Open AccessSource mechanism and surface wave excitation for two earthquakes in northern Baja California, Mexico
Author(s) -
Nava F. Alejandro,
Brune James N.
Publication year - 1983
Publication title -
geophysical journal of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0016-8009
DOI - 10.1111/j.1365-246x.1983.tb03343.x
Subject(s) - seismology , geology , aftershock , seismogram , epicenter , magnitude (astronomy) , seismic moment , geodesy , fault (geology) , physics , astronomy
Summary. Two earthquakes of comparable local magnitude from different regions of northern Baja California (the 1975 July 17 Pino Solo earthquake, M L = 5.1, from the Peninsular Ranges region, and the 1976 December 7 Mesa de Andrade earthquake, M L = 5.3, from the Colorado River Delta region) are studied in detail to determine possible causes for the observed stronger excitation of surface waves by earthquakes from the Delta region relative to earthquakes of comparable local magnitude in the Peninsular Ranges region. Data from distant stations are complemented with data from aftershock studies using local arrays and, for the Mesa de Andrade earthquake, with data from two strong motion stations. The strong motion records for the Mesa de Andrade earthquake show that it consisted of two events, 45 s apart, which produced maximum recorded horizontal accelerations of 0.21 and 0.24g, respectively, at a distance of 18 km from the epicentre. Synthetic seismograms are used to help determine the depth of these events and their source time functions. Both events are relatively simple. The second event is sharp and impulsive and could be thought of as an aftershock, since it has a considerably smaller moment than the first event, even though its recorded acceleration was higher. Comparison of measured parameters shows that while the local magnitude of the Mesa de Andrade earthquake is only 0.2 units larger than the Pino Solo earthquake and its source dimension is ∼ 2.2 times larger (source areas ∼ 4.8 times larger), its moment is larger by a factor of 6.2–8.5. This is approximately explained by the ω −2 scaling law. However, the near source spectra and accelerations recorded on the strong motion accelerograph at Riito, at a distance of about 18 km, are considerably larger than predicted by the ω −2 scaling law. This is probably a results of high stress drop asperities on the fault surface, although near field focusing (directivity) may also have contributed.