Teleseismic analysis of the 1980 Mammoth Lakes earthquake sequence
Bulletin of the SeismologicalSocietyof America,Vol. 72, No. 4, pp. 1093-1109,August 1982
T E L E S E I S M I C A N A L Y S I S OF T H E 1980 M A M M O T H L A K E S EARTHQUAKE SEQUENCE BY JEFFREY W. GIVEN, TERRY C. WALLACE, AND HIROO KANAMORI ABSTRACT The source m e c h a n i s m s of the three largest events of the 1980 M a m m o t h Lakes e a r t h q u a k e s e q u e n c e have been determined using surface waves recorded on the global digital seismograph network and the long-period body waves recorded on the W W S S N n e t w o r k . Although the fault-plane solutions from local data (Cramer and T o p p o z a d a , 1980; Ryall and Ryall, 1981) suggest nearly pure left-lateral strike-slip on north-south planes, the teleseismic w a v e f o r m s require a m e c h a n i s m with oblique slip. The first event (25 May 1980, 16 h 33 m 44 s) has a m e c h a n i s m with a strike of N12°E, dip of 50°E, and a rake of - 3 5 °. The second event (27 May 19 h 44 m 51s) has a m e c h a n i s m with a strike of N15°E, dip of 50 ° , and a slip of - 1 1 °. The third event (27 May, 14 h 50 m 57 s) has a m e c h a n i s m with a strike of N22°E, dip of 50 °, and a rake of - 2 8 °. The first: event is the largest and has a m o m e n t of 2.9 x 102s dyne-cm. The second and third events have m o m e n t s of 1.3 and 1.1 x 102s dyne-cm, respectively. The body- and surface-wave m o m e n t s for the first and third events agree closely while for the second event the body-wave m o m e n t ( a p p r o x i m a t e l y 0.6 x 102s dyne-cm) is almost a f a c t o r of 3 smaller than the surface-wave moment. The principal a x e s of e x t e n s i o n of all three events is in the a p p r o x i m a t e direction of N65°E which agrees with the structural trends a p p a r e n t along the eastern front of the Sierra Nevada.
INTRODUCTION Beginning 25 May 1980, a series of ML > 6 earthquakes occurred over a period of about 48 hr in the Mammoth Lakes region near the California-Nevadaborder. The sequence was part of a general increase in seismicity throughout California (McNa]]y, 1981) in the late 1970s. In addition, the earthquake sequence was preceded by 3 yr of local anomalous seismicity which Ryall and Ryall (1981) identified as precursory to strong earthquakes. The precursory phenomena as we]] as the interesting tectonic setting make this an important Californiaearthquake series. Figure I locates the epicenters of the three largest events and the extent of the aftershock area. Table 1 gives the epicenter parameters (taken from Cramer and Toppozada, 1980). In this study, we analyze the teleseismic records of the t h r e e largest e a r t h q u a k e s in the M a m m o t h L a k e s sequence. T h e y are all well r e c o r d e d worldwide on the global digital (IDA, SRO, and A S R O ) and analog ( W W S S N ) networks. We use the surface waves to construct an initial source m o d e l and to d e t e r m i n e the long-period seismic m o m e n t . T h e teleseismic P w a v e s were used to f u r t h e r constrain the source o r i e n t a t i o n and to isolate source complexity. TECTONIC SETTING T h e M a m m o t h L a k e s E a r t h q u a k e sequence occurred on the fault s y s t e m which b o u n d s the e a s t e r n front of the Sierra N e v a d a where it intersects the Long Valley Caldera. T h e geologic setting of the a r e a has b e e n discussed b y Bailey et al. (1976), and the m a j o r geologic features are s k e t c h e d in Figure 1. T h e caldera was f o r m e d b y a violent eruption 0.7 m.y. ago, and evidence indicates t h a t active volcanism has t a k e n place in the area as recently as 450 yr ago (Bailey et al., 1976). 1093
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T E L E S E I S M I C A N A L Y S I S OF T H E 1980 M A M M O T H L A K E S EARTHQUAKE SEQUENCE BY JEFFREY W. GIVEN, TERRY C. WALLACE, AND HIROO KANAMORI ABSTRACT The source m e c h a n i s m s of the three largest events of the 1980 M a m m o t h Lakes e a r t h q u a k e s e q u e n c e have been determined using surface waves recorded on the global digital seismograph network and the long-period body waves recorded on the W W S S N n e t w o r k . Although the fault-plane solutions from local data (Cramer and T o p p o z a d a , 1980; Ryall and Ryall, 1981) suggest nearly pure left-lateral strike-slip on north-south planes, the teleseismic w a v e f o r m s require a m e c h a n i s m with oblique slip. The first event (25 May 1980, 16 h 33 m 44 s) has a m e c h a n i s m with a strike of N12°E, dip of 50°E, and a rake of - 3 5 °. The second event (27 May 19 h 44 m 51s) has a m e c h a n i s m with a strike of N15°E, dip of 50 ° , and a slip of - 1 1 °. The third event (27 May, 14 h 50 m 57 s) has a m e c h a n i s m with a strike of N22°E, dip of 50 °, and a rake of - 2 8 °. The first: event is the largest and has a m o m e n t of 2.9 x 102s dyne-cm. The second and third events have m o m e n t s of 1.3 and 1.1 x 102s dyne-cm, respectively. The body- and surface-wave m o m e n t s for the first and third events agree closely while for the second event the body-wave m o m e n t ( a p p r o x i m a t e l y 0.6 x 102s dyne-cm) is almost a f a c t o r of 3 smaller than the surface-wave moment. The principal a x e s of e x t e n s i o n of all three events is in the a p p r o x i m a t e direction of N65°E which agrees with the structural trends a p p a r e n t along the eastern front of the Sierra Nevada.
INTRODUCTION Beginning 25 May 1980, a series of ML > 6 earthquakes occurred over a period of about 48 hr in the Mammoth Lakes region near the California-Nevadaborder. The sequence was part of a general increase in seismicity throughout California (McNa]]y, 1981) in the late 1970s. In addition, the earthquake sequence was preceded by 3 yr of local anomalous seismicity which Ryall and Ryall (1981) identified as precursory to strong earthquakes. The precursory phenomena as we]] as the interesting tectonic setting make this an important Californiaearthquake series. Figure I locates the epicenters of the three largest events and the extent of the aftershock area. Table 1 gives the epicenter parameters (taken from Cramer and Toppozada, 1980). In this study, we analyze the teleseismic records of the t h r e e largest e a r t h q u a k e s in the M a m m o t h L a k e s sequence. T h e y are all well r e c o r d e d worldwide on the global digital (IDA, SRO, and A S R O ) and analog ( W W S S N ) networks. We use the surface waves to construct an initial source m o d e l and to d e t e r m i n e the long-period seismic m o m e n t . T h e teleseismic P w a v e s were used to f u r t h e r constrain the source o r i e n t a t i o n and to isolate source complexity. TECTONIC SETTING T h e M a m m o t h L a k e s E a r t h q u a k e sequence occurred on the fault s y s t e m which b o u n d s the e a s t e r n front of the Sierra N e v a d a where it intersects the Long Valley Caldera. T h e geologic setting of the a r e a has b e e n discussed b y Bailey et al. (1976), and the m a j o r geologic features are s k e t c h e d in Figure 1. T h e caldera was f o r m e d b y a violent eruption 0.7 m.y. ago, and evidence indicates t h a t active volcanism has t a k e n place in the area as recently as 450 yr ago (Bailey et al., 1976). 1093
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