 34 0. 117 30.00 6.0 San Bernardino region
1860 3 15 19 0 39 30.00 119 30.00 6.5 Carson City, Nevada region
1861 7 4 011 37 48.00 122 0. 5.75 San Ramon Valley
1862 5 27 20 0 32 42.00 117 12.00 6.0 San Diego region
1864 2 26 1347 37 6.00 121 42.00 6.0 S. Santa Cruz Mountains
1864 3 5 1649 37 42.00 122 0. 5.75 E. of San Francisco Bay
1865 10 8 2046 37 0.00 122 00.00 6.5 S. Santa Cruz Mountains
1866 7 15 0630 37 30.00 121 18.00 6.0 W. San Joaquin Valley
1868 5 30 510 39 18.00 119 42.00 6.0 Virginia City, Nevada
1868 10 21 1553 37 42.00 122 6.00 7.0 Hayward fault
1869 12 27 155 39 24.00 119 42.00 6.25 Olinghouse fault, Nevada
1869 12 27 10 0 39 6.00 119 48.00 6.0 Carson City, Nevada region
1870 2 17 2012 37 12.00 122 6.00 6.0 Los Gatos
1871 3 2 21 5 40 24.00 124 12.00 6.0 Cape Mendocino
1872 3 26 1030 36 42.00 118 6.00 7.6 Owens Valley
1872 3 26 14 6 36 54.00 118 12.00 6.75 Owens Valley
1872 4 3 1215 37 0. 118 12.00 6.25 Owens Valley
1872 4 11 19 0 37 30.00 118 30.00 6.75 Owens Valley
1872 5 3 1 0 33 0. 115 0. 5.75 Imperial Valley (?)
1872 11 12 0 0 39 0. 117 0. 6.0 Austin, Nevada region (?)
1873 11 23 5 0 42 0. 124 0. 6.75 Crescent City
1875 1 24 1200 40 42. 120 30. 6.0 Honey Lake
1875 11 15 2230 32 30.00 115 30.00 6.25 Imperial Vly to Colorado R. delta
1878 5 9 425 40 6.00 124 0. 6.0 Punta Gorda region
1881 2 2 011 36 0. 120 30.00 5.75 Parkfield
1881 4 10 10 0 37 24.00 121 24.00 6.0 W. San Joaquin Valley
1882 3 6 2145 36 54. 121 12. 5.75 Hollister
1883 9 5 1230 34 12.00 119 54.00 6.25 Santa Barbara Channel
1884 1 28 730 41 6. 123 36. 5.75 Klamath Mountains
1884 3 26 40 37 6. 122 12. 6.0 Santa Cruz Mountains
1885 1 31 545 40 24. 120 36. 5.75 Susanville
1885 4 12 4 5 36 24.00 121 0. 6.25 S. Diablo Range
1887 6 3 1048 39 12.00 119 48.00 6.5 Carson City, Neveda region
1888 4 29 448 39 42.00 120 42.00 6.0 Mohawk Valley
1889 5 19 1110 38 0. 121 54.00 6.25 Antioch
1889 6 20 6 0 40 30.00 120 42.00 6.0 Susanville
1889 9 30 520 37 12. 118 42. 5.75 Bishop region
1890 2 9 12 6 33 24.00 116 18.00 6.5 San Jacinto (?)
1890 4 24 1136 36 54.00 121 36.00 6.25 Pajaro Gap
1890 7 26 940 40 30.00 124 12.00 6.25 Cape Mendocino
1891 7 30 1410 32 0. 115 0. 6.0 Colorado R. delta region
1892 2 24 720 32 33.00 115 38.00 7.0 Laguna Salada, B.C.
1892 4 19 1050 38 24.00 122 0. 6.5 Vacaville
1892 4 21 1743 38 30.00 121 54.00 6.25 Winters
1892 5 28 1115 33 12.00 116 12.00 6.5 San Jacinto or Elsinore fault region (?)
1892 11 13 1245 36 48.00 121 30.00 5.75 Hollister
1893 5 19 035 34 6.00 119 24.00 5.75 Pico Canyon
1894 7 30 512 34 18.00 117 36.00 6.0 Lytle Creek region
1894 9 30 1736 40 18. 123 42. 6.0 Cape Mendocino region
1894 10 23 23 3 32 48.00 116 48.00 5.75 E. of San Diego
1896 8 17 1130 36 42.00 118 18.00 6.0 SE Sierra Nevada
1897 6 20 2014 37 0. 121 30.00 6.25 Gilroy
1898 3 31 743 38 12.00 122 24.00 6.5 Mare Island
1898 4 15 7 7 39 12.00 123 48.00 6.5 Mendocino
1899 4 16 1340 41 0. 126 0. 7.0 W. of Eureka
1899 7 6 2010 37 12. 121 30. 5.75 Morgan Hill
1899 7 22 2032 34 18.00 117 30.00 5.75 Lytle Creek region
1899 12 25 1225 33 48.00 117 0. 6.4 San Jacinto and Hemet
1901 3 3 745 36 0. 120 30.00 6.4 Parkfield
1903 1 24 527 31 30.00 115 0.00 6.6 Colorado R. delta region
1903 6 11 1312 37 24.00 121 54.00 5.5 San Jose
1903 8 3 649 37 18.00 121 48.00 5.5 San Jose
1906 4 18 1312 37 42.00 122 30.00 8.25 Great 1906 earthquake
1906 4 19 030 32 54.00 115 30.00 6.2 Imperial Valley
1906 4 23 910 41 0. 124 0. 6.4 Arcata
1907 9 20 154 34 12.00 117 6.00 5.3 San Bernardino region
1908 11 4 837 36 0. 117 0. 6.0 Death Valley region
1909 10 29 645 40 30.00 124 12.00 5.8 Cape Mendocino
1910 3 11 652 36 54.00 121 48.00 5.8 Watsonville
1910 3 19 011 40 0. 125 0. 6.0 W. of Cape Mendocino
1910 5 15 1547 33 42.00 117 24.00 5.5 Glen Ivy Hot Springs
1910 8 5 131 42 0. 127 0. 6.6 W. of Crescent City
1911 7 1 22 0 37 15.00 121 45.00 6.5 Calaveras fault
1914 2 18 1817 39 30.00 119 48.00 5.5 Truckee region
1914 4 24 834 39 30.00 119 48.00 6.0 Truckee region
1915 5 6 12 9 40 0.00 126 0. 6.2 W. of Cape Mendocino
1915 6 23 359 32 48.00 115 30.00 6.0 Imperial Valley
1915 6 23 456 32 48.00 115 30.00 5.9 Imperial Valley
1915 10 3 652 40 30.00 117 30.00 7.3 Pleasant Valley, Nevada
1915 11 21 013 32 0. 115 0. 7.1 Volcano Lake, B.C.
1915 12 31 1220 41 0. 126 0. 6.5 W. of Eureka
1916 2 3 5 3 41 0. 117 48.00 5.9 N. of Pleasant Valley, Nevada
1916 10 23 244 34 54.00 118 54.00 5.3 Tejon Pass region
1916 11 10 911 35 30.00 116 0. 6.1 S. of Death Valley
1918 4 21 2232 33 48.00 117 0. 6.9 San Jacinto
1918 7 15 023 41 0. 125 0. 6.5 W. of Eureka
1922 1 26 931 41 0. 126 0. 6.0 W. of Eureka
1922 1 31 1317 41 0. 125 30.00 7.3 W. of Eureka
1922 3 10 1121 36 0. 120 30.00 6.3 Parkfield
1923 1 22 9 4 40 30.00 124 30.00 7.2 Cape Mendocino
1923 7 23 730 34 0. 117 18.00 6.0 San Bernardino region
1925 6 4 12 2 41 30.00 125 0. 6.0 W. of Eureka
1925 6 29 1442 34 18.00 119 48.00 6.3 Santa Barbara
1926 10 22 1235 36 37.00 122 21.00 6.1 Monterey Bay
1926 10 22 1335 36 33.00 122 11.00 6.1 Monterey Bay
1926 12 10 838 40 45.00 126 0. 6.0 W. of Cape Mendocino
1927 9 18 2 7 37 30.00 118 45.00 6.0 Bishop region
1927 11 4 1350 34 42.00 120 48.00 7.3 SW of Lompoc
1932 6 6 844 40 45.00 124 30.00 6.4 Eureka
1932 12 21 610 38 45.00 118 0. 7.2 Cedar Mountain, Nevada
1933 1 5 651 38 46.00 117 44.00 5.9 Cedar Mountain, Nevada
1933 3 11 154 33 37.00 117 58.00 6.3 Long Beach
1933 6 25 2045 39 4.00 119 20.00 6.1 Yerington, Nevada
1934 1 30 2016 38 18.00 118 24.00 6.3 Excelsior Mountain, Nevada
1934 6 8 447 36 0. 120 30.00 6.0 Parkfield
1934 7 6 2248 41 15.00 125 45.00 6.5 W. of Eureka
1934 12 30 1352 32 15.00 115 30.00 6.5 Laguna Salada, B.C.
1934 12 31 1845 32 0. 114 45.00 7.0 Colorado R. delta
1935 2 24 145 31 59.00 115 12.00 5.3 Colorado R. delta
1936 6 3 915 40 0. 125 30.00 5.9 W. of Cape Mendocino
1937 3 25 1649 33 24.00 116 16.00 6.0 Buck Ridge
1940 2 8 8 5 39 45.00 121 15.00 6.0 Chico
1940 5 19 436 32 44.00 115 30.00 7.1 Imperial Valley
1940 12 7 2216 31 40.00 115 5.00 5.5 Colorado R. delta
1941 2 9 944 40 42.00 125 24.00 6.6 W. of Cape Mendocino
1941 4 9 1708 31 0.00 114 0.00 5.3 Gulf of California
1941 5 13 16 1 40 18.00 126 24.00 6.0 W. of Cape Mendocino
1941 7 1 750 34 22.00 119 35.00 5.9 Carpenteria
1941 9 14 1643 37 34.00 118 44.00 5.8 Tom's Place
1941 9 14 1839 37 34.00 118 44.00 6.0 Tom's Place
1941 10 3 1613 40 24.00 124 48.00 6.4 W. of Cape Mendocino
1942 10 21 1622 33 3.00 116 5. 6.5 Fish Creek Mountains
1942 12 3 944 39 42.00 119 18.00 5.9 N. of Wadsworth, Nevada
1945 5 19 15 7 40 24.00 126 54.00 6.2 W. of Cape Mendocino
1945 9 28 2224 41 54.00 126 42.00 6.0 W. of Crescent City
1946 3 15 1349 35 44.00 118 3.00 6.3 Walker Pass
1947 4 10 1558 34 59.00 116 33.00 6.4 Manix
1948 12 4 2343 33 56.00 116 23.00 6.5 Desert Hot Springs
1948 12 29 1253 39 33.00 120 5.00 6.0 Verdi, Nevada
1949 3 25 456 41 18.00 126 0. 6.2 W. of Eureka
1949 5 2 1125 34 1. 115 41.00 5.9 Pinto Mountain
1951 10 8 410 40 15.00 124 30.00 6.0 W. Of Cape Mendocino
1951 12 26 046 32 48.00 118 18.00 5.9 San Clemente Island
1952 7 21 1152 35 0. 119 1.00 7.7 Kern County earthquake
1952 7 21 12 5 35 0. 119 0. 6.4 Kern County
1952 7 23 038 35 22.00 118 35.00 6.1 Kern County
1952 7 29 7 3 35 23.00 118 51.00 6.1 Bakersfield
1952 11 22 746 35 44.00 121 12.00 6.0 Bryson
1954 1 12 2333 35 0. 119 1.00 5.9 W. of Wheeler Ridge
1954 3 19 954 33 17.00 116 11.00 6.2 Arroyo Salada
1954 7 6 1113 39 25.00 118 32.00 6.6 Rainbow Mountain, Nevada
1954 7 6 22 7 39 18.00 118 30.00 6.4 Rainbow Mountain, Nevada
1954 8 24 551 39 35.00 118 27.00 6.8 Stillwater, Nevada
1954 8 31 2220 39 30.00 118 30.00 6.3 Stillwater, Nevada
1954 10 24 944 31 30.00 116 0. 6.0 W. of Santo Tomas, B.C.
1954 11 12 1226 31 30.00 116 0. 6.3 W. of Santo Tomas, B.C.
1954 11 25 1116 40 16.00 125 38.00 6.5 W. of Cape Mendocino
1954 12 16 11 7 39 19.00 118 12.00 7.1 Fairview Peak, Nevada
1954 12 16 1111 39 30.00 118 0. 6.8 Dixie Valley, Nevada
1954 12 21 1956 40 56.00 123 47.00 6.6 E. of Arcata
1956 2 9 1432 31 45.00 115 55.00 6.8 San Miguel, B.C.
1956 2 9 1524 31 45.00 115 55.00 6.1 San Miguel, B.C.
1956 2 14 1833 31 30.00 115 30.00 6.3 San Miguel, B.C.
1956 2 15 120 31 30.00 115 30.00 6.4 San Miguel, B.C.
1956 10 11 1648 40 40.00 125 46.00 6.0 W. of Cape Mendocino
1956 12 13 1315 31 0. 115 0. 6.0 W. shore, Gulf of California
1959 3 23 710 39 36.00 118 1.00 6.3 Dixie Valley, Nevada
1959 6 23 1435 39 5.00 118 49.00 6.1 Schurz, Nevada
1960 8 9 739 40 19.00 127 4.00 6.2 W. of Cape Mendocino
1966 6 28 426 36 0. 120 30.00 6.0 Parkfield
1966 8 7 1736 31 48.00 114 30.00 6.3 Gulf of California
1966 9 12 1641 39 25.00 120 9.00 6.0 Truckee
1968 4 9 228 33 11.00 116 8.00 6.5 Borrego Mountain
1968 6 26 142 40 14.00 124 16.00 5.4 Punta Gorda
1971 2 9 14 0 34 25.00 118 24.00 6.5 San Fernando
1973 2 21 1445 34 4.00 119 2.00 5.2 Point Mugu
1976 11 26 1119 41 18.00 125 42.00 6.3 W. of Orick
1979 8 6 17 5 37 7.00 121 31.00 5.7 Coyote Lake
1979 10 15 2316 32 36.00 115 18.00 6.5 Imperial Valley
1980 01 24 1900 37 50.00 121 47.00 5.8 Livermore
1980 5 25 1633 37 36.00 118 50.00 6.1 Mammoth Lakes
1980 5 25 1649 37 39.00 118 54.00 5.9 Mammoth Lakes
1980 5 25 1944 37 33.00 118 49.00 5.8 Mammoth Lakes
1980 5 27 1450 37 29.00 118 48.00 6.0 Mammoth Lakes
1980 6 9 328 32 12.00 115 5.00 6.4 Victoria, B.C.
1980 11 8 1027 41 7.00 124 40.00 7.2 W. of Eureka
1981 4 26 1209 33 8.00 115 39.00 6.0 Westmorland
1981 9 4 1550 33 40.00 119 7.00 5.9 N. of Santa Barbara Island
1981 9 30 1153 37 35.00 118 52.00 5.8 Mammoth Lakes
1983 5 2 2342 36 14.00 120 19.00 6.5 Coalinga
1983 7 22 239 36 14.00 120 25.00 5.7 Coalinga
1984 4 24 2115 37 19.00 121 39.00 6.1 Morgan Hill
1984 9 10 314 40 23.00 127 9.00 6.7 Mendocino Fracture Zone
1984 11 23 18 8 37 27.00 118 36.00 5.7 Round Valley
1985 8 4 12 1 36 8.00 120 10.00 5.9 North Kettleman Hills
1986 7 8 920 34 0. 116 36.00 6.0 North Palm Springs
1986 7 20 1429 37 34.00 118 26.00 5.6 Chalfant Valley
1986 7 21 1442 37 32.00 118 26.00 6.2 Chalfant Valley
1986 7 31 722 37 28.00 118 22.00 5.2 Chalfant Valley
1987 10 1 1442 34 3.00 118 5.00 5.8 Whittier Narrows
1987 11 24 153 33 4.00 115 47.00 6.2 Elmore Ranch fault
1987 11 24 1316 33 1.00 115 51.00 6.6 Superstition Hills
1989 10 18 0004 37 2.19 121 52.98 7.1 Loma Prieta
1991 8 16 2226 41 38.00 125 52.00 6.3 W. of Crescent City
1991 8 17 1929 40 17.00 124 14.00 6.2 Punta Gorda
1991 8 17 2217 41 41.00 126 3.00 7.1 W. of Crescent City
1992 4 23 450 33 58.00 116 19.00 6.1 Joshua Tree
1992 4 25 1806 40 20.00 124 14.00 7.2 Cape Mendocino
1992 4 26 741 40 26.00 124 36.00 6.5 Cape Mendocino
1992 4 26 1118 40 23.00 124 35.00 6.6 Cape Mendocino
1992 6 28 1157 34 12.00 116 26.00 7.3 Landers
1992 6 28 1505 34 12.00 116 50.00 6.2 Big Bear
1993 5 17 2320 37 9.00 117 50.00 6.1 Big Pine
1994 1 17 1230 34 13.00 118 32.00 6.7 Northridge
1994 9 01 1515 40 27.00 125 54.00 6.9 Mendocino Fracture Zone
1994 9 12 1223 38 49.00 119 37.00 6.0 Carter's Station, Nevada
1995 2 19 403 40 37.00 125 54.00 6.6 W. of Eureka
1995 9 20 2327 35 46.00 117 38.00 5.5 Ridgecrest
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Southern California Earthquakes
Below is a map of southern
California, with epicenters of historic earthquakes (as far back
as 1812) of particular note plotted over the background
topography. Shown, too, are major highways (in tan) and the
surface traces of major faults (in greenish-blue).
This map does not show the
epicenters of all earthquakes greater than magnitude 4.5
recorded in the southern California area since the 19th century.
It is meant as an overview of large and destructive, fairly
recent, or unusual earthquakes. The magnitudes given by the
scale are generally moment magnitudes (denoted Mw),
for earthquakes above magnitude 6, and local magnitudes (denoted
ML), for most earthquakes below magnitude 6 and for
earthquakes which occurred before accurate instrumental
measurements of magnitude were possible (i.e. before
1933).
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